Stress Concentrations in Graphite/Epoxy Model Composites during Creep at Room Temperature and Elevated Temperatures

Abstract

Micro Raman spectroscopy (MRS) was used to monitor the change in the local stress concentrations surrounding a single fiber break in 2-dimensional graphite fiber/epoxy composites during creep at room temperature and elevated temperatures up to 80 C. The experimental results were compared with a few shearlagbased multifiber composite models: Hedgepeth, which assumes perfect bonding between the linear elastic fiber and shear-only matrix; MSSL, which additionally accounts for the axial stiffness of the matrix; and VBI, which assumes a viscoelastic shear-only matrix. Instead of assuming time-independent stress concentrations around a fiber break as in previous models, this work investigated the change in the stress concentrations due to the local matrix creep. The VBI model uses parameters measured from unreinforced matrix creep tests to predict the time dependent strain– stress evolution in the composite around fiber breaks. The stress concentrations of the nearby intact fibers were found to change due to the combined effects of temperature sensitive matrix creep and creep-driven inelastic zone growth emanating from the fiber break, which were in good agreement with the VBI predictions in the absence of significant inelastic zones.