Crack growth in elastomers under biaxial stresses

The effects of biaxial stress fields on the rate of crack growth and the fatigue life of rubber have been investigated.;The second study involved the measurement of the fatigue life of N.R. and S.B.R. in equal-biaxial extension and comparing the results with the fatigue life in simple extension. Equal-biaxial extension was achieved by the inflation of a flat diaphragm. The samples were repeatedly inflated until the diaphragms ruptured. The fatigue life was measured at various maximum strain levels and compared with the results of uniaxial fatigue tests. In order to compare the fatigue life under equal-biaxial and uniaxial extensions on an energy availability basis, a method was developed to calculate the energy available for crack growth in equal biaxial extension. When the fatigue life as a function of the proposed available energy for equal biaxial extensions was compared with the fatigue life as a function of the available energy for uniaxial extensions, no significant differences were observed for natural rubber. However, there were some differences in the results for S.B.R., possibly due to the greater sensitivity of S.B.R. to the initial flaw size.;First, the rate of crack growth for a constant strain pure shear specimen was determined. The test consisted of first straining a pure shear test piece parallel to the direction of later crack growth and clamping the test piece in such a way that this strain was maintained constant even after a crack had grown along the sample. The sample was repeatedly stretched perpendicular to the static strain causing a crack to grow in the static strain direction. The rate of crack growth at various static and dynamic strains was measured and compared on an available tear energy basis. It was expected that the magnitude of the static strain would have a strong influence on the rate of crack growth, but this type of variation was not observed. There was no significant effect on the rate of crack growth in a biaxial stress field when the rate of crack growth was compared on an available energy basis.