Mechanisms-based failure laws for AS4/3502 graphite/epoxy laminates under in-plane biaxial compression

Failure mechanisms and stress–strain behaviors have been investigated for [±30]12s and [±45]12s graphite-epoxy (AS4/3502) laminates under in-plane biaxial compression by using a cruciform biaxial test frame and microscopy of load-interrupted samples. The loading confinement ratio R was varied from 0.24 to nearly 1.0 to measure the sensitivity of sample failure mechanisms and stress–strain behavior to different stress states. Failure modes with an increasing loading confinement ratio for both fiber orientations transitioned from the uniaxial failure mode of in-plane shearing to out-of-plane shearing and massive delamination. The shear failures were on one or more planes that traversed the entire sample thickness and thus encompassed alternate plies in which fibers were aligned and misaligned with respect to the shear plane. The local failure was found to be triggered either by the matrix shear in fiber-aligned plies or fiber-shear in the fiber-misaligned plies. A standard classical laminate theory in combination with the standard rule of mixture theory for composites was used to calculate the maximum matrix and fiber shear stresses. The failure data conformed nicely with a Mohr–Coulomb-type shear failure law. Most interestingly, both types of failures correlated rather remarkably to a single shear failure law when the stresses were smeared and expressed on the scale of the laminae. This latter failure law should have widespread use because of its simplicity.