Effect of Stitch Seam Orientation on the Mode I Fracture Energy of Stitched Composites

The use of carbon fiber-reinforced polymer (CFRP) composites has increased in the aerospace industry mainly due to their high stiffness and strength-to-weight ratios. However, these composites have low interlaminar strength that can result in delaminations when subjected to out-of-plane deformations. Through-the-thickness reinforcement, such as stitching, has been shown to increase interlaminar properties, resulting in improved crack-resistant characteristics. In this study, the influence of stitch seam orientation on the Mode I fracture energy, or strain energy release rate (SERR), of CFRP composites that are stitched using a one-sided modified chain stitching technique is investigated. The Mode I SERR was determined for stitched double cantilever beam specimens. A digital image correlation system was used to acquire surface strain measurements, and fracture surfaces were analyzed via an optical microscope. The results of the study indicated that SERR is dependent on stitch seam orientation. Therefore, the expected crack direction is crucial in designing stitch patterns within integrally stiffened composite structures.