Strain hardening-softening effects

These two examples illustrate two particular situations where strain rate effects are similar to temperature effects, the spurious hysteretic heating having been isolated. In most cases, strain rate effects (strain hardening, increase of the mechanical properties with speed) are mixed with heating effects due to mechanical energy absorption (softening, decrease of the material properties with temperature). 

Fig. 8.18 Compression stress-strain curves of nearly glassy PET (crystalline content 9%) for seven temperatures reaching Tg, showing strong yield phenomena and strain-softening effects that decrease with increasing temperature, having a relatively temperature-independent entropic strain-hardening contribution (from Zaroulis and Boyce (1997) courtesy of Elsevier).

Figures 10.9 to 10.11 illustrate how stretching curves and critical strains vary with temperature, again with results for PEVA12, and with the crystallinity here polyethylenes with different crystallinities are compared. Curves demonstrate a further general property of semicr3 talline pol5oners. While the stresses vary in systematic manner, there is no effect on the critical strains for softening (en 0.1) and hardening (en 0.6) and virtually no change in the elastic-plastic composition of the strains. Hence, tensile deformation of semicrystalline polymers is strain-controlled and changes the mechanism at two critical strains that are temperature and crystallinity invariant.

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