Abstract-In a simple impact experiment with a Hookean solid, such as a glass beam, the energy absorbed is simply related to the modulus of elasticity and fracture strength at short times. However, with plastics under nearly all conditions, the energy absorption due to pure elastic deformations is small and toughness is achieved through large deformations taking place after yield. Even in "brittle" plastics the energy required for crack propagation is mainly determined by the plastic deformation processes, such as crazing, which take place in a small volume of material at the crack tip. In such cases a major factor in determining the total energy absorbed by the test-piece is the volume of plastically strained material.Generally it is found that plastic deformation processes in polymers tend to be unstable leading to large localised strains instead of to uniform deformation. This is seen in necking under tension and in shear bands. A basic condition for such localization is the occurrence of strain softening and, in tension, material attenuation-as originally proposed by Considere. This conclusion is broadly in line with known stress-strain relations. For example, in the case of polycarbonate it can be shown that an annealing treatment leads to increased strain softening, a decrease in the volume of plastic strain during impact and a lower impact strength. This occurs without changes in relaxation behaviour.Since the energy absorbed in the brittle fracture of many plastics involves crazing, which is of course a plastic deformation process with high energy absorption, the multiplication of crazes also increases the volume of plastically strained material and therefore the amount of energy which can be absorbed by the test-piece. This is conventionally achieved by adding suitably dispersed and adhesive particles of rubber to an otherwise brittle polymer such as polystyrene.However, the introduction of an elastomer not only promotes crazing but, especially with the tougher types of matrix, it can initiate shear bands and even bulk yielding. These effects may be demonstrated in ABS. plastics by monitoring volumne changes and by a variety of other techniques. In this way the capability of the matrix to absorb energy during plastic deformation is spread through the test-piece so that products of very high impact strength can be made.The toughness of plastics is most conveniently measured by means of a suitable impact test. Such tests measure the energy required to fracture the test piece under an applied blow. The results obtained are generally regarded as qualitative only, though essential for many purposes, but their precise significance is often questioned. However, considerable progress has now been made in the case of brittle fracture, where the conditions defined by Tattersall and Tappin' apply. These may be defined in terms of the elastically recoverable energy stored in the test piece (U) and of the energy G required to propagate unit area of a crack (area A).Here -d U/dA = G. is the original Griffith-Irwin con...
