Rupture of rubber. XI. Tensile rupture and crack growth in a noncrystallizing rubber

A theory of tensile rupture in a noncrystallizing rubber, a particular instance of a more general theory of rupture in simple extension, is outlined. The theory assumes that failure takes place by growth of a crack from some imperfection in the material where the stress is high locally. The imperfections are considered as being equivalent, in terms of stress concentration effects, to small cracks initially present in the material, and the conditions for crack growth to occur are then treated on the basis of the tearing energy criterion of Part I. It is assumed, by analogy with tearing on a macroscopic scale, that the crack grows continuously with time at a rate, dc/dt , given by: dc/dt = AT n , where A and n are constants and T is the energy expended per unit increase in crack length, per unit thickness of specimen. The predicted relationships of the breaking time to the stored energy density and initial crack length for specimens tested by stretching at uniform rates and by holding at fixed extensions are first compared with the results of model experiments on test pieces containing small tears and cuts. Values of A and n derived from tear test data are used in the theoretical relationships, and it is shown that there is fair agreement with experiment. Results of tests on tensile test pieces containing no deliberately introduced tears or cuts are then shown to be consistent with a failure mechanism of the above type. It appears, however, that the tearing energies in tensile rupture are lower than those observed in tear tests, and reasons for this difference are discussed.