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In this study, the mechanisms governing compressive strength in quasi-isotropic carbon-fiber reinforced-plastic laminates were clarified to gain an understanding of the results of non-hole compress...

In this study, impact and compressive after impact (CAI) tests using varying impact energies were conducted using ply-level hybrid laminates composed of thin and standard ply clusters. In addition, fiber metal laminates (FMLs), which consisted of thin-ply prepregs and stainless layers with a thickness of 0.04 mm, were made in order to suppress the fiber breakage that caused a decrease in CAI strength. Next, impact and CAI tests were conducted on FMLs in order to investigate the effect of the inserting location in the thickness direction and the number of metal layers in failure mode, as well as CAI strength. The results confirmed that CAI strength was improved by both ply-level hybridization and the insertion of metal layers. In ply-level hybrid laminates, the incidence of fiber breakage and delamination was reduced by changing the ratio of thin-ply prepregs and their through the thickness location. In addition, plastic deformation of metal layers in FMLs led to a larger dissipated energy; thus, the failure occurred in a limited location, such as fiber breakage at the non-impacted laminate face.

In this study, flexural tests using both thermoset and thermoplastic carbon fiber reinforced plastics (CFRP) laminates with varied ply thicknesses were conducted in order to evaluate the effect of ply thickness and matrix resin on the flexural properties of thin ply composites. Results confirmed that the flexural properties were improved by decreasing the ply thickness in both thermoset and thermoplastic laminates. Furthermore, in-situ observation of specimens during the test revealed that short delamination occurred in the carbon fiber carbon fiber reinforced thermoplastics (CFRTP) thin ply laminate, while long delamination was observed in case of carbon fiber reinforced thermosets (CFRTS). This indicates that damage progressions of the laminates were influenced by the toughness of the matrix resin. On comparing the in-situ strength in the first ply failure calculated from the classical laminate theory in the thin and thick ply laminates, it was confirmed that high damage resistance was caused by higher in-situ strength in each layer in the thin ply laminate.

This study investigates the influence of local hybridization of Thin and Thick-Ply CFRP laminates on the open-hole and bearing properties. The area weight of the CFRP unidirectional prepregs used is 40 gsm in the case of Thin-Ply layers and 160 gsm in the case of Thick-Ply layers. The steel used is a 1.4310 stainless-steel foil with the same layer thickness as the prepregs. In the hybrid area, 90 ∘ layers were locally replaced by stainless steel patches. The local metal foil content varies from 6.25%, 12.5% to 25.0%. For notched laminates, the open hole tensile strength is significantly decreased with thinner layer thicknesses. The failure behavior changes from complex delamination dominated failure to brittle failure. By using stainless steel foils in the regions of stress concentrations, energy can be dissipated by plastic deformation of the steel foil and stresses can be deflected to neighbouring areas. For Thin-Ply samples with a local steel content of 25% the open hole tensile strength increases by 64% in comparison to the reference Thin-Ply specimens and the sensitivity towards stress concentrations decreases. The bearing strength of the hybrid CFRP laminates is increased by up to 54.6% in comparison to the reference material, due to the confinement of the steel foil and the resulting higher compressive strength and the plastic deformation at high stresses. The stress–strain diagrams and micrographs of the fibre metal samples reveal that damage is initiated before the maximum bearing strength. However, the damage offset bearing strength increase concerning the specific density of the material significantly.

The present study investigated the damage analysis based on the peridynamics (PD) theory, which can deal with damage in composites with arbitrary fiber orientation, as a basis for enhancing the strength properties of the composites with local fiber orientation. The damage of the composites was reproduced by setting the appropriate bond properties (bond constant and critical stretch). The advantages and disadvantages of the PD models for the damage in composites with curvilinear fiber paths were also discussed. The PD model was further extended to address the multiple-type and multiple-site damage in multi-ply laminated composites with arbitrary fiber orientation. Finally, in order to apply the PD model to composites with the AP-PLY (Advanced Placed Ply) configuration allowed by current fiber placement technology, the efforts were made to establish an integrated framework of forming and mechanical analyses for future composite manufacturing and design.

