© 2020 Elsevier Ltd Carbon fiber reinforced thermoplastics (CFRTPs) are becoming of interest to mass production industries. In this study, we investigated the characteristics of the direct bonding technique to join an aluminum alloy and a CFRTP laminate by fabricating a nanostructure on the aluminum alloy surface. The effect of the nanostructure on the fracture toughness and the damage mechanisms were investigated. The nanostructure improved the fracture toughness by about 2.6 times compared with that without the nanostructure. From observations of the fracture surface, ductile failure of the matrix owing to the nanostructure occurred, suggesting that plastic deformation improved the fracture toughness. From X-ray computed tomography observations, intralaminar failure caused by the nanostructure occurred, which appeared to be a factor for the improved fracture toughness.

© 2020 Elsevier Ltd The current study presents a novel test method to experimentally cancel out the thermal stresses in dissimilar material joints. For the commonly used double cantilever beam test the presence of thermal stresses results in a significant mode mixity at the crack tip, which varies with applied load even if the elastic properties of the adherends are similar. This is particularly a challenge for fibre reinforced plastics bonded to metals due to the large difference in thermal expansion coefficients. The presence of mode-II loading is likely to provide a higher fracture energy from experiments than if tested under pure mode-I loading, which can lead to non-conservative results when using standard test methods. To overcome this challenge a novel test method inspired by the mixed mode bending test is developed. It is shown that the thermal stresses can be cancelled by applying initial constant loads during testing, and that the fracture toughness under pure mode-I loading can be obtained under specific conditions. The test method is validated by carrying out virtual compliance calibration experiments using cohesive zone finite element modelling. As the test method relies solely on analytical calculations and can be used with standard test equipment, it is relatively simple to apply in practice.

In recent years, for the aim of weight reduction of transportation equipment, carbon fiber reinforced thermoplastics (CFRTPs), which have high recyclability and formability, are becoming suitable for mass production. Additionally, with the development of multi-material structures, excellent technologies for joining metal and CFRTPs are required. In present industry, joining between dissimilar materials include adhesive bonding and mechanical joining methods, however, these methods still have some problems, and therefore an alternative bonding method without adhesive and mechanical joining is required for joining CFRTPs and metals. Thus, this study focused on direct bonding between CFRTP and an aluminum alloy, by producing a nanostructure on the surface of the aluminum alloy. The nanostructure penetrates the CFRTP matrix causing an anchoring effect, which results in significant bonding strength improvement. The influence of the nanostructure on the fracture toughness for the directly bonded CFRTP and aluminum was evaluated by static double cantilever beam (DCB) testing. Due to the difference of the thermal expansion coefficients between the CFRTP laminates and the aluminum alloy, significant residual stresses are generated. The effect of the thermal residual stresses on the fracture toughness along with the resulting mode mixity (mode ? and ?) was calculated. It is found that the thermal stresses introduce a significant mode mixity of the fracture toughness.

In this study, few-layer graphenes (FLGs) were produced by new liquid phase exfoliation (LPE), exfoliation of graphite with weak acid salts. A high concentration dispersion of FLGs in low-boiling point solvents is successfully carried out, achieved by binding molecules with a dispersing function. And then graphene/epoxy nanocomposites were fabricated and tensile properties were evaluated in order to understand the effect of FLGs introduced into the resin. Graphene/epoxy nanocomposite showed improved mechanical properties. Tensile strength and fracture strain were increased by 11.5% and 55.6% compared with as-received one's. Especially, the improvement of fracture strain was outstanding, which indicated that adding FLGs had a positive impact on suppressing effect on crack propagation and improving fracture toughness of matrix resin. These beneficial results were derived from crack trapping by uniformly dispersed FLGs.

© Springer Nature Switzerland AG 2020. Fan blades are subjected to very high-cycle loadings during the design life, so it is essential to evaluate the giga-cycle fatigue characteristics of carbon fiber reinforced plastic (CFRP) laminates. In this study, the transverse crack initiation of the cross-ply CFRP laminates in very high-cycle fatigue region was evaluated using an ultrasonic fatigue testing machine. The fatigue tests were conducted at the frequency of f = 20 kHz and the stress ratio of R = −1. In order to suppress temperature rise of the specimen, the intermittent operation with the loading time of 200 ms and the dwelling time of 2000 ms was adopted. The fatigue life data to transverse crack initiation in very high-cycle fatigue region was compared with the data of the fatigue test which was conducted at the frequency of f = 5 Hz and the stress ratios of R = 0.1 and −1 using a hydraulic control fatigue test machine. It was evaluated considering the influences of the stress ratio and the thermal residual stress by using the modified Walker model. The fatigue life to the transverse crack initiation of the cross-ply CFRP laminates in the very high-cycle region exceeding 108 cycles was on the extension of the test data in the low cycle region.

© Springer Nature Switzerland AG 2020. The present work studies the cohesive behaviour of a previously proposed novel direct bonding method for dissimilar bonding between a carbon fibre reinforced thermoplastic (CFRTP) and aluminium. A nanostructure is manufactured on the aluminium surface and is directly bonded to the CFRTP by applying heat and pressure. Double cantilever beam (DCB) testing is carried out to evaluate the bonding properties and the initial results of a method for directly measuring the traction-separation behaviour from experiments is presented. The nanostructure is observed to improve the bonding properties significantly compared to two other considered bonding cases. Furthermore, the measured traction-separation behaviour is seen to be difference for each case. Nevertheless, the applied calculation method shows some challenges related to thermal stresses and plastic deformation that should to be taken into account in future studies.

© 2020 Trans Tech Publications Ltd, Switzerland. The energy absorbing performance in the progressive failure of glass long-fiber-reinforced polyamide was evaluated by using the split Hopkinson pressure-bar method. An impact compression test of glass long-fiber-reinforced polyamide was performed from –30 °C to 90 °C, and the temperature-independent energy absorbing performance was confirmed only for the progressive failure mode. To clarify this phenomenon, compression tests, interlaminar compressive shear tests and mode-I fracture-toughness tests were conducted under static and impact conditions. The compression strength and the shear strength of all specimens decreased with an increase in temperature. The toughness improved with temperature. In addition to the mechanical tests, failure-mode analysis was performed by using a three-dimensional X-ray microscope to clarify the absorbing mechanism. From the above, it was concluded that the temperature-independent energy absorbing performance results from a balance of these mechanical properties against the temperature change.

© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. The current study presents a workflow to import a fibre bundle structure of a non-crimp fabric based fibre composite obtained by X-ray CT to a solvable 3D model in the finite element software ABAQUS. The considered fibre composite is similar to that used for the load carrying parts of wind turbine blades, and each layer of the non-crimp fabric contains fibre bundles oriented in the 0◦, 90◦, and ±45◦ directions. The 3D fibre bundle geometry is first segmented in the software AVIZO and then imported to Geomagic Wrap where the geometry is smoothened and converted into a nurbs surface that can be imported into ABAQUS. The resulting stress distribution is qualitatively compared to previous experimental observations and discussed.

The effect of long-term seawater immersion on the mechanical properties and fatigue life of plain-woven carbon-fiber-reinforced plastic (CFRP) laminates was investigated. Under tension loading, the static strength and low-cycle fatigue strength of the CRFP laminates greatly decreased owing to the rapid growth of interface debonding and delamination. However, under compression loading, the fatigue strength of the CRFP laminates dramatically decreased in the entire fatigue region as fiber budding was likely to occur. These findings indicate that the fatigue strength degradation differed depending on the stress ratio and deterioration of the fiber/matrix interface strength. In addition, the fatigue life after seawater immersion was predicted using Epaarachchi and Clausen's model and constant life diagrams. These models were shown to accurately reflect the decrease in fatigue strength resulting from seawater immersion and can thus be used for fatigue life prediction of CFRPs after long-term immersion in seawater.

With recent design developments in the automotive industry, it has become necessary to join dissimilar materials such as aluminum and carbon fiber reinforced thermoplastics (CFRTPs). In this study, a nanospike structure is fabricated on the surfaces of aluminum plates and directly bonded to CFRTP laminates. The effect of the nanospike structure on the adhesion strength is evaluated by single-lap joint tests. It is found that the nanospike structure improves the adhesion strength. Furthermore, combining the nanospike structure with a silane coupling treatment results in failure in the aluminum part of the single-lap specimens with an overlap length of 12.5 mm, rather than in the joined region. The average adhesion strength of the single lap joint specimens with an overlap length of 5.0 mm is found to be 24.9 MPa. Scanning electron microscopy observations of the fracture surfaces of the joined region only showed cohesive failure. On the fracture surface of the CFRTP laminate, the matrix exhibits a hairy structure due to the presence of the nanospike structure in some regions and in other regions carbon fibers are exposed due to adherend failure. Thus, in addition to an improved joint strength, the results suggest that the nanostructure will also improve the fracture toughness by causing ductile failure of the matrix.

The method for mass production of nanosheets is essential for fully realizing their commercial use. Direct exfoliation of layered materials in liquid is the simplest and low-cost technique for exfoliation without introducing defects in basal plane. However, the technique requires toxic and high-boiling point solvents, which makes it difficult for further processing. In addition, solvent exchange involves time-consuming processes such as filtering and redispersion. A breakthrough is necessary to make 2D-crystal-based functional dispersions. Here, we find a new method to directly exfoliate layered materials in low-boiling point solvents. Addition of small amounts of salt prevents reaggregation of exfoliated nanosheets and improves electrical repulsion. In this process, the mechanical force applied by sonication generates active carbon species at the fractured surfaces, leading to a reaction between the active carbon and the salt in liquid. High concentration graphene dispersions (1 g/L) can be obtained in isopropanol with only 5 min of sonication. We also confirm salt-assisted exfoliation is effective for the other 2D crystals such as MoS2 and boron nitride in a wide variety of polar solvents. Direct exfoliation in processable solution opens up a range of large-area applications such as high performance nanocomposites and coatings. (C) 2018 Elsevier Ltd. All rights reserved.

<p>CuO nanowires are known to have excellent electrochemical properties and can be produced by various methods. Among them a method which directly heat either the Si wafer with Cu layer or pure copper plate so called the thermal oxidation method has lately been drawing attention. However, during its production, interfacial delamination between the Cu layer and the wafer has always been an issue. In this study, the mechanism of interfacial delamination was experimentally elucidated by comparing a Si wafer with Cu layer deposited by electron beam evaporation with a pure copper plate. As a result, it was found that the deposited Cu layer decreased with increasing heating time of the Si wafer production. Moreover, the disappearance of the Cu layer and Cu₂O layer on the Si wafer was found in the specimens with a heating time of 6 hours. It was suggested that this disappearance would form an opening between the CuO layer and the Si wafer leading to the delaminating of the interface between the oxide film and the wafer. The disappearance also suggested that the length and density of the CuO nanowires in the specimens with a heating time of 6 hours were minimized. As for the pure copper plate, the disappearance of the Cu layer and Cu₂O layer was not seen regardless of the heating time, therefore there was no interfacial delamination and it was able to produce CuO nanowires at a high density.</p>

<p>For further application of carbon fiber reinforced plastics (CFRPs), it is necessary to obtain fatigue properties up to very high-cycle fatigue (VHCF) regime. In this paper, CFRP laminates were subjected to fatigue test up to 10⁹ cycle using an ultrasonic fatigue test machine in order to evaluate the transverse crack initiation life of laminates and the propagation behavior. In this experiment, T800S/2592 prepreg used as the cross-ply [0/90₆]_s laminates. Using an ultrasonic fatigue test machine tension-compression fatigue tests were conducted with stress ratio of R= -1 at a frequency of 20kHz. The specimen dimension was determined by finite element analysis to resonance at the vicinity of the test frequency. It was found that the test was carried out as designed, since the temperature distribution obtained by IR-thermography camera and the displacement measurement by the Laser Doppler vibrometer showed close agreement with the finite element analysis result. It was confirmed that the slope of the S-N diagram would be gradual in the VHCF regime.</p>

<p>In this study, graphene oxide was grafted to carbon fibers using chemical bonding and Electrophoretic Deposition (EPD) to improve the properties of the fiber / resin interface. Graphene oxide grafted on the surface of fibers is expected to improve the mechanical properties of Carbon Fiber Reinforced Plastics (CFRPs) by improving the adhesion and its interlock to resin. At that time, the influence of the grafted graphene oxide on the mechanical properties was investigated. As a result, it was confirmed that the interface properties of the fiber were improved without the fiber strength decreasing by the grafting. In particular, the interface shear strength showed the highest value even under the preparation conditions where a uniform graft morphology was observed, and showed an increase of 41.5% compared to the untreated fiber. Finally, the improvement mechanism of the interface properties was examined. It was suggested that the main factor of the improvement was that graphene oxide was dispersed in resin around the fiber and the physical property of the resin is improved.</p>

<p>The current study investigates the damage and strengthening mechanisms of a novel direct bonding method for joining aluminium and carbon fibre reinforced thermoplastic laminates. The bonding properties are studied using double cantilever beam (DCB) and end notched flexure (ENF) tests. The specimens bonded using the proposed direct bonding method are found to show significantly stronger bonding properties than the considered adhesively bonded specimens. The interface properties are further investigated by extracting the cohesive traction-separation behaviour from DCB and ENF experimental tests and used in the finite element software ABAQUS to simulate the interface behaviour. Good agreement between experiments and simulation is found for the DCB tests, however poor agreement is found for the ENF tests due to significant plastic deformation of the aluminium adherend occurring prior to and during crack propagation. Thus, plasticity will have to be avoided or taken into account in future studies, which is ongoing work.</p>

<p>It is important to enhance the fatigue life of machines and structures to prevent unexpected fracture. The technique which has influence on plastic deformation in slip bands formed in fatigue loading has been developed. Fatigue test of brass, C2600, was conducted in two stress amplitude level (σ_A/σ_B=0.4, σ_A/σ_B=0.8), and slip offset transition was measured by atomic force microscope. In this study, to investigate the effect of electric pulse current, electric pulse current of 50A/mm² was applied to specimens in the interval of fatigue test. As a result, in case of no pulse current, slip offset transition was continued to grow and grow rapidly when reached to specific number of cycle in both of stress amplitude level. On the other hand, when pulse current was applied slip offset in low stress amplitude(σ_A/σ_B=0.4) was tended to decrease and slip offset in high stress amplitude(σ_A/σ_B=0.8) did not grow rapidly. A slip band was emerged when pulse current was applied. So that, it is suggested that electric pulse current affected plastic deformation of a slip band. This effect could be caused by compression stress of Joule heating around dislocations and shear stress by electron wind force.</p>

<p>Carbon nanotubes (CNT) have remarkable mechanical properties and low density. Since length of the CNT is limited, CNT yarn is regarded as a reinforced fiber of carbon fiber reinforced plastics. However, the CNT yarn does not have remarkable mechanical properties such as the individual CNT. The most popular way for improving the mechanical properties of the CNT yarn is to make composites with polymer such as polyvinyl alcohol or polyimide. If some functional groups such as carboxyl groups are introduced on the surface of CNTs, interaction between the CNTs and the polymer is improved and high mechanical properties will be obtained. In this study, untwisted CNT yarns were prepared by drawing vertical aligned CNTs through a die and functionalized with mixed acid. Mixed acid introduced not only the functional groups but also defects on the surface of CNTs. For reducing the defects, the CNT yarn was graphitized at a temperature of 2800°C before the mixed acid treatment. By the graphitization treatment, crystallinity of the CNT yarn was improved and amorphous carbon was removed. As a result of XPS analysis, a graphitized CNT yarn treated with mixed acid did not contain the functional groups. On the other hand, a graphitized CNT yarn treated with hot mixed acid (90°C) contained the functional groups. Crystallinity of this yarn was 4.5 times higher than the as-received CNT yarn. In addition, as a result of single fiber tensile tests, tensile strength of this yarn was increased by 79 % and Young's modulus was increased by 173 % compared to the as-received CNT yarn.</p>

<p>Carbon nanotube (CNT) yarn is enables CNTs to be used on macro scale. However, the mechanical properties of CNT yarns are smaller than CNT itself, and improvement of the mechanical properties is a challenge for practical application. In this study, untwisted CNT yarns were fabricated by a dry spinning method, and the yarns were graphitized and combined with polymer for the purpose of development of CNT yarns with high strength. As a result of the graphitized treatment to the as-received yarns under inert atmosphere at 2800°C, impure materials and defect structure on CNTs were removed and strength of the yarn was increased by 19%. After combining the as-received yarns with polyacrylic acid (PAA), the strength was increased by 174% and reached 2.3 GPa. Breaking form of the yarns were changed from pulling out of CNT bundles to rapture of CNT bundles by graphitization and combining with PAA, indicating an increase interactive force between the CNT bundles. However, the strengthening effect was limited when graphitized CNT yarns were combined with PAA. As a result of molecular dynamics simulations, it was revealed force transfer capability of PAA was low when the graphitized yarns was combined with PAA. There were functional groups on as-received CNT such as carboxyl groups. On the other hand, the functional groups were removed from CNTs after the graphitization treatment. Consequently, interaction such as hydrogen bond between as-received CNT and PAA was removed by the graphitization, and it lead to the decrease of the force transfer capability of PAA.</p>

<p>Carbon fiber reinforced plastics (CFRPs) are widely used as components of marine structures. Thus, it is important to understand the degradation of the mechanical properties and its mechanism under seawater immersion. The object of this study is the influence of seawater immersion on the mechanical properties of plain woven CFRP laminates. Static tensile test and tensile fatigue test were carried out on the CFRP immersed different time under seawater for 300, 2500 and 5400 hours. The mechanical properties immersed for 300 hours was almost the same value compared with those of no immersion. However, the tensile strength immersed for both 2500 and 5400 hours reduced by 22.5% compared with that of no immersion. Then, from the fatigue results, in the low-cycle fatigue region, the fatigue strengths decreased as immersion time was longer, on the other hand, in the high-cycle fatigue region, the fatigue strength did not change significantly regardless of immersion time. As a result, the inclination of S-N curves became gentle as immersion time was longer. From observation of fracture surfaces by scanning electron microscopy (SEM), it was shown that the fiber/matrix interface deteriorated remarkably after seawater immersion. Moreover, the difference of damage growth behaviors due to immersed in seawater under fatigue loading was investigated using soft X-ray photography. On specimen immersed in seawater, the accumulation of damage expanded more widely due to interface degradation compared with that of no immersion. Considering these results, it was suggested that the static tensile strength depended on load transmission efficiency between fiber and matrix, on the other hand, the fatigue strength in high cycle fatigue region depended on the strength of fiber along 0° that had small influences by seawater immersion.</p>

Healing technology for metallic materials is an important subject in terms of long-term reliability and durability of structural members, a healing technology to heal fatigue crack by applying heat treatment at annealing temperature level has been discovered. In this study, the influences of plasticity induced crack closure on healing were evaluated by obtaining the crack opening load during the pre-crack introduction and evaluating the fatigue crack propagation characteristics before and after the healing heat treatment, using compact tension specimens made of carbon steel with different test conditions. As a result, the specimen with high crack opening load showed high healing effect and were able to heal up to 95% of the pre crack length. This suggested that the residual compressive stress due to the plasticity-induced crack closure accelerates the solidstate diffusion bonding during the crack healing process and this leads to the improvement of the healing effect.

Research and development of SiC/SiC composite materials as structural members of aerospace engines is progressing. In order to manufacture SiC/SiC composites with excellent high-temperature characteristics, the SiC fibers which have high mechanical properties at high temperature are necessary; thus, further development of SiC fibers is considered a critical issue. In addition, the development of low-cost SiC fibers is necessary for the practical application of SiC/SiC composites. Here, the low-cost SiC fibers can be fabricated by dry spinning method. In the dry-spinning method, the raw material, Polycarbosilane (PCS) is dissolved in an organic solvent and the solution is spun at room temperature. As high-molecular-weight Polycarbosilane is prepared in advance, the infusible process conventionally required in the melt-spinning method is not required. In this study, to evaluate the differences among dry-spun SiC fibers fabricated under various conditions, monofilament tensile tests were conducted. Examination of the fracture surface and elemental analysis of arbitrary cross-sections were then performed to investigate the effects of the fabrication conditions. The tensile strength results indicated that defects were suppressed by excluding low-molecular-weight components and that heat treatment between 1300 degrees C and 1500 degrees C resulted in the maximum strength. Weibull analysis revealed that the dry-spun fibers exhibited lower tensile strength but smaller variation of fiber strength than that of the melt-spun fiber because the dry-spun fibers were more homogeneous. However, evaluation of the crystallinity indicated that the interference pattern derived from the crystal was unclear in the dry-spun fibers but clear in the melt-spun fiber. Therefore, it was suggested that the dry-spun fibers exhibited lower crystallinity than the melt-spun fiber. In addition, the dry-spun and melt-spun fibers exhibited similar C/Si ratios, whereas a large amount of oxygen was detected on the surface of the dry-spun fiber relative to that on the surface of the melt-spun fiber. Further improvement of the mechanical properties is expected upon increasing the molecular weight of the raw material and improving the microstructure.

The data published with this paper is obtained during fatigue testing of a unidirectional non-crimp fabric based glass fibre composite by means of ex-situ X-ray CT and in-situ transilluminated white light imaging experiments. The data experimentally shows the damage initiation and progression under fatigue loading both in terms of off-axis cracks in the thin supporting backing fibre bundles and fibre fractures in the load carrying fibre bundles. X-ray CT data comparing the loaded and unloaded state of damage regions by means of a tension clamp solution are also published with this paper. (C) 2018 The Authors. Published by Elsevier Inc.

© Published under licence by IOP Publishing Ltd. The current study presents a direct bonding method making it possible to obtain a high interface strength of aluminium joined to carbon fibre reinforced thermoplastic (CFRTP) plates by hot pressing. This is achieved by subjecting the aluminium to a combination of anodising, etching, and silane-coupling treatments prior to bonding. Different types of aluminium are subjected to different treatments and bonded to different types of CFRTP laminates. The effect of the surface structure on the static bonding strength and fatigue life measured by single-lap testing is compared and discussed. The bonding strength is found to be highly dependent on the anodisation conditions along with the type of thermoplastic resin.

Carbon fiber reinforced plastic (CFRP) laminates are used as main structural members in many applications. Transverse cracks that form in 90 degrees layers of CFRP laminates are mostly initial damage in the case where tensile loading is vertically applied to the 90 degrees layers of CFRP laminates, and they are the origin of more serious damage of delamination and fiber breakage. It is thus important to predict quantitatively the transverse crack initiation of CFRP laminates subjected to cyclic loading to ensure the long-term reliability of the laminates. The initiation and multiplication behaviors of transverse cracks strongly depend on the laminate configuration, thickness, and thermal residual stress. Therefore, a model based on the Walker model was proposed to predict transverse crack initiation in CFRP cross-ply and quasi-isotropic laminates under cyclic loading in the present study. The usefulness of the proposed model was verified with 10 different CFRP laminates formed from four different prepregs with epoxy resin matrices. The analysis results were in good agreement with experimental results. The fatigue life was expressed with three constants, which related to the fatigue strength reduction, the normalized fatigue strength at N = 1 cycle, and the contribution of stress amplitude to the fatigue life, and they are independent of the laminate configuration.

The current work studies the fatigue damage initiation and progression in a quasi-unidirectional non-crimp fabric based fibre composite used for wind turbine blades. This is done by combining in situ transilluminated white light imagining (TWLI) with ex-situ X-ray computed tomography (CT) experiments along with tension clamp X-ray CT experiments. TWLI is used to monitor the off-axis cracks in the thin supporting backing fibre bundles present in quasi-UD composites, and a crack counting algorithm is applied to automatically count the cracks in images obtained in situ during fatigue testing. It is found that off-axis cracks not only initiate at the specimen edges but also at isolated locations inside the specimen, which could be related to the microstructural features. In addition, a clear effect of strain level on the measured off-axis crack density is observed. From the Xray CT experiments, it is found that the UD fibre fractures initiate and progress from regions where the off-axis backing fibre bundles are 'in contact' with a UD fibre bundle. Damage is seen to first initiate at a cross-over region of the backing fibre bundles, and later at a region with only one backing fibre bundle. In addition, applying tension to the specimen during X-ray CT scanning is found to reveal additional UD fibre fractures that are not visible in scans performed the unloaded state. With load applied, a significant number of UD fibre fractures were observed earlier in the fatigue life than expected. Based on the observations of the study a damage progression scheme is presented for quasi-UD fibre composites.

