Crack-blunting mechanisms in impact tests on polymers

Some impact tests are described in which Gc, the fracture toughness, of three polymers was determined over a range of strain rates and tempera­tures. It was found that at high strain rates there was a substantial variation of Gc with rate. When this was investigated as a function of load­ing times, it was observed that the most likely explanation was in terms of notch tip blunting induced by adiabatic heating at the crack tip. A model of this process is presented which provides a good description of the observations and suggests some useful guidelines for practical impact testing.