Finite-strain elasto-viscoplastic behavior of an epoxy resin: Experiments and modeling in the glassy regime X. Poulain a , A.A. Benzerga a,b,⇑ , R.K. Goldberg c a Department of Aerospace Engineering, Texas A&M University, College Station, TX 77843, USA b Department of Materials Science & Engineering, Texas A&M University, College Station, TX 77843, USA c NASA Glenn Research Center, Cleveland, OH 44135, USA article info Article history: Received 24 August 2013 Received in final revised form 9 July 2014 Available online 22 July 2014 Keywords: B. Constitutive behavior Amorphous polymers B. Viscoplastic material Finite strain C. Mechanical testing abstract 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 macro- molecular 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 pre- dictions are excellent over the investigated ranges of temperature and strain rate. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Modeling the thermomechanical behavior of glassy polymers is a subject that has been tackled by many, see e.g., (Wu and Van der Giessen, 1993; Boyce and Arruda, 2000; Anand et al., 2009; van Breemen et al., 2011; Bouvard et al., 2013) and references therein. Seldom, however, is the identification of constitutive models based on discriminating experiments that include both tension and compression responses of the same material and for various strain rates and temperatures. In addition, most of the available data on polymers has been obtained on rectangular-prismatic specimens, which are notorious for promoting shear band formation and propagation. Under such circumstances, decoupling the intrinsic material behavior from structural and material instabilities becomes challenging. This paper deals with the identification of a physics-based constitutive model, hereafter referred to as macromolecular model, using new experiments. The latter consist http://dx.doi.org/10.1016/j.ijplas.2014.07.002 0749-6419/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Aerospace Engineering, Texas A&M University, College Station, TX 77843, USA. E-mail address: benzerga@tamu.edu (A.A. Benzerga). International Journal of Plasticity 62 (2014) 138–161 Contents lists available at ScienceDirect International Journal of Plasticity journal homepage: www.elsevier.com/locate/ijplas