Abstract
Tensile flow stress at the lower yield point of glassy poly (methyl methacrylate) was determined as functions of true strain rate and temperature. The stress was compared with that at the upper yield poini which had been considered to be a state of pure plastic flow. It was found that the glass transition of flow stress, which was well defined for the upper yield state, was not clear for the lower yield state. Phenomenological explanations by Zitek et al, and Brady et al. and molecular theories of Robertson and Argon for plastic flow of glassy polymers were examined both on the upper and the lower yield points. None of these explanations examined here reproduced the experimental variations at the upper yield point. On the other hand, when Eyring coefficients were treated as stress- and temperature-dependent variables, Robertson's theory showed an excellent fit to the experimental results at lower yield point.
