Geometric strain softening

There remains the question of whether the drop in load observed at yielding arises from the purely geometrical strain softening associated with a true-stress-strain curve of the form shown in fig. 8.4(c), where there is no drop in the true stress but merely a reduction in slope of the stress train curve, or whether there is actually a maximum in the true-stress strain curve as shown in fig. 8.4(d). Experiments on polystyrene and PMMA in compression, under which the geometrical effect cannot take place, show that a drop in load is still observed. Results from extensive studies of PET under a variety of loading conditions also support the idea that a maximum in the true-stress train curve may occur in a number of polymers. 

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