<p>Carbon nanotube yarn, which is an aggregation of Carbon nanotubes (CNTs), enables CNTs to be used on macro scale. However, tensile strength of the CNT yarn is much lower than CNT itself, and improvement of the mechanical properties is a challenge for practical application of the yarns. CNTs composing the yarns include some defect structures and impure materials, and their presence can cause a decrease in tensile strength of the yarn. In this study, untwisted CNT yarns were fabricated by dry spun method and graphitized at a temperature of 2800°C for the purpose of improving mechanical properties. Additionally, strength of a CNT composing the yarn and interactive force between the CNTs were evaluated to clarify strength development mechanism of the graphitized CNT yarn. Crystallinity of the CNT yarn was improved more than ten times, and defect structures and impure materials was removed by the graphitization treatment. As a result of single fiber tensile tests, strength of the yarn was increased by 20~35% after the graphitization. On the other hand, strength of the CNT composing the yarn was decreased. The breaking form of the CNT yarn was changed from pulling out of CNT bundles to rapture of the bundles by the graphitization, indicating an increase of interactive force between the bundles. In addition, as a result of pulling out simulations by molecular dynamics method, it was indicated that the pulling out of the CNT bundles were suppressed by increase in van der Waals force. Consequently, the improvement of mechanical properties of the untwisted CNT yarns was not due to the change in the strength of the CNT in the yarns but the main cause was the increase in the interactive force between the CNTs.</p>

<p>Tensile strength properties in the transverse direction of carbon fiber reinforced thermoplastic (CFRTP) laminates with in situ polymerizable phenoxy resin were evaluated. The unidirectional [90]14 laminates were formed using the prepreg, NS-TEPreg. By changing the conditions of fabricating CFRTP laminates, the void content in the laminates and the degree of polymerization of the matrix resin were varied. As the results of tensile tests and image analysis, it was revealed that when the void content inside the laminates is 0.5% or less, the influence of the void on tensile strength properties becomes limited. Also, as the results of measuring the average molecular weight of CFRTP laminates, it became clear that the transverse tensile strength σbT and transverse failure strain εbT increase as the average molecular weight of the matrix resin increases.</p>

<p>Establishment of fatigue crack healing technology for metallic materials is important to improve the safety and life of machinery and structures. Therefore, the authors developed a novel fatigue crack healing technique removing the oxide film on the crack surface by heat treatment and using plasticity induced crack closure generated during fatigue crack growth as a driving force of the healing. It was found that the resistance of the fatigue crack growth after crack healing was larger as the effect of the plasticity induced crack closure was larger.</p>

<p>The effect of stress ratio on through thickness fatigue properties of thick carbon fiber reinforced plastic (CFRP) laminates with toughened interlaminar layers was evaluated. The unidirectional (UD) [0₈₈] and quasi-isotropic (QI) [45/0/-45/90]_11S laminates were formed using prepregs (T800S/3900-2B) with toughened interlaminar layers. The spool shaped specimens were cut from the laminates. Static tensile and compressive tests were performed. As the results of the static tests on both laminates, the through thickness compressive strength was more than five times higher than tensile strength. The fracture morphology under compressive loading was difference between each laminate. Fatigue tests were performed under the stress ratio of R=0.1,-1,-3 and -6 on both laminates. As the results of the fatigue tests on both laminates, the fatigue life decreased as the stress ratio was lower. On the other hand, the remarkable difference of the fracture surface was not observed under each fatigue test condition by both macroscopic and microscopic observation in this study. The fatigue life of UD and QI specimens was able to be evaluated by the proposed model, the modified H-κ model based on strain energy approach. The predicted fatigue life was good agreement with the experimental results.</p>

<p>Fiber reinforced thermoplastics (FRTP) attracts attention as a lightweight material for mass-produced automobiles due to advantages such as excellent formability and recyclability in addition to high specific strength and specific rigidity. Many factors influence the improvement of the mechanical properties of FRTP. We have revealed that the tensile strength of injection-molded glass fiber/polyamide66 (GF/PA) improves as the reinforcement fiber diameter decreases in the wide strain rate range in previous researches. In addition, it was suggested that by using a strength prediction model of discontinuous fiber reinforcement, if the strength can be obtained for one diameter, the strength can be predicted for other diameters. In this study, we investigated the mechanism of the strength enhancement of the GF/PA associated with thinning of the reinforcement fibers and increase of the strain rate. To ascertain the interface properties, single-fiber pull-out tests were conducted on the GF/PA and it was confirmed that the interfacial shear strength (IFSS) of the GF/PA had strain rate dependency. The strength prediction of the GF/PA with the strain rate dependency of IFSS taken into account showed values closer to the experimental values than the values obtained from the prediction with an assumption that IFSS is constant regardless of the strain rate. Furthermore, investigating the factor of the increase in strength due to the thinning of the fibers suggested that the increase of the fiber strength is the dominant factor.</p>

<p>Carbon nanotube yarn, which is an aggregate form of carbon nanotubes (CNTs), is expected to be practically used as a lightweight wiring material. In recent years, CNT/metal composite yarn has been fabricated for making the CNT yarn highly conductive. While improving conductivity in the CNT/metal composite yarns, current capacity, which determines durable current value of the wiring material, has not reached practical value. In this study, composite material of untwisted CNT yarn and copper was fabricated by plating treatment for the purpose of creating a lightweight wiring with larger ampacity than metal wiring. In addition, CNT/copper composite yarns having different composite structures were fabricated and the relationships between the composite structure and electric characteristics were evaluated. One of the composite yarns had a two-layer structure in which copper was deposited on the surface of the CNT yarn, and the other had a structure in which copper precipitated to the inside of the CNT yarn. As a result of the plating treatment using a copper sulfate bath, current capacity of the composite yarn reached 6.87×10⁸ A/m² at the copper volume fraction of 28.9%, and the specific current capacity was 1.29 times larger than copper wire. From evaluation of the fracture mechanism, it was revealed that combustion of the CNTs and melting of the metal part were suppressed by combining the CNT yarn with copper, and it led to the large ampacity. In addition, it was possible to electroplate inside of the yarn by adding a dipping step to the plating process. In the case of the composite yarn plated the inside, an increase in resistance under large current was suppressed and further improvement of the current capacity was achieved.</p>

<p>The energy absorbing performance of glass fiber reinforced thermoplastics was evaluated by progressive crushing tests with the Split Hopkinson Pressure Bar (SHPB) apparatus. Two types of specimens, one was injection molded glass long fiber reinforced polyamide 66 (GF/PA66), the other was twill weave glass fiber reinforced polyamide 6 (GF/PA6), were prepared as specimens. As for injection molded plates of the GF/PA66, it was investigated the influence of the fiber orientation on the impact mechanical behavior of the test specimen cut out from the different position of the plate. Also, the impact behavior of injection molded the GF/PA66 plates and that of twill weave GF/PA6 laminates were compared in order to investigate the influence of the variation of reinforcing types. As a result of the progressive crushing tests at -30, 23, 90°C, it is revealed that the energy absorbing performance has no temperature dependency regardless of the fiber orientation and the reinforcing types. The reason that the SEA of the GF/PA66 increases with the mechanical properties was discussed from the comparison of the specific energy absorption (SEA) and the compressive strength. In the comparison of injection molded the GF/PA66 and the GF/PA6 laminates, the specimens were reinforced by the different mechanisms; therefore, it seems that influence of mechanical properties on the SEA is not critical because those fracture modes are different.</p>

<p>In recent years, applications of fiber-reinforced plastics (FRPs) to large marine structures are expected to improve their performances due to the high specific strength and stiffness and the corrosion resistance. However, the creep lifetime of FRPs in seawater environment has not been made clear so far. Objective of this study is to predict the creep lifetime of FRPs in seawater environment based on the accelerated test results and reveal the fracture mechanism. In this study, creep tests were carried out using the plain-woven GFRP and CFRP laminates under seawater environment and time-temperature superposition principle (TTSP) was used to predict the long-term creep rupture life. It was shown that the rupture time decreased with increase of the seawater temperature and the applied stress, and it was suggested that decrease of the strength was mainly caused by degradation of the interfacial shear strength between the fibers and the matrix. By using Larson-Miller parameter (LMP) as time-temperature parameter, the analytical results showed good agreement with the experimental results. For the GFRP laminates, behavior of the prediction curves varied in the lower applied stress because the glass fibers deteriorated in seawater. On the other hand, deterioration of carbon fibers was not observed in seawater so that the rupture time showed a consistent tendency to decrease for the CFRP laminates.</p>

<p>Ni-base superalloy Inconel 718 is used in structural members of aircraft engine parts. Its mechanical properties are deteriorated due to fatigue cracks at high temperature environment, but the processability of the material is poor and it is not easy to repair micro cracks. In this study, the fatigue crack healing technique for Inconel 718 was developed by controlling heating and cooling conditions in a furnace. Especially, the effect of the atmosphere on the fatigue crack healing was investigated. After the pre-fatigue crack was introduced using compact tension (CT) specimens, they were heated in vacuum or hydrogen atmosphere for the crack healing. The behavior of fatigue crack growth was evaluated before and after crack healing. As a result, the fatigue crack was successfully healed by heat treatment in hydrogen atmosphere whereas it was not healed in vacuum. The dimples which were characteristic fracture patterns in solid diffusion bonding were observed on the fracture surface of fatigue crack after crack healing treatment in hydrogen atmosphere. Thus, it was thought that the fatigue crack was healed because the hydrogen gas that has strong reduction power removed oxide film on the crack surfaces and solid diffusion bonding caused between the crack surfaces.</p>

<p>It is important to enhance the fatigue life of machines and structures to prevent their accidents. The healing technique of fatigue crack in metallic materials by heat treatment has been developed. A pre-fatigue crack was introduced using a compact tension (CT) specimen of austenitic stainless steel, SUS316, and the fatigue crack growth behavior was investigated before and after crack healing heat treatment. In this study, to investigate the effects of cooling atmosphere and rate on the crack healing treatment, air cooling, N₂ gas cooling, rapid cooling in vacuum and slow cooling in vacuum were applied to the specimens. As the results, the healing effect was improved in the specimen cooled rapidly in a vacuum furnace. It was thought that the fatigue crack was healed during cooling process because the fatigue crack healing by air cooling was not done enough due to the oxidization on the crack surfaces. The healed crack length under the condition of slowly cooling in vacuum was improved. It was thought that there was longer time for atomic diffusion between the crack surfaces. U-shape boundary was observed on the fracture surface of the fatigue crack after the crack healing. The detailed observation with scanning electron microscopy (SEM) revealed that the U-shape boundary corresponded to the healed area on the fracture surface. This result suggested that the plasticity induced crack closure caused by fatigue crack growth was one of the driving forces for the crack healing.</p>

<p>The influence of long-term seawater immersion on the mechanical properties of plain woven CFRP laminates was evaluated by static tensile and tension fatigue tests. These tests were carried out on the CFRP immersed different time under seawater: 2566 hours and 4967 hours. The mechanical properties of the specimen for 2566 hours immersion decreased drastically; the tensile strength, the elastic modules and the fatigue strength at 10⁶ cycles reduced by 21.3%, 12.3% and 30.7% compared with those of no immersion specimen, respectively. On the other hand, after 4967 hours immersion, it was found that the mechanical properties decreased slightly. From observation of the Scanning Electron Microscopy (SEM), it was clear that morphology of the fiber/matrix interface varied with the immersion time. However, there was a small difference on the interface fracture surface between 2566 hours and 4967 hours. These results indicated that degradation of the fiber/matrix interface progressed dramatically by 2566 hours, and after that, interface degradation gradually converged. Damage growth behavior under fatigue loadings was investigated by non-destructive inspections using soft X-ray photography and ultrasonic inspection. At first, matrix cracks occurred along 90° and ±45° fiber directions. After that, delamination occurred originating from the matrix crack. Then, these damages expanded as the number of cycles increased, and finally, the specimen broke as the longitudinal fiber in the 0°/90° layers damaged.</p>

<p>Recently, Indium tin oxide (ITO) has been widely used as transparent conductive films (TCFs) in various electronic devices. However, ITO has many drawbacks such as high cost and brittleness. Nowadays, graphene is the ideal alternative for ITO because of its excellent transparency, electrical conductivity and mechanical flexibility by the effect of sp² hybridized orbital. However, the electrical properties of graphene TCFs are inferior to those of ITO so additional processing to improve electrical properties is required. Achieving the purpose, there is a number of studies on composite graphene and conductive materials such as Ag. And now, new simple, low-cost methods are required. In this study, we used the liquid plasma method to combine graphene and Ag. This is a low-cost, simple method for the hybridization. Then we made graphene / Ag hybridized TCFs using thin layer graphene-supported Ag and evaluated some characteristics. Finally, we acquired transmittance and sheet resistance of the graphene / Ag hybridized TCFs.</p>

<p>The effects of environment temperature on initiation and multiplication of transverse crack in cross-ply carbon fiber reinforced thermoplastic (CFRTP) laminates have been investigated. Static tensile tests for the cross-ply laminates and the 90° unidirectional laminates were carried out at room temperature, 93 °C and 130 °C, respectively. The transverse cracks were observed by soft X-ray photography. The tensile strength and the failure strain in the cross-ply laminates and the 90° unidirectional laminates at high temperature decreased compared to the values at room temperature. It was also found that the behavior of initiation and multiplication of the transverse cracks in the cross-ply laminates was changed due to the environment temperature. The experimental results under different temperature were analyzed by Weibull distribution on the basis of probabilistic model. Next, the energy release rate was calculated due to formation of a new micro crack based on the Weibull distribution. The predicted transverse crack density by Weibull distribution was compared with the experiment result and the reasonability of using Weibull distribution to CFRTP cross-ply laminates under high temperature was verified. It was found that the critical energy release rate of CFRTP laminates has decreased at high temperature and the experimental results showed that the matrix strength was decreased at high temperature. Also, the fiber-matrix interfacial fracture on the fracture surface of the 90° unidirectional laminates was observed in some areas at high temperature whereas the matrix fracture was observed at room temperature. Therefore, it was suggested that the interface strength between polymer and fiber was decreased at high temperature.</p>

<p>Fatigue properties of the thick carbon fiber reinforced plastic (CFRP) laminates with toughened interlaminar layers in the out-of-plane direction (Z direction) and in the in-plane transverse direction (T direction) were evaluated experimentally. Spool specimens were machined from the thick mother plates which were laminated prepregs of T800S/3900-2B unidirectionally. The specimens were attached to metal tabs to apply loads in the thickness direction of the specimen. The tensile strengths in Z and T direction were measured by static tensile tests and S-N curves were obtained by fatigue tests at a stress ratio of R=0.1. As the results, the tensile strength in Z direction was 24% lower than that in T direction. Fatigue strength in Z direction at 10⁶ cycles was also 25% lower than that in T direction. It was observed using a digital microscope that the fracture occurred in intralaminar layers in both static tensile tests and fatigue tests in Z direction. The thermal residual stress which was generated during the fabrication process and the stress distribution by mechanical loadings in spool specimens were calculated by finite element analysis. The calculated results showed that compressive residual stress applied in intralaminar layers in T direction by restraining the thermal deformation. It is found that the static tensile and fatigue properties in Z direction were almost the same as those in T direction by evaluating with the stresses applied in the nearest intralaminar layer to the minimum cross-section in the spool specimen.</p>

Carbon fiber reinforced plastics (CFRP) pipes are expected to substitute for steel drive shafts to improve motorcar's fuel efficiency and driving performance. The static torsional strength of CFRP pipes formed by a modified simultaneous multi ply winding method is 20% higher than that of CFRP pipes formed by a filament winding method owing to few initial flaws. As the results of static torsional tests regarding [90/-45/+45]6 pipes, it was revealed that the delamination from the prepreg end occurred in the innermost layer and propagated in the interlaminar area of the -45°/+45° plies before the final failure. It is expected to design stacking sequence for preventing the delamination. In this study, effects of stacking sequence on the static torsional properties of the CFRP pipes were investigated. [902/-45/+45]6 and [90/-45/90/+45]6 pipes were formed to investigate effects of lamination angle difference between adjacent plies. Maximum lamination angle difference of the [90/-45/90/+45]6 pipe is smaller than that of the [902/-45/+45]6 pipe. In case of the [90/-45/90/+45]6 pipes with small lamination angle difference, the initiation of the delamination was delayed because the interlaminar stress was reduced. Furthermore, [(90/-45/90/+45)6/90] pipes were formed to investigate effects of an application of a 90° layer on the innermost layer. The delamination from the prepreg end did not occur before the final failure by the application of a 90° layer on the innermost layer since the applied load on the prereg end was reduced. Finally, the static torsional strength of the [(90/-45/90/+45)6/90] pipes was 25% higher than that of the [902/-45/+45]6 pipes due to improvement of delamination resistance.

© 2016, European Conference on Composite Materials, ECCM. All rights reserved. The fatigue life of thick carbon fiber reinforced plastic (CFRP) laminates with toughened interlaminar layers in the out-of-plane direction was evaluated at different stress ratios. The spool shaped specimens were cut out the unidirectional thick CFRP laminates which pile up 88 plies of the prepreg with toughened interlaminar layer, T800S/3900-2B. The fatigue tests were conducted under the stress ratios of R = 0.1, -1, -3 and -6 to evaluate the effect of the stress ratio. As the results of the fatigue tests, the fatigue life of the specimens became shorter as the stress ratio became smaller, i.e. as the absolute values of the compressive stress became higher. It was found that the fatigue properties of the CFRP laminates in the out-of-plane direction are affected by the stress amplitude from the experimental results. In addition, the fatigue life under the different stress ratio was evaluated equivalently using the proposed model, modified H-κ model, which considers the strain energy. The analytical results showed good agreement with experimental results. In addition, the fatigue properties of the thick CFRP laminates in the out-of-plane direction were evaluated with the constant fatigue life diagram derived from the proposed model.

© 2016, European Conference on Composite Materials, ECCM. All rights reserved. Carbon nanotube (CNT) is increasingly applied as a reinforcement of polymer matrix composite because of extremely high mechanical properties. Among several forms of CNT reinforcement, CNT yarn can be a next-generation reinforcement which enables the use of CNTs in the macro-scale. In this study, untwisted CNT yarns were fabricated by the dry spinning method using a die and graphitized in order to develop CNT yarns with high strength and stiffness. Impure materials and defective structures on MWCNTs were removed after graphitization treatment. G/D ratio was improved more than 10 times. Stress transfer between constituent MWCNTs of CNT yarns became more effectively by removing impure materials. Consequently, macroscopic mechanical properties of CNT yarns were improved by the graphitization treatment in addition to improvement of MWCNT itself.

<p>The effects of environment temperature on initiation and propagation of transverse crack in cross-ply carbon fiber reinforced thermoplastic (CFRTP) laminates have been investigated. Static tensile tests for the cross-ply laminates and the 90° unidirectional laminates were carried out at room temperature and high temperature (366K). The transverse cracks were observed by soft X-ray photography. In consequence, the tensile strength and the failure strain in the cross-ply laminates and the 90° unidirectional laminates at 366K decreased compared to the values at room temperature. It was also found that the behavior of initiation and propagation of the transverse cracks in the cross-ply laminates was changed due to the environment temperature. Fiber-matrix interfacial fracture on the fracture surface of the 90° unidirectional laminates was observed in some areas at high temperature whereas matrix fracture was observed at room temperature. Therefore, it was suggested that the interface strength between polymer and fiber was decreased at high temperature.</p>

<p>The energy absorption mechanism on progressive crushing of injection molded fiber reinforced thermoplastics (FRTP) have been investigated with triggered coupon specimens. These materials consist of glass fiber and polyamide 6,6. Progressive crushing tests were carried out at -30, 23 and 90°C. The energy absorption performances on progressive crushing were constant independently of the temperature nevertheless the impact compressive strength decreased as the temperature rises. In the observation of fracture surface, matrix was stretched in the direction of shear. Furthermore, in the observation of fracture morphology using high speed camera, plural crack were formed during crushing. According to the above results, it was suggested that mode I/II fracture toughness affect the energy absorption performance.</p>

<p>Transparent conductive films (TCFs) are widely used in various electronic devices. In addition, due to the excellent transparency (T=97.7%) and electrical conductivity by the effect of sp2 hybridized orbital, using graphene for the materials of TCFs is ideal. Typical method for manufacturing graphene TCFs is chemical vapor deposition (CVD) method. However, CVD method takes a high cost. On the other hand, liquid phase exfoliation (LPE) is the method for obtaining thin-layer graphene by peeling graphite in the organic solvent. In the LPE, method of making graphite oxide and peeling graphite is often used, but this method has problem that falling down conductivity due to structural defect in graphene. In this study, we used pressure homogenizer to obtain thin-layer graphene without using chemical treatment. Then, we made TCFs by using thin-layer graphene and evaluated some characteristics. Finally, we investigated transmittance and sheet resistance of the TCFs.</p>

<p>Carbon nanotube (CNT) is attracted a lot of attention for new conductors because of its high current capacity, comparing with copper. In past study, carbon nanotube-copper (CNT-Cu) composite sheet was developed, which exhibiting similar conductivity as copper, but with a 100-times higher current capacity. However, since the length of CNT-Cu conductors was limited to several hundred μm long, the problem that using CNT with high electrical properties macroscopically remains to be solved. Therefore, in this study, untwisted CNT yarn was prepared by drawing multiwall carbon nanotube through die and was electroplated to realize metal CNT composite yarn with high electric conductivity and current capacity. For the influence of copper oxide layer, the electrical property of copper CNT composite yarn (CNT-Cu) showed a poor improvement from the untreated CNT yarn. On the other hand, both conductivity and current capacity of gold CNT composite yarn (CNT-Au) showed a higher value by the heat treating at 800°C, comparing with that of untreated CNT-Au.</p>

<p>Infrastructures built in the period of high economic growth have been used over their design life and the accidents due to aging and fatigue fracture become a serious problem. Therefore, the technique of healing fatigue cracks in metallic materials was developed by heat treatment. In this study, the effect of the cooing rate during crack healing treatment on the properties of fatigue crack growth was investigated using austenitic stainless steel, SUS316, and low carbon steel, S25C. The specimens were cooled with nitrogen gas or oil to evaluate the effect of cooling rate. As a result, the crack healing effect of SUS316 was improved by rapid cooling with oil. Although the delay of crack growth of S25C was observed, the cause was due to the change of the metallographic structure by rapid cooling. The dimples were observed on the fracture surface of fatigue crack after crack healing treatment. Thus, it was thought that the crack healing was caused by solid diffusion bonding.</p>

Applications of fiber reinforced plastics have been expanding due to improvement of not only fuel efficiency but also the motion performance of some recent vehicles. Especially, the demand for injection-molded fiber reinforced thermoplastics is expected to increase because of their superior moldability, productivity and recyclability. In this study, the influence the fiber diameter has on the impact tensile properties of long glass-fiber reinforced polyamide (GF/PA) is investigated using the split Hopkinson pressure bar method. Prior to the tensile tests, an investigation of the fiber-orientation distribution was conducted in order to cut out specimens with the same fiber orientation angle from the injection molded plate. Two types of specimens, referred to as specimens with high- and low orientation angle, were manufactured using glass fibers with average diameters of 13, 17, 23 μm. In the tensile test, the GF/PA with smallest fiber diameter showed the highest tensile strength and the most significant strain rate dependency on the strength. These effects were more significant for the specimens with high orientation angle. From SEM observations on the fracture surface and an average fiber length measurement, it was observed that the interfacial fracture and the fiber breakage were dominant failure modes under the considered tensile loading conditions. It was suggested that decreasing the stress acting on the fiber/matrix interface by reducing the fiber diameter affected the improvement of the GF/PA strength. Using the modified linear rule of mixtures, the tensile strength was predicted. The predictions showed good agreement with experimental results. Therefore, it is believed that the decrease of critical fiber length is the reason that the impact tensile properties are higher for the samples with smaller fiber diameter.

Application of carbon fiber reinforced plastics (CFRP) pipes to torque transmission shafts makes it possible to improve automotive driving performance as well as fuel efficiency. A modified simultaneous multi ply winding method was developed as a new forming method for CFRP pipes using prepregs. The CFRP pipes formed by the method have fewer initial flaws, such as voids and fiber waviness. It resulted in 20 % increase in the static torsional strength than conventional CFRP pipes formed by a filament winding method. In this study, effects of stacking sequence on the torsional fatigue properties of the CFRP pipes formed by the modified method were investigated. Torsional fatigue tests were conducted under load control. All tests were conducted at the test frequency f=1 Hz, the maximum applied torque Tmax=1.0 kNm and the stress ratio R=0.1. [902/-45/+45]6 and [90/-45/90/+45]6 pipes were formed to investigate effects of lamination angle difference between adjacent plies. Maximum lamination angle difference of the [90/-45/90/+45]6 pipe is smaller than that of the [902/-45/+45]6 pipe. When the CFRP pipes had smaller lamination angle difference, the initiation and growth of the delamination from the prepreg end in the innermost layer were delayed because interlaminar stress was reduced. Moreover, [(90/-45/90/+45)6/90] pipes were formed to investigate effects of an application of a 90° layer on the innermost layer. As the result, the delamination hardly initiated since the shear stress on the prereg end was reduced. Consequently, the fatigue life of the [(90/-45/90/+45)6/90] pipes increased seven-fold than that of the [902/-45/+45]6 pipes due to improvement of delamination resistance under cyclic torsional loading.

