Impact resistance of thermoplastics - Prediction from bulk properties

Although neither the 'impact strength' nor the brittle-tough transition tem perature measured using notched impact bend (Charpy and Izod) tests can be used for quantitative design, both are widely accepted as realistic indices on which to assess and select plastics. However, polymer suppliers cannot easily tailor materials for impact strength, since it is not a single prope rty but a convolution of several. The thermal decohesion criterion allows t wo of these properties - resistance to the initiation and rapid propagation of brittle fracture - to be predicted explicitly from bulk material proper ties. This paper demonstrates the strengths and the limitations of impact t est simulation, using this criterion, to predict an inferred G(c). Methods of isolating and measuring a third property, shear lip resistance, and of e valuating its contributions to toughness and transition temperature, are di scussed. A potential scheme for building this into the simulation is assess ed using a simplified analytical model. Using data from the 'inverted Charp y' test, this model itself yields a useful account of the brittle-tough tra nsition temperature in impact.