Failure mechanisms in bioinspired helicoidal laminates

Previous research has shown that stacking unidirectional laminates helicoidally with small interply angles resulted in improvements in transverse load resistance. Guided by computational simulations, damage evolution within helicoidal and cross-ply laminates was tracked with further experiments to offer an insight into key differences responsible for their distinct load bearing characteristics. Under transverse loads, the first form of damage is delamination. Unlike cross-plys, which suffer multiple delamination of about the same size throughout the thickness, delamination is harder to initiate in helicoidal laminates due to the small angle between each ply. A large delamination eventually formed at the mid-plane. Transverse cracks then appear on the tensile surface of both types of specimens and propagate upwards with increasing loads. Load drops occur whenever the transverse cracks propagate to the delamination immediately above them. In the case of cross-plys, where there are multiple delamination, multiple load drops occur. Load drops in helicoidal laminates are delayed until the cracks reach the large delamination in the mid-plane. Helicoidal specimens do not experience multiple loads drops and can attain high peak load before catastrophic failure. It is shown that even higher peak load is achieved by selectively seeding delamination in helicoidal laminates to further delay the merging of transverse cracks with the dominant delamination.