EVIDENCE OF MIXED MODE FAILURE IN 2D WOVEN COMPOSITES UNDER UNIAXIAL COMPRESSION

Woven fiber-reinforced composites exhibit excellent damage resistance and high shear strength due to the interweaving structure of fill and warp yarns. Due to their heterogeneous microstructure involving interlacing tows, compression induced failures at stress concentrations often include multiple failure mechanisms, such as shear failure and kink band formation. This might cause an external mode I loading to induce a mixed-mode failure at a pre-existing crack tip. This possibility is investigated and quantified here experimentally for an epoxy/carbon twill woven composite. Single-edge-notched compression (SENC) specimens are tested under uniaxial compressive loading whilst being clamped in a combined loading compression fixture. The full-field in-plane near crack tip displacement fields are characterized via the digital image correlation (DIC) approach. These displacement fields are then utilized in the determination of the mode I and mode II stress intensity factors using an over-deterministic least squares approach. The findings clearly suggest the induction of a non-negligible mode-mixity in the failure behavior, despite a far field uniaxial compressive loading applied. This is also supported by the observed lack of symmetry in the displacement fields about the crack ligament. This mode-mixity must be accounted for when calculating the relevant fracture energy associated with compression kink band propagation.