FAILURE MECHANISMS OF LAMINATED CARBON-CARBON COMPOSITES .2. UNDER SHEAR LOADS

Failure mechanisms under both interlaminar and in-plane shear loading are determined for two-dimensional carbon-carbon composites by using a direct shear set-up. This set-up is applicable for both types of shear loading, ''as manufactured'' laminate thickness can be tested without the need to make long samples by gluing different pieces together. A detailed finite element analysis, which considers the microstructure of the composite shows that for woven laminates, the initial crimp angle morphology does not allow the composite to deform in a state of simple shear. In fact, normal tensile and compressive stresses of almost twice the magnitude of the peak shear stress are produced in the vicinity of the crimped bundles. Consistent with these predictions, we observed the shear fault following the crimp boundaries in 0-degrees/90-degrees and quasi-isotropic laminates. Therefore, experimental techniques which can secure a state of pure shear stress in aligned, unkinked, uniaxial fiber composites cannot do so in woven laminated composites.