In this study, void distributions in unidirectional carbon fiber-reinforced plastic (CFRP) laminates were measured using X-ray computed tomography (CT), and the stress concentrating effects of the voids on the reduction of the CFRP matrix-dominated strength were modeled. We first investigated the statistical features of the void sizes, geometrical shapes, and locations by analysis of the X-ray CT three-dimensional images. The natural logarithms of the void sizes showed a three-dimensional normal distribution; consequently, the void shapes and aspect ratios were correlated. Next, using the measured results regarding the voids, the stress concentration effect on the reduction of transverse tensile strength was modeled based on the Weibull model considering the volume dependence of strength. From the results, we attributed the decrease in the matrix-dominated strength with increases in void volume fraction to the increase in stress concentration volumes accompanying the increased number of voids.

In this study, the viscosity of unidirectional CFRTP laminates near the melting point was evaluated to discuss the relationship between the thermal fusion conditions and the flow of fiber and matrix. The laminates were regarded as a fluid flowing in a direction perpendicular to the fiber. The microscopic flow of the fiber and matrix near the melting point was observed during a parallel plate compression test. Based on this observation, viscosity and flow properties were evaluated by the finite element method. The results of the parallel plate compression test showed that the test conditions and the decrease in the thickness of the laminates are related to the apparent viscosity of the unidirectional plate. In addition, cracks in the direction of the maximum shear stress were detected via in situ observation near the melting point of the matrix. The occurrence of this phenomenon depended on the apparent viscosity of the unidirectional laminates and it was found that no cracks occurred when the apparent maximum viscosity of the unidirectional laminates was lower than a certain threshold.

The present study investigated nonlinear elastic-plastic stress-strain relationships of discontinuous carbon fiber-reinforced thermoplastics (CFRTPs) using experiments and numerical simulations. In the experiments, we conducted uniaxial tensile tests and three-point bending tests for two types of CF/PA6 (carbon fiber/polyamide 6) specimen: injection-molded specimens with short fiber length and aligned fiber orientation, and compression-molded specimens with long fiber length and random fiber orientation. Comparison of the experiment results indicated that the injection-molded specimens exhibited a nonlinear stress-strain response, while the compression-molded specimens exhibited an almost linear response. These results implied that long discontinuous fibers effectively increased the yielding point of composites, even if the composites had random fiber orientation, which eliminated orientation dependence on mechanical properties of the composites. Furthermore, we attempted to simulate elastic-plastic stress-strain relationships of discontinuous CFRTPs in an effort to understand the effect of the microstructure, including fiber length. For this purpose, we employed fiber-based simulations to deal with the microstructure of fibers and matrix and the constitutive law of the matrix. The simulated results indicated that fiber length influences the nonlinearity of the stress-strain relationships of discontinuous CFRTP composites.

There are two types of flow mechanism in prepreg processing: shear flow, in which resin and fibers flow together, and percolation flow, in which resin flows out of the gaps between fibers. In this study, we experimentally evaluated how these flow mechanisms change during cure including the gelation stage. Results of compression tests at various initial degree of cure at the start of compression showed that the maximum amount of shear flow is almost constant before matrix gelation. In the gelation region of matrix resin, shear flow still occurs at the gel point whilst percolation flow has completely stopped. Furthermore, it was suggested that the limit of the shear flow in the gelation stage can be described by the combined effects of the shear modulus of the fiber-bed and the shear storage modulus of matrix resin.

This paper presents a method to simultaneously estimate the interfacial stiffnesses between the adhesive and the adherends of an adhesive joint, and the adhesive thickness by combining the experimental measurement based on the ultrasonic reflection method and theoretical analysis. The interfacial stiffnesses and the adhesive thickness were evaluated for an aluminum adhesive joint to study the effects of contamination of the release agent. The optically observed thickness of the adhesive is in good agreement with the theoretical results, indicating that the estimation is appropriate. The interfacial stiffness of the interface contaminated by the release agent was estimated to be approximately 24% of the properly bonded interface. In addition, double-cantilever beam tests were performed on different specimens to evaluate the mode I fracture toughness. The results showed that the mode I fracture toughness decreased to 10% of that of the properly bonded interface owing to the contamination. The results suggest that the reduction in the mode I fracture toughness induced by adherend contamination can be detected by identifying the decrease in the interfacial stiffness based on the proposed method.

In this study, fiber metal laminates (FMLs), which consisted of thin-ply prepreg and stainless steel layers with a ply thickness of 0.04 mm, were fabricated with the aim of suppressing the fiber breakage that decreases the compression-after-impact (CAI) strength. Then, static flexural tests of FMLs were conducted to investigate the effects of the number of metal layers on the failure mode and mechanical properties during static out-of-plane loading, and the results were compared with those of our previous impact and CAI tests. The results confirmed that the plastic deformation of metal layers improved the energy dissipation and thus failure occurred in a limited range of locations. Furthermore, the investigation of the in situ stress during flexural tests by finite element method (FEM) analysis revealed that a hybrid material that both suppresses ply failure and has a high modulus of the metal can be realized owing to the constraining effect of thin-ply laminates.

