The interlaminar fracture toughness of composite laminates have been evaluated mainly with unidirectional or cross-ply laminates. However, it is difficult to estimate the realistic interlaminar fracture toughness and failure process with these laminates, since quasi-isotropic or angle-ply CFRP laminates consisting of various fiber orientation angles are mainly used in industrial structures such as aircraft. In this paper, we focused on the effects of fiber orientation angle on the interlaminar fracture toughness. The static mode I interlaminar fracture toughness was measured by using three types of CFRP laminate, in which the neutral interlaminar consisted of 0º/45, ±22.5º and 0º/0º layers. The results were evaluated by both experimental and analytical approaches. Both the (0º/45º) and (±22.5º) laminates exhibited an energy release rate almost twice larger than that of the unidirectional laminate. Moreover, the fracture mechanism between (0º/45º) and (±22.5º) laminates was significantly different due to the effect of fiber bridging.

The mechanical properties of three thermoplastic prepreg systems: carbon fiber/polypropylene (CF/PP), polyamide 6 (CF/PA6), and polyphenylene sulfide (CF/PPS) are studied and analytical formulas introduced for the extraction of matrix and interface properties from composites. Since a semi-crystalline thermoplastic matrix is sensitive to crystallization, the influence of cooling rate on strength is statistically evaluated. While the 0º tensile strength is independent of the cooling rate, the 90 tensile strength is strongly influenced by the matrix type and cooling rate. The matrix modulus increases as the cooling rate becomes lower; the degree of crystallinity also increases. The matrix residual stress, interfacial shear strength, and mode II interlaminar fracture toughness are also dependent on the cooling rate, with the trends different for different matrices.

Carbon fiber tape reinforced thermoplastics (CTT) are highly promising materials due to their high formability, mechanical performance, and recyclability; however, CTT is known to have a larger standard deviation of mechanical properties and lower out-of-plane shear modulus than metal materials. In this study, a topology optimization was conducted considering the mechanical property deviation and anisotropy of CTT. A CTT tensile simulation model was used to formulate the relationship between the material size and coefficient of variation. The simulation result showed that a lower specimen length and thickness caused a larger variation of elastic modulus due to the non-uniformity of mechanical properties. A penalty parameter was introduced in the topology optimization model based on the idea of effective length and thickness in the structure. Different optimal structures with thicker parts were obtained when a penalty parameter was considered; however, some rotation angles in the optimum structure were complex and a large threshold for penalty parameter caused bad convergence.

Strain rate and temperature dependence of carbon fiber reinforced thermoplastic composites (CFRTP) were evaluated. CFRTP composites are superior not only in terms of specific strength and stiffness, but also in terms of formability. They are, therefore, expected to be widely applied in mass production of automobiles, thereby resulting in significant vehicle-weight reduction. To incorporate CFRTP composites in automobile structures, novel techniques in vehicle-structure design must be developed to ensure safety in the event of collision accidents. It is also necessary to clarify impact properties of the structure under various temperature conditions that an automobile may be subjected to. To this end, the 3-point bending impact test on chopped carbon fiber tape reinforced thermoplastics (CTT) was performed at various operating temperatures. Fracture behavior was also observed using a high-speed video camera, along with investigations concerning strain rate and temperature dependence. Using the abovementioned bending test, the strain rate and temperature dependence of CTT were confirmed in terms of flexural strength, and the fracture behavior was categorized into three failure modes based on temperature conditions. Concurrently, the viscoelastic properties of the matrix resin were evaluated, and the strain rate and temperature dependence of the flexural strength of CTT were predicted using the master curve derived based on the time–temperature superposition principle. The prediction results were found to be consistent with those obtained via experiments.