Yield Processes and Fracture

Deformations of a polymeric solid may include, in addition to the reversible part, an irreversible flow. This plastic flow or yielding sets in, when the stress becomes large enough and surmounts the yield point . For polymers, this threshold is low compared to metals or ceramics. Moderate forces are often sufficient to induce yield processes. Temperature plays a big role and there are also effects from the environment when fluids or gases are present which permeate into the polymer. 

The amount of flow before fracture determines the ductility of a polymer sample. In tough materials, a considerable amount of energy is dissipated by yield processes prior to fracture. In contrast, brittle samples break without showing much preceding flow. The difference becomes apparent in the fracture 

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In contrast to the simplicity of elastic deformation, plastic deformation occurs in diverse ways. Figure 1.9 illustrates the stress-strain curves for two typical elastoplastic materials (hardened metal and polymer). Both materials show similar linear relationships between stress and strain for the elastic deformation (i.e., before yield strength) but quite different correlations in the yielding processes before fracture. 

Creation of high strength states (increase of yield stress and fracture stress), based on new achievements of materials science and on application of the innovative technologies of material producing and processing increase of material stiffness (increase of its elastic moduli). 

The fact that even the rapid deformation of a glassy polymer under concentrated stresses entails considerable local plastic deformation immediately suggests that the molecular properties, which influence yielding and flow, are also effecting Gc and thus the impact strength. The data compiled in Table 9-II reveal this dependency of Gc on temperature, rate of deformation, and molecular properties. A possible relation between molecular relaxation processes and fracture surface energy of polymers has been pointed out in many of the cited references (e.g. [14, 19, 22, 24, 25, 54,63,... 

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