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.
The effects of bypass and bearing forces on the failure behaviors of mechanical joints of carbon fiber reinforced CFRP laminates were experimentally studied. Two types of specimens were tested: single-shear/single-fastener (SS) and single-shear/double-fastener (SD) specimens. In addition to specimens prepared according ASTM standards, narrower specimens than those designated by ASTM standards were prepared. The strengths of the resulting fastener joints were limited by the bearing strength of the fastener portion, and were comparably constant regardless of the specimen width when the width exceeded a critical value which depended on the number of fasteners. The narrow specimens, however, tended to fail in a tensile brittle manner at the side edge of the hole after the accumulation of the bearing damage. The ductility of the fastener joints was discussed from the reduced equivalent width of the plates.
In this study, we applied a modified cruciform test to the evaluation of the interfacial tensile strength (IFTS) of the carbon fiber (CF)/polycarbonate (PC) interface and evaluated the effectiveness of the evaluation method via in situ observation. Although the interfacial shear strength has been extensively analyzed in previous works with thermosetting plastics, few studies have evaluated IFTS of thermoplastics, for instance, PC. However, previous reports have stated that the cruciform test is not suitable when the fiber transverse modulus is much lower than the matrix modulus. Therefore, in the present study, the modified cruciform test was carried out with specimen in which loading arms are reinforced with CF/PC cross-ply laminates in order to induce stress concentration at the fiber surface. Debonding was observed and was found to instantly propagate along the fiber, and the IFTS was calculated as 63.1±12.6 MPa on average. Thus, this investigation has revealed the IFTS of the CF/PC composite, and the effectiveness of the modified test to evaluate thermoplastic composites was confirmed.
In this study, we performed digital image correlation (DIC) using fiber cross sections as random markers, focusing on damage initiation and propagation in 90º layers in carbon fiber/epoxy cross-ply laminates. By comparing these results with mesoscopic in-situ observations, we confirmed the validity of DIC as a method for evaluating such mesoscale damages. This method is independent of the judgement of the observer. Both the locations and values of strain concentrations measured by DIC analysis showed good agreement with the initial in-situ observation of cracks at 0.54 [%] average nominal strain. In addition, a higher strain distribution was confirmed with DIC before the crack was recognized by in-situ observation. Thus, it is expected that the detection and prediction of damages can be realized by the measurement of mesoscopic strain distributions by DIC using fiber cross sections as random markers, as proposed in this study.
In recent years, automotive bodies are required to be lightweight because of the need to reduce travel resistance. Consequently, researchers are attempting to apply high-strength and lightweight FRP (fiber reinforced plastic) and FRTP (fiber reinforced thermoplastic) to automobile structures. Besides reducing the weight, the collision safety of automobiles, which is also a very important issue, should be ensured though investigation of the dynamic properties of FRP and FRTP. Both FRP and FRTP have been applied to automotive parts in small volumes so far, and dynamic fracture simulations of strain-rate-dependence cases are few compared to those for steel materials. In this study, the strain rate dependence results of tensile, compressive, and shear characteristic values of CFRP and CFRTP were experimentally obtained. Using these results and LS-DYNA, which is a general-purpose finite element solver, CFRP cylinders under axial impact compression were analyzed. These CFRP cylinders are assumed to be the member that absorbs impact energy in automotive structures. In addition, the FEM impact results were compared with the experimental results of the CFRP cylinders, and the validity of the FEM analysis was thus demonstrated.