Herein, epoxy resin is cured by coupling quantum chemical calculations with molecular dynamics simulations that enable the prediction of material characteristics with fewer artificial parameters. A polymer network is formed by the reaction between base resin and curing agent. The reaction uses activation energy and heat of formation data obtained by first-principle calculations coupled with global reaction route mapping algorithms. Density, glass-transition temperature, Young’s modulus, and curing conversion are used to validate the procedure. Experimental and simulation results indicate that base resin with multi-functional reaction groups increase glass-transition temperature and Young’s modulus because of cross-linking at the molecular scale.

The present study investigated the effect of reinforcement on the viscoelasticity of shape-memory polymer composites (SMPCs) from two different scale views utilizing two different techniques. From a nano-scale view, we conducted dynamic viscoelastic simulations using molecular mechanics simulation models proposed by our research group [1]. These nano-scale simulations could reproduce a clear glass transition from glassy state to rubbery state in the frequency domain; therefore, they are suitable for studying the filler effect including interface properties. From a macro-scale view, we conducted experiment dynamic mechanical analysis using woven fabric based SMPCs. Through comparison of multi-scale analyses, we investigated the importance of nano-scale analysis for the composites’ design of viscoelasticity.

The purpose of the present study is to model shape fixity and time-dependent deployment in shape-memory polymer composites (SMPCs) and to evaluate the effect of textiles’ tensile and bending moduli on these properties. We constructed an SMPC model by combining SMP layers and a reinforcing layer. We also considered the thermo-viscoelasticity of SMP and the difference in values between the tensile and bending moduli of the reinforcing layer. Employing this model, we simulated deployment under pure bending conditions. Comparison with experimental results confirmed that our proposed model is able to simulate shape fixity and time-dependent deployment in SMPCs. We also confirmed that the bending modulus is an important factor for shape fixity and time-dependent deployment, whereas the tensile modulus has nothing to do with these properties.

Intralaminar and interlaminar fatigue crack growth behaviours under mode I loading were investigated with conventional and interlayer toughened unidirectional CFRP laminates. For intralaminar crack growth tests, initial defects were introduced using “intralaminar film insertion method”, in which a release film is inserted inside a single lamina prepreg. A fatigue test under a constant maximum energy release rate, Gmax, was carried out using DCB specimens. It was found that the intralaminar fatigue crack growth property of the interlayer toughened CFRP laminates was the same as that of the conventional CFRP laminates. For the interlayer toughened CFRP laminates, the Gmax with a given crack growth rate, da/dN, was much lower for intralaminar crack growth than for interlaminar crack growth. The da/dN-Gmax curve at zero crack extension, Δa = 0, which was estimated by extrapolating the da/dN-Δa relationship, was not affected by bridging fibres, and most conservative for the interlayer toughened CFRP laminates.

The present study presented a numerical model for the tensile damage in fiberreinforced composites with an open hole, using a peridynamics theory. Especially, peridynamics theory can deal with the damage in the composites having arbitrary fiber orientations by setting the bond properties (bond constant and critical stretch) appropriately. We compared two approaches for the peridynamics analysis of fiberreinforced composite materials, proposed by Oterkus and Madenci [1] and Ghajari et al. [2] and discussed the advantages and disadvantages of the peridynamics modeling for the damage in composites with curvilinear fiber paths. Finally, we reproduced the multiple-type and multiple-site damage in multi-ply laminated composites with arbitrary fiber orientations, using peridynamics simulations.

A novel initial crack insertion method, “intralaminar film insertion method”, was proposed to investigate the fracture toughness of unidirectional carbon fiber reinforced plastic (CFRP) laminates when the crack propagates inside the ply and not in the interlayer resin-rich area. Here, a release film was inserted inside a single lamina during the resin impregnation process of prepreg manufacturing. Mode I intralaminar fracture toughness tests were carried out for conventional CFRP laminates and interlayer toughened CFRP laminates. For comparison, two conventional methods were used to introduce initial cracks. One is the “interlaminar film method”, where a release film is inserted between two prepreg plies during the lay-up process. The other is the “machined slit method”, where a slit notch is machined in parallel to the layer of CFRP laminates. It was demonstrated that the proposed “intralaminar film method” can correctly evaluate the intralaminar fracture toughness of both conventional CFRP laminate and interlayer toughened CFRP laminate from the initial value to the propagation value. For this range, it was also found that the intralaminar fracture toughness of interlayer toughened CFRP laminate was the same as that of conventional CFRP laminate. Thus, the intralaminar fracture toughness was not influenced by interlayer toughening.

