Finite-strain elasto-viscoplastic behavior of an epoxy resin: Experiments and modeling in the glassy regime

• We generate a discriminating set of experimental data on the mechanical behavior of Epon 862.
• We motivate important modifications to a widely accepted constitutive model for amorphous glassy polymer.
• We demonstrate that the rate-sensitivity exponent must be treated as a free parameter, within thermodynamically consistent bounds.
• We propose a robust procedure for material parameter identification.

The finite deformation response of an epoxy resin is investigated in the glassy regime using a constitutive relation that accounts for thermally activated yielding, pressure-sensitivity, strain softening and molecular chain reorientation. A previous formulation of this macromolecular model is modified so as to decouple the onset of yielding from the peak nominal stress, and enable accurate modeling of temperature and strain-rate effects concurrently. The latter cannot be modeled adequately with existing models when the temperature dependence of elastic moduli is accounted for. Tension and compression experiments are carried out on Epon 862 across a range of temperatures and strain rates. Special care is taken to extract the intrinsic material behavior from the recorded mechanical responses using a new technique of video-based extensometry, which is well adapted to cylindrical geometries. Key features of the data include a temperature dependence of elastic moduli and a tension–compression asymmetry that goes beyond differences in peak yield. The experimental data is divided in two sets for model calibration and assessment. The first set contains sufficient data to identify all model parameters following a procedure outlined in the paper. The second data set is used to assess the predictive capabilities of the model for test conditions not used in the calibration step. It is shown that when the tension and compression cases are treated separately, with respect to post-peak softening, model predictions are excellent over the investigated ranges of temperature and strain rate.