A model for the combined effects of elevated temperature and stress ratio on the LEFM fatigue threshold condition

Abstract

The effects of moderate elevated temperature and stress ratio on the value of long-crack (LEFM) fatigue threshold stress intensity range ΔK 0 are analysed in the light of a new approach that concentrates on events in a critically stressed material volume V c ahead of the crack tip. A model developed to incorporate the effect of temperature is based on the main concepts of previous models describing the effects of stress ratio, yield strength, and gaseous environment on ΔK 0 at room temperature. The upper- and lower-bound intrinsic thresholds that played important roles in those models are seen to play a similar role in the present model. It is shown that ΔK 0, equalling the upper-bound threshold at room temperature, decreases to a value close to the lower-bound threshold of the material at a characteristic elevated temperature T crit. Good agreement is shown to exist between theoretically predicted and experimental ΔK 0 vs R and ΔK 0 vs temperature curves for several low-strength steels tested in the laboratory air environment. The model may be useful in obtaining a quick, fairly accurate and conservative estimate of ΔK 0 for material selection and design.