Yielding localised

Elastic stress concentrations cannot explain most product failures, since yielding nearly always occurs before crack initiation. However, they indicate locations where yielding is likely to occur first. Therefore, the failure stress in Charpy impact tests (Section 9.5.1) should not be calculated using the notch q value. Craze formation is another form of (localised) yielding, which also modifies the stress distribution in the product. Section 9.4.4 shows that craze breakdown may occur at a critical opening displacement, rather than at a critical stress. Hence, elastic-plastic analyses must be used for most polymer product failures. 

The fracture behaviour of some tough plastics can be inconsistent. For example, polycarbonate sometimes fails by yielding, but, on other occasions, a crack initiates and brittle fracture follows (Fig. 9.5). The explanation lies in the analysis of localised yielding. Section 8.2.4 showed that the indentation pressure could be three times the uniaxial compression yield stress. The yield stress can change by a similar factor during localised tensile deformation. 

Fig. 6.2 Possible forms of the load-extension curve for a polymer (a) low extensibility followed by brittle fraction (b) localised yielding followed by fracture, (c) necking and cold drawing, (d) homogeneous deformation with indistinct yield and (e) rubber-like behaviour.

When a triaxial stress is applied to a brittle polymer, microvoiding is initiated in the matrix because it cannot easily deform. The localised yielded region therefore consists of an interpenetrating system of voids and polymer fibrils (1-pm diameter) called a... 

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