Time-dependent micromechanical behavior in graphite/epoxy composites under constant load: a combined experimental and theoretical study

We present an integrated theoretical and experimental study on the localized creep behavior around fiber breaks in model unidirectional graphite fiber/epoxy matrix composites under constant axial stress at room temperature. Micro Raman spectroscopy (MRS) and classic composite shear-lag models were coupled to examine the time evolution of fiber and matrix strain/stress distributions around a single fiber break in planar low volume fraction graphite fiber-epoxy matrix composites. In-situ MRS micro-scale measurements show that strain redistribution around the fiber fracture is time-dependent and localized. We observe decreases in peak interfacial shear stress and concomitant increases in load recovery length and interfacial inelastic zones from the fiber fracture point. These results showing the time dependence of load transfer are related to creep tests on the monolithic matrix material at various stress levels. The translation of monolithic to in-situ matrix creep is achieved using two viscoelastic matrix composite models, a multi-fiber and a single fiber model. MRS results show that the load recovery length increases at the rate of (T/T-c)(alpha/2) and the maximum interfacial shear stress relaxes at the rate of (T/T-c)(-alpha/2), where T is time, T-c and alpha are parameters obtained from matrix creep tests. These results are in good agreement with the multi-fiber model predictions. The single fiber model gives similar results for these samples where the fiber spacing is relatively large (5similar to7 fiber diameters). (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.