Rate transitions in the fatigue crack growth of elastomers

It has been observed previously that cracks in elastomers under repeated strain initially grow rapidly from sharp cuts, but as the cracks proceed, the rate decreases to a steady value. This work examines these transitions from a sharp crack tip, introduced by a razor blade into a pure shear test piece, for a range of elastomers. The changes in the rate of crack growth are also reflected by changes in the fracture surface appearance, which roughens as the crack develops. This phenomenon is of practical significance in engineering applications, in which an initially fast crack growth rate from a sharp cut in a rubber product can result in significantly lower fatigue life than anticipated from the steady-state value, which is usually presented and quoted. The change in the rate with the number of cycles can be represented by an empirical relation whose parameters are functions of the strain energy release rate. The roughening of the crack can be envisioned as the splitting of a sharp tip into several tips with a consequent sharing of the strain energy release rate between them, leading to a reduction in the crack growth rates. This representation of the roughening process has been analyzed with a FEA approach. (C) 2008 Wiley Periodicals, Inc.