Characterizing Nonlinear Rate-dependent Behaviors of Graphite/Epoxy Composites using a Micromechanical Approach

This research aims to characterize the nonlinear rate-dependent behaviors of graphite/epoxy composites using micromechanical analysis. In this analysis, graphite fibers are considered as elastic solids, while the surrounding epoxy matrix exhibiting rate sensitivities are described using the three-parameter viscoplasticity model. By using Aboudi's generalized method of cells (GMC), the incremental form of the constitutive relations of the composites are expressed in terms of the constituent properties as well as the geometry parameters of the representative volume element. After a numerical iteration, the corresponding stress and strain relations of composites at different strain rates are generated. In order to verify the model predictions, off-axis graphite/epoxy composite specimens are tested at strain rates from 10 ^—4 to 550/s. For strain rates less than 1/s, the experiments are conducted using a hydraulic MTS machine, while high-strain-rate tests were carried out using a split Hopkinson pressure bar. Experimental results indicate that the stress and strain curves are quite sensitive to the strain rate; moreover, when the strain rates increase, the material stiffens. A comparison of model predictions with experimental results indicates that the rate-dependent behaviors of composites can be characterized effectively with the GMC model in conjunction with the viscoplastic properties of epoxy.