This study investigates the effectiveness of through-thickness chain stitching in enhancing mode-II interlaminar fracture resistance in carbon fiber/epoxy resin-infused composites. Quasi-isotropic end-notched flexure specimens were fabricated using SAERTEX Class-75 ± 45 ° non-crimp carbon-fiber fabrics and API 1078 epoxy via vacuum-assisted resin transfer molding, with matched stitched/unstitched configurations geometrically scaled at three levels. A combined global-local damage analysis framework was developed, comprising load–displacement characterization, size effect-based fracture energy analysis using a novel equivalent load conversion for quasi-isotropic layups, and through-thickness separation measurements based on digital image correlation. The unstitched specimens exhibited pseudo-ductile behavior with 28.2% higher fracture energy than a unidirectional prepreg baseline but significantly smaller effective fracture process zone size, indicating more brittle behavior. Through-thickness stitching did not affect pre-peak response but provided substantial post-peak enhancement: load drops were reduced by 43%–95%, effective fracture energy increased by 39.7%, and effective fracture process zone size increased nearly four-fold, shifting failure toward ductile behavior. Separation values at final snapdown increased by 52%–111%, demonstrating that stitch bridging enables further cohesive zone development. This work establishes an experimental framework for evaluating stitching effectiveness under mode-II loading and contributes to damage-tolerant composite designs for high-speed aircraft. • Mode-II fracture in stitched resin-infused composites was characterized. • A combined global-local damage analysis framework was developed and applied. • A novel method for size effect analysis of quasi-isotropic layups was proposed. • Through-thickness stitching enhanced the effective fracture energy by 39.7% • Through-thickness stitching shifted the failure mode from brittle to ductile.
Kim et al. (Sun,) studied this question.