Stress-Strain Behavior of Glassy Epoxy Network Subjected to a Stepwise Change of Tensile Rate

Stress-strain relations of a cured epoxy network subjected to uniaxial stretching during which the tensile rate changed stepwise were observed at a temperature below the glass transition. When the tensile rate was increased or decreased to a prearranged identical value, the stress-strain curves after the tensile rate change finally coincided with each other in the post-yield strain-hardening region, after showing transient variations. This behavior was observed independently of the amount of strain where the strain rate changed. Thus, the stress-strain relation in the strain hardening region at a given strain rate did not depend on preceding strain rate history. Since the strain-hardening behavior in this system is ascribable to an increase of the rubbery stress originated from the chain orientation, the strain-hardening behavior independent of strain rate history means that the intensity of glassy stress, which is another stress component of glassy polymers, is uniquely determined only by the strain rate at the moment. Thus, the glassy stress in the strain range well beyond the yield point proved to be a steady flow stress in the network polymer.