THE COMBINED EFFECTS OF STRESS RATIO AND YIELD STRENGTH ON THE LEFM FATIGUE THRESHOLD CONDITION

Abstract— Long‐crack (LEFM) fatigue threshold, Δ,K₀ values are predicted which include the commonly observed effects of stress ratio, R, and yield strength, σ_y. It is assumed that the yield strength effect on threshold is indirectly related to grain size and so is not an independent variable. Two intrinsic thresholds of a material are invoked to explain the observations of higher ΔK₀ values and a higher R‐ratio sensitivity of ΔK₀ in low strength materials compared to high strength materials. The paper shows that ΔK₀ is almost independent of both yield strength and stress ratio at high values of the stress ratio. Quantitative relations are developed to estimate curves of (i) ΔK₀ versus R and (ii) ΔK₀ versus σ_y. These curves show good agreement with experimental data for steels and aluminium alloys. A method is presented that may be used as an alternative procedure for obtaining quick and conservative estimates of ΔK₀ for design applications.