Self-healing composites using microvascular channel networks have been intensively studied. Microvascular networks in composite in-plane directions are normally utilized to deliver healing agents to damaged areas. However, this channel configuration is not optimal to form robust networks and to infuse large damaged areas. This study proposes a self-healing system using multiple short microvascular channels in the through-thickness direction. First, the impregnation advantage of using this channel configuration is confirmed through delamination infiltration tests. Next, the feasibility of this channel configuration is evaluated by tensile tests and four-point bending tests. Double cantilever beam tests were then conducted to confirm its healing function. Finally, this system is applied to composite stiffened panels and the healing performance is evaluated using compression testing following panel indentation. The healing system almost fully recovered the original strength, confirming its high potential under practical conditions. Additionally, the weight reduction potential of the self-healing system was discussed through a strength comparison of specimens with different thicknesses.

Glass fiber reinforced plastics (GFRP) have been used in a wide range of industrial fields due to the high strength and failure strain of glass fibers; their mechanical properties and price are well balanced as well. To investigate the effect of the glass fiber tex (i.e., mass measured in grams per 1000 m of bundle) on the torsional properties of GFRP shafts, five kinds of glass fiber rovings (280, 800, 1150, 2220, and 2300 tex) were used as reinforcement, and GFRP shafts were fabricated by the filament winding method. As a result, it was determined that high strength GFRP shafts with excellent surface properties could be fabricated using glass fiber rovings with lower tex values. Furthermore, the shear strengths of GFRP shafts using 280, 800, and 1150 tex glass fiber rovings were higher than that of the GFRP shaft using HME-glass fiber (high strength glass fiber) roving for filament winding.

The fiber/matrix interfacial strength in discontinuous carbon fiber reinforced thermoplastics was evaluated by a push-out test using a nanoindenter. Two types of composites were prepared. They were molded using polypropylene reinforced by discontinuous carbon fibers using different surface treatments. Thin polished specimens with a thickness of 20 μm were prepared. The load–displacement curves were obtained by extruding carbon fiber with a custom-made flat-ended push rod attached to the nanoindenter. The process of interfacial debonding fracture was numerically simulated by the cohesive zone model, using the mixed-mode critical energy release rate as a criterion. In addition, interfacial shear strengths were evaluated using the feature points of the measured load–displacement curve. It can be concluded from the results that the higher the interfacial shear strength, the higher the macroscopic tensile strength. Subsequently, Kelly–Tyson's theory was used to predict the macroscopic tensile strengths. The predicted tensile strengths of the composites were almost consistent with the practical values.

A reliable numerical model for composite structures in vehicle crash simulations is needed because passive safety requirements are stricter than ever. The objective of the present study is to evaluate the prediction capability of a finite element (FE) model for bending and axial compression deformations, which are the main modes of vehicle structures in a crash scenario. First, we performed four-point bending simulations of a carbon fiber reinforced polymer (CFRP) laminated component using a multi-layered shell model, where the laminate consists of homogenized plies and interfaces between plies. The continuum damage mechanics (CDM) model is introduced to the intra-ply, and the damage parameters in the longitudinal direction are identified from a crack growth resistance curve (R-curve). The predicted fracture behavior and the force–displacement relationship are in good agreement with the test results. Next, we propose a discrete FE modeling scheme where cohesive elements (CEs) are inserted at all possible crack locations in axial progressive crushing. Because there is no mass loss due to element deletion, it is possible to accurately simulate the load transmission effect. Numerical studies with different laminate configurations reveal that the proposed model can capture the crushing mode under different circumstances.

Evaluation of the interface strength is important in the design of composite materials such as carbon fiber reinforced plastic (CFRP). Molecular simulation considers aspects such as chemical structures, and can be used to evaluate the interface strength in composites. Therefore, in this study, the interface energies between graphene considering the electric charge state and resin (TriA-X polyimide, DGEBA, Triethylenetetramine, Vinyl ester and PA6) were evaluated via molecular dynamics simulation. First, the interface energy and interface strength obtained using the experiment were compared. Subsequently, R² was calculated using linear approximations from the interface energy and interface strength. Based on this, it was conjectured that there exists a relationship between the interface strength and interface energy. Furthermore, the validity of the magnitude relationship between the interface strength obtained using the experiment and the interface energy obtained using the simulation was evaluated. Moreover, considering graphene oxide, the interface energies between the resin and three forms of graphene (functionalized with OH, COOH, and O groups) were obtained, and the effect of various oxidation surface treatments of graphene on the corresponding interface strengths was discussed.