Influence of stress ratio on the threshold level for fatigue crack propagation in high strength steels

Abstract

Centrally cracked specimens of JIS SM58Q and HT80 steels were fatigued. The fatigue crack growth rates, da/ dn, and the stress intensity threshold levels, ΔK th were measured over the range of stress ratio, R, from −1 to 0.8 by the use of an automatic method of continuously decreasing stress intensity factor with crack extension. The measured ΔK th was well represented as | ΔK th /2| R =(1− R) γ| ