Impact strength

Abstract

It is well known that plastics components are often more prone to failure under impact than under slowly-applied or constant load. This tendency is promoted by low temperatures and the presence of a sharp notch, and is of particular concern for tough, unreinforced crystalline thermoplastics, in which a sudden blow can precipitate brittle fracture more typical of a glassy polymer like PMMA. Impact tests (usually Charpy or Izod — see Impact and Rapid Crack Propagation) are widely used for such materials, and ‘impact strength’ data are widely quoted in material specifications. This is partly because such materials are often selected for components such as bumper bars or blow-moulded containers, which are likely to suffer impact. The popularity of impact strength data owes more, however, to the ease and speed with which tests can be conducted, and to a widespread (and incorrect) belief that impact performance somehow characterises the overall susceptibility of a polymer to brittle behaviour. In fact, it is important to treat impact strength data warily even for their primary purpose.