Impact and dynamic fracture resistance of crystalline thermoplastics: Prediction from bulk properties

The behavior of tough, crystalline thermoplastics in notched impact tests leads to the definition of crack initiation resistance and propagation resistance as two distinct properties, G c and G D . It is shown here that a single criterion—adiabatic thermal failure of a crack‐tip cohesive zone—can be applied to predict both. Dynamic fracture resistance G D emerges as a geometry independent, though crack speed and temperature dependent, material property, whose minimum value G D, min depends only on temperature and bulk physical properties. G D , min can be measured using a simple pressurized‐tube test. Crack initiation resistance G c , however, is inherently influenced by geometry and impact speed, although its lower bound is also G D, min . Craze extension and failure of a notched impact specimen, and hence G c , can be predicted for a specific temperature, given bulk thermal property data and a dynamic stress/strain curve measured by impact bending of an unnotched beam. For materials that comply with the model, sharp‐notched Charpy type impact tests will not arrive at a unique G c value, while Izod type tests, for which a revised compliance calibration is presented, may fail to establish any G c value at all.