A method for statistical prediction of the long-term creep failure time of CFRP using the statistical static strengths of CFRP at various temperatures and the viscoelasticity of matrix resin is proposed based on Christensen's model of viscoelastic crack kinetics. The tensile strength along the longitudinal direction of unidirectional CFRP constitutes important data for the reliable design of CFRP structures. The authors developed a reliable method for testing creep and fatigue strengths as well as static strength at elevated temperatures for resin-impregnated carbon fiber strands (CFRP strands) as unidirectional CFRP. Two kinds of CFRP strands with two types of PAN-based carbon fibers were examined in this study. The statistical static strengths of these CFRP strands and the creep compliances of matrix resins were measured at various temperatures. The tensile creep failure times of these CFRP strands are predicted statistically based on a prediction method using measured data. The predicted creep failure times of these CFRP strands were compared with the creep failure times of these CFRP strands measured experimentally and statistically. Additionally, the statistical temperature-dependent static strengths are also discussed for CFRP strands with pitch-based carbon fibers.
We have proposed a general and rigorous accelerated testing methodology (ATM) which can be applied to the life prediction of CFRP laminates and structures. The most important condition for ATM is the fact that the time and temperature dependence on the strength of CFRP is controlled by the viscoelasticity of matrix resin and that the long-term strength of CFRP can be predicted by using the short-term strength measured at elevated temperatures based on the time-temperature superposition principle for the viscoelasticity of matrix resin. The viscoelastic behavior of matrix resins depends on the water absorption, therefore the long-term strength of CFRP also depends on the water absorption. The static strengths for typical four directions of unidirectional CFRP were measured under various temperatures at a single loading rate for Dry and Wet conditions. As results, the master curves of long-term static strengths in these four directions of unidirectional CFRP were obtained for Dry and Wet conditions. The effect of water absorption on these long-term static strengths was characterized by the viscoelastic behavior of matrix resin and the failure mode.
Accumulation of microscopic damage, which occurs in fiber-diameter scale, causes failure of carbon fiberreinforced plastics. Therefore, accurate evaluation of microscopic damage is important to predict failure of structure made from composite materials. In this study, effect of macroscopic nonlinear behavior and failure criteria for prediction of initial matrix cracking are investigated using a multiscale approach which consists of macroscopic scale and microscopic scale analyses. On the macroscopic analysis, laminate is modeled as a homogeneous body, and comparison between elastic and elasto-plastic constitutive laws is performed to explore effect of nonlinear behavior on microscopic damage. On the microscopic analysis, the 3D periodic unit cell (PUC) analysis considering microscopic structure of the material is conducted using the strain history obtained from the macroscopic analysis as external force. The multiscale approach is applied to tensile tests of unidirectional laminates under off-axis loading, and initial cracking strains of each loading condition are predicted. Several failure criteria are employed for matrix resin in the PUC analysis, and are compared each other to reveal important factors of failure prediction in CFRP laminates. Comparison of stress states at failure shows that hydrostatic stress plays an important role in failure prediction.
In this paper, the microscopic damage behavior of a carbon fiber reinforced plastic laminate with interlayers subjected to low-velocity impact was experimentally and numerically investigated. Impact tests were conducted by using a guided drop-weight test rig, whereas in numerical analysis, finite element (FE) software was employed to simulate low-velocity impact damage on the laminate. The base ply and the interlayer were modelled as an orthotropic elastic material and an isotropic elastic material, respectively. Cohesive elements were introduced not only between the base ply and the interlayer but also inside the base plies to simulate the interlaminar and intralaminar delaminations. From the experimental and numerical results, it is proved that the crack and the dent on the front surface of the laminate was due to compressive stress in the fiber (0°-) direction. Both interlaminar and intralaminar delaminations extend in the 0°- and transverse (90°-) directions. Especially, the both delaminations should be considered to reproduce the overall impact behavior including damage in the FE analysis.
