General Fracture Behaviour of Plastics

If the defect or crack in the plastic is very blunt then the stress intensification effect will be small and although failure will originate from the crack, the failure stress based on the net section will correspond to the failure stress in the uncracked material. If the stress on the materi is based on the gross area then what will be observed is a reduction in the failure stress which is directly proportional to the size of the crack. This is shown as line A in Fig. 2.68. 

however, the defect or crack is sharp then the picture can change significantly. Although ABS and MDPE are special cases, where the materials 

Polycarbonate is perhaps the most notoriously notch-sensitive of all thermoplastics, although nylons are also susceptible to ductile/brittle transitions in failure behaviour caused by notch sharpening. Other plastics such as acrylic, polystyrene and thermosets are always brittle - whatever the crack condition. 

For brittle failures we may use the fracture mechanics analysis introduced in the previous sections. From equations (2.96) and (2.99) we may write 

From this therefore it is evident that the failure stress, o/, is proportional to This relationship is plotted as line B on Fig. 2.68. This diagram is now very useful because it illustrates the type of ductile/brittle transitions which may be observed in plastics. According to line B, as the flaw size decreases the failure stress tends towards infinity. Clearly this is not the case and in practice what happens is that at some defect size (oq) the material fails by yielding (line A) rather than brittle fracture. 

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