Combining composites with nanoscale materials such as Carbon Nanotubes (CNTs) has attracted much attention, because various synergic effects can be realized easily. Especially, grafting CNTs onto carbon fibers and reinforcing the interface between resin and the fibers are promising approaches in enhancing the mechanical properties. In this study, the effects of CNTs grafted onto carbon fibers on the mechanical properties of multiscale CFRP were investigated. CNTs were grafted on carbon fibers by chemical vapor deposition (CVD) using a Fe-Cu catalyst system at temperature <600 ℃. As a result, degradation of the properties of the carbon fibers was suppressed. Interfacial shear strength (IFSS) was increased by 45.8%. After that, static tensile test of unidirectional CFRP (0° and 10°) was performed. The elastic modulus was increased by CNTs around the carbon fibers. In case of the 10° specimens, the shear yield stress increased by 41.9%. It was the result from improvement of the yield strength of matrix resin around the carbon fibers by grafting CNTs.

In this study, we synthesized cup-stacked carbon nanotubes (CSCNT) supported platinum nanoparticles (PtNPs) using liquid phase plasma treatment, and morphology, particle size distribution and supported amount of the PtNPs supported onto the CSCNT were investigated. PtNPs were supported onto the CSCNT surfaces using liquid phase plasma treatment after adsorption of Pt precursor onto the CSCNT surfaces. First, observation of the CSCNT surfaces with field-emission transmission electron microscopy (FETEM) were conducted to examine the morphology of the PtNPs supported onto the CSCNT surfaces. PtNPs were well anchored and uniformly dispersed on the CSCNT surfaces without any dispersion stabilizer due to an exposure of the dangling bonds of graphene sheet which has high chemical reaction field. Moreover, the PtNPs size distribution of the CSCNT supported PtNPs were investigated with FETEM images of the CSCNT surfaces. The average size of PtNPs supported onto the CSCNT surfaces was about 2.2 nm, which was smaller than the PtNPs of conventional electrode catalysts for PEFC (3〜5 nm). Thermogravimetric analysis (TGA) of the CSCNT supported PtNPs were performed to investigate the supported amount of Pt of the CSCNT supported PtNPs. The supported amount of Pt was about 14.7 wt%, which was smaller than the PtNPs of conventional electrode catalysts for PEFC (50〜60 wt%).

An effect of compressive loads on the out-of-plane fatigue properties of thick CFRP laminates with the toughened interlaminar layers was investigated. The unidirectional [O_<88>] and quasi-isotropic [45/0/-45/90]_<11s> laminates were fabricated using T800S/3900-2B prepreg. This prepreg is constituted of a fiber layer and an interlaminar toughened layer in which polyamide particles are dispersed uniformly. The spool shaped specimens which were machined from the thick laminates were loaded in the out-of-plane direction of the specimen. The fatigue tests were performed at three types of stress ratios of R= 0.1, -1 and -3. As the results of the fatigue tests, it was conformed that the fatigue life was reduced with the compressive load under same maximum stress. In addition, the effect of the stress ratio on the fatigue life was dependent on the laminate configuration. When the cyclic loading at R= -3 were applied in the unidirectional specimen, the shear fracture mode was observed in the high-cycle region.

In this article, the dimensional stability of epoxy- and cyanate-based laminates is discussed, focusing on the thermal deformation, moisture-induced deformation, and deformation induced by relaxation of thermal residual stress. Each of the deformations was calculated independently based on the laminate theory. The material properties of the unidirectional laminates were obtained by conducting thermal mechanical analysis, moisture absorption tests, and tensile creep tests. These material properties were adopted to the laminate theory to predict the deformation of quasi-isotropic laminate, and it was calculated that each type of deformation induced micron-level dimensional instability. The moisture-induced deformation was an order of magnitude larger than that caused by the other factors. The validity of the calculations was confirmed by comparing the calculated results with the experimental ones. It is important to control moisture absorption even if cyanate resin, which has high moisture resistance, is used.

© 2015 International Committee on Composite Materials. All rights reserved. The effect of the stress ratio on fatigue characteristics in the out-of-plane direction of thick carbon fiber reinforced plastic (CFRP) laminates with toughened interlaminar layers was evaluated. The spool shaped specimens cut out the unidirectional thick CFRP laminates, which pile up 88 plies of the prepreg with toughened interlaminar layer, T800S/3900-2B, were used. The fatigue tests were conducted under the stress ratios of R = 0.1 and -1 to evaluate the effect of the stress ratio. As the results of the fatigue tests, the fatigue life of the specimens at R = -1 was shorter than that at R = 0.1. It was found that the fatigue properties of the CFRP laminates in the out-of-plane direction are affected by the stress amplitude from the experimental results. In addition, the fatigue life under the different stress ratio can be evaluated equivalently using the Walker model which can consider the mean stress effect.

© 2015 International Committee on Composite Materials. All rights reserved. Long Fiber reinforced Thermoplastics (LFT) have been used in a lot of industrial fields such as automotive industries because of their excellent moldabilities, productivities, and high mechanical properties compared with injection molded Short Fiber reinforced Thermoplastics (SFT). On the other hand, mechanical properties of LFT are significantly low compared with continuous fiber reinforced plastics. So, improvements of them are still important study subjects. In past studies, there were some reports about the absorbed impact energy of LFT, and LFT showed higher energy absorption property than SFT. However, there have been few studies focused on strength, stiffness and their strain rate dependencies of LFT in impact loading condition. These quantitative evaluations are essential to material design for developments of impact-resistant LFT. In this study, mechanical properties and strain rate dependency of injection molded long glass fiber reinforced thermoplastics under impact loading were investigated. The effectiveness of longer residual fibers to improvement of impact properties of injection molded composites was indicated. LFT showed higher mechanical properties compared with SFT at any strain rate in this study. Increasing rate of tensile strength in LFT was also much higher than that in SFT, and significant improvement of impact properties of injection molded composites were achieved by longer residual fibers. As a result of observation of micro structures and fracture surfaces after impact tensile test, it was confirmed that the fracture occurred mainly in matrix and fiber/matrix interface in the case of SFT. On the other hand, in LFT specimens, impact tensile loading was effectively transferred to reinforcement glass fibers, and they were broken after impact tensile test. Consequently, it was revealed that strain rate dependency of glass fiber strength resulting from the slow crack growth development led to high impact tensile properties of LFT.

© 28th ICAF Symposium, ICAF 2015. All rights reserved. A method has been proposed to predict the fatigue life to the transverse crack initiation in cross-ply and quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates. The six kinds of the laminates were formed of the two kinds of the prepregs, T800S/3900-2B with toughened interlaminar layers and T800H/3631 without toughened interlaminar layers. The unidirectional and cross-ply laminates of the stacking sequence of [90]12, [0/904]S and [0/906]S were formed of the T800S/3900-2B prepreg, and the cross-ply and quasi-isotropic laminates of the stacking sequence of [0/902]S, [0/906]S and [45/0/-45/90]S were formed of T800H/3631 prepreg. Tensile fatigue tests were performed under load control using a hydraulic fatigue testing machine. The transverse crack initiation in the laminates under fatigue loading was evaluated by modifying the Smith-Watson-Topper (SWT) model, which can evaluate the mean stress effect on the fatigue life. Finally, it was found that the initiation of transverse crack in the laminates with the various laminate configuration can be predicted using the modified SWT model, and that means that it is possible to predict the initiation of transverse crack in the laminates with the various laminate configuration using S-N curve of unidirectional laminates in 90° direction.

Carbon nanotubes (CNTs) have very high specific strength and stiffness. The excellent properties make it possible to enhance the mechanical properties of polymer matrix composites. However, it is difficult to use CNTs as the reinforcement of long fibers because of the limitation of CNT growth. In recent years, a method to spin yarns from CNT forests has developed. We have succeeded in manufacturing the unidirectional composites reinforced with the densified untwisted CNT yarns. The untwisted CNT yarns have been manufactured by drawing CNTs through a die from vertically aligned CNT arrays. In this study, the densified untwisted CNT yarns with a polymer treatment were fabricated. The tensile strength and the elastic modulus of the yarns were improved significantly by the treatment, and they were 1.9 GPa and 140 GPa, respectively. Moreover, the polymer treatment prevented the CNT yarns from swelling due to impregnation of the matrix resin. Finally, the high strength CNT yarn composites which have higher volume fraction than a conventional method were successfully fabricated.

A fatigue life to the initiation of transverse cracks in cross-ply carbon fiber-reinforced plastic (CFRP) laminates has been predicted using properties of the fatigue strength of unidirectional CFRP in the 90 degrees direction. In the experiments, unidirectional [90](12) laminates were used to obtain a plot of maximum stress versus number of cycles to breaking, and two types of cross-ply laminates of [0/90(4)](s) and [0/90(6)](s) were used to evaluate the initiation and multiplication of transverse cracks under fatigue loading. Transverse cracks were studied by optical microscopy and soft X-ray photography. Analytical and experimental results showed good agreement, and the fatigue life for transverse crack initiation in cross-ply laminates was predicted successfully from the fatigue strength properties of the unidirectional CFRP in the 90 degrees direction. The prediction results showed a conservative fatigue life than the experimental results. (C) 2014 Elsevier Ltd. All rights reserved.

Carbon nanotubes (CNTs) with superior mechanical properties have been of interest as reinforcement for polymer composites. However, the length of individual CNTs is limited. As a solution, yarns spun by twisting together multi-walled carbon nanotubes (MWCNTs) have been reported. In this study, untwisted CNT yams were prepared by a non-conventional method drawing CNTs through a die. The MWCNTs in these yarns are held together by strong van der Waals forces that arise due to the interactions on the long and smooth surfaces of the MWCNTs. Here, mechanical properties of untwisted CNT yarn were studied by tensile tests. The strength of the CNT yarn was increased by increasing the apparent density of the yarn. The CNT yarns showed high tensile strength of 1 GPa and elastic modulus of 79 GPa at a yam diameter of 35 gm. The interfacial shear strength between the CNT yarn and epoxy resin was studied by the microdroplet method, and it was very low. The wettability of the CNT yarn was affected by a type of curing agent. A unidirectional composite of epoxy resin and CNT yam was prepared by the pultrusion molding method. Mechanical properties of the unidirectional composite were affected by the type of curing agent. (C) 2014 Elsevier Ltd. All rights reserved.

In this study, fatigue strength properties of interlaminar toughened CFRP laminates in the out-of-plane direction, or through thickness, were investigated. Thick laminates whose stacking sequence was unidirectional were formed with 88 plies of T800S/3900-2B prepreg. The T800S/3900-2B prepreg is constituted of fiber layer and interlaminar toughened layer in which polyamide particles are dispersed. The material properties of the thick laminates were measured by compression test and 4-points shear test. Spool specimens machined from the thick laminates were loaded in the out-of-plane direction. Stress distributions of spool specimens were evaluated by FE analysis. Fiber layer and interlaminar toughened layer in the each ply were modeled separately in the analysis. In comparison to the out-of-plane direction, the properties of in-plane transverse direction were investigated with 90° thin laminates. To evaluate the fatigue strength properties quantitatively, an analytical equation was introduced for the results of fatigue test. The fracture surfaces of the specimens after static and fatigue tests were observed by SEM. From the observation of the fracture surfaces after fatigue test, it was found that the interfacial debonding between fiber and matrix was occurred due to cyclic loading in both of specimens. In addition, it was observed that crack generated from debonding grew in the fiber layer until ultimate fracture. Moreover, the experimental and analytical results showed that, in comparison to the in-plane transverse direction, the fatigue life in the out-of-plane direction is shorter.

Initiation and growth behaviors of a transverse crack occurred in interlaminar toughened CFRP cross-ply laminates under cyclic loading were evaluated. Specimens whose stacking sequence is [0/904]S and [0/906]S were formed with T800S/3900-2B prepreg. The specimen edges were observed with an optical microscope and a laser microscope to investigate the behavior of transverse crack initiation and growth. To observe the edge surface of the specimen at arbitrary loading cycles, a replica technique was used. In addition, soft X-ray photography was used to observe internal damage. The number of cycles to transverse crack initiation was predicted quantitatively by applying the normalized modified Paris law, which shows the relationship between transverse crack density growth rate and normalized energy release rate range associated with transverse crack formation. Analytical results showed good agreement with experimental results. It was found that transverse crack initiation in interlaminar toughened CFRP laminates can be evaluated by applying the normalized modified Paris law. Moreover, in comparison to the laminates with and without toughened layers, the fatigue life to transverse crack initiation was prolonged due to the toughened layers. From damage observation, it was cleared that a transverse crack path to the thickness direction of the laminate was prevented by polyamide particles in the toughened layers. Therefore, it was found that the toughened layers dispersed polyamide particles to prevent delamination are effective for obstructing initiation of a transverse crack under cyclic loading.

This study aims to investigate the effects of a hydrothermal environment on the creep behavior of woven glass fiber reinforced plastics (GFRPs) and to propose a method for predicting their lifetime. Toward this end, experiments were carried out in air and deionized water at 40, 60, 80 and 95 °C. Static tensile tests of woven GFRP were conducted in air and in deionized water to evaluate its mechanical properties and to determine suitable experimental conditions for subsequent constant tensile load tests. The mechanical properties of the woven GFRP decreased with an increase in temperature and with water immersion. Constant tensile load tests were also conducted in air and in deionized water to investigate the creep behavior and fracture time. The fracture time decreased with an increase in stress and water temperature and demonstrated the possibility of a threshold stress for fracturing. In addition, the fracture time during each constant tensile load test was predicted using a modified Reiner-Weissenberg (R-W) criterion, which is a failure criterion for linear viscoelastic materials based on the accumulation of dissolved energy within the GFRP. In this study, the R-W criterion was modified to consider the effects of degradation and its acceleration, which are due to the applied stress and immersion in a solution. The predicted results were in good agreement with the experimental data when considering the effects of hydrothermal aging. © 2012 Springer Science+Business Media, B. V.

In this paper, time-dependent dimensional change in quasi-isotropic laminates induced by relaxation of thermal residual stress and physical aging was predicted by the classical lamination theory. CFRP with pitch-based carbon fiber and cyanate ester resin was chosen for the study. Viscoelastic properties were investigated by performing tensile creep test for unidirectional laminates in the transverse direction. In addition, shrinkage strain induced by physical aging was studied by measuring the strain change of unidirectional laminates as well. Shrinkage strain in off-axis layers was calculated by using the coordinate-transform method. Shrinkage strain in 60° and 45° laminates were measured and the results were compared with the calculation. From the comparison, it was found that shrinkage strain of off-axis layers can be calculated by using the coordinate-transform method. Experimental results were applied to the classical theory in order to predict the time-dependent dimensional change of quasi-isotropic laminates. The strain change in quasi-isotropic laminates was obtained experimentally, and the result was compared with the prediction. It was verified that the time-dependent deformation of quasi-isotropic laminates can be predicted with a με-order by using proposed prediction method. © 2013 The Japan Society of Mechanical Engineers.

The effect of ply thickness on the formation of first transverse crack caused in cross-ply carbon fiber reinforced plastic (CFRP) laminates was evaluated under fatigue loading. In addition, the initiation process of the transverse crack was observed with an atomic force microscopy (AFM) in detail. The formation of the first transverse crack was evaluated quantitatively with the power law between the transverse crack density growth rate and the normalized energy release rate range associated with transverse crack formation. The analytical results showed good agreement with the experimental results. Moreover, from the analytical results with the cross-ply [0/906]s and [02/9012]s laminates, it was shown that the fatigue life to the formation of the first transverse crack in [0/906]s is approximately 100 times longer than that in [02/9012]s. Furthermore, as the results observed the process of the transverse crack initiation with AFM, it was cleared that matrix resins around fibers were uplifted on the laminate edge surface due to cyclic loading. The observation results indicate that the micro cracks are initiated at the interface between fiber and matrix resin by the stress concentration due to the uplift of matrix resins, that the micro cracks grow to the thickness direction in 90° plies with concatenating the interfacial cracks and that the transverse crack is formed finally. © 2013 The Japan Society of Mechanical Engineers.

In this paper, we investigated the lifetime of delayed fracture in woven glass fiber reinforced plastics (GFRP) laminate under corrosive environment, which possess high corrosion resistance. Corrosive environments discussed in this paper were deionized water and hydrochloric acid (1.0mol/l) at 80℃, and in air for comparison. First, static tensile test of GFRP was conducted in order to evaluate its mechanical properties after immersion for 0-2000h into corrosive environment. The mechanical properties of GFRP in deionized water didn't decrease, but, that in hydrochloric acid decreased slightly. Also, the mechanical properties of GFRP after immersion decreased toward certain values with increasing immersion time regardless of the solution. Finally, constant tensile load test of GFRP was conducted in all environments to investigate its lifetime of delayed fracture. The fracture time shortened with the increase of the applied stress. In addition, the fracture time in hydrochloric acid was shorter than that in deionized water. Furthermore, we predicted the lifetime using Reiner-Weissenberg (R-W) criterion. The predicted results showed good agreement with the experimental data while considering the degradation of GFRP in corrosive environments.

It is well known that the strength of glass fibers increases with increasing strain rate. Consequently, impact strength of glass fiber is competitive with that of carbon fiber. This strengthening phenomenon is well recognized for bulk glass. Strain-rate dependence of the strength for bulk glass was described by considering slow crack growth in glass. The analytical model that considered the slow crack growth of glass is proposed to predict the strength of glass fibers. The proposed model considered the stress corrosion limit and a constant crack velocity region. Calculations showed almost same results with the previous model, however, some differences were confirmed. To discuss the validity of the analysis, tensile tests of E-glass fiber bundles were conducted at various strain rates. It was observed that the fracture behaviors differ with the strain rates. Experimental results showed that the strength of E-glass fibers increased with increasing strain rate. Furthermore, we confirmed that the analytical results were in good agreement with the experimental results. The strain-rate dependence of the strength of glass fibers was successfully predicted by considering the slow crack growth in glass.

A simple method for obtaining viscoelastic parameters from the results of static tensile tests is presented herein. Viscoelastic parameters were obtained by fitting experimental results and calculated results based on the power law model and linear viscoelasticity. The static tensile tests were carried out at various pre-aging times and the effect of physical aging was determined. The data confirmed that the physical aging process has a significant effect on the viscoelastic behavior. A creep test was conducted in order to discuss the validity of the prediction using the results of the static tensile test. It was confirmed that the predictions based on the viscoelastic parameters obtained from static tensile tests cannot adequately model actual viscoelastic behavior. The effective time theory was incorporated into the prediction in order to account for the progress of physical aging. It was verified that incorporating effective time theory into the prediction allows for the precise prediction of the long-term viscoelastic behavior.

The formation of a first transverse crack in cross-ply CFRP laminates was predicted under fatigue loading and the fatigue limit of transverse crack initiation was evaluated quantitatively. Transverse cracks induce more serious damage, such as delamination or fiber breakage. It is essential to understand the mechanism of the transverse crack initiation for improving long-term durability of CFRP laminates. Therefore, a method was proposed to predict the number of cycles to transverse crack initiation in cross-ply CFRP laminates under fatigue loading. Two types of cross-ply CFRP laminates, [0/90 6]s and [02/9012]s, of different thickness were used for fatigue tests. As the results, we were successful in predicting the number of cycles to transverse crack initiation under fatigue loading and evaluating the fatigue limit of the transverse crack initiation by the proposed analysis. Moreover, it was found that the fatigue life to transverse crack initiation in [0/906]s laminate was approximately 100 times longer than that in [02/90 12]s laminates. © 2012 by Asian-Australian Association for Composite Materials (AACM).

This study presents an analytical model to predict compressive strength of unidirectional FRP. Proposed model considers the effect of strain-rate dependency on mechanical properties of constituent materials. The model is based on the elastic foundation model and the microbuckling model of fiber which has initial misalignment in matrix. Compressive deformation of unidirectional FRP is considered by dividing into fiber microbuckling region and plastic kinking region. Additionally, to take into consideration the change in compressive deformation mode accompanying fiber volume fraction or fiber microbuckling, A mode function is introduced. The predictions from the proposed model are compared with experimental results of unidirectional E-glass/Epoxy and T700SC/Epoxy evaluated by using the conventional split Hopkinson pressure bar method. Incorporating strain-rate dependency on compressive modulus of reinforcement calculated from composite mixture law, the predictions are found to be in good agreement with experimental results of strain-rate dependency on compressive strength. Accuracy of the prediction is improved by changing the mode function. © 2012 The Japan Society of Mechanical Engineers.

A method to predict quantitatively the first of transverse cracks accumulated in the various types of [0 m/90 n] s cross-ply carbon fiber reinforced plastic (CFRP) laminates subjected to fatigue loading was proposed. On the basis of the assumption that the mechanism of transverse crack initiation is equivalent to that of transverse crack increase in the earlier stage of fatigue within low transverse crack density, the cycles at which a transverse crack initiates are calculated by applying the normalized modified Paris law, which shows the relationship between the transverse crack density growth rate and the normalized energy release rate range associated with the transverse crack formation. When the constants of the normalized modified Paris law are given with an arbitrary cross-ply laminate, the proposed method makes possible to predict the initiation of a transverse crack in the other various types of cross-ply laminates under fatigue loading by only measuring the stress at which a transverse crack initiates under static tensile loading. © 2012 The Japan Society of Mechanical Engineers.

The tensile properties of E-glass, which is the most popular reinforcement fiber in composite materials, were determined from the experimental results of fiber bundle testing under a high strain rate. The tests were performed by using two types of experimental methods. One is the tension-type split Hopkinson bar system and the other is the universal highspeed tensile-testing machine. In the results, it was demonstrated that the tensile strength and fracture strain of E-glass fiber increased with the strain rate. The absorbed strain energy, therefore, significantly increased. It was also shown that the strain rate dependency of E-glass fiber tensile strength was strongly affected by fiber diameter. The smaller diameter of E-glass fiber has the stronger strain rate dependency. Finally, the impact tensile strengths of high-strength glass and carbon fibers were investigated. It was confirmed that the tensile strength of the high-strength glass fiber also increased with the strain rate, but the tensile properties of carbon fiber were almost independent of the strain rate.

This paper aims to clarify the off-axis delayed fracture characteristic of woven GFRP laminate by the constant tensile load test in hydrothermal environment. Experimental condition discussed in this paper are air and deionized water at 40 -C. Static tensile tests of woven GFRP laminate were conducted to examine the off-axis directionnal mechanical properties and to determine the stress level for the constant tensile load test. It turned out that the maximum stress, elastic modulus and Poisson's ratio slightly decreased and the fracture strain decreased greatly in deionized water. Constant tensile load tests of woven GFRP laminate were conducted to investigate the delayed fracture characteristics: the strain response and fracture time. The stress level of constant tensile load test was determined to be 15, 30 and 45% of the maximum stress obtained from static tensile tests. As a result of constant tensile load test, it was ascertained that the strain increase with the increase of the applied load and the immersion in deionized water.

A method to predict quantitatively the initiation of a transverse crack caused in the various types of [0・90,1 cross-ply carbon fiber reinforced plastic (CFRP) laminates subjected to fatigue loading was proposed. On the basis of the assumption that the mechanism of transverse crack initiation is equivalent to that of transverse crack increase in the earlier stage of fatigue within low transverse crack density, the cycles that a transverse crack initiates are calculated by applying the normalized modified Paris law, which shows the relationship between the transverse crack density growth rate and the normalized energy release rate range associated with the transverse crack formation. Once the constants of the normalized modified Paris law are given with an arbitrary cross-ply laminate, the proposed method makes possible to predict the initiation of a transverse crack in the other various types of cross-ply laminates under fatigue loading by only measuring the stress at which a transverse crack initiates under static tensile loading.

The change of fatigue damage behavior depending on an applied stress level in carbon fiber reinforced plastic (CFRP) laminates was evaluated quantitatively in this study. To evaluate damage growth, the energies released due to transverse crack propagation and delamination growth per unit length with consideration of transverse crack propagation were derived. Moreover, the transverse crack propagation and the delamination growth were evaluated using a modified Paris law that gives the relationship between the damage growth rate and the energy released due to damage growth. As a result, it was found that the growth of the transverse crack and the delamination could be evaluated with the unique Paris law constants, respectively. Finally, it was concluded that the change of the fatigue damage growth behavior was caused due to the difference of the growth rate of the transverse cracks and delamination at an applied stress level. (C) 2011 Elsevier Ltd. All rights reserved.

