A formula relating the upper yield stress δ y, following stress aging, in a constant strain rate test to the delay time for yield td([sgrave]r) at a constant nominal reference stress δ r has been derived, viz., where ε is the imposed strain rate, E is Young's modulus of the polymer, V* is a shear activation volume, kB is the Boltzmann constant, and T is the absolute temperature. Measurements of the upper yield stress following stress aging are reported, which, together with previous measurements of delay times, confirm the form of this equation. It is also demonstrated that stress aging cannot be due to a change in the shape of the shoulder of the neck. Ringing and oscillation of the stress after stress aging are shown to be the result of alternate cycles of thermal runaway of the neck, which causes rapid stress drops, and of further stress aging during reloading, which causes yield points.
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Richards et al. (1972) studied this question.
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