Amorphous strain rate sensitivity

Thin films of materials tested included two semi-crystalline polymers PE and PP above their corresponding glass transition temperatures Tg, and three fully amorphous polymers PS, PC, and PMMA well below The authors concluded The maximum penetration depth jc ax was found to scale linearly with the residual depth X, for all six polymers, irrespective of material strain rate sensitivity. The ratio Ix is related directly to the ratio of total impact energy W IW" dissipated via an elastic deformation of the material. 

The extreme temperature and rate sensitivity of polymers means that they can display a wide range of mechanical behaviors depending on the precise conditions imder which they are tested. Figure 1 shows a set of typical engineering stress-strain curves that an amorphous pol5uner might be expected to exhibit as a fimction of rate or temperature in a uniaxial tensile test (10,11). At low temperatures or high rates polymers tend to fail in a brittle manner (Curve A)— the strain... 

There can be no doubt as to the importance of plane strain conditions for the fracture of plastics especially where sharp notches and thick sections are concerned. Such conditions nearly always lead to brittle or semi-brittle fracture. Vincent has shown that the notch sensitivity in a braod range of amorphous and crystalline polymers is increased as the testing temperature is lowered and the loading rate is increased. Before fracture occurs, amorphous plastics often craze under these conditions. The complex questions of craze initiation, propagation and transformation into a crack have been treated extensively for amorphous polymers in the first three chapters of this book (see also The problem becomes more complicated when... 

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