Interfacial debonding on a fiber/resin interface is known as an initial failure followed by a transverse crack or delamination in composite materials. Thus, interfacial tensile strength is an essential material property for designing composites. Although several studies have been conducted on thermosetting resin-based composites, interfacial properties of thermoplastic resin composites have not been sufficiently studied. Furthermore, interfacial tensile strength between carbon fiber and matrix has also not been evaluated. In this study, the cruciform test method was used to experimentally evaluate the interfacial tensile strength in pitch-based carbon fiber/matrix. The process of interfacial debonding between pitch-based carbon fiber/epoxy resin was successfully observed, and an interfacial tensile strength of 41.3 [MPa] was obtained. The interfacial tensile strength of the composite with a polyamide 6 matrix was found to be 36.8 [MPa].

In carbon fiber reinforced thermoplastics (CFRTP) employing polyamide 6 (PA6) as the matrix resin, we found that the reduction in CFRTP fiber length due to mechanical recycling not only decreased the impact strength but also accelerated the thermal oxidation of the matrix resin. In a heating test of an injection molded CFRTP specimen prepared by crashing and remolding, a portion of PA6 was insoluble in the solvent and in a state in which carbon fibers were entrapped. With the results of a heating test employing model samples with different fiber length, we also confirmed that thermal oxidation was accelerated as the fiber length was reduced. There was no significant correlation between the length of the carbon fibers and the number of radicals; but the shorter the fiber length, the greater the number of acidic functional groups on the surface of the carbon fiber. Therefore, we inferred that the insolubilization was mainly caused by the reaction between functional groups in the carbon fiber and PA6.

During typical compression resin transfer molding (CRTM), resin is first injected into a gap between the mold and fiber preform, and the preform is then impregnated in the out-of-plane direction. CRTM thus has an advantage of reduced molding time because of the short impregnation distance. This study predicts resin flow during CRTM of composite structures by performing a resin impregnation simulation using the finite-element method. In the case of the CRTM of a thin curved plate, resin penetrated the preform beneath the gate by the resin pressure, when the injection of the resin into the gap was finished. Reduction of the molding time cannot be achieved in such a case, because impregnation proceeds over a long distance in the in-plane directions. Furthermore, in CRTM with multiple gates and multi-axial compression for an L-shaped component connecting two plates, impregnation was concentrated near the connecting part, and a non-impregnated area remained after finishing compression of the preform. The molding time by CRTM was always longer than that of vacuum-assisted resin transfer molding even at an increased compression speed. These results indicated that optimization of the molding conditions is necessary to achieve the benefits of CRTM.

This study focuses on permeability in the molding process of fiber reinforced plastics. Microscopic resin permeation behavior is governed by the capillary number, which is assumed to be dependent on temperature and pressure. We experimentally evaluated the temperature and pressure conditions at which capillary numbers became identical when different resins were impregnated into an identical glass cloth. We observed that when the capillary numbers were identical, different resins exhibited almost identical permeability values.

In this study, impact and compressive after impact (CAI) tests using varying impact energies were conducted using ply-level hybrid laminates composed of thin and standard ply clusters. In addition, fiber metal laminates (FMLs), which consisted of thin-ply prepregs and stainless layers with a thickness of 0.04 mm, were made in order to suppress the fiber breakage that caused a decrease in CAI strength. Next, impact and CAI tests were conducted on FMLs in order to investigate the effect of the inserting location in the thickness direction and the number of metal layers in failure mode, as well as CAI strength. The results confirmed that CAI strength was improved by both ply-level hybridization and the insertion of metal layers. In ply-level hybrid laminates, the incidence of fiber breakage and delamination was reduced by changing the ratio of thin-ply prepregs and their through the thickness location. In addition, plastic deformation of metal layers in FMLs led to a larger dissipated energy; thus, the failure occurred in a limited location, such as fiber breakage at the non-impacted laminate face.