This study investigated the effects of fiber cutting angle on the fatigue behavior of open-holed carbon fiberreinforced plastic (CFRP) laminates with initially cut fibers (ICFs). Three kinds of fiber cutting angles (θ= 22.5, 45, and 90°) were applied to two types of quasi-isotropic ICF laminates: one was fabricated using autoclave (ICF-A) and the other was fabricated using press molding (ICF-P). First, fatigue tests were conducted to obtain S (maximum stress)-N (the number of cycles to failure) curves in order to reveal fatigue strength. The fatigue tests of one specimen for each configuration were interrupted at three prescribed numbers of cycles to observe damage progress. Finally, a semi-empirical equation was proposed to predict delamination area just before the final failure against normalized applied stress. It is found from the experiment result that the fatigue strength in both laminates is the highest when the θ is the smallest (22.5°) although the static strength is the highest when the θ is 90°.In contrast, both laminates with the biggest θ(90°) exhibit the greatest delamination growth around the hole by cyclic loading.
In this study, elastic properties of the plain-weave fabric composite laminates are investigated through finite element analysis. In the analysis, the intra-lamina inhomogeneity caused by the architecture of woven yarns is taken into consideration. Using the homogenization procedure, the effective plate stiffness of the fabric composite laminate is calculated. Assuming the number of laminated plies is infinite, the effective three-dimensional continuum elastic properties of the fabric composite are also calculated. Then, the effect of the number of plies on the in-plane (in-plane tension and in-plane shear) elastic properties and out-of-plane (bending and twisting) elastic properties of the composite laminate is investigated. Through the numerical investigation, it is found that the inplane tensile stiffness and the twisting stiffness of the composite laminate are overestimated, particularly for a small number of laminated plies, when they are evaluated with the assumption that each lamina is homogeneous with the effective three-dimensional continuum elastic properties.
The subject of this study is to understand the continuous impregnation process of carbon fiber reinforced thermoplastic (CFRTP) sheet using fixed rollers double belt press (DBP). Fixed rollers DBP has a complicated mechanism of pressure process. The pressure changes along the belt move under the rollers. In this study, theoretical impregnation model was developed about carbon fiber woven fabric / PA6 films materials with heat and cool process. The permeability of this material was calculated by comparing to the experimental result. Then, this model was applied to DBP impregnation process. In order to understand DBP process, material temperature profile and applied pressure profile was measured during the process. And based on these results, theoretical DBP impregnation model was developed. This model was evaluated with the experimental result and showed a certain level of validity. These understanding and the impregnation model is helpful to optimize the DBP process and to improve the machine capability for CFRTP sheet production.
The permeability is one of the most important manufacturing parameters in fiber reinforced plastics (FRP) in which fibers are impregnated by resin. Although the permeability is defined by the Darcy's law, wettability between fibers and resin is not considered as a constituent parameter. In this study, we focused on the correlation between wettability and permeability in FRP. Wettability was controlled by the concentration of silane coupling solution for the surface treatment of fibers.
In this study, the interaction between resin and fiber was simulated based on the coarse grained molecular dynamics with a model of resin and fibers with various surface structures. The main target of this simulation is to evaluate the effect of surface structures on the contact angle between resin and fiber. In the result, there was a tendency to be small the contact angle between fiber and resin when a simulated silane coupling treatment was applied on the surface of a fiber model. Therefore coarse-grained molecular simulation shows the same trend as the contact angle measurement test, suggesting that it is possible to evaluate the change in wettability by surface modification. on the coarse grained molecular dynamics.
As a next step in self-healing materials, it is desired that the repeatable self-healing function would be realized. In this case, a biomimetic approach, where the functions or structures of living tissues should be mimicked, might be effective. In this study, we had devised the application of stress-activated micro-channels, which can be opened and closed by applied stresses, on microcapsules, by mimicking the ion-channels on cellular membrane. We tried to apply small holes to microcapsule membrane utilizing the removal of “islands” from the “sea-island structure” composed of membrane material and second phase, as the first step to realize stress-activated micro-channels. Here, microcapsules were prepared using the coacervation method, and glass beads were used as “islands”. Then, we tried to optimize the membrane thickness of microcapsules and the size of holes on the microcapsule membrane applied by the dropout of glass beads, through the in-situ observation of leakage and enclosure behavior of microcapsules with holes using a simplified compression device. As results, it was suggested that both the membrane thickness and hole diameter of 10 [μm] are suitable to apply stress-activated channels to microcapsules. Finally, we demonstrated the function as microcapsules with stress-activated channels by the in-situ observation of the tensile leakage and enclosure behavior of microcapsules embedded in a resin strip specimen.