Size effect in the J–R curves for the nonlinear shear failure of woven composites

The objective of this study is to investigate the specimen size dependence in the nonlinear shear failure of woven composites. The selected test geometry involves the recently developed modified lap shear test, particularly suited for the characterization of the in-plane shear failure of specimens of multiple sizes. The test involves uniaxial compression of a double-cracked S-shaped specimen, resulting in a predominantly shear induced failure in the specimen ligament. An epoxy/carbon twill woven composite was used to fabricate geometrically scaled specimens of three different sizes. The modified lap shear test on these specimens resulted in a geometrically similar shear failure of all specimens. The behavior was observed to be pseudoductile with a marked nonlinearity and a prolonged post-yield hardening response, followed by specimen self-contact and friction, terminating the test. The propagation of the failure was characterized using the J–R curve approach, generated via J-integral and compliance calibration approach. The calculated J–R curves are found to exhibit a strong dependence on specimen size, with a smaller size exhibiting a smaller toughness for a given effective crack extension. An approximate framework to infer a size-independent J–R curve is proposed, which is verified via cohesive crack simulations and validated via Iosipescu shear failure experiments. These findings emphasize the importance of accounting for this size effect in the J–R curve in designs and numerical models for composite structures subjected to intralaminar shear failure.