To clarify biaxial fracture properties of the unidirectional carbon-fiber-reinforced plastic (CFRP) composites, a cruciform specimen with two symmetric rectangular flat dents in the thickness direction was developed, and tensile tests were carried out under 0º and 90º loadings. The specimen showed three fracture modes, namely transverse crack (TC), fiber break (FB), and a mixture of TC and FB modes (TC&FB). According to the strain measurement result, regardless of the magnitude of the strain occurring in 0º direction, TC- and TC&FB-modes occurred when the strain occurring in 90º direction was positive, and FB-mode occurred when it was negative. It was concluded that when the strain along the 90º direction was zero, the straight line with slope ν₂₁ (Poisson’s ratio in the 1-direction when stressed in the 2-direction) can divide the fracture stress data occurring in the FB-mode and other two modes. A coefficient determining the elliptical rotation angle of the failure envelope was introduced, assuming that FB-mode occurs at levels comparable to the uniaxial tensile strength. The failure envelopes given by the coefficient correspond with average fracture stress data.

This study proposes a genetic algorithm (GA)-based optimization method for fiber orientation angle and examines its effectivity. Determining the optimal fiber orientation angle involves searching for a solution from multitudinous combinations, which makes it difficult to attain the optimal solution by simple GA. In addition, the generally used principal stress design cannot give an optimal solution when combined loads are applied. Therefore, an optimization method is developed that does not directly treat the fiber orientation angle as a design variable and attains the optimal solution with lesser design candidates than simple GA. Herein, stiffness maximization is formulated as a strain energy minimization problem, and optimization is performed using the proposed method, wherein the orientation angle of each element of the target FEM model is treated as a design variable. The resultant design candidate (with the flat plate and B-pillar models under combined loads) optimized by the proposed method yields a higher stiffness than that optimized by its conventional counterparts. Thus, it is shown that the proposed method obtains a better design candidate than that obtained by simple GA and principal stress design methods.

This study aimed to examine the nonlinear material behavior of unidirectional carbon fiber reinforced thermoplastics (UD-CFRTP) under off-axis loading using numerical material tests (NMTs). A method to identify the appropriate material properties of an assumed macroscopic anisotropic constitutive law is proposed. To identify the macroscopic material properties using an optimization method, seven macroscopic deformation modes (three vertical directions, three shear directions, and a deformation pattern in the off-axis direction of 45º) were studied for preparing the virtual material responses. Identification accuracy was verified by comparing the predicted macroscopic material responses with those obtained using the actual off-axis tensile test. The extensive use of the NMTs with seven macroscopic deformation modes enabled the successful confirmation of the material properties of the assumed macroscopic constitutive law for UD-CFRTP. This was identified by comparing the NMT results obtained using the off-axis macroscopic deformation and standard six modes with those obtained using only six modes.

Impact damage characteristics and compression after edge-on impact (CAEI) strength of CFRP laminates were experimentally investigated. CFRP laminates with various thicknesses and stacking sequences were prepared. Impact tests with a constant energy level were performed using the special impact fixtures. The damaged areas and their shapes after the impact on the CFRP laminates were evaluated using an ultrasonic C-scanning machine. It was observed that the CFRP thickness affected the impact delamination shape, which was expressed by the longitudinal (w) and transverse-direction (d) extensions. The delamination was propagated in the longitudinal direction when the CFRP laminates were thinner. After the C-scope observations of the delamination, compression strength tests were performed. The CAEI strength degraded when the delamination shape factor (w/d) was increased. A correlation between the CAEI strength and increased volume of the CFRP laminates was also observed after the edge-on impact.