Comparison of mode II delamination behaviours in multidirectional and unidirectional composite laminates

Multidirectional (MD) composite laminates are extensively employed in structural applications owing to their superior mechanical characteristics. Nevertheless, the evaluation of the fracture toughness of composite laminates primarily relies on tests using unidirectional (UD) specimens. This study evaluates the reliability of characterizing mode II delamination behaviour in MD laminates by using UD specimens. The quantification of delamination area through Digital Image Correlation (DIC) analysis is integrated with a physical Energy Release Rate (ERR) method to ascertain the fracture resistance, which is compared with the ERR derived via a modified J-integral method and the standardized compliance methods. Fractographic analysis reveals similar fracture mechanisms in specimens with identical interfaces. The physical ERR increases notably due to large-scale fibre bridging induced by fibre nesting at 0 ∘ //0 ∘ interfaces. Conversely, in 0 ∘ //90 ∘ interfaces, large-area matrix cracking enhances the intrinsic fracture resistance, excluding the extrinsic toughening provided by fibre bridging. • An equivalent strain energy density method is developed to track delamination growth. • Different layups and interfaces show distinct mechanisms, altering mode II toughness. • Mode II fracture toughness is decomposed into intrinsic and extrinsic parts. • The total ERR and J-integral methods determine the upper and lower bounds of G I I c .