Nonlinear stress relaxation in an epoxy glass and its relationship to deformation induced mobility

The mobility evolution in an epoxy glass during constant strain rate uniaxial deformation was examined via the stress relaxation response observed at various locations along the stress–strain curve, including the pre-yield, yield, and post-yield regions. Experiments were performed in the temperature range from Tg-30 °C to Tg-5 °C in both uniaxial extension and compression. At the faster loading strain rates the initial rate of stress relaxation is the slowest in the pre-yield region and the fastest in the post-yield region. However, at the slower loading strain rate the ordering is unexpectedly reversed so that the initial rate of stress relaxation is the fastest in the pre-yield region and the slowest in the post-yield region. These findings challenge the key assumption made in most existing constitutive models for glassy polymers that yield is the straightforward consequence of deformation induced increase in the mobility.