A test method to measure fatigue crack growth rate of rubbery materials

Crack growth characteristics of rubbery materials are an important factor determining the strength and durability of the materials. It is necessary to define the means of measuring the fatigue crack growth rate of rubbery materials. In this paper, a test method is introduced using a newly designed test machine. Measurements were made in order to characterize the fatigue crack growth behavior under repeated load expressed as a crack growth rate, i.e. crack length per cycle, as a function of the tearing energy, determined by a broad range of the strain energy density in a test piece of pure shear geometry. Measurements were also made to observe the effects of test temperature and loading frequency. A high speed CCD camera is utilized to allow in situ measurements following the path of the crack growth simultaneously. The power law dependency between crack growth rate and tearing energy was confirmed by the new test methodology. Comparison of the fatigue crack growth behavior between NR and SBR rubber compounds was made, and the exponent α values of the NR compound were found to be, in general, lower than those of the SBR compound, indicating that the NR compound is more resistant to fatigue crack growth than the SBR compound.