This paper discusses the accuracy of ply angle alignment and how it relates to out-of-plane deformation in carbon fiber reinforced plastics (CFRP) laminates. We investigated the deformation of symmetrical cross-ply laminates under hot and humid conditions. In spite of the symmetrically stacked laminates, unpredictable out-of-plane deformation occurred over time due to ply angle misalignment. The deformation was unstable and disproportionate to the absorbed moisture. A Monte Carlo simulation based on laminate theory was performed to quantify the deformation induced by the ply angle misalignment. Symmetrical cross-ply laminates were found to twist as they absorbed water when they underwent ply angle misalignments. By comparing the analytical results with experimental results, we concluded that a standard deviation of approximately 0.4 exists as ply angle misalignment in the laminates used in this study and that this slight ply angle misalignment can be a significant factor in out-of-plane deformation of cross-ply laminates. (C) 2010 Elsevier Ltd. All rights reserved.

This paper aims to clarify the delayed fracture mechanism for glass fiber reinforced plastics (GFRP) in corrosive environments. The GFRP under study is composed of plain NCR-glass cloth and vinylester resin, which both possess high corrosion resistance. In this study, the experimental conditions were performed in air, deionized water, and hydrochloric acid at 40°C. Static tensile tests of woven GFRP were performed to evaluate the mechanical properties and determine the experimental conditions for the constant tensile load tests in each environment. The mechanical properties of the woven GFRP decreased with its immersion into deionized water and hydrochloric acid. The stress-strain curve decreased intensely after the knee point especially in hydrochloric acid, which is possibly because of the damage accumulation generated by the solution and applied stress. Constant tensile load tests of the woven GFRP were performed to investigate the creep behavior and fracture time in each environment. The strain and strain rate increased in the tests in deionized water and hydrochloric acid, which are the result of decrease in the stiffness owing to immersion in each solution. In addition, delayed fracture occurred in deionized water and hydrochloric acid, and the lifetimes in hydrochloric acid were shorter than those in deionized water. Moreover, it was suggested from fracture surface observations that the delayed fracture of the woven GFRP under a constant tensile load in a corrosive environment is dominated by degradations in the fiber reinforcement and fiber/matrix interface.

In this study, transverse crack initiation in cross-ply CFRP laminates under fatigue loading was evaluated focusing on the transverse crack growth and saturation. The number of cycles that a transverse crack initiates was predicted by the analytical model on the basis of the modified Paris law. In addition, the lower threshold of the transverse crack formation was researched by a modified Paris-law, and the fatigue limit of the transverse crack initiation was evaluated by calculating the stress applied in 90° layer where the increase of the transverse cracks was saturated. Moreover, transverse crack was observed by using scanning electron microscope (SEM) in order to investigate the mechanism of transverse crack initiation. As the results, the analytical results for predicting the transverse crack initiation showed good agreement with the experimental results. Moreover, it was shown that the lower threshold of the transverse crack formation existed, and that the stress applied in 90° layer at saturation state was almost equivalent to the fatigue limit of the transverse crack initiation from the experimental results. From the observation with SEM, it was found that the initiation of the transverse crack under fatigue loading depended on the interface property between fiber and matrix. © 2011 The Japan Society of Mechanical Engineers.

Mechanical properties of glass fiber reinforced plastics (GFRP) mainly depend on the mechanical properties of the glass fiber, which decreases with stress and corrosion. Thus, the calculation method to predict the residual fiber strength after exposure to stress and corrosion is required. In this paper, constant strain test of single fiber composite (SFC) was conducted in hydrothermal environment (deionized water) at 40• •and 75• •to apply the stress and the hydrothermal aging. The residual fiber strength after constant strain test was evaluated by fiber fragmentation test. Besides, the residual strength of glass fiber was predicted using subcritical crack growth model. In the subcritical crack growth model, the surface flaw on the fiber surface which arises while manufacturing was assumed as an ideal crack. The crack growth rate was expressed by the combination of Paris law and Arrhenius model, and the strain history of constant strain test was integrated into the calculation. The residual strength of the glass fiber was calculated based on the fracture mechanics discussing the crack length. The predicted results of the fiber strength showed good agreement with the experimental data at various experimental conditions and the validity of the proposed model was ascertained in this paper. © 2011, The Japan Society of Mechanical Engineers. All rights reserved.

In this paper, time-dependent dimensional change in a symmetrical cross-ply laminates was predicted by transverse properties of the CFRP laminates. CFRP with pitch-based carbon fiber and cyanate ester resin was chosen for the study. Viscoelastic property was obtained by performing tensile creep test for unidirectional laminates in the transverse direction. In addition, shrinkage caused by physical aging was obtained by measuring the strain change for unidirectional laminates as well. Experimental results were applied to the classical lamination theory in order to predict the time-dependent dimensional change of a symmetrical cross-ply laminates. The strain change in a symmetrical cross-ply laminates was obtained experimentally using an extensometer, and the result was compared with the prediction. From the comparison, it was concluded that the proposed prediction method is appropriate. It was also found that physical aging shrinkage must be compensated in order to evaluate the relaxation modulus from tensile creep test result. The effect of physical aging shrinkage must also be considered in prediction of time-dependent deformation in cross-ply laminates. © 2011 The Japan Society of Mechanical Engineers.

In this study, transverse crack initiation in cross-ply CFRP laminates under fatigue loading was evaluated focusing on the transverse crack growth and saturation. The number of cycles that a transverse crack initiates was predicted by the analytical model on the basis of the modified Paris law. In addition, the lower threshold of the transverse crack formation was researched by a modified Paris-law, and the fatigue limit of the transverse crack initiation was evaluated by calculating the stress applied in 90° layer where the increase of the transverse cracks was saturated. Moreover, transverse crack was observed by using scanning electron microscope (SEM) in order to investigate the mechanism of transverse crack initiation. As the results, the analytical results for predicting the transverse crack initiation showed good agreement with the experimental results. Moreover, it was shown that the lower threshold of the transverse crack formation existed, and that the stress applied in 90° layer at saturation state was almost equivalent to the fatigue limit of the transverse crack initiation from the experimental results. From the observation with SEM, it was found that the initiation of the transverse crack under fatigue loading depended on the interface property between fiber and matrix.

Strict geometrical stability is required for the precise structures like telescopes. Unpredictable out-of-plane deformation is a serious problem when we use CFRP (Carbon Fiber Reinforced Plastic) laminate to the precise structure. This out-of plane deformation of symmetrical CFRP laminate mainly arise from combination effects of ply angle misalignment and temperature change. We discussed here is effective stacking sequence of CFRP laminate that mitigate the deformation caused by the ply angle misalignment. The analysis based on laminate theory was performed to calculate the thermal deformation. In this analysis, the random numbers were added to each layers as ply angle misalignments. The analytical results were obtained statistically by Monte Carlo method. Mohr's curvature circle was also incorporated to evaluate the deformation as P-V (peak to Valley) values. We performed the analysis with various stacking sequence. It was calculated that the symmetric cross-ply laminates deformed 10 times larger than the other quasi-symmetric laminates. In the case of the total ply number is less than 12, the stacking sequence in the laminate has a significant effects on the thermal deformation. However, if the total number ply number is more than 24, effect of stacking sequence on the thermal deformation becomes negligible. We also discussed the geometrical stability of CFRP mirror by considering unavoidable ply angle misalignment. It was presumed that the CFRP mirror can be used for wide range of wave length when the back structure was attached to CFRP laminates.

This paper proposes a strength prediction method for unidirectional glass fiber reinforced plastics (GFRPs) after hydrothermal aging: immersion in deionized water at 80 degrees C. First, the strength degradation of the constituents (i.e., the glass fiber and the fiber/matrix interface) of unidirectional GFRP after hydrothermal aging was evaluated from the fiber strength and the interfacial shear stress by using a single-fiber composite (SFC). Both the fiber strength and the interfacial shear stress had a tendency to decrease in the early stage of hydrothermal aging and to saturate toward certain values with long-term aging. In addition, the tensile strength of the unidirectional GFRP was measured after hydrothermal aging. The residual strength of the unidirectional GFRP also had a tendency to decrease sharply in the early stage of hydrothermal aging and to saturate toward a certain strength with long-term aging. Finally, the residual strength of the unidirectional GFRP after hydrothermal aging was predicted using the global loading sharing (GLS) model, by considering the degradation of both the glass fiber and the fiber/matrix interface. The predicted results indicated good agreement with the experimental data while considering not only the degradation of the fiber reinforcement but also the fiber/matrix interface adhesion. It was concluded that the GLS model applied considering the degradation of the GFRP constituents would be a suitable and a simple model to predict the residual strength of the unidirectional GFRP after hydrothermal aging. (C) Koninklijke Brill NV, Leiden, 2011

We performed a numerical simulation of a time-dependent interfacial failure accompanied by a fiber failure, and examined their evolution under shear and compressive loads in single-fiber composites. The compressive load on the interface consists of Poisson&apos;s contraction for matrix resin subjected to longitudinal tensile load. As time progresses, compressive stress at the interface in the fiber radial direction relaxes under the constant longitudinal tensile strain condition for the specimen, directly causing the relaxation of the interface frictional stress. This relaxation facilitates the failure of the interface. In this analysis, a specific criterion for interface failure is applied; apparent interfacial shear strength is enhanced by compressive stress, which is referred as quasi-parabolic criterion in the present study. The results of the stress recovery profile around the fiber failure and the interfacial debonding length as a function of time simulated by the finite element analysis employing the criterion are very similar to experimental results obtained using micro-Raman spectroscopy.

The initiation of a transverse crack in CFRP laminates under fatigue loading was investigated. Transverse cracks cause serious damage, such as delamination or fiber breakage. It is essential to understand the mechanism of the transverse crack onset for improving long-term durability and reliability of CFRP laminates. Therefore, a method to evaluate and predict the initiation of a transverse crack in cross-ply CFRP laminates under fatigue loading was researched. Fatigue tests at the various stress levels were performed in order to evaluate the initiation and propagation of the transverse cracks. The analytical method on the basis of Paris law in order to predict the number of cycles of transverse crack initiation was proposed. As the results, we were successful in predicting the initiation of the transverse crack under fatigue loadings by the proposed analysis.

This paper reports a development of space telescope mirror made by light and thermally stable CFRP. We first compare thermal stabilities of a mirror made by CFRP with a conventional that made by glass material. The superior point regarding the thermal stability of the CFRP mirror to the glass mirror is described. One of the most critical issues for CFRP mirror is the mirror-surface roughness deterioration induced by "fiber-print through". We made a prototype mirror consisting of only CFRP with a gel-coating on the mirror surface, addressing the issue. The gel-coated mirror-surface roughness was 20 nmRMS just after fabrication. Durability of the surface roughness under various hostile conditions is examined in the present study.

We measured residual stress relaxation in epoxy-based CFRP laminates and cyanate-based CFRP laminates. We estimated the residual stress in symmetric cross-ply laminates by measuring the curvature of unsymmetric cross-ply laminates and investigated the relaxation of the residual stress by measuring the curvature of unbalanced laminates in a nitrogen environment. Our experimental results demonstrated that the residual stress in both kinds of laminate can decay by approximately 15% after 500 hours at 40°C. The stress relaxation can be modeled by the power law model. Viscoelastic parameters were obtained by a quasi-static tensile test to predict stress relaxation. The validity of the prediction is discussed.

This paper presents an experimental investigation into the effects of water temperature and immersion times on the tensile strength degradation of plain-woven glass fiber reinforced plastics (GFRP) laminates. GFRP specimens were tested as-received and after hydrothermal aging in deionized water at 40 °C, 80 °C, 95 °C to evaluate their tensile properties. The strength and rupture strain had a tendency to decrease drastically in the early stages and to saturate toward certain strength with long-term aging, regardless of the water temperature. The fracture surfaces were examined by scanning electron microscopy (SEM) to study the fracture mechanisms of woven GFRP after hydrothermal aging. While the strength of the glass fiber decreased, the fracture surfaces of the E-glass fibers flattened, and the mirror zones on the fracture surfaces enlarged. Interfacial degradation was confirmed by fiber pull-out, and the debonded fibers showed no resin matrix adhesion to the fiber surfaces. These experimental results suggest that the degradation in the strength of woven GFRP is dominated by degradation of their fiber reinforcement and the fiber/matrix interface.

This study focused on the initiation of a transverse crack in cross-ply CFRP laminates under fatigue loading. Transverse cracks induce more serious damages, such as delamination or fiber breakage. It is essential to understand the mechanism of the transverse crack onset under fatigue loading for improving long-term durability and reliability of CFRP laminates. Therefore, a method to evaluate and predict the initiation of a transverse crack in cross-ply CFRP laminates under fatigue loading was proposed. Fatigue tests at various stress levels were performed to evaluate the initiation and multiplication of the transverse cracks. Fatigue tests were interrupted at arbitrary loading cycles to observe damage in cross-ply CFRP laminates. The transverse cracks caused in the laminates were observed by an optical microscope and soft X-ray photography. The fatigue limit of transverse crack initiation in this fatigue conditions was indicated by calculating the stress applied in 90° layer of the laminates at the saturated state of the transverse cracks with a variational approach. Moreover, the analytical method on the basis of Paris law in order to predict the number of cycles to transverse crack initiation under fatigue loading was proposed. As the results, the predicted number of cycles to transverse crack initiation shows good agreement with experimental results.

FBG (Fiber Bragg Grating) sensors were embedded into CFRP unidirectional composite laminates in a direction perpendicular to the carbon fiber. Residual strains after curing were evaluated by the reflection spectrum from the FBGs. The CFRP laminates were kept at 100 degrees C for monitoring of the residual strain change measured by the FBGs. Without mechanical loading, occurrence of physical aging was confirmed by the residual strain changes. The relationship between the influence of physical aging on residual strain and the thickness of the CFRP was almost linear.

CFRP (Carbon Fiber Reinforced Plastics) is the ideal material for space based mirror due to its low thermal expansion, and high specific modulus. To expand the use of CFRP, we investigated the long-term stability of CFRP under humid environment. CFRP mirror was made as precise as possible by using special class of material and adopting particular design techniques. Dimensional stability of CFRP mirror was evaluated by nano-scale measurement. The factors which cause out-of-plane deformation of the mirror is discussed.

This study aims development of space telescope mirror made by light and thermally stable CFRP. For that, we must solve a problem called "print-through"; the fiber pattern of the carbon fiber appears on the surface. We investigated the temperature-induced surface roughness variation experimentally and analytically.

Thermal deformation analysis was performed on a carbon fiber reinforced plastic (CFRP) mirror with sandwich structure. To obtain unexpected asymmetry of the surface sheet, we investigated the deformation of a quasi-isotropic laminate under hot and humid conditions. Despite the symmetric lay-up, quasi-isotropic laminate deforms into twisted saddle shape with time, and this deformation could be simulated by assuming ply angle misalignment. Then, the elastic moduli of honeycomb cores were calculated theoretically. A honeycomb sandwich mirror model was constructed by adopting a sheet model and using honeycomb elements. The thermal deformation analysis was performed considering the ply angle misalignment. The test results clarified that the deformation of the surface sheet was a critical factor in the dimensional stability of the CFRP mirror.

High-cycle fatigue features of over 108 cycles, particularly the initiation and propagation of edge delamination considering the effects of transverse cracks, were investigated using quasi-isotropic carbon-fiber-reinforced plastic (CFRP) laminates with a stacking sequence of [45/0/-45/90](s) in this study. In the relationship between a transverse crack density and initiation and growth of edge delamination, it was found that fatigue damage growth behavior varied depending on applied stress. It was observed that edge delamination initiated and grew at parts where transverse cracks were dense at ordinary applied stress, whereas it was observed that edge delamination grew before or simultaneously with transverse crack propagation at a low applied stress and high-cycle loading. In addition, the critical transverse crack density where delamination begins growing was calculated to evaluate the interaction between transverse crack and edge delamination growth. (C) 2009 Elsevier Ltd. All rights reserved.

The interfacial strength is the most important mechanical property among those of constituents in Polymer Matrix Composite : PMC. In this study, we conducted the tensile tests on Single Fiber Composite specimen and observed initiation of interfacial debonding at different loading rates. As a result, we recognized dependency of the interfacial strength on strain rates. According to the elasto-plastic analysis assuming that matrix's yield stress varies depending on strain rates, the stress distribution of the interface becomes close to the elastic analysis and it can be concluded that the dependency of interfacial strength on strain rates is caused by matrix's dependency on strain rates.

The strengthening mechanism of the glass-fiber at cryogenic temperature has not been fully studied so far. In the present study, tensile tests of a single E-glass fiber with heat treatment in air and liquid nitrogen were conducted to reveal the strengthening mechanism. The strength of glass fiber in liquid nitrogen was twice as high as the strength in air. Based on the area of the mirror zone in the fracture surface, the mirror constant was detemined. Besides, the mirror constant of the glass fiber were equal regardless of the heat treatment condition and the testing temperature. From the observation of surface crack, it was clarified that the mirror zone was the mark of surface flaw propagation and therefore it was suggested that the area of mirror zone doesn't have a direct effect on fiber strength.

Long-term durability of glass fiber reinforced plastics (GFRP) under water environment is strongly influenced by the strength degradation of its fiber reinforcement. Constant strain test in water, in which stress corrosion cracking (SCC) is initiated, was conducted for the single fiber composite (SFC) in order to investigate the strength degradation of E-glass fiber within the SFC. The strain applied to the glass fiber during the constant strain test was formulated by taking account of the fiber strain history, such as water absorption, thermal expansion, and the strain applied to the SFC. After the constant strain test, fragmentation test was conducted in order to estimate the residual strength of the embedded fiber. It was clarified that the degradation of the fiber strength progresses at higher applied strain and longer test time, moreover, the degradation of the fiber strength accelerated drastically at higher temperature.

In this study, we focused on the geometrical change of cross-ply laminates that are widely used for the composite structures. The behavior of time-dependent out-of-plane deformation under high temperature and humidity was examined. The CFRP plates were exposed at 80°C, 90% RH and their change in geometry was measured at arbitrary elapsed time. The initial shape of the specimens was twisted saddle shape, and the shape gradually became flat with time. Up to 500 μm of out-of-plane deformation arose during moisture absorption for CFRP symmetric plate with the size of 280×280×2.4 mm. The displacement in the thickness direction was the most along ±45° directions in which reinforcing fiber was not aligned. Finite element analysis (FEA) was performed to simulate the out-of-plane deformation of cross-ply laminates. The diffusivities D and coefficient of moisture expansion β, which were obtained other experiments in authors' previous work (Arao et al.: Adv. Compos. Mater., 17 (2008), 359-372), were used. The ply angle misalignment was considered as factors of generating out-of-plane deformation. For the analytical results, it was confirmed that the model the surface layer of which was rotated 1° deformed approximately 700 μm, and the plate geometry became twisted saddle shape by moisture absorption.

Time-dependent out-of-plane deformation of UD-CFRP (unidirectional CFRP laminate) caused by subjection to a humid environment was examined and analyzed. The UD-CFRP plate showed unpredictable geometrical variation with time in a humid environment, like asymmetric materials. The unpredictability was caused by non-uniform fiber distribution in the thickness direction of the specimen. A three-dimensional diffusion-stress coupling analysis considering the non-uniform fiber distribution was conducted based on finite element analysis. The analytical results showed very good agreement with the experimental results. Furthermore, the relationship between the non-uniform fiber distribution and the out-of-plane deformation with time was obtained quantitatively. (C) 2008 Elsevier Ltd. All rights reserved.

The high-cycle fatigue characteristics focused on the behavior of the transverse crack growth up to 10(8) cycles were investigated using quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates whose stacking sequence was [-45/0/45/90](s). To assess the fatigue behavior in the high-cycle region, fatigue tests were conducted at a frequency of 100 Hz in addition to 5 Hz. In this study, to evaluate quantitative characteristics of the transverse crack growth in the high-cycle region, the energy release rate considering the free-edge effect was calculated. Transverse crack growth behavior was evaluated based on a modified Paris law approach. The results revealed that transverse crack growth was delayed under the test conditions of the applied stress level of sigma(max)/sigma(b) = 0.2. (c) 2008 Elsevier Ltd. All rights reserved.

The long-term micro-dimensional stability of a carbon fiber reinforced plastic (CFRP) mirror was investigated in terms of creep deformation, moisture swelling and self-shrinkage. A 4-point bending creep test was carried out using specimens made from pitch-based high-modulus CFRP laminates to obtain a creep constant based on linear viscoelasticity, and we then investigated the weight change and geometrical change during a moisture absorption test using a CFRP specimen. The anisotropic diffusivities and coefficients of moisture expansion (CMEs) in CFRP laminates were obtained by fitting analytical data into the experimental data. Finally, the shrinkage behavior caused by physical aging of the polymeric material was examined using a fiber Bragg grating (FBG) sensor embedded in the neat resin specimen. Applying these results, we analyzed the geometrical changes in a CFRP mirror that resulted from time-dependent deformation by the mirror&apos;s weight, moisture absorption and physical aging, respectively. We discuss which factor is dominant in the deformation of CFRP mirrors under various conditions.

Impact tensile properties of unidirectional fiber-reinforced composites are evaluated using a tension-type split Hopkinson bar technique. In this study, thermoplastic epoxy resin and three kinds of reinforced fiber such as carbon, E-glass, and T-glass are employed as a matrix and reinforcement, respectively. The experimental results indicate that strain rate has no effect on tensile properties in case that the carbon fiber is employed as reinforcement. On the other hand, the tensile strength and strain of E-glass and T-glass fiber composites are obviously affected by the strain rate. Although both fibreglasses and thermoplastic epoxy resin are shown to be sensitive to the strain rate, the fibreglasses contributes more significantly to the strain rate effects of the maximum stress and strain.

As CFRPs (Carbon Fiber Reinforced Plastics) are being sought for the greater use in aerospace and aircraft structures, it is necessary to understand the mechanical behavior for long-term usage. Furthermore, its low thermal expansion and high specific stiffness has brought attention to the application of CFRPs for the next-generation materials of the ultra-precise structure, such as antenna and mirror. In spite of the advantages of CFRPs, it is well known fact that the long-term dimensional stability of CFRP laminates are affected by several factors, such as moisture absorption, residual stress relaxation, and physical aging. Among these factors, residual stresses are due to the mismatch of thermal expansion rate and cannot be eliminated. This research paper introduces an experimental method of using unsymmetrical cross-ply laminates to determine the behavior of residual stress relaxation. Experimental results are compared to the analytical results derived by classical lamination theory (CLT).

Present paper deals with the creep behavior in woven GFRP underwater at elevated temperature. The tensile test of GFRP after water immersion was conducted to evaluate the residual strength after immersion at various water temperatures. The residual strength of GFRP decreased with the increase in the water temperature and the immersion time. In fact, the transition in failure mode with water immersion was ascertained from SEM observation of the fracture surface. Additionally, creep test in air and under hot water at 95℃ was conducted. The creep rupture time decreased drastically with water immersion, in contrast, creep rupture wasn't observed from the creep test in air. It was clarified that the water immersion generated the transition in the failure mode of GFRP and therefore led to the acceleration of the strength degradation.

Double notched shear (DNS) test were performed with unidirectional CFRP specimen varying the shape of the test specimen. In this study, DNS tests using a unidirectional CFRP specimen having various notch shape and temperature were performed. Results of the DNS test indicated that the in-plane shear strength decreased while test temperature rose. In addition, curvature of notch shape is the smaller, the deterioration of in-plane shear strength is larger. These showed that traditional evaluation using an average shear stress is not quantitative due to stress singularity. Therefore, the in-plane shear strength is evaluated considering stress singularity by means of point stress criterion.

In this study, initiation of transverse crack for CFRP cross-ply laminates was investigated at various stress levels during high-cycle fatigue tests. Internal damages were observed with soft X-ray photography at arbitrary loading cycles. A variation of transverse crack density and the local delamination length was evaluated as the fatigue damage progression. The stress in 90° layer considering local delamination length was calculated with a variational approach from the experimental data. It was clear from the results that as the applied maximum stress is set to lower, the saturated transverse crack density and 90° layer stress also lowers. Moreover, the fatigue limit of transverse crack initiation is indicated in this fatigue test conditions.

The behavior of transverse crack growth and delamination growth under high-cycle fatigue loadings was investigated with cross-ply CFRP laminates, [0/90₂]_s and [0/90₆]_s, and quasi-isotropic CFRP laminates, [45/0/-45/90]_s. As a result, it was observed that the behavior of damage growth was different depending on the applied stress level. The growth of local or edge delamination was exacerbated under the test conditions of a low applied stress level and high-cycle loadings, because the areas of stress concentration were applied with high-cyclic loadings. On the other hand, when the fatigue tests were conducted under the applied stress level of 40% of the transverse crack initiation, the growth of transverse cracks was hardly observed until 10⁸ cycles with [0/90₂]_s, [0/90₆]_s and [45/0/-45/90]_s laminates.

