The effect of a dwell period on fatigue crack growth rates in filled SBR and NR

Loading conditions for rubber components are often more complex than the constant amplitude signals used in material characterization. During a series of uniaxial fatigue crack growth experiments on filled SBR under variable amplitude loading conditions, test signals that included a dwell period produced higher crack growth rates than corresponding constant amplitude test signals without a dwell period. These test signals alternated periods of cyclic loading with dwell periods ranging from 1 to 100 seconds at a near zero stress level. Dwell period tests produced average experimental crack growth rates up to 30 times greater than constant amplitude crack growth rates in filled SBR. The length of dwell time and the number of applied cycles between dwell periods were the most significant influences on the crack growth results. An empirical model was developed that captured the dwell effect based on these parameters. Dwell periods also produced increased crack growth rates in natural rubber, but the effect was less significant. It is proposed that the effect is caused by the time-dependent recovery in the rubber microstructure at the crack tip producing a localized and temporary elevated stress-state during loading events immediately following a dwell period. Current fatigue prediction methods do not account for the dwell effect.