A MODEL FOR THE EFFECT OF MEAN STRESS ON THE THRESHOLD CONDITION FOR FATIGUE CRACK-PROPAGATION

A long-crack fatigue threshold model which also includes the effect of mean stress or stress ratio R on ΔK0 in the range -1 to +1 is proposed. The model is based on the hypothesis that near-threshold crack growth can occur by a combination of the micromechanisms of submicroscopic cleavage and reversed shear. The former is controlled by Kmax and the latter by ΔK, and a transition between the two occurs at a critical stress ratio Rcrit. The model leads to theoretical ΔK0 vs. R curves characterizing the mean-stress-influenced fatigue limits of cracked specimens in a manner similar to the well-known Goodman diagrams for uncracked specimens. Excellent agreement of the predicted curves with a large amount of available experimental data for various metallic materials is shown. The model can be used as an alternative procedure for obtaining quick and conservative estimates of ΔK0 for practical design applications, in preference to crack closure which may be difficult to measure or predict quickly and accurately. © 1990.