A study of the stress ratio effects on fatigue crack growth using the unified two-parameter fatigue crack growth driving force

A unified two-parameter fatigue crack growth driving force model was developed to account for the residual stress and subsequently the stress ratio effect on fatigue crack growth. It was found that the driving force should be expressed as a combination of the maximum stress intensity factor, Kmax, and the stress intensity range, ΔK, corrected for the presence of the residual stress. As a result, the effects of residual stresses manifest themselves in changes of the applied maximum stress intensity factor and the applied stress intensity range. A two-parameter function of the maximum total stress intensity factor, Kmax,tot, and the total stress intensity range, ΔKtot, was proposed to model the fatigue crack growth rate data obtained at various R  -ratios. Based on the analysis, the unified two-parameter driving force, Δκ=Kmax,totpΔKtot(1-p), was derived accounting for the mean stress or the stress ratio effect on fatigue crack propagation. It was shown that the two-parameter driving force, Δκ=Kmax,totpΔKtot0.5, was capable of correlating fatigue crack growth data obtained under a wide range of load ratios and fatigue crack growth rates spanning from the near threshold to the high growth rate regime.The model was successfully verified using a wide range of fatigue crack growth data obtained for Al 2024-T351 aluminium alloy, St-4340 steel alloy and Ti–6Al–4V titanium alloy with load ratios, R, ranging from −1 to 0.7.