A variational approach is presented to evaluate the 3-dimensional stress state in cross-ply laminates of the form [(S)/90(n)](s) that contain transverse cracks in the 90, plies, where (S) is any orthotropic sublaminate. Admissible stress states that satisfy equilibrium and all boundary and interface conditions are constructed, and the principle of minimum complementary energy is employed to find an optimal approximation of the composite strain energy. Using this method of analysis, we can express the stress state by considering the free-edge effect, which causes edge delamination in cracked laminates. The calculated results using the proposed model showed good agreement with the results calculated by the finite element method. (C) 2008 Elsevier B.V. All rights reserved.

Moisture absorption behavior of CFRP and its effect on dimensional stability was examined. Moisture diffusivity in CFRP was determined by measuring specimen weight during the moisture absorption test. Three types of CFRP specimens, unidirectionally reinforced, quasi-isotropically laminated and cloth-laminated composites were prepared. Each CFRP was processed into two geometries; a thin plate for determination of diffusion constants and a rod with square cross-section for the discussion of two-dimensional diffusion behavior. Coefficient of moisture expansion (CME) was also obtained from specimen deformation involving moisture absorption. During moisture absorption, the specimen surfaces strongly deformed especially near the edges because of three-dimensional moisture concentration distribution. This deformation was reasonably predicted by the finite element analysis using diffusion constants and CME, which have been experimentally determined. For unidirectional CFRP, an effect of fiber location distribution on CME is discussed by a micromechanical FEA.

A comprehensive model that determines tensile strengths for various systems of unidirectional composites is presented. The model derives the strength of unidirectional composites as a function of single-fiber strength distribution, interfacial shear strength and matrix strength. The point of this model is to consider that a fiber group considered to be experiencing simultaneous fiber failures triggered by neighboring fiber failure is assumed to fail when the weakest fiber in the fiber group fails. A method to determine the magnitude of the fiber group for various systems of composites is discussed based on whether a crack located near a bi-materials interface penetrates into another material or deflects along the interface. The comprehensive model is established by integrating the magnitude of the simultaneous fiber failure into the conventional Global Load Sharing model.

The moisture absorption behavior of carbon fiber-reinforced plastic (CFRP) and its effect on dimensional stability were examined. Moisture diffusivity in CFRP was determined by measuring a specimen&apos;s weight during the moisture absorption test. Three types of CFRP specimens were prepared: a unidirectionally reinforced laminate, a quasi-isotropic laminate and woven fabric. Each CFRP was processed into two geometries - a thin plate for determination of diffusivity and a rod with a square cross-section for the discussion of two-dimensional diffusion behavior. By solving Fick&apos;s law expanded to 3 dimensions, the diffusivities in the three orthogonal directions were obtained and analyzed in terms of the anisotropy of CFRP moisture diffusion. Coefficients of moisture expansion (CMEs) were also obtained from specimen deformation caused by moisture absorption. During moisture absorption, the specimen surfaces showed larger deformation near the edges due to the distribution of moisture contents. This deformation was reasonably predicted by the finite element analysis using experimentally determined diffusivities and CMEs. For unidirectional CFRP, the effect of the fiber alignment on CME was analyzed by micromechanical finite element analysis (FEA) and discussed. (C) Koninklijke Brill NV, Leiden, 2008

Long-term durability of FRP under hostile environment is strongly influenced by fiber strength degradation. In this paper, the fiber strength degradation in the FRP in water under constant loading has been investigated using a single fiber composite (SFC). In order to estimate the fiber degradation due to aging in water, distributions of remaining fiber strength were obtained by fragmentation test. The remaining strength distributions have been quantified on the basis of microscopic stress corrosion cracking of initial defects on the fiber surface. Because the initial defect growth depends on the fiber stress history, increase of SFC strain in water has been also formulated. The remaining strength distribution obtained by the proposed model shows a good agreement with the experimental results. Additionally, it has been exhibited that the proposed model is available to predict increase of fiber failure caused by the fiber degradation in the SFC under constant loading in water.

Various composites are reinforced by high strength fiber. The contributing efficiency of the reinforcing fiber to composite strength is different in GFRP, CFRP, MMC, C/C, CMC, respectively. It depends on whether a fiber fracture penetrates in the matrix or deflect into interface in the class of omaterials that the composite strength is lower than bundle strength. On the other hand, in the class with poor interface, the tensile strength increases with an increase of interfacial strength. Furthermore, there exist a class of material in which both of mechanisms coexist. This study provides a parameter that determines the contributing efficiency of reinforcing fiber for the various composites.

Long-term durability of fiber reinforced plastics (FRP) under hostile environment is strongly influenced by the degradation in the fiber strength. The degradation of the fiber strength has been quantified on the basis of stress corrosion cracking (SCC) of the initial defects on the fiber surface. In this paper, the strength degradation of the embedded fibers under constant loading in water environment has been investigated using the single fiber composite (SFC). Creep test was conducted under water environment using the SFC specimen, to represent the stress corrosion cracking of GFRP. By taking account of the strain concurrent with the water absorption, creep strain, and the thermal expansion, the strain history of the fiber was formulated. From the strain history, the remaining strength distribution was measured by the fragmentation test. The model containing the Paris Law and the Arrhenius model has been developed to predict the remaining strength of fiber embedded in FRP.

Moisture absorption behavior of CFRP and its effect on dimensional stability was examined. Moisture diffusivity in CFRP was determined by measuring specimen weight during the moisture absorption test. Three types CFRP specimens, unidirectional, quasi-isotropic and cloth-laminated composites were prepared. Each CFRP was processed into two geometries; a thin plate for determination of diffusion constants and a rod with square cross-section for the discussion of two-dimensional diffusion behavior. Coefficient of moisture expansion (CME) was also measured from specimen deformation involving moisture absorption. During moisture absorption, the specimen surfaces deformed especially strongly near the edges because of three-dimensional moisture concentration distribution. This deformation mechanism was reasonably predicted by the finite element analysis using diffusion constants and CME, which have been experimentally determined.

The high-cycle fatigue characteristics, especially the initiation and propagation of edge delamination, were investigated with quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates with a stacking sequence of [45/0/-45/90]S. To investigate the influence that transverse cracks have on the initiation and propagation of edge delamination, two types of specimens are used. One was a specimen where transverse cracks were arbitrarily introduced by static tensile loading before conducting the fatigue tests. The other was an undamaged specimen as new. As a result, it was found that the single transverse crack introduced before the fatigue test did not seriously affect the initiation of edge delamination. Moreover, the difference of the fatigue damage growth behavior depending on the applied stress level was observed. Under the test conditions of low-applied stress level and high-cyclic loadings, it was observed that the edge delamination grew before, or simultaneously with, the transverse crack propagation.

The aim of this study is to measure the cure shrinkage of epoxy resin system used in the CFRP. First, the cure kinetics of Bisphenol A type epoxy resin system was studied by Differential Scanning Calorimetry measurement. From the dynamic DSC measurement, the total heat of cure of the epoxy was determined as 293 [kJ/g]. Also, curing condition and the extent of cure were determined by the isothermal DSC and TGA measurement. By controlling the curing condition, immature cured epoxy bar-shaped specimen was moulded. The cure shrinkage was measured by using the laser confocal displacement meter without any contact with the bar-shaped specimen. Comparing the specimen size after each cure, cure shrinkage of the bar-shaped specimen was measured.

High-cycle fatigue characteristics of quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates [-45/0/45/90](8) up to 10(8) cycles were investigated. To assess the fatigue behavior in the high-cycle region, fatigue tests were conducted at a frequency of 100 Hz, since it is difficult to investigate the fatigue characteristics in high-cycle at 5 Hz. Then, the damage behavior of the specimen was observed with a microscope, soft X-ray photography and a 3D ultrasonic inspection System. In this study, to evaluate quantitative characteristics of both transverse crack propagation and delamination growth in the high-cycle region, the energy release rate associated with damage growth in the width direction was calculated. Transverse crack propagation and delamination growth in the width direction were evaluated based on a modified Paris law approach. The results revealed that transverse crack propagation delayed under the test conditions of less than sigma(max)/sigma(b) = 0.3 of the applied stress level.

In this study the fatigue characteristics of quasi-isotropic carbon fiber reinforced plastics laminates subjected to variable amplitude cyclic two-stage loading were investigated. The cumulative damage was evaluated by considering residual strength as a parameter since the Linear Cumulative Damage rule, i.e., the Palmgren-Miner rule, did not show good agreement. Further, the internal microscopic damage was observed with an optical microscope. As a result, it was found that cumulative damage subjected to variable amplitude cyclic loading could be expressed by considering residual strength. Additionally, we were able to predict the fatigue life of laminates subjected to variable amplitude cyclic two-stage loading. (c) 2006 Elsevier Ltd. All rights reserved.

Fiber-matrix interfacial adhesion in composites is traditionally evaluated by means of a stress-based parameter. Recently, an interfacial energy parameter is suggested to be a valid alternative. However, the energy-based approaches overestimated the energy release rate to initiate the interfacial debonding (interfacial energy), since the plastic deformation in the vicinity of the debonding was neglected for simplicity. An effect of the plastic deformation on the interfacial energy of a fiber-reinforced polymer matrix composite is studied to evaluate the initiation of the interfacial debonding. The fragmentation tests with a model of glass fiber-reinforced vinylester matrix composite were performed, and the interfacial energy with the energy balance method taking into account an energy dissipation of the plastic deformation was calculated. The following results are confirmed; the plastic deformation has a significant influence on the interfacial energy, and the energy balance scheme taking into account the plastic energy dissipation leads to the constant interfacial energy without reference to the amount of the released potential energy. The differences between our model and the previous one are discussed.

Interfacial debonding propagation is one of the most important problems for long-term creep in unidirectional composites because the strength of the unidirectional composites decreases with increase of the interfacial debonding length. In this study, time-dependent interfacial failure was investigated by using the Micro Raman Spectroscopy. The specimen was single fiber composite consisting of one carbon fiber embedded in the vinylester resin. It was subjected to a constant strain to assume the long-term creep in the unidirectional composites. To discuss the timedependent interfacial debonding propagation behavior, a time-dependency of a criterion for interfacial debonding propagation in the SFC must be determined. In this paper, the time-dependent boundary condition of the interfacial debonding was obtained from experimental results, and the interfacial debonding propagation was formulated. Here, the compression stress applied to the interface in the fiber radius direction was formulated as a function of time on the consideration of Poisson contraction and thermal residual stress. The interfacial debonding propagation and variation of the fiber axial strain distribution as a function of time were formulated by using the following three parameters : a relaxation of the frictional stress, an interfacial shear viscoelastic behavior and the time-dependent boundary condition at the interfacial debonding tip. These formulations showed a good agreement with the corresponding experimental results.

Tests for evaluating delayed fracture of an E-glass fiber were conducted under water environment. Loaded E-glass fibers were immersed in purified water, and time-to-fracture was measured. Since the delayed fracture was confirmed under the water, the moisture was the only environmental factor for the delayed fracture. The relationship between the chemical reaction and the fiber strength was evaluated for estimating moisture-absorption effect on the delayed fracture. The E-glass fiber surface was observed by XPS and AFM. These observations showed that roughness on the glass surface increased due to the moisture-absorption. Furthemore the fiber-bundle strength decreased with increasing immersion time. The results of these observations indicated that microscopic cracks accelerated the delayed fracture under the water. It is concluded that the delayed fracture of the E-glass fiber occurred by microscopic crack growth caused by moisture-absorption on the glass surface.

Viscoelastic property of CFRP is one of most critical parameters which dominate its dimensional stability. In order to investigate the time-temperature dependency of micro-scale deformation, four point bending creep tests were conducted. It is reported that extremely slow cure causes microscopic-scale deformation. So the influence of aged shrinkage upon secular change of dimension was investigated and creep behaviors with two different curing conditions were compared.

Delayed fractures in carbon-fiber-reinforced polymer matrix composites can be observed even at room temperature due to an accumulation of micromechanical damages, such as time-dependent interfacial debondings or fiber breakage. In this study, the creep-rupture time in unidirectional composites consisting of carbon fiber reinforcement and vinylester resin is predicted under consideration of the progress of interfacial debonding which decreases rupture strain of the composites due to increase of the stress recovery length in time. The decrease of the rupture strain in unidirectional composites is formulated based on the global load sharing, which assumes that lost load distribution involved a fiber break is uniformly redistributed to all the intact residual fiber in the range of the stress recovery length. Time-dependency of the stress recovery length including the interfacial debonding length is analytically and experimentally investigated using carbon/vinylester single-fiber composite as a function of time and specimen strain. The creep rupture prediction is completed by means of putting the increase of stress recovery length into it. The validity of this creep rupture prediction is discussed by investigation of the residual strength after quasi-creep test, which must be derived by the formulated creep rupture prediction.

An exact theory is developed to evaluate propagation of an interfacial debonding in single fibre composite by means of the energy release rate due to crack growth. The debonding propagation during fragmentation test is significantly affected by frictional slip at the interface in the debonding region. Hence, frictional dissipation of the energy release rate occurs along with the debonding propagation. An analysis considering the effect of the interfacial friction is conducted. The fragmentation tests are performed varying the amount of fibre pre-stress, and the debonding growth is observed directly. To validate the present analysis, the debonding propagation predicted by the present theory is compared with the experimental one. The theoretical prediction is proved to be in the excellent agreement with the experimental results, which illustrates the importance of including the frictional effect in the energy release rate. An application of the present theory to fibre fragmentation in single-fibre composite test that can evaluate the fibre strength is described.

Long-term durability of polymer-matrix composites (PMCs) in hostile environments is described. Characteristics of a stress-corrosion crack in glass fiber reinforced plastics (GFRP), creep fracture in unidirectional carbon fiber reinforced plastics (CFRP) and predictions of a variable loading fatigue life in CFRP laminates are the main topics of this paper. To enhance the performance of the PMCs as structural materials, many improvements are achieved so far. Characterization of mechanical properties under the hostile environments is still need for the studies. © 2005 Elsevier B.V. All rights reserved.

This study investigates an effect of time-dependent debonding propagation of fiber/matrix interface and that of stress concentrations to adjacent fibers from broken fibers on creep rupture lifetime in unidirectional composites. Above two phenomena are concerned to reduce materials strength and effect on creep rupture lifetime in unidirectional composites. A creep rupture model that takes into account the decrease of rupture strain due to time-dependent inter facial debonding propagation, based on the GLS rule is proposed and the rupture lifetime is predicted. It was found that the rupture lifetime is strongly dominated by the interfacial debonding propagation rate, and rupture lifetime is shortened considerably in case that time-dependent debonding propagation is remarkable. Subsequently, the change in the rupture strain due to the stress concentration without interfacial debonding propagation is estimated using a 3D shear-lag model and the rupture lifetime taking into account stress concentration is predicted using the McLean's creep model. Prediction result is compared with the result of Curtin-McLean model in which the stress concentration is not taken into account and the effect of stress concentration is investigated.

The effect of water on crack propagation resistance by fiber bridging of glass fiber reinforced plastic (GFRP) has been investigated. Specimens were immersed and weighed just before conducting the crack propagation test. Decrease of critical energy release rate by the water was measured as a function. The critical energy release rate of the matrix decreased with increase of the water absorption. The crack resistance of a bridged crack varied with the water absorption. To investigate the variation in the bridging mechanism under the water environment, we calculated the moisture concentration distribution on the cracked plane. As a result, the moisture concentration affected the crack propagation resistance. The increase of the moisture concentration caused the decrease of crack propagation resistance.

This paper presents the characterization of nonlinear viscoelastic behavior in vinylester resin. The description of nonlinear viscoelastic behavior is based on well-known Schapery's constitutive equation. Cardon proposed the method that can estimate nonlinear parameters in Schapery's constitutive equation, separately by creep-recovery tests in consideration of permanent deformation nonexistent in original Schapery's constitutive equation. However, formulation of permanent deformation has almost never been discussed in these articles, In this study, the irreversible strain is formulated and nonlinear viscoelastic constitutive equation taking into account the permanent deformation is proposed. It is found that the behavior of irreversible strain is nonlinear and it has a threshold stress value between the linear behavior and the nonlinear behavior same as nonlinear parameters in Schapery's constitutive equation. Tensile tests in which crosshead speed is changed as time-dependent factor is performed to verify that the proposed constitutive equation can predict time-dependent behavior. Fine agreement between the experimental results and prediction is observed and the validity of the constitutive equation is confirmed.

The effect of water on fiber bridging of polymer matrix composites (PMCs) has been studied for stress-corrosion cracking (SCC). The fiber bridging during crack propagation in the PMC was observed directly using a transparent polymer matrix. Specimens were immersed and weighed just before conducting the crack propagation test. The contribution of mechanical degradation caused by the water to the bridging was measured as a function of weight gain due to water absorption. The critical energy release rate of the matrix decreased with increase of the water absorption. The energy release rate of a bridged crack varied with the water absorption. To investigate the variation in the bridging mechanism under the water environment, we calculated the bridging stress the σ_t and the bridging contribution to crack propagation resistance ΔG based on a single-fiber bridging mechanism with the interfacial debonding length and the interfacial debonding energy. The debonding length was measured directly, and the debonding energy was obtained in a fragmentation test. The calculation showed that the increase in the interfacial debonding length influenced the fiber bridging mechanism.

This paper describes the high-cycle fatigue characteristics of quasi-isotropic CFRP laminates [-45/0/45/90] s up to 10 8 cycles. To investigate the fatigue behavior in the high-life region, the fatigue tests were conducted with the frequency of 100Hz since it's difficult to conduct the tests with 5Hz. Then, the damage behavior of the specimen was observed with a microscope, a soft X-ray photography and a 3D ultrasonic inspection system. In this study, to evaluate quantitative characteristics of each the transverse crack propagation and the delamination growth in the high-cycle region, the energy release rate associated with damage growth to the width direction was calculate. The transverse crack propagation and the delamination growth to the width direction were evaluated based on a modified Paris-law approach. As the results, it was found that transverse crack propagation didn't depend on the frequency within the small temperature change and it was observed that delamination growth delayed with the test condition of less than σ max/σ b=0.3 of the applied stress level.

Recently the crack propagation properties of GFRP on the stress corrosion cracking (S.C.C) are investigated, and the threshold stress intensity factor K-ICC is verified in some environmental solution. From the investigation, it was found that GFRP reinforced by C-glass fiber has a superior acid resistance. However the microscopic crack propagation mechanisms caused by the material corrosion are not verified, and the microscopic mechanisms are necessary to assure the durability. Therefore the degradation mechanisms of the inner fiber and the matrix and the fiber/ matrix interface should be quantified. In this study, the degradation of the fiber strength and the fiber/matrix interfacial shear strength are investigated using a single fiber composite previously immersed into environmental solutions, distilled water and acid solution. The effects of solution diffusion into the matrix resin on the fiber strength and the interfacial shear strength have been evaluated as a function of immersion time by fragmentation test in the room air. It is found that the diffusion of distilled water influences the degradation earlier than the acid solution. And the diffusion behavior is confirmed by Fickian diffusion analysis. The calculated concentration distribution showed that the water concentration around the fiber is saturated much earlier than the saturation of the acid ion due to the lower diffusion coefficient. Furthermore the crack propagation mechanisms are discussed based on the degradation estimated by the fragmentation test.

Over the past several years stress corrosion cracking of the FRP plant applications have been a serious problem for durability. So, the elucidation of the fracture mechanism is indispensable to adopt GFRP as a structure material. The purpose of this study is to clarify the threshold property of crack propagation of C-glass/vinyl ester composite. Load descending test is performed for the unidirectional GFRP (90゜specimen). As a result, the existence of the threshold properties is observed, and it is confirmed that the cause is the fiber bridging by the FEM analysis by which the fiber bridging and the cohesive stress are considered. Additionally, the prediction of initiation of the damage is considered in view of time to failure of the fiber by stress corrosion cracking.

This paper describes creep rupture model considering interfacial debonding and relaxation on interfacial shear stress in broken fibers in unidirectional phenol matrix composites reinforced with glass fibers. Interfacial debonding is accompanied with fiber breaks under the tensile load. It depends on the strain of the composites and is represented as a function of the strain by considering the energy balance equation of interface. Interfacial debonding length, fiber failure probability and interfacial shear stress are measured experimentally on fragmentation test and then formulated as a function of the strain. Interfacial debonding length is taken in Curtin's model that is considering break of fiber and creep rupture time is predicted. Relaxation on interfacial shear stress is also taken in creep rupture model in a long time.

Properties of the fiber/matrix interface in SiO2/epoxy and SiC/epoxy composite are investigated using the slice compression test (SCT) for the single fiber, where the specimen is loaded and unloaded between a plate which has different mechanical properties. It is found that the interfacial debonding proceeds from the polished surface at a soft plate side and that the fiber protrusion occurs after unloading. The fiber-protrusion length is directly measured at each applied stress level using a scanning electron microscope. Interfacial shear-sliding stress is obtained based on the constant shear-sliding stress analysis employing the obtained protrusion length.
It is demonstrated that the value of interfacial shear-sliding stress shows good agreement with that obtained from another technique, the push-out test, on the same system. The relation between the fiber-protrusion length and applied stress is proportional to a certain extent. From this result, it is analytically pointed out that the applied stress has a limiting value in this SCT because of Poisson's effect. Also, two interfacial debonding criteria, which are determined analytically for the PMC, are discussed.

Objective of this study is to clarify the crack-propagation properties of corrosion resistant GFRP laminates under various acid stress environments, an effect of the constituents on fracture and the threshold properties of the crack propagation. This experiment was performed under constant loading in HCl solution and the crack propagation behavior was depicted on a K_1-da/dt diagram. The fracture surfaces of the specimens were observed by scanning electron microscope. As the acid concentration increased the crack propagation was accelerated, however, the effect of the acid concentration was not so dominant. The crack propagation behavior in the water was largely enhanced in comparison with that in the air and it was nearly as promoted as that in the acid solution. Therefore it was noted that the failure of the C/VE systems was occurred by not fiber corrosion but absorption and swelling derived from the diffusion of the water into the matrix and a changing of the stress shearing on the fibers at crack tip.

Carbon/Carbon(C/C) composites have attractive mechanical properties such as superior specific strength and high elastic modulus at high temperature exceeding 2000 degrees C in an inert atmosphere. However, mainly due to lack of knowledge of design criteria, C/C composites have not been used in primary heat resistant structures. For example, almost no unified explanation has been given about the fracture behavior of C/C composites. The objective of this paper is to examine the adequacy of the linear elastic fracture mechanics (LEFM) as the fracture criterion of notched C/C composites. Thus the LEFM was tried to be applied to fracture behavior in tensile tests of double-edge-notched and compact tension specimens and in four-point bending tests of single-edge-notched specimens. It was found that the results of three kinds of fracture tests can be consistently and rationally explained in terms of the LEFM concept with the aid of R-curve behavior if the pre-crack length is long enough to be able to neglect the notch tip radius. From fractographic observation it was found that R curve behavior of C/C composites was mainly attributed to the fiber-bridging effect near the notch tip.

Carbon/Carbon (C/C) composites have attractive mechanical properties such as superior specific strength and high elastic modulus at high temperature exceeding 2000°C in an inert atmosphere. However, mainly due to lack of knowledge of design criteria, C/C composites have not been used in primary heat resistant structures. For example, almost no unified explanation has been given about fracture behavior of C/C composites. The objective of this paper is to examine whether the linear elastic fracture mechanics (LEFM) is able to be applied to a C/C composite. The LEFM is tried to apply to fracture behavior in tensile tests of the double-edge-notched and compact tension specimens and in the four-point bending test of the single-edge-notched specimens. It was found that the results of three kinds of fracture tests can be consistently and rationally explained by the LEFM concept with the aid of R-curve behavior if the pre-crack length is long enough to be able to neglect the notch tip radius. From fractographic observation it was found that R-curve behavior of the C/C composite was mainly attributed to the fiber-bridging effect near the notch tip.

In order to establish design criteria for a turbine disk made of Carbon-Carbon (C/C) composites, spin burst tests were performed on quasi-isotropically laminated C/C composite disks. Un-notched and notched flat disks were prepared to evaluate the effect of stress concentration on fracture behavior.
Strain measurements during rotation tests revealed that deformation of the C/C composite disks with or without notches was in agreement with analytical or finite element calculations in the elastic region, which suggests that the test fixtures of the spin tester have a negligible effect on the deformation of C/C composite disks.
In the spin burst tests, the un-notched C/C composite disks burst at an average rotation speed of 25000 r.p.m. with a peripheral velocity of 393 mis. A comparison between the mean hoop stress averaged over the radius and the tensile strength of smooth specimen suggests that the burst of the un-notched C/C composite disks obey the mean stress criterion. On the other hand, the fracture toughness criterion was shown to be applied for the notched specimen. The difference in the fracture criterion between the un-notched and the notched C/C composite disks is believed to correspond to that between the double edge notched specimens and specimens with a hole in the static tensile tests, (which was explained by the difference in the magnitude of the damaged zone where the stress concentrations would be relaxed).

