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 unidirecti onal fiber composite under steady axial tension are investigated. Shear def ormation of a Newtonian viscous matrix material or interface is assumed to govern the local creep response, while the fibers have time-independent ela stic properties. Under these assumptions, the recently developed computatio nal-mechanics technique called viscous break interaction (VBI) is used to e fficiently compute the time-dependent stress and strain redistribution in t he fibers and matrix in response to any number and spatial configuration of fiber breaks. As typically observed in the intermediate stages of composit e creep failure, bridged cracks, cracks with process zones, and mis-aligned , staggered breaks are studied using VBI. Asymptotic relations are develope d for the time evolution of opening displacements of large cracks and bridg ed cracks, whose predictions agree well with VBI results. For staggered bre aks, some important differences emerge. Results show how time-growing inter actions between staggered breaks and the spatial arrangement and number of such breaks influence local creep rate, fiber tensile stress redistribution , and macroscopically, the timescales of multiple creep stages in overall c omposite strain. This VRI technique and insight developed here into the tim e-dependent deformation and stress concentration behavior are particularly useful in modeling the statistical evolution of creep failure mechanisms an d for incorporating into computational codes for predicting time-to-failure . (C) 2000 Elsevier Science Ltd. All rights reserved.