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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...

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.

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.

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.

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.

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.

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.

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.

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.

We presented a numerical simulation to address the impact-induced deformation and damage of composite plates subjected to soft-body, high-velocity impacts for application to the bird-strike problem of composite fan blades. A new stabilized contact algorithm was developed based on the Lagrange multiplier method to predict appropriate impact forces applied to the plate, in order to solve soft-body impact at high velocity without causing severe numerical instabilities. The bird-strike impact on composite fan blade was simply modeled by discussing the damage characteristics of a unidirectional composite plate. Combining the model of a soft-body impactor with an appropriate contact algorithm, we could capture the transition from the global bending mode at low velocity to the local deformation mode at high velocity, enabling a discussion of the ballistic limit using the damage analysis of the laminate. As the impact velocity increased, the damage in the composite changed from bending-induced matrix-cracking to an intensive fiber-breakage mode causing local shear perforation. The damage mode transition allows us to detect the transition velocity as a ballistic limit, which is one of the critical factors for discussing the bird-strike resistance of composite fan blades.

This study proposes a technique for enhancing strength and uniformity in unidirectionally arrayed chopped strands (Enhanced UACS). Enhanced UACS is made by introducing slits at small angles to the fiber direction into a unidirectional prepreg. As the angle becomes smaller, the stress concentration around the slit decreases. Therefore, delamination initiated from the slit is effectively suppressed and the final failure is mainly caused by fiber breakage. As a result, the Enhanced UACS laminate achieves excellent tensile, compressive, flexural, fatigue and impact strength, comparable to continuous fiber composites. Moreover, we demonstrate that the laminate can be uniformly stretched without a slit opening when it is formed.

This paper investigated the damage transition mechanism between the fiber-breaking mode and the fiber-avoiding crack mode when the fiber-length is reduced in the unidirectional discontinuous carbon fiber-reinforced-plastics (CFRP) composites. The critical fiber-length for the transition is a key parameter for the manufacturing of flexible and high-strength CFRP composites with thermoset resin, because below this limit, we cannot take full advantage of the superior strength properties of fibers. For this discussion, we presented a numerical model for the microscopic damage and fracture of unidirectional discontinuous fiber-reinforced plastics. The model addressed the microscopic damage generated in these composites; the matrix crack with continuum damage mechanics model and the fiber breakage with the Weibull model for fiber strengths. With this numerical model, the damage transition behavior was discussed when the fiber length was varied. The comparison revealed that the length of discontinuous fibers in ...

The statistical strength distribution of Carborundum polycrystalline SiC fibers was derived from the fragmentation process in a single fiber/epoxy composite. We conducted Monte Carlo simulations of the fragmentation process using an elastic-plastic hardening shear-lag model. The Monte Carlo simulation with the estimated fiber strength distribution reproduces the fragmentation seen in the experiments very well. We also compared these simulated results with those calculated by the elastic shear-lag model with the shear-lag parameter beta tuned as proposed in Curtin et al. [Curtin WA, Netravali AN, Park JM. Strength distribution of Carborundum polycrystalline SiC fibers as derived from the single-fiber-composite. J Mater Sci 1994;29:4718-28] and with an elastic-plastic finite element model. The fiber axial stress distributions in all three models are in close agreement, with the characteristic length beta(-1) used in Curtin et al. consistent with the plastic length around a fiber break in an elastic-plastic hardening shear-lag model. (c) 2008 Elsevier Ltd. All rights reserved.

A numerical model of the progressive damage in cross-ply laminates (e.g., transverse cracks, interlaminar delaminations, and fiber breaks) is proposed. In this model, the embedded process zone (EPZ) model is used for the transverse cracks and interlaminar delaminations; the truss elements are used to express the fiber breaks. First, we describe the formulation and algorithm of this model. Second, we calculate the transverse cracking stress in CFRP [0/90] s laminates and compare it with the experiments by Boniface et al. The comparison validates that our model can appropriately simulate the onset and accumulation of transverse cracks for an arbitrary thickness of the 90° ply to the 0° ply with a set of parameters. Finally, this model is applied to our experiments for GFRP [90/0] s laminates. The simulated results reproduce the complicated progressive damage in GFRP [90/0] s laminates very well.

The tensile strength of unidirectional carbon fiber-reinforced plastic (CFRP) composites was predicted by numerical simulation plus size scaling. The fiber strength distribution used in the numerical simulation was determined from the fragmentation process in a single fiber composite. Since the experimental data obviously did not fit the normal Weibull distribution, we fitted them with the Weibull of Weibull model, considering the statistical distribution of scale parameters of fiber strength in the normal Weibull model. Moreover, the constitutive law of the matrix was derived from the stress–strain curves of the angle ply laminates, utilizing the micromechanics approach proposed by Tohgo et al. [9]. Based on these parameters, we simulated the tensile fracture of unidirectional CFRP composites with the spring element model (SEM). The predicted tensile strength by numerical simulation plus size scaling agreed well with the experimental data. The results also confirmed that the Weibull of Weibull model is i...

This paper examines the stress distribution around a fiber break in alumina fiber-reinforced aluminum matrix (Al 2 O 3 /Al) composites using finite element analysis and predicts their tensile strengths using tensile failure simulations. In particular, we discuss the effect of matrix hardening on the tensile failure of the Al 2 O 3 /Al composites. First, we clarify the differences in the stress distribution around a fiber break between an elastic–perfect plastic matrix and an elastic–plastic hardening matrix using finite element analysis. Second, the procedures for simulating fiber damage evolution in the Al 2 O 3 /Al composites are presented. The simulation incorporates the analytical solution to the stress distribution of a broken fiber in the spring element model for the stress analysis of the whole composite. Finally, we conduct Monte Carlo simulations of fiber damage evolution to predict the tensile strength of the Al 2 O 3 /Al composites. Coupled with a size-scaling analysis, the simulated results express the size effect on the strengths of the composites seen in experimental results.

This study proposes a numerical method for analyzing and simulating the failure of unidirectional fiber-reinforced composites using the spring-element model (SEM). We compare the stress distribution calculated by this method with that of the 3D finite-element method (FEM). A Monte Carlo simulation is performed to simulate failures by this method. The computational efficiency is discussed in comparison to our previous SLM. Additionally, we demonstrate a hybrid (SEM/FEM) analysis to show the compatibility for the structural analysis. We found this method accurate and efficient for simulating and analyzing the failure process in the damaged composite.