Although both the crack nucleation and growth stages of the fatigue failure process in rubber are manifestations of the same characteristic material behavior, the nucleation stage deserves special attention. In this case, continuum mechanical parameters may be used to characterize the driving forces of small cracks, without reference to the geometry of the test piece. The ability to estimate crack driving forces from continuum mechanical parameters during the growth process of small cracks has been investigated by correlating three different parameters (maximum principal strain, strain energy density, and cracking energy density) to rates of crack growth observed photographically during fatigue tests on initially uncracked specimens. Significant scatter in crack growth rates was observed resulting from high crack density and crack interactions. These results are also compared to crack growth measurements made on a pure shear (planar tension) test piece. The difference between continuum parameters that refer to a specific material plane, and those that do not is emphasized. Generally, the maximum principal strain and cracking energy density parameters provided similar levels of correlation. The strain energy density parameter consistently gave the poorest correlation. An advantage of the cracking energy density is that it considers the experiences of specific planes embedded in the material (i.e. it is a plane-specific parameter).

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