Failure locus of fiber-reinforced composites under transverse compression and out-of-plane shear

The failure locus a fiber-reinforced composite lamina, made up of 50 vol.% of carbon fibers embedded in an epoxy matrix, is computed under transverse compression and out-of-plane shear, a stress state whose experimental reproduction is highly complex. The mechanical response was obtained by the finite element method of a representative volume element of the lamina, which explicitly takes into account the fibers and the matrix in the lamina. The actual deformation and failure mechanisms experimentally observed in the matrix, fibers and interfaces were included in the simulations through the appropriate constitutive equations. Two sets of simulations were performed, assuming that the fiber/matrix interface was either strong or weak. The corresponding failure loci were compared with those given by three failure criteria for composites (Hashin, Puck and LaRC) which provide reasonable predictions in other multiaxial stress states. The estimations of the failure criteria were largely consistent with the numerical simulations in the composites with a strong interface but overestimated the composite strength when the interface was weak because the effect of interface decohesion (which becomes dominant) was not taken into account. These results point out the need to include interface fracture in the failure criteria for composites.