The Development of Fracture Mechanics for Elastomers

The energy balance, or fracture mechanics, approach has proved successful in treating a number of fracture phenomena and unifying them in terms of what is believed to be a basic characteristic, the crack growth behavior when expressed in terms of the energy release rate T. It has also enabled some of the underlying physical factors to be identified and incorporated into appropriate theories. There are, however, some important limitations and outstanding problems which remain. For example, we do not at present have any quantitative understanding of what determines the precise form of the crack growth characteristic under repeated stressing. During the development of the approach, numerous checks were made of its validity by comparative experiments on different test piece geometries. This is important, as it is not certain that such comparisons will invariably give equivalent results and that the T vs. rate of tear relation is a true material characteristic. For example, if a test piece, such as the “split” test piece in Figure 2(d), is subjected to large pre-extensions in the direction of tearing, the tear resistance in some cases may apparently be much reduced from its unprestrained value. (Anisotropy is produced which is believed to be important in knotty tear development). Gent and Kim found a similar effect with laterally pre-strained pure shear test pieces. Also, if tear measurements are made with an edge crack test piece of natural rubber and the strains become great enough to produce crystallization in the main bulk of the test piece, the stick-slip behavior is suppressed and the tear strength appears to be increased. Thus it appears that in some cases the assumption that the strains just around the tip at the instant of tearing are independent of the bulk deformation is not true. Fortunately these cases seem to be the exception rather than the rule, but their occurrence demands continuing caution. Some peel tests of rubber-to-metal bonds for example show apparently anomalous force dependencies on peel angle. The current interest in the understanding and prediction of strength and durability of elastomer articles stems from the increasing importance attached to avoiding premature failure in service. The growing availability of finite element programs capable of dealing with large strains has already meant that force-deflection behavior can be at least approximately designed for; but a corresponding failure design procedure based on fracture mechanics is still in the stage of development.