Fiber-reinforced thermoplastics (FRTPs) have superior strengths and elastic moduli compared to most other materials and are lighter than metal and ceramic materials. The mechanical properties of FRTPs are broadly dominated by those derived from the material and from the interface between the fibers and plastics. In recent years, research on the strength of this interface has been active, and various evaluation methods have been proposed. These methods target a single fiber and do not assume a dispersed state in which multiple fibers are present inside the composite material. In particular, using the evaluated values to predict the strengths and impact resistances of composite materials is difficult because the interaction force generated between fibers is ignored. We propose a short beam method to determine the interfacial shear strength of FRTP injection-molded products, and we verify the validity of the method. Analysis of the obtained results based on the theory proposed in this study found that, in terms of fiber content, the interfacial shear strength has a small dependence, whereas the interfacial interaction force and solidification temperature have a considerable dependence. It is also clarified that the solidification temperature of polypropylene/glass fiber is the crystallization completion temperature.

Interlaminar fracture toughness in carbon fiber reinforced plastics has been studied primarily using unidirectional or symmetric laminates. However, even in quasi-isotropic laminates, delamination caused by an out-of-plane impact could result in asymmetric, partial laminates. Thus, evaluating the interlaminar fracture toughness in asymmetric laminates, particularly that of Mode II associated with impact resistance, is necessary. Asymmetric laminates result in coupling effects, which cause warpage or twisting due to the thermal shrinkage derived from the molding process. In this study, based on the reports of Verchery et al., we manipulated the stacking sequences to generate zero coupling stiffness. Subsequently, Mode II interlaminar fracture toughness was experimentally and analytically examined using laminates with zero or non-zero coupling components. The results demonstrated that Mode II interlaminar fracture toughness was slightly higher in the laminates with zero coupling components than in those with non-zero coupling components. In addition, the crack tip geometry was significantly influenced by the coupling effects.

In recent decades, the CFRTP press-forming process has drawn intense research interest because of its use in automobile and industrial equipment. However, it is still very challenging to cost-effectively fabricate large, long, and thick CFRTP parts, such as infrastructure and general-purpose structures. Therefore, the authors propose the partial bend-forming of thick CFRTP laminates via partial heating. While bend-forming has the potential for low-power heating and low-load pressing, this process always causes wrinkles in the bent area. The purpose of this paper is to clarify the effect of wrinkles in the bent area for the development of a new process that applies bend-forming. The wrinkles caused by a partial bend-forming experiment were observed, and strength and stiffness were evaluated by a 4-point bending test using L-shaped specimens containing wrinkles. Forming simulation of the partial bend-forming was also attempted. It was found that the bent section with wrinkles had lower strength but comparable stiffness compared to the bent section without wrinkles. When running forming simulations of wrinkling with delamination, it was found that the definition of the interlayer separation stress was important. The simulation results were in good agreement with the experimental results.

When analyzing the fracture behavior of unidirectional carbon fiber-reinforced polymer (CFRP), it is important to consider the interfacial strength between the reinforcing fiber and the base resin, and the strength of the base resin. Therefore, the adhesiveness of the base material and the compatibility with the sizing material and fibers are important design parameters in the development of CFRPs. However, a quantitative method for estimating the interfacial strength and the strength of the base resin has not been established. In this study, we propose a method to evaluate the interface strength of unidirectional CFRPs by creating learning data through a series of numerical material tests and by constructing a neural network that outputs the interface strength based on a homogenization method from the results of off-axis tensile tests. We adopt a general feed forward neural network whereby parameters are learned by employing a backpropagation method. The interfacial strength and the matrix resin strength is predicted and evaluated from the results of the off-axis tensile test to demonstrate the effectiveness of this system.