Compression tests on GF/Nylon6 tubes were performed to clarify the mechanism of the initial failure process and the energy absorption behavior. It was found that all the specimens were crushed in a progressive crushing mode regardless of the trigger geometry, but that the specimens with an asymmetric trigger demonstrated better absorbing characteristics than the symmetric specimens. The formation mechanism of the debris wedge was examined through a stepwise morphology analysis. It was found that changes in the trigger geometries affected the performance of the initial failure process which was characterized by the formation of a debris wedge formation process. It was also shown that at high testing speeds in the range of practical use, GF/Nylon6 tubes exhibited similar energy absorbtion behavior to that of a CF/PEEK system.

The effect of stress concentrations on tensile fracture behavior of carbon-carbon (CIC) composites was investigated using circularly holed specimens and double-edge-notched (DEN) specimens.
As for the circularly holed specimens, the tensile fracture stress was much higher than that estimated from the maximum stress criterion, which suggest that major stress relaxation mechanisms should exist. On the other hand, the linear elastic fracture mechanics can be applied to the DEN specimen, which means the damaged zone should be small enough compared with the notch length. In order to discuss the magnitude of the stress relaxation, damaged regions of the two kinds of testing geometry were estimated using the point stress criterion. The estimation led to remarkable difference in the size of the damaged regions, which will explain the difference in the magnitude of the stress relaxation.
Through the observations of fractured specimen, it was deduced that not only the shear deformation but delamination along fiber bundles and opening of transverse crack would relax the stress concentrations. The other mechanism was also proposed based on the testing results, that is strength increase in the damaged region.

Compression tests on CF/PEEK tubes were performed using a MTS servo-hydraulic material testing machine and a crush testing machine to clarify the mechanism of the initial failure process and the characteristic of the energy absorption. It was found that all the specimens were fractured in a progressive crushing mode and the specific energy absorption indicated the dependence of testing speed in the range of our study. Morphological observation was conducted to examine the influence of testing speed on the specific energy absorption. The mechanism in the steady state stage which takes place after the maximum load is characterized by the interlaminar cracks and lamina breakage at the crush zone. In the initial failure process, the formation mechanism of the debris wedge was examined through the step-wise morphology analysis. The angle and the dimension of the debris wedge were the essential factors in the specific energy absorption.

This paper presents stress corrosion cracking (SCC) of notched GFRP laminates under an acid environment. Based on a fractographic analysis, it is found that the stress-corrosion cracking a is governed by a breakage of the warp fiber strand. The crack propagation rate is possible to estimate from a microscopic fracture model of the warp fiber strand. To obtain the crack propagation rate, a fracture model is proposed on the basis of some assumptions as follows : ( 1 ) A relation between an applied stress and a mirror zone radius (in the fracture surface of the warp fiber) obeys the Jaras's equation, ( 2 ) Shape of the warp strand's shape is almost an ellipse, ( 3 ) The crack is a self-similar one during the propagation. The crack propagation rate is obtained as a function of the stress intensity factor. It is found that its value agrees the experimental value, and confirmed that the proposed microscopic fracture model is appropriate for evaluating the crack propagation rate in an acid environment.

Static compression tests of unidirectional AFRP with a circular hole are carried out. The experimental result shows that the compressive strength of unidirectional AFRP containing a circular hole depends on the hole diameter. As the hole diameter increases, the compressive strength tends to be lower in spite of a decrease in the stress concentration factor α_r. A good agreement between the experimental and the predicted compressive strength is observed by the application of two failure criteria. For the specimen with small hole diameter, the strength can be predicted by the Soutis' theory based on the stress distribution around the hole and linear fracture mechanics. For the specimen with large hole diameter, the strength can be predicted by the point-stress criterion(d_0=1.5mm). Finally, the failure mechanism of the unidirectional AFRP subjected to compressive loads is discussed.

In-plane Mode II fracture toughness tests of unidirectional GFRPs are conducted using a four-point shear loading test method. In the previous paper, the effect of the pre-crack length on the fracture toughness was investigated, and the advantage of this test method was also discussed. In this study, the same test method is used to evaluate the fracture toughness of the unidirectional GFRP. The normalized stress intensity factor concerning an orthotropic body of the unidirectional GFRP is calculated by a finite element method. The mode II fracture toughness is found to be constant regardless of pre-crack length. Using the finite element method, the propagation of the damage zone at the initial failure process is obtained. The analytical mode is shown to be quite effective and the initial failure process is well simulated by this modelization.

To evalute fracture toughness of C/C composites, four-point bending tests (SEN) are performed. The C/C composites are carbonized from the CFRPs, which are fabricated by means of two different methods (a hot-press method and a vacuum-bagging one). It is found that each fabrication method leads to a different distribution of bending strength. The vacuum-bagging method is preferable to obtain uniform bending strength properties. Crack growth resistance curves are obtained based on the compliance matching method. These curves have an inclination of convergence at a nearly equal constant value regardless of fabrication method. Existence of the stress shielding mechanism in the C/C composites is suggested by the R curves. From a fractographic observation, crack propagates three-dimensionally, and then pullouts of the fiber bundle occur at a large scale in the fiber-bridging mechanism.

In-plane Mode II fracture toughness tests on unidirectional GFRP are conducted using a four-point shear loading test method.In the previous paper,the four-point shear loading test method was examined as a simple Model II fracture toughness test.Then the effect of pre-crack length on fracture toughness was investigated.In this study,the same test method is applied to evaluate the Mode II fracture toughness.The dependence of the pre-crack length and the initial failure process are clarified.The results obtained are summaruzed as follows;(1) It is revealed that the crack opening displacement is much less than the crack relative sliding displacement in the four-point shear loading test.(2) From an analysis of the damage propagation length,it is found that the initiation of the main crack is close to the maximum load point.The initial failure behavior of the unidirectional GFRP subjected to shear loading is clarified.(3) The normalized stress intensity factor concerning an orthotropic body of the unidirectional FGRP is calculated by a finite element method.The mode II fracture toughness is constant irrespective of the pre-crack length.

Mode II fracture toughness tests of unidirectional GFRP were conducted using a four-point shear loading test method This test method was a modified Iosipescu test method which was identified to be the most reliable one for obtaining a shear stress-strain relation in composite materials. In this study, a leverage ratio of the loading and a pre-crack length ratio of the specimen were examined to perform the fracture toughness tests. Fracture behavior of the unidirectional GFRP under shear loading was investigated through a fractographic observation. The results obtained are summarized as follows: (1) The four-point shear loading test method can be assessed at a simplified in-plane mode II fracture toughness test. To prevent premature failures without reinforcement attached to the specimen, it is necessary for the pre-crack length ratio, a/W to be larger than 0.3 under the definite leverage ratio. (2) The mode II fracture toughness evaluated in terms of stress intensity factors at the maximum load points was found to be dependent on the pre-crack length ratio. (3) The Hackle patterns were confirmed at a stable crack propagation region which are typical fracture traces on the matrix under the shear loading. The formation mechanism of the Hackle at different parts was investigated in detail, and then the physical meaning of the fracture toughness obtained in this experiment was discussed.

Rail-shear tests were carried out on the glass cloth/epoxy laminates, and then the mode II fracture toughness value K_<IIin> was obtained as the pre-crack length, 2a/ W, was altered from 0.2 to 0.45. In addition, the spread of damage was obtained under simulation by the finite element method and then compared with the experimental results. The following results were obtained; (1) In this test method, the recommendable pre-crack length existed in order to obtain the fracture toughness value. (2) The material used in this study was assumed to be a homogeneous an isotropic material of a tetragonal system, and then the finite width correction factor of the specimen geometry was calculated. Consequently, K_<IIin> became almost constant in the range of 2a/ W=0.25〜0.45. (3) The spread of damage was simulated by the finite element method, and it was found that the analytical result agreed with the initial failure process obtained in the experiment. In addition, the J_<II> value was calculated from the path integral, and discussed.

Tensile tests were conducted on specimens of plain woven glass cloth/epoxy laminates with a notch. The effect of the notch root radius p on fracture was investigated. In the previous study, the three-point bending tests were carried out to make clear the effect of the notch root radius on fracture. In this study, the experimental results were compared with those of the previous study. The F.E.M. simulation was performed, and compared with the experimental results. The following results were obtained: (1) As the notch root radius increased, the smaller ρ specimen showed the fracture behavior which was caused mainly by the propagation of a stable crack, but the larger ρ specimen showed the fracture behavior which was caused mainly by the spread of the damage. (2) The spread of the damage zone was different from that of the three-point bending test, but the effect of the notch root radius on strength remained unchanged for the tension and the bending test. (3) The F.E.M simulation explained the experimental result of the larger ρ specimen well.

Three point bending tests were conducted on specimens of plain woven glass cloth/epoxy laminates (fiber volume fraction≒0.4) with notch and R curves were obtained. The effects of the thickness and the size of the specimens on the fracture toughness were investigated. The crack propagation was experimentally observed with the ink method. The electric potential method and the acoustic emission method were carried out to detect the initiation of the crack. In this study, the validity of these nondestructive inspection method was examined corresponding to the load-displacement curves. The following results were obtained: (1) the effect of the thickness on the fracture toughness was less than that of the specimen size, (2) the outputs in the electric potential method were correlatable with the initiation of the damage and the propagation of macroscopic crack, and classified into three distinct patterns and (3) AE counts gave so much information for the fracture behavior in such a way that the initiation of the damage and the proportional limit corresponded to the initiation and the rapid increase of the AE event counts, respectively.

Fiber reinforced plastics is increasingly used to improve fuel efficiency and motion performance of vehicles by weight reduction. Especially, short fiber reinforced thermoplastics is expected to expand its demand because of its superior moldability, productivity, and recyclability. In this study, influences of glass fiber diameter and cross sectional shape on the impact properties of short glass fiber reinforced polyamide were investigated using split Hopkinson pressure bar. In the tensile test, specimen with smaller fiber diameter showed higher tensile strength. Glass fiber reinforced polyamide with flat glass fiber also showed higher strength than that with normal glass fiber. On the other hand, a significant influence of fiber diameter and cross sectional shape on mechanical properties was not found in the compressive test. As a result of the fracture surface observation using scanning electron microscope and the average fiber length measurement, it was revealed that interfacial fracture was a dominant fracture mechanism under tensile loading whereas matrix fracture was dominant under compressive loading. Consequently, the influence of fiber diameter and cross sectional shape on impact properties was different under compressive and tensile loading.

Application of CFRP tubes for torque transmission shafts is expected to improve driving performance and natural frequency as well as fuel efficiency. CFRP tubes manufactured by modified simultaneous multi ply winding method have less voids and fiber waviness compared with conventional CFRP tubes manufactured by filament winding method, and the modified CFRP tubes showed 20% improvement of the static torsional strength. We investigated the fracture mechanisms of CFRP tubes by modified simultaneous multi ply winding method. As a result, a delamination from the prepreg end occurred at the innermost layer and it progressed through ±45° interlayer just before the failure of the specimen. In order to apply CFRP tubes to torque transmission shafts, it is expected to design the stacking sequence appropriately for preventing the delamination from the prepreg end in terms of long-time reliability. Thus in this study, we investigated an influence of the stacking sequence of CFRP tubes on their torsional strength. According to static torsional tests, it was revealed that the specimen with less lamination angle differences caused less delamination due to relaxation of interlaminar shear stress. Moreover, torsional strength was improved moderately mainly because the small lamination angle differences delayed the initiation of the delamination.

Carbon nanotubes (CNTs) grafted on the fiber surface improves fiber/matrix interfacial adhesion, and form the CNT-resin local composite around each fiber. This study reports the fabrication method of CNT-grafted unidirectional glass fiber reinforced plastics (GFRP) and their mechanical properties. In this study, CNTs were grafted on the T-glass fiber surface using thermal chemical vapor deposition (CVD) at 750℃. First, quality of grafted CNTs and the mechanical properties of T-glass fiber were evaluated. From the results of investigations and analyses, CNTs grafted onto the fiber surface had relatively-many defects, but were uniform and dense. Moreover, mechanical properties of CNT-grafted T-glass fiber decreased drastically compared with as-received fiber. This degradation was caused by the relaxation of thermal residual stress at fiber surface. Finally, static tensile test were performed to investigate the tensile properties of GFRP. According to static tensile test, it was revealed that fracture strain of CNT-grafted GFRP was decreased by 80% compared with as-received one, and it showed brittle fracture. This fracture was mainly due to the degradation of fiber strength and the improvement of fiber/matrix adhesion.

In the automotive industry, reducing vehicle body weight is one of the most effective ways to increase fuel economies. Fiber Reinforced Thermoplastics (FRTP) have several advantages over FRP or thermoplastics such as high specific strength and stiflBiess, high moldability. Fiber reinforced polyamide is one of FRTP and in engine parts (e.g. cylinder head covers, intake manifolds) . However, its applications for structural materials or energy absorption ones are limited due to the strict design standards. Therefore, it is necessary to evaluate impact properties of the material. The objective of this paper is to investigate strain rate effects on flexural properties of glass fiber reinforced polyamide. Reinforcement morphology of the materials were two types, twill weave (TWGF) and short fiber (SGF) . Drop weight impact test was used to determine impact flexural properties. The results showed that both TWGF and SGF had strain rate dependencies on flexural properties. SGF had higher strain rate dependency on flexural modulus owing to its higher viscoelastic property, while TWGF had higher one on flexural strength and specific absorbed strain energy owing to its superior tensile property. Furthermore, it was revealed that both TWGF and SGF had high impact absorption properties.

In recent years, carbon nanotubes (CNTs) have attracted a lot of interest as an additional component in fiber reinforced plastics (FRP) to improve the properties of the fiber/matrix interface. An improvement of the apparent interfacial shear strjpnth (1SS) was achieved by grafting CNTs onto reinforcement fibers instead of dispersing CNTs in the matrix. In one study, composites containing CNT-grafted fibers and epoxy resin demonstrated 26% ISS improvement over the baseline composites0). However, few studies are focused on glass fibers, due to their low heat resistance. In this study, the effects of grafting CNTs onto T-glass fibers were investigated, especially focusing on the mechanical and interfacial properties of CNT-grafted T-glass fiber reinforced plastics. We used the chemical vapor deposition (CVD) method to graft CNTs onto T-glass fibers. As a result, CNTs were grafted relatively uniformly and cylindrically onto the fibers, which indicates that the CNT-grafting process was appropriate. The CNT-grafted fiber/epoxy resin model composites showed a significant (46-67%) increase of interfacial shear strjpnth. The formation of an interfacial region containing CNTs was observed around each fiber.

2Waseda University Dept. of Applied Mechanics and Aerospace jpnineering, Okubo 3-4-1 Shinjyuku-ku, Tokyo Grafting carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber-matrix interfacial adhesion enhance the composite delamination resistance, and possibly improve its toughness and matrix-dominated elastic property as well. In this study, CNTs were grafted upon Tyranno fibers by the chemical vapor deposition (CVD) at temperatures of 750℃. The ljpnth of the CNTs was controlled by the surface treatment of the fibers and the grafting time. Tensile tests of the single fiber were conducted to examine the thermal effect during the CVD process. It is well known fact that the high heat resistance of Tyranno fiber, it was clear from the results that CNTs grafting process didn't decrease the mechanical property of Tyranno fiber. Moreover the interfacial shear strjpnth of CNTs grafted Tyranno fiber in an epoxy resin was evaluated using the single fiber composite (SFC) fragmentation test. Results of the SFC fragmentation tests indicate an improvement in interfacial shear strjpnth with the addition of the CNTs grafting.

This study reports the off-axis delayed fracture characteristic of woven glass fiber reinforced plastics (GFRP) laminate under constant tensile load in hydrothermal environment. In this study, static tensile test and constant tensile load test were performed in air and water at 40℃. Mechanical properties of GFRP were evaluated by static tensile test. Static tensile test was conducted in water for 200, 500 and 1000 hours, to find out the effect of hydrothermal environment on mechanical properties of GFRP. Result of static tensile test, the maximum stress decreased and the strain at maximum stress decreased greatly in water. In addition, the maximum stress decreased in short term immersion and converged after 200 hours. Constant tensile load test of GFRP was conducted to investigate compliance and fracture time of GFRP. The applied stress level of constant tensile load test was determined as 15%〜75% of the maximum stress obtained from static tensile test. As the result of constant tensile load test, the compliance increased obviously and delayed fracture occurred earlier in water than air. In this study, prediction was conducted for off-axis woven GFRP laminate under constant tensile load in 40°C water using the Reiner-Weissenberg(R-W) criterion; which is based on the non-linear viscoelasticity response. The prediction data was compared well with the experimental data by considering the degradation of GFRP in 40°C water.

Accurate geometrical stability is required for the precise structures like telescopes. It was reported that symmetrical CFRP (Carbon Fiber Reinforced Plastics) laminates show unpredictable deformation due to the ply angle misalignment and temperature change. This ply angle misalignment is unavoidable. One of the answer to mitigate the deformation due to the ply angle misalignment is to chose effective stacking sequence. We discussed here the effective stacking sequence to reduce the thermal deformation.

This study aims to quantify the effect of physical aging progression on viscoelastic property in carbon/epoxy composites. Understanding viscoelastic behavior of carbon fiber reinforced plastics (CFRPs) has become an essential issue for further application of CFRPs to aerospace structures, which require high dimensional accuracy. In this research, the viscoelastic property was evaluated by quasi-static tensile tests in transverse direction for unidirectional laminates. The progression time of physical aging versus viscoelastic parameters was obtained. It was found that the viscoelastic behavior becomes less significant as the physical aging progresses. Considering the quantitative result, creep compliance function, which is conventionally difficult to predict due to the unknown physical aging effect, was predicted as functions of creep time and preceding aging time. This prediction was verified by actually performing a long-term creep test in transverse direction for unidirectional composites. Creep test result and the calculated results using viscoelastic parameters obtained from quasi-static tensile tests showed similar results up to the aging time given to the specimen before the test, but they showed different trends along with time progression. By taking into account the progression of physical aging, the calculated lines are likely to show better correspondence with the creep curves.

The interfacial strength is especially the important property amongst the other components', reinforcing fiber and matrix which consist of PMC. For the further use in industrial application, the evaluation of interfacial strength under dynamical loading is requested. In this study, we conducted the tensile test on SFC specimen and observation of nucleation of interfacial debonding at different strain rates, and recognized the dependency of interfacial strength on strain rates. According to elasto-plastic analysis with the assumption that matrix's yield stress be the variant of strain rates, stress distribution of the interface asymptotically approaches the elastic analysis, and it can be concluded that the dependency of interfacial strength on strain rates caused by matrix's dependency on strain rates.

The cause of strength increase in the glass-fiber at cryogenic temperature has not been fully studied so far. In the present study, tensile tests of a single E-glass fiber in air and liquid nitrogen were conducted to reveal the mechanism of strength increasement. From the mirror zone measurement on the fracture surface, the mirror constant's value was determined. It was clarified that the mirror constant's values of the fiber were equal regardless of the heat treatment condition and testing temperature. These experimental results suggest that an increase of the glass-fiber strength in cryogenic environment is attributed to not the molecule network changing but the crack blunting of the fiber.

In this study, we focused on the geometrical change of symmetric laminates that are widely used for the composite structures. The behavior of time-dependent out-of-plane deformation under high temperature and humidity was examined with high accuracy. The geometries of the CFRP plates were measured after constant time. It was obvious that the main factor which causes geometrical change was moisture absorption at 80°C, 90%RH environment. The geometrical change was calculated based on Finite Element Analysis considering fiber misalignment. Coupling analysis including stress and diffusion analysis was performed to reproduce the time-dependent deformation. In this analysis, we rotated the layer of the symmetric laminate about 5 degrees, so specifically the model was asymmetric, and the coupling effects that arose bending by tension took place due to moisture absorption. Quasi-isotropic laminate model including small fiber misalignment deformed into saddle shape and twisted saddle shape. It was predicted that it must be needed to control fiber alignment strictly when we product CFRP laminate with high dimensional stability.

In order to quantify the dimensional stability in CFRPs at humidity environment, the water diffusion considering an anisotropy and moisture expansion behavior are investigated. Water diffusivity coefficients were determined by measuring the specimen weight during the moisture absorption test. The diffusivity coefficients of three main axial directions are obtained from three shapes specimen&#039;s result and solving simultaneous equations. A 3-dimensional locator using laser-displacement measurement was employed to measure micro deformation of the CFRP specimens caused by moisture absorption. The curvature of surface was observed due to the distribution of moisture contents. The coupling analysis which includes mechanical deformation and thermal diffusion was conducted. The numerical results are in good agreement with the test results.

In order to quantify the dimensional stability in CFRPs, the water diffusion considering an anisotropy and moisture expansion behavior are investigated. Water diffusivity coefficients were determined by measuring the specimen weight during the moisture absorption test. The diffusivity coefficients of three main axial directions are obtained from three shape specimen&#039;s result and solving simultaneous equations. A 3-dimensional locator using laser-displacement measurement was employed to measure micro deformation of the CFRP specimens. The curvature of surface was observed due to the distribution of moisture contents. The coupling analysis, which includes mechanical deformation and thermal diffusion, was conducted. The numerical results are in good agreement with the test results.

A numerical study was performed on the fracture toughness test to obtain the critical strain energy release rate (G_<IC>) of Glass Fiber Reinforced Plastic (GFRP) laminates in wind turbine blade. Two-dimensional numerical analysis, based on an elastic finite element method (FEM) using the static explicit method and the cohesive damage model, was performed to simulate the fracture toughness test. The cohesive damage model used to predict the delamination occurrence and growth is based on a special developed interface finite element including an exponential damage process. The numerical prediction showed good agreement with the experimental results.

Damage growth behavior of transverse crack and delamination in cyclic loading was studied with cross-ply laminates of carbon fiber reinforced plastics (CFRPs) composite. Damage of the specimens during the fatigue test was observed by soft X-ray photography. It was found that delamination growth behavior depended on the inter-laminer stress and the out-plane stress in thickness direction caused by free-edge effect. Quasi-isotropic, [45/0/-45/90]_s, laminates, are applied large peering stress at free edges of the specimen when the specimen is subjected to tensile stress. Therefore they showed edge delamination. On the other hands, cross-ply, [0/90_6]_s laminates, are applied large inter-laminer shear stress at transverse crack tips, and show local delamination originated by a transverse crack. Under the high-cycle loading, it was observed that depending on the applied stress level delamination growth behavior differed.

Long-term durability of fiber-reinforced-plastics (FRP) under hostile environment is strongly influenced by the degradation of fiber strength. The degradation of the fiber strength has been quantified on the basis of microscopic stress corrosion cracking (SCC) of the initial defects on the fiber surface. In this paper, the fiber strength degradation in the single-fiber-composite (SFC) has been investigated under constant strain in water to represent the SCC of GFRP. By taking account of the fiber strain history, such as water absorption, thermal expansion, applied strain, the growth of the initial defect after the constant strain test was formulated. After the constant strain test, fragmentation test was conducted in order to estimate strength degradation of the embedded fiber. The remaining fiber strength distribution obtained from the fragmentation test was predicted using the model containing the Paris Law and the Arrhenius model presented by K. Liao.

This paper describes in-plane shear mechanical properties of woven fabric reinforced plastics by the static and dynamic ±45° tension test method. Using the tension-type SHB apparatus, the dynamic tension tests were performed. 8-types of specimens were used. 2-types of epoxy resin (thermo-set and thermo-plastic) were combined with 4-types of fibers (E-glass, T-glass carbon, and aramid fiber). Comparing their mechanical properties, especially, the shear of glass fiber specimens more dependent on strain rate than other specimens.

Applying CFRP to precision machinery components and aerospace materials, CFRPs long-time dimensional accuracy must be secured. Matter for consideration about the dimensional accuracy, moisture absorption, chemical shrinkage, and visco-elastic deformation such as creep are presumable to be the leading factor for the deformation. These deformations are very small, so we need to evaluate quantitative analysis with these factors individually. In this study, we deal with the chemical shrinkage. First, we conducted dynamic DSC measurement to determine the total heat of reaction. Heat of reaction during the cure was measured by isothermal DSC measurement and was quoted to define the extent of cure. A Kinetic model, the Arrhenius model was applied to predict the extent of cure. Our aim is to evaluate the dimensional change concurrent with the cure of epoxy by using laser confocal displacement meter.

