Impact resistance of thermoplastics Prediction from bulk properties

Abstract

Although neither the ‘impact strength’ nor the brittle–tough transition temperature 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 property but a convolution of several. The thermal decohesion criterion allows two of these properties – resistance to the initiation and rapid propagation of brittle fracture – to be predicted explicitly from bulk material properties. This paper demonstrates the strengths and the limitations of impact test simulation, using this criterion, to predict an inferred G_c. Methods of isolating and measuring a third property, shear lip resistance, and of evaluating its contributions to toughness and transition temperature, are discussed. A potential scheme for building this into the simulation is assessed using a simplified analytical model. Using data from the ‘inverted Charpy’ test, this model itself yields a useful account of the brittle–tough transition temperature in impact.