Time-dependent micromechanical behavior in graphite/epoxy composites under constant load at elevated temperatures

Time dependent deformation at the individual fiber level was investigated in graphite fiber/epoxy composites at elevated temperatures using micro Raman spectroscopy (MRS) and a time dependent shear-lag based single fiber composite model (SFM). The modeling parameters were obtained from the creep response of the unfilled epoxy at several stress levels and at temperatures up to 80°C. An effective fiber spacing was used in the model predictions to account for the radial decay of the interfacial shear stress from the fiber surface. Good agreement was observed between the model predictions and MRS data when the temperature dependence of _p (the shear stress in the matrix yielded zone) and _c (the critical shear strain for the onset of inelasticity) were taken into account. Overall, the inelastic length growing from the fiber fractures increases with temperature and time. This leads to a wider stress concentration profile in the neighboring intact fibers, which increases the chance of failure in the intact fibers and facilitates the creep-rupture process of the composite.