Delayed fracture occurs in unidirectional composites as a result of accumulation of micro-damages, such as fiber breakage or time-dependent interfacial debonding. In this study, the creep rupture lifetime in unidirectional composites is predicted under consideration of the propagation of interfacial debonding and its effect on rupture lifetime and damage accumulation is examined. Predictions were performed for two cases that stress concentration on intact fiber adjacent to broken fiber is not taken into account or taken into account. For former case, the decrease in rupture strain due to time-dependent propagation of interfacial debonding is formulated. For latter case, Monte-Carlo simulation under consideration of interfacial debonding propagating fiber breakage point is performed. As a result, the propagation of stress concentration region is observed and it is thought to be dominant factor for time-dependent damage accumulation in unidirectional composites.

This paper describes strain rate effect on mechanical properties of CFRP tube. The investigated relations of CFRP tubes (fiber orientation 0, 30, 60, 75deg) between bending stress and bending strain under lateral impact loading using SHPB (Sprit Hopkinson's Pressures Bar) method. The experimental result pointed that bending stress and bending modulus depended on strain rate. Furthermore, it demonstrated that the higher fiber-oriented specimens produced, the more significant contribution to the strain rate effect.

This paper describes fiber strength degradation of FRP under constant loading in water was investigated using a single fiber composite (SFC) specimen. In experiments, the fiber strength degradation in the SFC specimen was accelerated by water and applied stress. The fiber strength was evaluated by a fragmentation test. We assumed the degradation was caused by microscopic crack propagation and the propagation was accelerated by water and applied stress. Therefore the degradation mechanisms based on the microscopic crack propagation was established to predict the degradation in the SFC specimen. Finally, the experimental results were predicted using the degradation mechanisms.

This paper studies the effects of edge stitching on tensile fatigue properties of CFRP laminates. Fatigue damage development was investigated through experimental observation. From experimental observation results, it was shown that introduction of the stitch threads suppressed the initiation and propergation of the delamination. But the stitch thread broke in earlier stage under the high stress level cyclic loading. As a result, fatigue lives of the stitched laminates were shorter than that of the unstitched laminates. On the other hand, it was remarkable that the fatigue lives between the both laminates were almost equal due to the difference of internal damage propergation.

Delayed fractures in carbon-fiber-reinforced polymer matrix composites could be observed even at room temperature due to an accumulation of micromechanical damages, such as interfacial debondings or fiber breaks. In this study, the creep-rupture time of unidirectional composites consisting of carbon fiber and vinylester resin is predicted under consideration of the progress of interfacial debonding, which corresponds to an increase of stress recovery length, and which is accompanied by a fiber break. The time-dependent rupture strain of unidirectional composites is calculated based on the global load sharing theory, which takes into account an increase of the stress recovery length, which is determined using carbon/vinylester single-fiber composite. The creep rupture time of unidirectional composites is formulated, and the prediction of the time-dependent rupture strain decrease are verified by a comparison between the residual strength of the unidirectional composite after a fixed displacement test and the calculated results at that condition.

This paper describes high-cycle fatigue characteristics of quasi-isotropic CFRP laminates [-45/0/45/90]_s to 10^8 cycles. To investigate the fatigue behavior in the high-life region, the fatigue tests were conducted with frequency of 100Hz since it's difficult to conduct its with 5Hz and the damage behavior of the specimen was observed with a microscope, a soft X-ray photography and a 3D ultrasonic inspection system. In this study, to evaluate quantitive characteristics of transverse crack propagation in the high-cycle region, the energy release rate associated with transverse crack growth to the width direction was calculated and the transverse crack growth to the width direction was evaluated based on a modified Paris-law approach.

An effect of the plastic deformation on the interfacial energy of a fiber-reinforced polymer matrix composite is studied to evaluate the initiation of the interfacial debonding. We performed the fragmentation test with a model glass fiber-reinforced vinylester matrix composite, and calculated the interfacial energy with the energy balance method taking into account an energy dissipation of the plastic deformation. The following results are confirmed; the plastic deformation has a significant influence on the interfacial energy, and the energy balance scheme considering the plastic energy dissipation leads to the constant interfacial energy without reference to the amount of the released potential energy. The differences between our model and the previous one are discussed.

Single fiber fragmentation test is one of the methods to evaluate strength of a fiber embedded in matrix resin. This method has two advantages; the Weibull parameter can be obtained from a few tests and an actual strength of the embedded fiber in practice can be obtained. This test evaluates relationship between the fiber strength and the fiber length. However, the fiber length is defined by the length of the gage length divided by the number of fragment points, and the fiber length is overestimated. Therefore, in the conventional evaluation, the fiber strength is not independent to interfacial strength and test speed. To assume the exact fiber length, an interfacial debonding length and a stress recovery length must be considered. In this study, the new corrected evaluation to generalize the fiber strength is suggested. To discuss the validity of the method, several fragmentation tests are performed under the condition of the various interfacial strength and the various test speed.

Interfacial debonding propagation is very important problem for long-term creep in unidirectional composites because material's properties of the unidirectional composite decrease with increase of the interfacial debonding length. In this study, time-dependent interfacial failure was investigated by using the Raman Micro Spectroscopy. The specimen was single fiber conposite consisting one carbon fiber embedded in the vinylester resin and it was subjected to constant strain condition to assume the long-term creep in the unidirectional composites. The interlace between the carbon fiber and the vinylester resin is relatively weak; therefore interfacial frictional stresiss is one of the most important factors, which dominated the interfacial debonding propagation. In this paper, the compression stress applied to the interface in the direction of fiber rudius direction is calculated as a function of time on the consideration of Poisson compression and thermal residual stress. The interfacial frictional stress is assumed to be dominated by the Coulomb's friction and compared with the Experimental results. The interfacial debonding propagation is discussed by using the relaxation of the frictional stress. Long-term behavior of the interfacial debonding propagation is predicted in the single fiba composites.

This paper describes the results of an experimental investigation of cumulative damage development in quasi-isotropic [0/+45/-45-90]_s alumina FRP laminates subjected to tension-tension fatigue loading. The onset and growth of matrix cracks and delamination were monitored with an optical microscope. The matrix crack density and the normalized length of delamination were measured as a function of the normalized number of stress cycles by edge observation. It was found that a decrease of the stiffness was observed in an early stage due to the initiation of the matrix crack, and that a different pattern of the damage evolution was observed in each lamina under tension-tension fatigue. Results of the study include a detailed description of the chronology of the damage development prior to failure. Additionally, the mechanism of the fracture of quasi-isotropic alumina FRP laminates under tension-tension fatigue was evaluated from the result such that the static strain of [0]_8 Alumina FRP laminates and fatigue failure strain of quasi-isotropic alumina FRP laminates are equivalent.

Fragmentation tests have been conducted to investigate the degradation mechanism using a single fiber composite. The effects of environmental solution diffusion into the resin on the fiber strength and the interfacial shear strength have been investigated as a function of immersion time. As a result, the fiber strength did not chang in respect to an environmental solution containing water and acid. It was suggested that only water would diffuse into the resin and reach the inner fiber, and that acid would not reach the fiber. A Fickian diffusion analysis showed that the water concentration around the fiber already reached a saturation limit at the beginning of the acid diffusion. In this study, the importance of water diffusion on each material degradation was discussed. It is suggested that only water would diffuse into the resin and reach the inner fiber, and acid would not reach the fiber. Therefore, it is concluded that the water diffusion influences the degradation of materials, fiber, matrix and interface immediately before acid corrodes the fiber and interface.

The primary aim of present study is to clarify the fatigue fracture mechanism of high strength steel at the high cycle region. Oil-tempered high strength steel wire (SWOSC-B) is prepared for fatigue test by using monopodium fatigue testing machine at frequencies of 1kHz. The S-N diagram has a knee point at about 2×(10)^6 cycle. From scanning electron microscopy (SEM) observation of the fracture surface at the long cycle fatigue range, nonmetallic inclusion is identified at the origin of fracture and optically dark area (ODA) is formed around the inclusion. On the other hand, a constitution of the nonmetallic inclusion is measured by using electron probe micro analyzer (EPMA). As a result, the inclusions are decided as Al_2O_3-CaO and TiN. Fatigue fracture from Al_2O_3-CaO is occurred earlier than that of TiN. Based on these results, finite element analysis (FEA) is conducted for analyzing the stress field in vicinity nonmetallic inclusion. Then, the effect of mechanical property of inclusion on fatigue failure behavior and ODA are discussed.

The primary aim of present study is to clarify the fatigue fracture mechanism of high strength steel at the high cycle region. Oil-tempered high strength steel wire (SWOSC-V) is prepared for fatigue test by using Nakamura rotating-bending fatigue testing machine. The S-N diagram had a knee point at about 10^5cycle. From scanning electron microscopy (SEM) observation of the fracture surface at the long cycle fatigue range, nonmetallic inclusion was identified at the origin of fracture and it formed "fish-eye". On the other hand, a constitution of the nonmetallic inclusion is measured by using electron probe micro analyzer (EPMA). As a result, Aluminum, calcium and oxygen were detected mainly at the inclusion. Based on these results, finite element analysis (FEA) was conducted for analyzing the stress field and equivalent plastic strain field in vicinity of the nonmetallic inclusion. Then, the effect of mechanical property of inclusion on fatigue failure behavior was discussed.

This paper describes fatigue properties of a quasi-isotropic [0/+45/-45/90]s Alumina. FRP laminates. Matrix crackgrowth behaviors in the laminates under static and fatigue loadings have been studied. Detailed experimental observations were made on the initiation and the growth of individual cracks in 90, -45 and 45 deg plies. Distinctly different crack growth behavior was obtained in each lamina under fatigue loading. At the first of fatigue process, reduction of the stiffness in the laminates due to the 90 deg transverse matrix cracking was obtained. After that, the reduction of that didn't occur, but the transverse cracks in the 90 deg layer were saturated. And then, some delaminations were initiated after the -45 and +45 deg layer's cracks were observed. A higher density of each ply clacks and delaminations would be a characteristic of the fatigue damage in quasi-isotropic Alumina FRP laminates.

This paper describes the characteristics of tensile fatigue in GFRP tube(±45deg) under dynamic cyclic loading at room temperature and 77K. Dynamic cyclic loading tests were conducted at three levels of the maximum stress and the cyclic frequency 10Hz. The relationship between the visco-elastic plastic properties and the internal fatigue damage was investigated at each stress level. The fatigue strength at cryogenic temp, was greatly improved in the range of this study. However, change of the apparent stiffness and hysteresis loss of the specimen were similar behavior.therefor ,similar fatigue damage mechanisms were conjected at each temperature.

The primary aim of present study is to clarify the fatigue fracture mechanism of high strength steel at the high cycle region. Oil-tempered high strength steel wire (SWOSC-V) is prepared for fatigue test by using Nakamura rotating-bending fatigue testing machine. The S-N diagram had a knee point at about 10^5cycle. From scanning electron microscopy (SEM) observation of the fracture surface at the long cycle fatigue range, nonmetallic inclusion was identified at the origin of fracture and it formed "fish-eye" failure. On the other hand, a constitution of the nonmetallic inclusion is measured by using electron probe micro analyzer (EPMA). As a result, Aluminum, calcium and oxygen were detected mainly at the inclusion. Based on these results, finite element analysis (FEA) was conducted for analyzing the stress field around the neighborhood of the nonmetallic inclusion. Then, the effect of mechanical property of inclusion on fatigue failure behavior was discussed.

Single fiber fragmentation test have been conducted to examine the degradation of inner fiber strength surrounded by the resin using E-glass/Vinylestr model composite. Stressed and unstressed specimens under the environmental attack by water and acid have been prepared. The effect of diffusion phenomenon into the resin on inner fiber strength is investigated as a function of immersion time. As a result, the strength of fiber in stressed resin was lower than that in unstressed resin. This result indicates that stressed polymer have the diffusion phenomenon accelerated. In reference to each environment conditions (water and acid), the fiber degradation behavior was unchanged. It seemed that the water would only diffuse into the resin, and the acid would not. These experimental results were explained analytically based on Fickian diffusion analysis. It is expected that water diffusion affect the mechanism of the fiber degradation in the earlier stage than acid diffusion.

Over the past several years or so a Stress Corrosion Cracking of the FRP plant applications has been a serious problem. Under such conditions an establishment of a life prediction has been one of the great interests for a long time. A threshold property of the crack propagation, K_<ISCC>, is regarded as one of the fracture criteria for estimating life span. In this study, fracture mechanism in the threshold properties was investigated. C-glass/Vinylester composites were tested under constant load in the hydrochloric acid, sulphuric acid and water. The crack propagation behavior was depicted on the K_I-da-dt diagram. It was found that diffusion behavior of the acid solution ion into the matrix plays an important role in the process of the threshold failure. Especially, judging from the diffusion coefficient, there is a possibility that molecular of water diffuses precedent to hydrogen ion at the crack tip. As the time passes, the resin absorbs the water and this leads to a swelling of the resin. In addition, molecular of water and belated hydrogen ion corrode the C-glass fiber. Therefore it is suggested that the fracture of the C/VE systems was occurred by absorption and swelling derived from the diffusion of the water into the matrix and a changing of the stress sharing on the corroded fibers at the crack tip.

The purpose of this study is to establish an internationally-acceptable methodology of fatigue testing for prosthetic heart valves, based on the improvement process of a Chinese-made accelerated fatigue test apparatus. This test system had four parallel water cylinders, in which valves were incorporated. Open and closing motion of each valve was controlled at a repetition speed of 10-20 Hz with a displacement of 1-2mm. The Jerryfish (JF) valve was used in this study. It had the longest implantation record in animal trials. In this case, the polyurethane membrane with the thickness of 50-70 mum was ruptured due to a creep effect observed at the middle portion between two spokes after 300days in goat.First of all, in vitro fatigue test machine has been innovated in order to reproduce the above mentioned in-vivo fatigue test results. Dynamic conditions which enabled the identical forces between accelerated and normal cycles were determined : maximum load during valve closure was 5.2N (increased by 20% to the normal), and impulse was 2.5*10^<-2>N・s. Under this condition, valves were ruptured for 1.0*10^7 cycles, but no distinct creep was noticed. Secondary, a temperature of the test-fluid was changed from 20ﾟC to 50ﾟC,expecting a creep effect to the membrane shape. Change in shape of the membrane was observed after 3.2*10^6 cycles, however, its distortion value was only 50% of the animal experimental results. For the next step, a material of the membrane was changed from polyurethane to polystirene to achieve bigger change in shape. As a result, the identical distortion level was successfully reproduced, but a location of the ruptured area was different between in vitro and in vivo results. In order to clarify the above mentioned difference, FEM analysis was performed. The calculated result exhibited that there are three major areas of the maximum elastic strain between two spokes. Moreover, it was confirmed that the ruptured areas observed by both in vitro/in vivo results could be fully superimposed to the areas of the calculated maximum elastic strain.Further investigation should by carried out to determine more reliable methodology for fatigue testing, based on the in vitro and FEM analysis data

Studies on crack propagation behaviors and threshold properties of GFRP Laminates under an acid stress environment.GF/epoxy specimens, which are made of E-glass fiber and epoxy resin, are investigated, and effects of the loading conditions arid the environmental conditions on crack propagation behaviors and threshold properties in HCI solution are researched.Constant and cyclic loading tests are performed, so that the stress ratio and the frequency are changed for the cyclic loading tests. As a result, it is found that crack propagation depends on a time and the maximum stress intensity factor, K_<lmax>. There is no acceleration of crack propagation due to cyclic loading in a K_<lmax>-da/dt diagram. Furthermore, it is verified that the threshold stress intensity factors, K_<ISCC>, under both loading condition have a distinguished difference in the range of our study. From fractographic observations, interfacial crackings and pull-outs of fibers are remarkable under the cyclic loading condition.As for the environment condition, changes in the crack propagation behaviors due to the alternations of acid concentration and temperature of the HCI solution are researched. As the acid concentration increases, the K_<ISCC> seems to be a constant under the constant loading conditions and the solution temperature of 303K.And it is noted that the K_<ISCC> takes its minimum value at lmoh/l under the cyclic loading conditions and the solution temperature of 303K.Increase of the solution temperature lead to the quickening of crack propagation. Arrhenius' equation can be adopted in a stable crack propagation region according to Paris' power low. As for the threshold properties, there is little effect of temperature under the constant loading conditions. On the other hand, the K_<ISCC> decreases and its minimum value vanished under the cyclic loading conditions and the solution temperature of 343K

The objective of this study is to clarify the crack-propagation of the corrosion-resistant GFRP that laminates under various acid stress environments, an effect of the constituents on fracture and the threshold of the crack propagation in the C/VE and E/VE systems. Two kinds of experiments of the stress corrosion cracking (SCC) test and the single fiber fragmentation test were performed.SCC tests were performed under constant loading in an HCl solution, H_2SO_4 solution, water and air, and the crack-propagation behavior was depicted on the da/dt-K_1 diagram. The crack-propagation behavior in water was largely enhanced in comparison with that in the air and it was almost as promoted as that in the acid solution. The failure mechanism of C/VE under the acid stress environment differed from E/VE with which the glass fiber was corroded. As expected, the main causes were the interfacial strength decrease and the water absorption of the matrix.Single fiber fragmentation tests were conducted to examine the degradation mechanism using a single fiber E-glass/Vinylestr model composite. Stressed and unstressed specimens under the environmental attack by water and acid were prepared. The effect of diffusion phenomenon into the resin on the fiber strength and the interfacial shear strength were investigated as a function of immersion time. As a result, the strength of fiber in stressed specimens was lower than that in unstressed specimens. This result indicates that the stressed polymer have the diffusion of the corrosive solution acceralated. The fiber degradation behavior was not changed regardless of each environmental condition, such as water and acid. It seemed that only the water would diffuse into the resin, and the acid would not. A Fickian diffusion analysis has shown that the moisture concentration around the fiber already reached a saturation limit at the beginning the acid diffusion

第3年度は,人工弁・人工心臓などの開発の早期に血液適合性をある程度評価でき,また,設計改良の指針となるデータを得ることが可能な,より信頼性のあるin vitro血液適合性評価試験回路の開発を目指した.第一に,回路内での血液と空気の接触面を完全に排除した4つの同一形状の旋回渦流型血液ポンプで構成される一巡閉鎖回路を開発した.4つの血液ポンプのうち1つのみを駆動し,他の1つはコンプライアンス,残りの2つはリザーバの役割を持たせることで,平均大動脈圧100mmHg,平均ポンプ拍出量2.5L/minといった実際の使用環境に近い試験環境にすることができた.また,washout性能の極めて良好な旋回渦流型血液ポンプを回路の構成要素として用いることで,流れの停滞領域のない試験回路を開発することができた.第二に,チューブやコネクタなど試験回路の構成パーツに対する血小板の活性能を同一にするために,血液ポンプと同一材料で回路内面全体をコーティングした.なお,本研究では新開発の高分子製二葉弁を血液適合性評価試験対象とした.開発した回路内をヘパリン化した新鮮牛血液で満たし,その後プロタミンで中和してACTを300sec〜400secに調節した後に試験を開始した.採取した血液中の血小板の活性能時間を考慮して,試験は4時間で終了した.試験後の弁葉表面をSEMによって観察した結果,(1)弁流出側の弁葉表面では血球成分の付着及びフィブリン網などが全く観察されず血液適合性は良好であったが,流入側の弁葉表面では弁閉鎖時にスポークと接触する部分の吸着タンパク質層に磨耗跡が観察された.また,(2)弁座と弁葉の接着部分に血小板の付着が観察された.以上から,新たに開発したin vitro血液適合性評価試験回路は製品開発の早期に問題点を明確にするのに極めて有効であることが明らかとなった

The stress corrosion cracking mechanism of glass fiber reinforced plastics (GFRP) were investigated in this work. First of all, an effect of the constituents on the fracture and the threshold properties on the crack propagation of woven C-glass/vinyl ester laminates. The propagation behavior was evaluated in HCl solution, in water and in air. As a result, the behavior was plotted on the da/dt-K_I diagrams. The crack-propagation rate da/dt distribution in water was largely enhanced in comparison with that in the air, and it was nearly as promoted as that in the HCl solution. Therefore the crack-propagation properties of unidirectional FRP were investigated to confirm the role fiber direction. The threshold properties the 90° unidirectional FRP were appeared on the da/dt-K_I diagrams and the it is confirmed that the cause is the fiber bridging by the FEM analysis.Single fiber model composites were applied to evaluate the interfacial degradation mechanisms. The degradation was described by interfacial debonding energies in this study. And the interfacial debond length near the fiber failure point was measured to calculate the energies. The matrix plastic zone was also measured, therefore the plasticity was considered in the energy balance model to decide the interfacial debonding energies. Furthermore it was suggested that the interfacial debonding energy kept the constant value.In order to estimate the fracture behaviors of the fiber and interface, the double cleavage drilled compression (DCDC) test was applied. And one fiber bundle was embedded in the specimen in order to estimate the fracture mechanisms. The crack was deflected around the fiber bundle and the fiber/matrix interface was debonded. As a result, it was found that the debond length depended on the interfacial degradation. The debonding length and interfacial debonding energy decided the bridging stress on the crack surface. And the bridging stress determined the traction on crack surface, although the traction decreased due to the fiber failure. Finally the toughening effect that depended on the traction was discussed

Fiber reinforced plastics (FRP) are widely used in structural applications because of their good specific stiffness and strength. In addition, corrosive resistance of the FRP is superior to conventional materials. Therefore use of the FRP under corrosive environment has been extended, such as a chemical solution storage tank. Under the environments, stress corrosion cracking is a typical problem for the conventional materials. Also, the stress corrosion cracking is a principle problem for the FRP under the corrosive environment.Strength and stiffness depends on fiber strength, therefore strength degradation of single E-glass fiber under the water environment was investigated. The fiber strength is characterized by Weibull shape and scale parameter. As a result, the scale modulus decreased as a function of immersion time in the water. In addition, time-to-failure under constant loading in the water was measured. Distributions of the time-to-failure were predicted assuming initial defect growth on the fiber surface. The result showed that the strength degradation depended on initial strength and applied stress.Fiber bridging of water-absorbed FRP is principle in crack propagation, therefore studied in order to estimate crack propagation resistance under water environment. The crack propagation resistance depended on the weight gain due to water absorption. Therefore the absorbed water was an essential factor to decrease the crack propagation resistance. Finally, increased resistance by the fiber bridging was predicted using bridging stress distribution that was obtained by measurements of debonding length of the bridging fiber. The propagation resistance obtained by the fiber bridging showed a good agreement with experimental results. It was found that the crack propagation under the water environment depended on the fiber/matrix interfacial properties

Polymer matrix composites(PMC) such as Fiber Reinforced Plastics(FRP) have numerous variations in its components, and therefore it is possible to design its mechanical properties corresponding to its application. Previous researches dealing long-term reliability of FRP in corrosive environment mostly focus on macroscopic damage failure such as crack propagation and mechanical properties degradation, but not on its failure mechanism. The failure of FRP occurs from the accumulation of microscopic damage in its components. Thus present research focus on microscopic damage such as degradation in mechanical properties of resin matrix, strength degradation of fiber reinforcement, and interfacial degradation, and furthermore, evaluated these degradation behavior quantitively.Firstly, neat matrix resin specimen was immersed in Deionized water and was statically〜in air to measure its mechanical properties such as stiffness and rupture strain. The stiffness remained unchanged but the rupture strain increased in vicinity of immersion.Secondly, E-glass fiber was embedded in the epoxy resin to mold Single Fiber Composite(SFC) specimen. Constant strain test of SFC specimen was conducted under water environment(40, 75[℃]. Fragmentation test was conducted after the constant strain test to estimate fiber residual strength. It was clarified that the fiber strength degrades by constant strain test, and the degradation accelerates with higher applied strain, immersion time, and higher water temperature. Furthermore subcritical crack growth model was applied to predict the fiber strength degradation.Finally, the length of interfacial debonding accompanied with embedded fiber failure was measured to estimate the maximum shear strength. Also, from the energy balance in vicinity of fiber failure, Energy Release Rate(ERR) was quantified. Wagner's model and Morais's model were applied to estimate maximum shear strength and ERR. It was clarified that both values degrades and showed a tendency to saturation

The aim of this research is to investigate the long term durability of GFRP in hydrothermal environment and to propose its lifetime prediction model. The GFRP under study is consisted with plain E-glass cloth and vinylester resin. The constant tensile load tests were conducted in hydrothermal environment (i.e. deionized water) and in air. The fracture occurred only in hydrothermal environment and the fracture time decreased with the increase in the applied stress and the temperature. The fracture time was predicted based on Reiner-Weissenberg criterion and the predicted results showed good agreement with the experimental data by considering the degradation induced by the hydrothermal aging and its acceleration by the increasing temperature

