Fracture Behaviour of Unreinforced Plastics

If a plastic moulding fails in the performance of its normal function it is usually caused by one of two factors - excessive deformation or fracture. In the previous sections it was pointed out that, for plastics, more often than not it will be excessive creep deformation which is the limiting factor. However, fracture. 

The second approach to fracture is different in that it treats the material as a continuum rather than as an assembly of molecules. In this case it is recognised that failure initiates at microscopic defects and the strength predictions are then made on the basis of the stress system and the energy release processes around developing cracks. From the measured strength values it is possible to estimate the size of the inherent flaws which would have caused failure at this stress level. In some cases the flaw size prediction is unrealistically large but in many cases the predicted value agrees well with the size of the defects observed, or suspected to exist in the material. 

In this chapter the various approaches to the fracture of plastics are described and specific causes such as impact loading, creep and fatigue are described in detail. 

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