Damage Behavior in Unidirectional CFRP Laminates with Ply Discontinuity

This study presents a comprehensive quantitative investigation into the material properties and damage behavior of unidirectional carbon fiber reinforced plastic laminates with ply discontinuities. Laminates with varying resin gap lengths were evaluated. In situ edge observations and X-ray computed tomography were employed to assess the damage processes in the area of the ply discontinuity. These methods examined initially occurring cracks, secondary cracks, and delamination. Some unexpected cracking behavior was observed in this study. For instance, initial cracks were observed to form not at the interface but at a distance from the resin–ply interface, likely due to a constraint condition caused by the chemical shrinkage of the resin during the curing process. Furthermore, all cracks in the resin region curved toward the center, with successively formed cracks having higher curvatures. This behavior can be attributed to the redistribution of principal stresses within the resin pocket following the initial cracks. The matrix cracking was followed by delamination that started from the tips of the matrix cracks and progressed toward the ends of the specimen. Additionally, the number and locations of cracks in the resin pocket were influenced by the length of the resin gap and the uniformity of the thickness and width of the specimen. In addition to the experimental investigation, a stress-based variational analysis was employed to predict crack onset stresses in the laminates. The analysis successfully calculated the energy release rate for laminates with varying resin pocket lengths and provided predictions for crack onset stresses. The analytical predictions demonstrated good agreement with the experimental observations, enhancing the understanding of resin pocket behavior in CFRP laminates.