The relationship of annealing and shape memory effect of uniaxially oriented shape memory polyurethane was studied; meanwhile a new method of adjusting shape recovery ratio by annealing was proposed for further consideration. Experiments were designed to compare the influence on length change from annealing and shape memory effect with shape memory polyurethane film at 65°C. We found that for shape memory polyurethane which had residual strain from material processing procedure, annealing and shape memory effect have the same effect on its length change if they are both carried out at the same temperature. It is because annealing and shape memory effect have the same mechanism, which is the change of state from low conformational entropy states to the recovery of a stable high entropy state in the polymer. Moreover, it is proved by experiment that shape recovery ratio of shape memory polyurethane can be adjusted by annealing.

This study presents a simple approach to model unidirectionally arrayed chopped strand (UACS) laminates via the extended finite-element method to represent slits (i.e., fiber cutting lines). This enables the introduction of slits independent of the finite-element mesh and reduces the effort to represent a complex discontinuity pattern when compared to standard modeling using double nodes. The laminated structure was represented by stacking two-dimensional layers, and cohesive elements were inserted into the layer interfaces to predict the extension of delamination. Damage progress in CFRP quasi-isotropic laminates with diagonal continuous slits were analyzed via the present approach, and the effect of the angle between the slit line and fiber direction on the tensile strength was investigated. The predicted strength was in agreement with the reported experiment results within the examined slit angle range. A numerical study revealed that the strength of the UACS laminates was enhanced by a low inclined slit angle.

In particle accelerators, a very effective way to shield an external magnetic field, which affects the beam inside the vacuum chambers, is to manufacture the vacuum chambers using soft magnetic materials with high permeability. We selected a permalloy and a ferritic stainless steel as candidates of those magnetic materials. However, until now the vacuum performance of vacuum chambers made of magnetic material does not yet have a proven track record. Therefore we made a list of items to be examined and verified. These items are listed as follows. 1. General vacuum performance of the magnetic materials. 2. Magnetic annealing condition and its effect on each performance. 3. Magnetic and vacuum characteristic of a vacuum chamber made of soft magnetic material. The outgassing rate of the magnetic materials was equivalent to that of an austenitic stainless steel. A good magnetic shielding performance is obtained by annealing at higher temperatures than 850 °C for both magnetic materials, although the performance declines, if the annealing temperature is too high. Furthermore the magnetic annealing in good vacuum is combined with vacuum heat treatment. The TDS spectrum showed that the reduction of hydrogen outflux from the bulk material near the surface was achieved by this process. Finally, we successfully produced a vacuum chamber with such a magnetic material, which will be used in a beam line at the 3 GeV J-PARC synchrotron, because satisfying magnetic shielding and vacuum performance were obtained.

To safely use turbo-molecular pumps (TMPs) in a magnetic field, it is necessary to reduce the eddy current induced in a rotating rotor. Instead of adding a magnetic shield facility to the TMPs available in markets, we developed a TMP with a magnetic shield function by replacing the housing material of the TMP with a ferromagnetic substrate SUS430. Before and after machining, the SUS430 was vacuum-fired at 700 °C for 10 h in order to attain a good vacuum quality and recover its high magnetic permeability. The magnetic shield efficiency of the TMP with SUS430 housing was then examined. When a perpendicular magnetic field of 9 mT was applied, the field inside the TMP was reduced to less than 0.3 mT. Next, we confirmed that the developed TMP shows good performance in achieving an ultrahigh vacuum in magnetic fields of up to 9 mT. The limit of the magnetic shield efficiency for the developed TMP is also discussed.

The effect of current peening techniques that modify the surface layer of metallic materials on the fatigue strength of those materials has been enhanced by employing a micro-size, high-rate shot impact. The present paper evaluates the effect of impact velocity and impact size on work hardening on the surface of polycrystalline metals subjected to peening. A single-shot impact was modeled based on a polycrystal plasticity finite-element analysis in order to address the effect of grain-order work hardening. Using the finite-element analysis, the effect of the relative size of the shot and the individual grains on the surface work hardening of polycrystalline metal was investigated. Simulated results reveal that the deformation progresses preferentially along the grain boundary rather than inside the grain after a large shot impact, while peening with a small shot can introduce intense work hardening inside the grain just beneath the surface. Moreover, we compared a shot impact and a static indentation whose dimples were almost the same size and confirmed that a high-rate shot impact can generate a significantly work-hardened layer beneath the impacted surface.

