Experimental characterization of deformation damage in solid polymers under tension, and its interrelation with necking

In many polymers, including glassy thermoplastics and reinforced blends, it has been shown qualitatively that damage processes (crazing and cavitation) contribute to the apparent plastic deformation in addition to shear yielding. The aim of this paper is to determine more quantitatively their influence on the constitutive equation and/or on the kinetics of plastic instability. By using a novel video-controlled testing system, the evolution of volume strain is determined in polyethylene terephtalate (PET) and high-impact polystyrene (HIPS) by measuring in real time the three principal strain components in a small volume element, while the specimens are deformed under uniaxial tension at constant true strain rate. The contribution of volume strain to the overall true strain is 50% in the case of PET and nearly 100% for HIPS. Observation of sample geometry during complementary stretching tests at constant elongation rate show that necking is moderate in PET and completely absent in HIPS, although both polymers undergo stress drop at yield and nearly no strain hardening. This unexpected plastic stability is shown to be due to damage. In this scope, the classical theory of diffuse necking in polymers is revisited in order to take explicitly into account the damage rate, D, which expresses the slope of the volume strain vs. true strain curve