Prediction of delay cycles due to instant of single overload cycles: Kumar, R. Eng Fract Mech42 3 (June 1992) pp 563–571

Abstract

The effects of crack closure on fatigue crack growth rate in 7050 Al were studied with the aid of small-foil strain gages glued at positions near to the crack plane. The crack closure stress was determined from the change in the slope of the local strain-nominal stress curves.The crack closure stress was found to increase slightly with crack length under a cyclic loading total amplitude of Δσ = 88 MPa and R ratio R = 0.95. The crack closure stress σc was found to increase moderately with increasing R-ratio (R = σminσmax) for th1e same cyclic stress level of 88 MPa. Above a critical R-ratio value of 0.25 the crack closure stress is smaller than the minimum applied stress. The modified crack growth law, which states that crack growth rate (dadN) is proportional to the nth power of the effective stress intensity factor (ΔKeff = Kmax-Kin), cannot account for the dependence in growth rate on the R-ratio. For cyclic loadings with R-ratios less than 0.25 the dadN vs ΔKeff curves did not emerge as expected from the modified law. For cyclic loadings with R-ratios larger than 0.25 the crack growth rate still increased with R-ratio even though the value of ΔKeff remained the same.The retardation of crack growth rate that occurred after tensile overloads was accompanied by an increase in crack closure stress. A log-log plot of dadN vs ΔKeff showed that the modified crack growth law was reasonably obeyed. But it was found that after a single overload the minimum value of ΔKeff generally was reached only after the minimum value dadN had occurred.Cyclic compressive stresses which decreased the retardation effect of tensile overloads were found to depress the crack closure stress to a low value. A large compressive spike also depressed the crack closure stress and increased the crack growth rate after a tensile overload.