Keisuke Iizuka

Carbon fiber-reinforced plastics (CFRPs) offer excellent in-plane mechanical performance, but their relatively low interlaminar fracture toughness makes them vulnerable to delamination, particularly around intralaminar discontinuities such as resin-rich regions or fiber gaps. This study investigates the effectiveness of polyamide (PA) mesh inserts in improving interlaminar toughness and suppressing delamination in CFRP laminates with such features. Two PA mesh configurations were evaluated: a fully embedded continuous layer and a 20 mm cut mesh strip placed between continuous and discontinuous plies near critical regions. Fracture toughness tests showed that PA mesh insertion improved interlaminar toughness approximately 2.4-fold compared to neat CFRP, primarily due to a mechanical interlocking mechanism that disrupts crack propagation and enhances energy dissipation. Uniaxial tensile tests with digital image correlation revealed that while initial matrix cracking occurred at similar stress levels, the stress at which complete delamination occurred was approximately 60% higher in specimens with a 20 mm mesh and up to 92% higher in specimens with fully embedded mesh. The fully embedded mesh provided consistent delamination resistance across the laminate, while the 20 mm insert localized strain redistribution and preserved global mechanical performance. These findings demonstrate that PA mesh is an effective interleaving material for enhancing damage tolerance in CFRP laminates with internal discontinuities.

The effects of initial defects, such as fiber breakage, on the strain measurement of 3D-printed carbon fiber-reinforced plastic (CFRP) are evaluated herein. The generation of defects is difficult to avoid during the 3D printing process of CFRP, resulting in degradation of the tensile strength. However, the effects of these initial defects on the elastic modulus have not been investigated. The elastic modulus is generally determined by performing strain measurements using strain gauges and extensometers, as defined by testing standards. Strain gauges and extensometers are attached to the specimen, and therefore, the measurement is affected by the specimen surface, which contains initial defects. In this study, the effects of initial defects on the strain measurement are examined by performing tensile tests, evaluating the elastic modulus, and comparing the results of the 3D-printed CFRP with those of a homogeneous material, namely aluminum. The elastic modulus is evaluated using strain gauges, extensometers, and digital image correlation (DIC). The strain in homogeneous materials is independent of the strain gauge position, but strain variation is observed in CFRP depending on the position, presumably owing to the defect distribution. Furthermore, the strain obtained from extensometers with longer gauge lengths is also affected by the initial defects. Moreover, the variation is larger for the strain obtained by DIC than that obtained using strain gauges or extensometers. The strain distribution obtained by DIC may be useful for identifying the positions where the effects of initial defects are significant.