Deformation of thermosetting resins at impact rates of strain. Part 2: constitutive model with rejuvenation

The constitutive responses of three glassy thermoset polymers at impact rates of strain and slower, together with measurements of adiabatic heating, were reported earlier by the authors. The results are interpreted here in the context of a constitutive model proposed previously for amorphous polymers, expanded to incorporate strain-softening and the adiabatic heating deficit. In terms of the model, both features are a natural consequence of strain-induced evolution of the glass structure, as represented by Tool's "fictive temperature"-the phenomenon of structural rejuvenation. A representation is proposed for the evolution of fictive temperature with plastic strain, motivated by an approximate treatment of the kinetics of physical ageing/rejuvenation. Formulated in this manner, the model agrees reasonably well with experimental results across the wide range of strain rates of the previous experiments, 10(-3) to 4.5 x 10(3) s(-1), and across most of the range of strain to failure in compression. At the highest strains, however, an additional adiabatic heating deficit appears that is not predicted by the model, either suggesting the onset of structural breakdown possibly associated with the appearance of cracks or reflecting a need for better physical understanding of large deformations in glassy polymers. (C) 2004 Elsevier Ltd. All rights reserved.