Notch nonlinearities in pseudo-ductile composite laminates: A novel LE/HE sublaminate design

Abstract

Attempts to achieve pseudo-ductility in quasi-isotropic (QI) thin-ply laminates have traditionally relied on stacking [LE/HE/LE] sublaminates, with LE representing low-elongation and HE high-elongation. However, the increase in effective ply thickness led to reduced unnotched strength. Alternatively, in this study, we define a new sublaminate configuration ([LE/HE]) to minimise the increase in ply-block thickness and compare experimentally such hybrid QI thin-ply laminate with a conventional thin-ply QI ($\Pi /4$) laminate. The hybrid specimens demonstrated consistent but modest pseudo-ductile properties (ultimate-to-pseudo-yield strength ratio, ${\sigma }_f/{\sigma }_y$=1.1; pseudo-ductile strain, ${\varepsilon }_d$=0.3%). Using Digital Image Correlation (DIC) and advanced dark-field X-ray imaging, we detected earlier and more pronounced deviations from linear strain fields in hybrids compared to the reference laminates. The hybrid laminates showed an 11.7% reduction in unnotched strength but a 4% increase in notched strength in Open-Hole Tension (OHT) specimens. Thus, the proposed hybridisation introduces new damage mechanisms facilitating stress redistribution, thereby recovering more nominal strength with a reduced impact on the unnotched strength. Our findings suggest viable approaches to integrate pseudo-ductility into thin-ply laminates whilst preserving the inherent advantages of ply thinness.