Effects of negative stress ratios on crack propagation behavior after an overload

A study of the effect of an overload on fatigue crack propagation behavior has been carried out at baseline R-values of 0, -1, and -1.5. The usual delayed retardation event was observed at R = 0. At R = -1 the degree of retardation was reduced. At R = -1.5 the fatigue crack growth rate was observed to accelerate after the overload. These behaviors were related to the extent of crack closure, which existed following the overload. The rate of fatigue crack propagation for all loading conditions was correlated with the parameter ΔK_eff - ΔK_effth, where ΔK_effth is the effective value of the stress intensity factor range at threshold.