Stress Softening and Strain Softening of Poly(Methyl Methacrylate) in Yielding under Constant Load

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Abstract
The strain rate of poly(methyl methacrylate) during yielding under constant load was determined experimentally for various tensile loads at temperatures from 70° to 100°C. In the constant‐load test, the polymer strains uniformly beyond the yield point up to several times the yield strain. The yield point is evident as a minimum of the strain rate. Intermittent superposition of load increments affects the strain rate reversibly. The observed strain‐rate increase under constant load in the uniform‐strain post‐yield region, results essentially from strain softening with some additional softening from the stress increase due to specimen thinning. The stress influence on the strain rate is expressible as a stress‐shift factor of the strain rate. A theoretical stress‐shift factor is derived from Doolittle's viscosity equation and the assumption that Poisson's volume dilatation contributes entirely to the free volume. Good agreement between the predicted and the observed stress‐shift of the strain rate is found. An apparent anomaly of the stress‐shift factor at 100°C is probably caused by recovery from strain softening in the vicinity of the glass transition temperature.