Plastic Deformation and Stretching

The stress-strain curve in Fig. 7.24b first of all exhibits elastic and preplastic behaviour. It then reaches a maximum whose sharpness depends on the polymer and also the deformation rate. Beyond this point, the stress remains almost constant over a certain region, before suddenly increasing to fracture. This is brittle fracture, perpendicular to the load. Many semi-crystalline polymers, such as polyethylene, polypropylene, polyamide 6 and polyamide 6,6 exhibit this type of behaviour at ambient temperature. However, among amorphous polymers in the glassy state, polycarbonate is one of the rare examples to behave in this way. 

Curve b in Fig. 7.24 has several new features which we shall examine in greater detail. 

Removing the load from a sample after various degrees of deformation, we find that for the value corresponding to the maximum stress, a permanent deformation appears. The material has undergone plastic deformation and the maximum corresponds to the plasticity threshold (or plastic yield point). This is characterised by a stress ay and a strain Sy. 

The reduction in stress occurring beyond Sy is called intrinsic strain softening. It engenders a strain instability which, under tensile stress, is manifested as a neck of reduced sample thickness, as shown in Fig. 7.25a. The structure stabilises when local deformation reaches a certain value, called the natural stretching level. This depends on the polymer and load conditions (i.e., temperature T and deformation rate). 

During deformation at more or less constant stress, the neck propagates along the sample (see Fig. 7.25b). Propagation stops either when the neck arrives at the clamp holding the sample, or when it reaches the wider part for a dumbbell-shaped sample. 

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Solid Polymer Behaviour Under Longitudinal Load 249 7.7.2 Plastic Deformation and Stretching... 

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