Fracture Mechanics Approach to Impact

In recent years impact testing of plastics has been rationalised to a certain extent by e use of fracture mechanics. The most successful results have been achieved by assuming that LEFM assumptions (bulk linear elastic behaviour and presence of sharp notch) apply during the Izod and Charpy testing of a plastic. 

During these types of test it is the energy absorbed at fracture, Uc, which is recorded. In terms of the applied force, Fc, and sample deformation, b, this will be given by 

from equation (2.91) we have the expression for the toughness, Gc, of the material 

It is apparent from equation (2.123) that a graph of BD0 against fracture energy Uc (using different crack depths to vary 0) will be a straight line, the slope of which is the material toughness, Gc. 

Example 2.23 A series of Charpy impact tests on uPVC specimens with a range of crack depths gave the following results 

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