Effects of micro voids on the early stage of transverse crack formation in unidirectional composites

A study of the early stage of crack formation in unidirectional composites subjected to transverse tension in the presence of micro voids in the matrix is conducted by computational simulation. The focus of the study is brittle cavitation leading to fiber/matrix debonding followed by debond crack growth and kink-out into the matrix. These failure events are studied considering a nonuniform fiber distribution and a uniform hexagonal fiber pattern in the cross section of a unidirectional composite with voids in the vicinity of the initiated cracks. A dilatational energy density criterion is used for initiation of debonding while the debond crack growth and kinkout into the matrix are studied by the energy release rate, which is calculated by the virtual crack closure technique (VCCT). Results show that while the debond initiation site is not affected significantly, the debond growth and kink-out are affected by circular voids in the close vicinity and when the diameter of these voids is a significant fraction of the fiber diameter. The effect of fiber distribution nonuniformity is manifested in the interfiber distances that governs the critical conditions for the debonding and kink-out processes.