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An experimental study was carried out on the phenomena of upper and lower yield points observed with hard, amorphous polymer solids below the glass-transition temperature. The effects of temperature, strain-rate, specimen shape, working strains, and annealing on the appearance of the upper and lower yield points were examined on epoxy cast resin (a three-dimensional cross-linked polymer) and on Celluloid (a linear polymer). It has been concluded that upper and lower yield points represent transient behavior, at a given moderate strain-rate, associated with mechanical heterogeneity, expressed as a distribution of relaxation times. Some applications of this yielding mechanism are discussed.
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Katsuhiko Ito (1971) studied this question.
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