Tensile failure of unidirectional composites from a local fracture plane

Final failure in composite structures often occurs from fiber failures in a local region. This paper aims to study the formation of a critical fracture plane that simulates the local failure region in a unidirectional composite. The nucleation site for the critical plane is assumed to be a broken fiber. An axisymmetric finite element model is constructed to study the progression of failure from the broken fiber placed at the center and surrounded by intact fibers. Two scenarios are considered: one, where the broken fiber results from a manufacturing process, and two, where a fiber fails at a weak point under loading. In the first case, a matrix crack is found to initiate from the broken fiber end and grow normal to the fiber axis, while in the second case, a matrix crack kinks out of an (assumed) short fiber/matrix debond crack and grows out towards the neighboring fibers. The consequent stress enhancement in the neighboring fibers is analyzed to determine their probability of failure. The influence of the initial debond length on the formation of the critical fracture plane is also studied.