大電流容量・高導電率を有する配線用CNT金属複合繊維の創製に向け，母材となる高配向CNT糸に黒鉛化処理ならびに高密度化処理を施し，各処理が繊維の電気特性に及ぼす影響の定量評価を行った．CNT糸の黒鉛化処理では，不活性ガス環境下における高温熱処理によって結晶性が改善し，それに伴う比導電率・比電流容量の顕著な増加が確認された．一方，ポリマー溶液浸漬による高密度化処理では，繊維の収縮によってCNTバンドル間の空隙が減少し，導電率が大幅に向上した．ポリマー処理によって線密度の増加が見られたことから，ポリマーがCNT糸に含浸し，残留していることが示唆された．

本研究では層間高靭性化CFRP積層板(T800S/3900-2B)の面外方向(Z方向)疲労特性に及ぼす内部欠陥の影響を評価するため，応力比R=0.1にてZ方向疲労試験を実施し，ボイド等の内部の初期欠陥が積層板のZ方向疲労寿命に及ぼす影響の調査を行った．成形条件を変えることで積層板の空洞率や最大ボイド長さを変えた2種類の積層板を用いた．なお，空洞率や最大ボイド長さは試験片の端面観察および3次元X線CT装置を用いた内部観察により測定した．Z方向疲労試験を行った結果，本研究の範囲では初期欠陥の条件による疲労寿命の差異は見られないこと，試験片の表面性状を変化させた場合でも疲労寿命に差異は見られないことが明らかとなった．破壊力学的には脆性材料である本試験片の疲労寿命は最大ボイドの長さに大きく影響を受ける．しかしながら，本研究で用いた積層板はいずれも最大ボイド長さが2mm以上と大きい．したがって，内部欠陥や表面性状が疲労寿命に及ぼす差異が確認されなかったと示唆された．

　本研究では層間高靭性化CFRP積層板(T800S/3900-2B)の面外方向(Z方向)疲労特性に与える応力比の影響を評価するため，複数の応力比条件(-1, -3，-6)にてZ方向疲労試験を実施し，圧縮荷重が積層板のZ方向疲労寿命に与える影響の調査を行った．その結果，応力比が小さくなるにつれて疲労寿命も低下していることから，負荷される圧縮荷重の大きさが疲労寿命に影響を与えることが明らかとなった．損傷観察により，圧縮荷重の増加に伴い繊維/母材樹脂界面でのはく離は促進されるが，破壊形態に変化は与えないことが示唆された．有限要素解析により算出された面外方向応力と修正H-kモデルを用いることで疲労寿命に与える圧縮荷重の影響を定量的に評価し，等寿命線図による疲労寿命予測を行った．

本研究では，大電流容量・高導電率を有する金属複合繊維の作製を目的とし，繊維配向性の高いCNT無撚糸を硫酸銅メッキ浴に浸漬させた後，電気メッキ処理を行うことで，繊維内部に銅が析出した銅複合CNT無撚糸の作製を行った．作製した繊維の繊維長手方向には部分的に銅が連結するような析出形態をしていることが確認された．また，作製した繊維に対して，電気特性評価を行ったところ，銅複合CNT無撚糸の電気特性は繊維外周部に堆積した金属層の影響が支配的であるという結果を得た．そこで，繊維内部に析出した銅の電気特性に対して与える影響の調査を行ったところ，繊維内部に銅が析出することにより，電流密度増加に伴う抵抗率の増加が緩やかになることで，電流容量が増加することが確認された．

　本研究では層間高靭性化CFRP積層板（T800S/3900-2B）の面外方向（Z方向）疲労特性の評価のため，Z方向引張-圧縮疲労試験(R=-1)を行い，Z方向引張-引張疲労試験と比較，応力比および繰り返し圧縮荷重が層間高靭性化CFRP積層板に与える影響を調査した．また同一スプール形状試験片を使用した面内トランスバース方向（T方向）引張-引張疲労試験(R=0.1)を行い，Z方向との比較を行った．その結果，疲労試験にて得られたS-N線図にて整理を行い，疲労強度に及ぼす圧縮応力の影響，およびZ方向試験片はT方向試験片に比べ疲労寿命が短いことが示唆された．有限要素解析によって試験片内の応力分布を評価した．圧縮熱残留応力の影響により，繊維層に作用する応力が異なることを明らかにした．

本研究では，層間高靱性化CFRP積層板の面外方向における疲労強度特性の評価を行うことを目的とした．層間に熱可塑性粒子を含んだ層間強化層を持つプリプレグ，T800S/3900-2Bを一方向に88 ply積層し，成形された厚肉CFRP積層板よりスプール形状の試験片を作製した．試験片の両端を精度よくタブと接着し，面外方向の負荷を可能とした．また，層間強化層を考慮した有限要素解析を行い，スプール形状試験片の応力状態を評価した．さらに，得られた試験結果に解析式を適用し，面外方向の疲労強度特性を定量的に評価した．以下に得られた成果，結論を述べる．（1）スプール形状試験片および接着冶具を用いることで，接着時に試験片とタブの接着が精度よく行えるため，面外方向特性の厳密な評価が可能となることが示された．さらに，接着後のタブ付き試験片の接着部に補強を行うことで，過去に報告の少ない面外方向の疲労試験を行うことに成功した．（2）静的試験結果について，面外方向垂直応力の応力集中を考慮し評価した結果，面外方向の静的強度特性は，面内トランスバース方向の静的強度特性と比べて低下することが示唆された．また，層間高靱性化CFRP積層板に面外方向の荷重が作用した際には，繊維層での破壊が支配的であることが示唆された．（3）面外一方向材の疲労試験結果について，面外方向応力の応力集中を考慮して評価した結果，面内トランスバース方向の疲労強度特性と比較して，面外方向の疲労強度特性は低下する傾向が見られた．また，静的試験以上に繊維層での破壊が支配的であることが示唆された．（4）疲労試験結果にH-k則を適用し，解析的に疲労強度特性を比較した．S-N線図へのフィッティングにより得られた各材料パラメータは面外方向がH＝1.76，k＝9.47，面内トランスバース方向がH＝0.991，k＝16.6となった．さらに，面内トランスバース方向の材料パラメータを用いて面外方向の疲労寿命を予測したところ，長寿命側に予測した．この結果より，面外方向の疲労強度特性が低くなることが解析的に示された．（5）静的試験後の破面と比較し，疲労試験後の破面では繊維への樹脂の付着量が少ないことから，繰り返し荷重による繊維/樹脂界面はく離の発生が示唆された．したがって，面外方向，面内トランスバース方向ともに，疲労荷重下においては，繊維/樹脂界面はく離を起点として発生したクラックが，臨界き裂長さに達した時点で破断することが推察された．さらに，面内トランスバース方向ではクラックの進展方向に破壊靱性値の高い熱可塑性粒子を含む層間強化層をクラックが進展する必要があることから，面外方向は面内トランスバース方向と比較して，疲労強度特性が低下するものと結論付けた．

繊維強化プラスチック(FRP)の更なる長寿命化を図るために，「FRPの表面に金属薄膜を被覆して強化する」という新しい発想に基づく研究を行った．金属薄膜の目的は，FRP内部への水分拡散を防ぐものであり，水分バリアーとして有効に機能するか検討を行った．　本年度は2011年度に行っためっき技術および吸水試験の結果の妥当性について調査した．・めっき技術について　めっき被膜は2011年度と同様に，耐食性，耐摩耗性に優れるNi-Pとした．基板は平織GFRP積層板（母材樹脂：ビニルエステル樹脂，強化繊維：Eガラスクロス）である．2011年度では，通常のめっき工程ではめっき不良が生じたため，めっき工程に物理的エッチング（＃120～＃2000）を導入することでめっき不良を改善した．さらに，得られるめっき被膜の膜厚および均一性を考慮し，＃1200に決定した．2012年度は，3D形状測定顕微鏡を用いた観察から，めっき膜厚の標準偏差と表面粗さに相間関係があることがわかり，2011年度の結果の妥当性を確認した．・吸水試験について　2011年度において，試験片全面を水中に浸漬させる通常の吸水試験方法ではめっき被膜の防水性を評価することは困難であったため，片面のみを浸漬させるように片面吸水試験機を創作し，評価を可能とした．その結果，めっき膜厚の増加に伴い，吸水飽和時の重量増加率および拡散係数の低下が確認された．2012年度では，吸水試験後の試験片表面の観察を行い，前年度の結果を視覚的に考察した．試験片表面の観察から，①めっき時間の増加（めっき膜厚の増加）に伴うめっき粒子の粒径が拡大すること，②めっき被膜は層状の組織を成しており，吸水試験後においてはめっき被膜にき裂が生じ，徐々に剥がれていくことが判明した．めっき膜厚が薄い場合にはき裂がGFRPにまで達してしまい，早い段階で吸水が起きてしまうと推察された．き裂が生じる原因として基板のGFRPとめっき被膜の熱膨張率の差が挙げられる．以上の結果より，2011年度の結果の妥当性が示された．　以上より，金属皮膜が水分バリアーとして機能し得ることが確認され，より最適なめっき被膜の選定，密着性の向上によって更なる防水効果が期待できる．

本研究課題では，ハイブリッドナノ複合材料の材料創製に先駆け，ガラス繊維へのカーボンナノチューブ(CNT)の析出による表面改質の確立・評価を目的とした．CNT析出による表面改質は化学気相成長法(CVD法)を用いて行った．まず，析出温度，析出時間，触媒量，炭素源量をパラメータとした様々な条件下でTガラス繊維へのCNTの析出を行った．その中で析出量に差が出た2条件についてTガラス繊維の機械的性質の変化および樹脂中に埋蔵した場合の界面接着性の測定および定量的な評価を行った．また，比較のため未処理のガラス繊維および熱処理のみを施した繊維も同様に試験を行った．Tガラス繊維の機械的性質についてはCNT析出により引張強度および破断ひずみに約60%の低下がみられた．この原因はCVD法での繊維の加熱および装置からの取り出し時の環境ガスによるものと考えられる．そこで環境ガスを空気から窒素に変更して同様の条件で作製したTガラス繊維の機械的性質を調査した結果，機械的性質の低下が抑制された．次に，CNTを析出させたTガラス繊維をエポキシ樹脂中に1本埋蔵したモデル試験片を作製し，界面接着性の測定および定量的評価を行った．熱処理のみを施した繊維については界面接着性の低下が確認されたものの，CNTを析出させた2条件については界面接着性の向上が確認された．また，得られた結果から界面せん断強度を算出すると，CNTを析出させた場合，未処理のものと比較して約45%以上の上昇が確認された．これは，CNTの析出したTガラス繊維を樹脂中に埋め込むことでCNTに樹脂が含浸してCNT層を形成するためであると考えられる．この層の弾性率をナノインデンテーション試験により測定した結果，CNT層の弾性率はエポキシ樹脂よりも高いことが明らかとなった．これより，CNT析出繊維の界面接着性の上昇の要因はCNT層形成による繊維周囲の弾性率の変化であることが示唆された．以上の結果をもとに弾性軸対称モデルを用いて数値計算を行った結果，CNT層の弾性率の増加を考慮した場合，界面せん断強度が上昇することが解析的にも確認された．ただし，本研究で用いたモデルは実際の実験的事実を考慮していない簡素的なモデルであるため，今後はそれらの要素を導入したモデルの構築が課題となると考えられる．

繊維強化プラスチック(FRP)の更なる長寿命化を図るために，「FRPの表面に金属薄膜を被覆して強化する」という新しい発想に基づく研究の提案を行った．金属薄膜の目的は，FRPの弱点である水分拡散を防ぐものであり，水分バリアーとして最も有効に機能するか検討を行った．具体的には，GFRPへのNi-Pめっき技術の確立，吸水試験による水分バリアー機能の評価方法の検討，Ni-Pめっきの膜厚変化による吸水特性のモデル化の三点について研究を行った．GFRPへのNi-Pめっき技術の確立：母材をビニルエステル樹脂，強化繊維をEガラスクロスとした平織GFRP積層板(以降平織GFRPと表記)へNi-Pめっきを施す技術の確立を行った．通常の工程でめっきを行った際，亀裂やフクレなどのめっき不良が生じたため，基板である平織GFRPにあらかじめ物理エッチング(サンディング)を施すことでめっき不良が改善された．物理エッチングは＃120～＃2000まで行い，＃1200が最適であると決定した．ビニルエステル樹脂および平織GFRPともにめっき浴浸漬時間と膜厚の関係を調査し，膜厚制御を可能とした．吸水試験による水分バリアー昨日の評価方法の検討：通常の吸水試験を行った場合，同程度のNi-P膜厚に対しての重量増加率の変化の挙動がビニルエステル樹脂と平織GFRPで大きく異なった．この原因として，平織GFRPの端部(側面)をSEMで観察を行ってみたところ，ガラス繊維がむき出しとなっていたため，端部からの吸水が顕著に表れたからであると示唆された．そこで，片面のみから吸水するような試験機(片面吸水試験機)の創作を行った．片面吸水試験を行った結果，ビニルエステル樹脂と平織GFRPの重量増加率は似た挙動となった．Ni-Pめっきの膜厚変化による吸水特性のモデル化：吸水試験の結果から，Ni-Pめっきの膜厚が厚くなるほど，吸水飽和したときの重量増加率が低下することが判明した．このような傾向は吸湿試験において試験片に対する相対湿度が変化すると，飽和したときの重量増加率が変化するという実験結果と酷似している．そこで，Ni-Pコーティングを施すことで，基板(ビニルエステル樹脂，平織GFRP)周囲の水分環境が変化すると仮定した．湿度環境下にさらされた際の吸水飽和したときの重量増加率の理論式の変形を行い，Ni-P膜厚に対する吸水飽和したときの重量増加率の予測が可能となった．今後の展望として，水分バリアー機能がさらに向上されると考えられる多層めっきを施す必要があると考えられる．

　本研究では，炭素繊維強化プラスチック（CFRP）積層板の超長疲労特性について調査を行った．一般に，CFRP積層板の疲労損傷挙動は次のような3過程を経て破断に至る．（I）層内樹脂割れと呼ばれる微小クラックが積層板内部に多数発生する．（II）層内樹脂割れを起点として層間剥離が発生し，繰返し数が増すとともに進展する．（III）強化繊維が破断することによって試験片の破断に至る．層間剥離の発生は，積層板の圧縮強度を大幅に低下させる原因となる．そこで本研究では，超長寿命域におけるCFRP積層板の層内樹脂割れを起点とした層間剥離の発生挙動について調査を行うことを目的とした．　初期損傷の影響を考慮した層間剥離の発生，進展挙動を調査するために，疲労試験前に予め層内樹脂割れを任意に生じさせた試験片と未損傷状態の試験片の2種類を用意した．その2種類の試験片を用いて繰返し数108回を超える疲労試験を行った．その結果，負荷応力レベル&#61555;max/&#61555;b=0.4~0.6の範囲内では，初期損傷の有無に関わらず，層間剥離は層内樹脂割れが多数集中する箇所から発生が観察され，層内樹脂割れが疎な箇所からの層間剥離の発生は観察されなかった．この負荷応力レベルの範囲内では初期損傷として与えた層内樹脂割れが少数である場合は，層間剥離の発生には大きな影響を及ぼさないことが分かった．負荷応力レベル&#61555;max/&#61555;b=0.3で行った疲労試験では，層内樹脂割れに先行して層間剥離が発生，進展することが観察された．この原因として，試験片端部のエッジ効果と呼ばれる応力特異場の影響が大きく関与していることが考えられる．この現象は従来の疲労損傷挙動とは異なる結果である．CFRP積層板の疲労に対する設計は層内樹脂割れが発生しないような応力レベルで設計されているが，超長寿命域側では層内樹脂割れが発生しなくとも層間剥離が発生する可能性があり，従来の設計基準では危険となる可能性が示唆された．一方，負荷応力レベル&#61555;max/&#61555;b=0.2で行った疲労試験では繰返し数2×108回を超えても層内樹脂割れ及び層間剥離の発生及び進展は観察されなかった．　以上まとめると，超長寿命域のCFRP積層板の疲労特性を調査した結果，初期損傷として存在する層内樹脂割れが少数であれば，層間剥離の発生には影響を及ぼさないことが分かった．しかしながら，低応力レベルの超長疲労域においては，層内樹脂割に先行し，層間剥離が生じることが明らかとなり，従来のなされている層内樹脂割れの発生を基準とした設計ではなく，層間剥離の発生を基準とした設計を行わねばならないことが示唆された．

　応力腐食環境におけるガラス繊維強化プラスチック（GFRP）の長期信頼性評価を目的として，水環境下において材料の吸水により生じるGFRP構成機材の機械的劣化とその劣化によるき裂進展抵抗の変化の関係について調査を行った．き裂進展抵抗としては，繊維架橋による高じん化機構における界面劣化の影響を実験的に調査するため，エポキシ樹脂にECRガラス繊維束を2つ埋蔵し試験片とし，吸水による重量変化率の増加による見かけの破壊じん性値の変化を求めた．　見かけの破壊じん性値は繊維がき裂面間で架橋することによって増加する．実験において得られる高じん化への寄与分の決定には，試験片の吸水による重量増加率が同一となる母材樹脂単体の破壊じん性値と見かけの破壊じん性値の差をとり，母材吸水率と高じん化分の関係を求めた．この高じん化への寄与分は母材の吸水初期状態では増加するが，極値をとり減少を示した．この高じん化分と吸水状態の関係を，繊維束埋蔵位置における水分量の時間変化を用いて評価した．水分量の変化はフィックの第二拡散則に基づいて有限要素解析により求めた．得られた水分量の時間変化と高じん化分の時間変化を比較すると，繊維束部の水分量の増加が開始する時間において，高じん化への寄与は極値をとることが明らかとなった．その時間以後は水分量の増加に従って高じん化分は減少を続けるが，繊維束部における水分量が飽和に至ることで高じん化分の減少も終了し，一定値を示すと考えられる．　構造材料においては使用される環境への露出時間や環境の状態により，内部における水分量は分布を示すと考えられる．本実験で得られた水分量と高じん化機構の関係は，このような構造におけるき裂進展抵抗を考慮する上で重要である．水分量が飽和することで一定値をとることは，水分の影響を強く受ける環境における長期使用において材料の信頼性を確立することが期待される．

横方向繊維の一方向材（90°材）を供試材として応力腐食割れ試験を行った．その結果，クロス材と同様にda/dt-KI線図は低応力拡大係数域と定常き裂進展域に分類され，下限界値の存在も確認された．また，低応力拡大係数域においてはKI値の変動幅が大きく，き裂成長に伴いKI値が上昇していることがわかった．一般に，一方向材で繊維方向に沿い開口型き裂が進展する場合，繊維の架橋現象により閉口応力が作用するとされている．KI値上昇はこの架橋繊維の本数増加が原因であると考えられる．さらに水環境，大気中で試験を行った試験片破面を観察したところ，水環境では大気中に比べ繊維の露出率が高く架橋繊維が形成されやすい状況であり，特に水環境の低応力拡大係数域においては界面先行型の破壊形態であることがわかった．また，架橋繊維を円柱両端固定はりと仮定し算出した閉口応力を用いFEM解析により，各環境下におけるき裂成長に伴うき裂進展抵抗の上昇を再現した． また，微視的観点から劣化メカニズムを評価するため，純水中に浸漬させた単繊維モデル試験片を用いたフラグメンテーション試験を行った．界面はく離，クリープの影響に着目し，これらが繊維強度・界面強度に及ぼす影響について調査し，これらの強度評価モデルについてより厳密な考察を行った．浸漬に伴う試験片の物性変化の影響を正確に評価するため，吸水膨張ひずみ，クリープひずみによるプリテンション応力を算出し，繊維強度に対して評価の補正を行った結果，強度低下の負荷応力依存性は無いことが明らかとなった．また，フラグメント過程で発生する界面はく離を考慮したモデルにより界面せん断応力の分布を検討した結果，界面はく離は界面強度に大きく影響を及ぼすことが明らかとなり，最大界面せん断応力ではなく系全体のエネルギバランスモデルを用いた界面強度の評価方法の有効性を示した．

　塩酸応力環境下におけるGFRP織物積層板のき裂進展挙動及び下限界特性の解明を目的に研究を行った。本年度は新たに母材樹脂中への水溶液溶質の拡散現象を考慮した。拡散モデルを構築し、構成基材に依存しない統一的な破壊機構の提案を試みた。　応力腐食割れ試験において、耐食性に優れたガラス繊維及び樹脂を構成基材としたCガラス/ビニルエステル積層板（C/VE）と従来のEガラス/エポキシ積層板（E/Epoxy）とを用い、き裂進展挙動及び下限界特性の環境依存性を明らかにした。E/Epoxy試験片では、塩酸温度上昇よりも塩酸濃度上昇の影響を大きく受けKⅠSCCが低下する。また、濃度上昇が直接関係する水素イオンの影響を顕著に受けているため、破壊機構は水素イオン拡散及びそれに伴う繊維腐食が支配的であると考えられた。一方、C/VE試験片において、塩酸環境下のき裂進展挙動は空気中よりは大きく促進されているが純水環境下と比較してもあまり大きな変化が見られない。すなわち、耐酸性の高いCガラスを用いた試験片においては、水素イオンの拡散による繊維の劣化よりも、水溶液（主に水）の拡散吸着によるき裂先端近傍の樹脂の吸水膨張等による繊維荷重分担率増加に伴う繊維破断による破壊が支配的である事が示唆された。　拡散現象について、母材中の水素イオン拡散がき裂進展に及ぼす影響を明らかにするため、ひずみエネルギ依存の拡散係数及び化学ポテンシャル勾配による拡散方程式を用いた2次元拡散シミュレーションを行った。その結果、外力の負荷により無負荷の時には見られない、き裂先方での高濃度領域が現れた。応力拡大係数の増大に伴いこの領域が顕著に拡大されることを確認し、この領域の拡大がき裂進展挙動に大きく影響を及ぼしていることがわかった。

GFRP（ガラス繊維強化プラスチック）はその汎用性の高さから広範な分野で利用が広まっているが，強化材であるガラス繊維が腐食環境に接触し，応力腐食割れ（SCC）を起こすことが知られており，その破壊形態の解明は重要である。また，工学設計上重要と思われる，応力腐食割れにおけるき裂進展下限界応力拡大係数（KISCC）についての報告例は少なく，また環境条件がKISCCに及ぼす影響など，解決していない課題は多い。 そこで本研究では広範な酸応力腐食環境中での本材料の破壊機構・き裂進展挙動を調査するため，切欠きを有するGFRP織物積層板の応力腐食割れ荷重漸減試験を行い，KISCCの定量化と破壊機構の解明を行う。また破壊メカニズムを基礎とした数値計算モデルを提案し，き裂進展速度の推定を行う。 応力腐食割れ試験の結果，巨視的き裂進展挙動は遷移点を有し，パリス則に沿う定常き裂進展域と，KISCCへの漸近領域である低KI域へと大別できた。この両領域には微視的破壊機構に差異が見られ，特にKISCC近傍では荷重線方向の縦繊維周囲のマトリックスに特徴的な破面である，ポリゴナルラインが観察され，酸の拡散抵抗となっていることが分かった。これに対し定常き裂進展域においてはマトリックスは脆性的な破面を呈しており，ガラス単繊維のSCCにき裂進展挙動が依存していると考えられる。 これらの破面観察の結果から，定常き裂進展域に対してガラス単繊維の腐食モデルを基盤とした，き裂進展速度の予測モデルを提案し，その妥当性を検証するため，理論的観点から数値計算的手法を用いて巨視的き裂進展速度の予測を行った。ガラス繊維表面の初期欠陥が酸による腐食を受け，特徴的な破面であるミラーゾーンが成長し破断に至るモデルである。応力腐食割れにおける反応速度はArrheniusの式で示され，これを積分することによって繊維の破断時間を算出できる。また，これら単繊維の集合体である繊維束は楕円形状にて近似し，繊維配列を考慮することによって縦繊維束の定量的モデルがたてられる。繊維束側面に初期腐食領域を与え，腐食繊維からSCCが開始する。繊維束全体の破断時間を算出することにより，巨視的き裂進展速度が算出できる。 本モデルにより算出されたき裂進展速度は定常き裂進展域において実験値と良い一致を示し，繊維束内のき裂進展シミュレーションにより得られる縦繊維束内のき裂進展過程に関しても実験でのフラクトグラフィと一致し，本モデルの妥当性が示され，本モデルが，GFRP織物積層板のき裂進展速度予測に関して有効であると思われる。 以上より，実験的側面より詳細な破壊機構を解明し，これを基にして，き裂進展速度予測及びき裂進展シミュレーションが可能でかつ妥当性を有する数値計算モデルが提案され，この分野での新しい材料設計へのアプローチが示された。