The Assessment of Internal Stresses in Plastics by a Stress Relaxation Method

Abstract
When plotting the slope of stress relaxation curves (stress vs. log time) against the initial stress, straight lines are obtained intersecting the axis at a stress value which appears to be associated with the internal stress level of the sample. For injection molded samples, the internal stresses are negative, i.e. frozen-in compressive stresses. They disappear on annealing. Both tensile and compressive stresses can be introduced into stress-free samples by suitable thermal treatment when in a state of compression or tensile strain.

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