Cryogenic Mode I Fracture Behavior of Stitched Composites

Cryogenic polymer matrix composite structures are highly susceptible to interlaminar damage due to thermal contraction, matrix embrittlement, and residual stress buildup during service. In Mode I-dominated regions, such as multi-lobe cryogenic tank Y-joints, unstable delamination can lead to catastrophic leakage and structural failure. Through-thickness stitching has emerged as a promising reinforcement strategy to suppress interlaminar cracking; however, its fracture mechanics performance under true cryogenic conditions remains insufficiently quantified. In this study, the Mode I fracture behavior of stitched and unstitched carbon/epoxy laminates is experimentally investigated at both room temperature (RT) and cryogenic temperature (CT, −196 °C). Double cantilever beam (DCB) testing combined with digital image correlation is used to obtain full load-displacement responses, crack growth behavior, and Mode I fracture resistance curves. Results show that unstitched laminates exhibit low, nearly flat R-curve behavior at both temperatures, with cryogenic exposure promoting highly unstable, brittle delamination. In contrast, stitched laminates display a strongly rising R-curve with fracture resistance values approximately 300-500% higher than unstitched counterparts. Most importantly, the stitched cryogenic response closely matches the room-temperature stitched behavior in terms of peak load, displacement capacity, and sustained crack-bridging resistance. These findings demonstrate that through-thickness stitching effectively preserves Mode I damage tolerance under cryogenic conditions and provides a robust fracture-mitigation mechanism for composite cryogenic tank structures.