Fracture mechanics applied to plastics

These equations show that the local stress is proportional to the applied stress and varies with the square root of the crack size. is independent of the material from which the plate is made, provided that it is linear elastic. does depend on the size and location of the flaw (and on the applied stress). The location of the crack in relation to the plate geometry is incorporated in a modifying factor, Y. 

The considerations so far have not taken account of the properties of the material under stress, the only requirement being that it is linear elastic. The material will fail by very rapid crack growth when K, reaches a critical value for the material often called the fracture toughness. Typical values for fracture toughness for different plastics subjected to short-term loading are given in Table 2.4. 

However, it is not possible for the applied stress to exceed the failure stress, (jp, for the material. If is applied to the product which does not contain a 

It would seem that if there were a crack in a structure there would be no reason to be concerned, provided its size was smaller than the critical crack size. This is not true-any crack in a body under stress will gradually increase in size with increasing speed, until catastrophic failure occurs when K, = X,c. The crack growth rate, a, is given by  

The time taken for a crack to grow can be found by integrating the crack growth equation between the original size of the crack and the new size of the crack. 

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