In this study, we investigated the influence of base resin and curing agent and their mixture on the curing characteristics by performing experiments and molecular simulations. In the curing experiment of epoxy resin, we used differential scanning calorimetry (DSC) to obtain the conversion by mixing curing agents and base resins. We used the molecular orbital method (MO) and the molecular dynamics method (MD) to simulate the curing reaction in molecular scale and investigate the effect of differences in resin composition on the curing characteristics. This simulation took into consideration activation energy, heat of formation, and polarization in the curing reaction. The simulation captures the trend of curing reaction obtained by the experiment. We found that the selection and mixture of curing agents are very important when controlling the curing characteristics of epoxy resin.

This study proposes the layer-wise method (LWM) as a new approach for predicting the tensile strength of discontinuous fiber-reinforced composites that have arbitrary fiber orientation angles. The LWM assumes the discontinuous fiber-reinforced composites are identical to laminates that are composed of unidirectional fiber-reinforced plies and have the same distribution of fiber angles over the entire laminate. We applied the LWM to discontinuous carbon fiber polypropylene composites and evaluated the effect of fiber length on tensile strength and fracture mode. Simulated results agreed well with those of experiments. In addition, we proposed a simple analytical model based on micromechanics. This analytical model can correctly evaluate the strength and the fracture mode as effectively as the LWM. We also compared these models with a rule of mixture considering the failure criterion of fiber breakage and examined the limitation of the rule of mixture in predicting composite strength.

In this paper we propose an experimental method for estimating the depth to which the surface of a material is modified after being treated by cavitation peening. The estimate is made on the basis of two theories: plate theory and beam theory, in which the plastic strain in the modified layer is considered. As the depth of the compressive residual stress is an important factor for the fatigue strength and for stress corrosion cracking, a simple and straight-forward method to estimate the depth of the modified layer is needed. In the proposed method, measurements of the surface residual stress and the radius of curvature generated as a result of the plastic deformation introduced by cavitation peening are combined with either plate theory or beam theory. The most appropriate theory depends on the thickness of the specimen. The plate theory is more accurate than beam theory in the case of both a thin and thick specimens. The beam theory should be applied only in the case of a thick specimen.

Discontinuous fiber-reinforced composites have better productivity and formability than continuous fiber-reinforced composites. However, their strength is remarkably low. Thus, there is an urgent need to improve the strength of discontinuous fiber-reinforced plastic composites. In this study, we utilized a unit-cell model that considers microscopic damage including matrix cracking and fiber breaking, and incorporates constitutive laws of thermosetting resin or thermoplastic resin for the matrix. The tensile damage and strength of the composite were investigated for various fiber lengths and/or matrix properties. We compared the simulated strengths with experiments for carbon fiber-reinforced polypropylene. The effect of deformation rate on mechanical behavior was also investigated.

For this study we used finite element analysis to show how the residual stress affects the hydrogen concentration around a crack tip in a plastically deformable material after a fatigue process. Following a 9 cycle fatigue process, hydrogen diffusion analysis was carried out at the highest applied fatigue stress. This showed hydrogen invading the crack surface and diffusing into the material. The concentration of hydrogen was higher close to the crack tip and its behavior was largely affected by the residual stress in the material. Tensile residual stress accelerated the hydrogen invasion and increased its concentration, while compressive residual stress simulated as the stress induced by peening clearly suppressed them. This is due to the affect the residual stress has on the hydrostatic stress around the crack tip which is a dominant factor in the hydrogen diffusion behavior. Peening, which is a surface treatment used to introduce compressive residual stress to enhance the mechanical properties of a material, such as its resistance to stress corrosion cracking and its fatigue strength, may, therefore, suppress the embrittlement caused by hydrogen.

In the present study, we conducted periodic-cell simulations of the thermomechanical cycle of thermally activated shape memory polymer (SMP)-based composites. The present simulation utilizes a micromechanical model for reproducing the discontinuous fibers and SMP. We analyzed the effect of fiber volume fraction, fiber aspect ratio, and fiber end position on the shape fixity and shape recovery of the composite. The simulated results revealed that fiber elasticity is a key factor for the shape fixity of the composite, while both strain concentration near the fiber ends and fiber elasticity play important roles in the shape recovery properties of the composite.

