Interfiber/interlaminar failure of composites under multi-axial states of stress

Unidirectional and textile carbon/epoxy composites were characterized under multi-axial states of stress. In-plane and through-thickness tensile, compressive, and shear tests were conducted at various orientations with the principal material axes. The stress–strain behavior, failure modes, and strengths were recorded. Results were compared with three types of failure criteria in three dimensions, limit criteria (maximum stress), fully interactive criteria (Tsai-Hill, Tsai-Wu), and failure mode based and partially interactive criteria (Hashin–Rotem, Sun, NU). The latter, a new interfiber/interlaminar failure theory developed by the authors, was found to be in excellent agreement with experimental results, especially in cases involving interfiber/interlaminar shear and compression. Of special note was the failure mode in transverse compression, where the failure plane was not predictable by conventional composite failure theories. The orientation of the failure plane was more in line with predictions by a Mohr–Coulomb failure model.