Stress fields around cracks with a viscous matrix and discontinuous fiber bridging

In this work, effects of pre-existing fiber fractures on the time-dependent stress redistribution and opening displacements within a planar unidirectional fiber composite under steady axial tension are investigated. Shear deformation of a Newtonian viscous matrix material or interface is assumed to govern the local creep response, while the fibers have time-independent elastic properties. Under these assumptions, the recently developed computational-mechanics technique called viscous break interaction (VBI) is used to efficiently compute the time-dependent stress and strain redistribution in the fibers and matrix in response to any number and spatial configuration of fiber breaks. As typically observed in the intermediate stages of composite creep failure, bridged cracks, cracks with process zones, and mis-aligned, staggered breaks are studied using VBI. Asymptotic relations are developed for the time evolution of opening displacements of large cracks and bridged cracks, whose predictions agree well with VBI results. For staggered breaks, some important differences emerge. Results show how time-growing interactions between staggered breaks and the spatial arrangement and number of such breaks influence local creep rate, fiber tensile stress redistribution, and macroscopically, the time-scales of multiple creep stages in overall composite strain. This VBI technique and insight developed here into the time-dependent deformation and stress concentration behavior are particularly useful in modeling the statistical evolution of creep failure mechanisms and for incorporating into computational codes for predicting time-to-failure.