The effect of fiber arrangement on transverse tensile failure in unidirectional carbon fiber reinforced composites with a strong fiber-matrix interface was studied using a unit-cell model that includes a continuum damage mechanics model. The simulated results indicated that tensile strength is lower when neighboring fibers are arrayed parallel to the loading direction than with other fiber arrangements. A shear band occurs between neighboring fibers, and the damage in the matrix propagates around the shear band when the interfacial normal stress (INS) is sufficiently high. Moreover, based on the observation of Hobbiebrunken et al., we reproduced the damage process in actual composites with a nonuniform fiber arrangement. The simulated results clarified that the region where neighboring fibers are arrayed parallel to the loading direction becomes the origin of the transverse failure in the composites. The cracking sites observed in the simulation are consistent with experimental results. Therefore, the matrix damage in the region where the fiber is arrayed parallel to the loading direction is a key factor in understanding transverse failure in unidirectional carbon fiber reinforced composites with a strong fiber/matrix interface.

The present study proposed a method to evaluate the equibiaxial compressive residual stress of a metal surface by means of a depth-sensing indentation method using a spherical indenter. Inverse analysis using the elastic–plastic finite-element model for an indentation test was established to evaluate residual stress from the indentation load–depth curve. The proposed inverse analysis utilizes two indentation test results for a reference specimen whose residual stress is already known and for a target specimen whose residual stress is unknown, in order to exclude the effect of other unknown mechanical properties, such as Young’s modulus and yield stress. Residual stress estimated by using the indentation method is almost identical to that measured by X-ray diffraction for indentation loads of 0.49–0.98 N. Therefore, it can be concluded that the proposed method can effectively evaluate residual stress on metal surface.

In order to eliminate stress corrosion cracking, a method of introduction of compressive residual stress using cavitation impacts was proposed, without use of shots. The cavitation impacts were successfully produced by a cavitating jet in air, without the requirement of a water-filled chamber. The injection condition of the jet was optimized and the introduction of compressive residual stress into stainless steel was demonstrated using the jet. The maximum compressive residual stress introduced by the optimized jet was 500 MPa on the surface, while the thickness of the modified layer was up to 400 μm. A method for estimation of the introduced compressive residual stress by the jet as a function of processing time was proposed, considering the stochastic phenomena of the cavitation impacts. Both the intense impact at 0.2 Hz and relatively weak impact at 4.5 Hz affect the introduction of compressive residual stress. The value of the residual stress and the thickness of the modified layer can be estimated by the proposed experimental equation.

We have developed a novel stampable sheet composed of discontinuous carbon mono-fibers and thermoplastic resin for press molding of composite materials. This sheet enables in-plane random orientation and dispersion of carbon fibers in composites. Therefore, complexly shaped products, such as rib structures, are easily fabricated. Moreover, composites having superior mechanical properties can be obtained because the fiber length in composite is easily controlled to be long. This study addressed the effect of fiber length on strength and fracture modes of composites made of the sheet, using tensile tests and micromechanical analyses. To investigate the influential damage for the fracture mode, our analysis utilized Duva-Curtin model for fiber breakage. We incorporated this damage model into an equivalent inclusion model combined with the Mori-Tanaka theory to predict the tensile strength of the composites. The predicted results agreed well with those of the experiments when the fiber length and orientation in the composites were appropriately considered. Our analyses also indicated that 3 mm as the lower-limit fiber length in the composite, below which the damage mode is no longer dominated by fiber breakage and superior strength of the composites cannot be obtained.

This paper experimentally investigates damage mechanisms which is closely related with the bearing failure in bolted joints of CFRP laminates and proposes a finite element model to reproduce those damage mechanisms. First, the damage was observed in the bearing failure of bolted joints, and the main factor of the large stiffness degradation in bearing failure is the fiber kinking damage. In addition, the fiber kinking is suppressed by the bolt clamping, which increases the bearing strength of bolted joints. Then, three-dimensional finite element model was developed for addressing the bearing failure in bolted joints based on the experimental observation. In this model, the initiation and the propagation of the fiber kinking damage which plays an important role in the bearing failure of bolted joints were reproduced and the stress failure criteria were investigated. The variation of the load at the kinking onset due to the bolt clamping was simulated and consequently the load history during the evolution of the fiber kinking damage was simulated.