Near-threshold fatigue crack propagation in ultra-high strength steel: Influence of load ratio and cyclic strength

Fatigue crack propagation behavior of an ultra-high strength steel (300-M) has been investigated in humid air over a very wide spectrum of growth rates from 10^-8 to 10^-1 mm/cycle. Particular emphasis has been devoted to the influence of mean stress (or load ratio R = K_min/K_max) and microstructure on fatigue crack growth near the threshold stress intensity for crack propagation, ΔK₀. Increasing the load ratio from R = 0.05 to 0.70 was found to lead to increased near-threshold growth rates, and a decrease in the threshold stress intensity. Similarly, increasing material strength, by varying the microstructure through quench and tempering and isothermal transformation, resulted in higher near-threshold growth rates, and a marked reduction of ΔK₀. These effects are contrasted with behavior at higher growth rates. The influence of strength on ΔK₀ is rationalized in terms of the cyclic hardening or softening response of the material, and hence it is shown that cyclic softening can be beneficial to fatigue crack propagation resistance at very low growth rates. The results are discussed in the light of crack closure and environmental contributions to fatigue crack growth at low stress intensities.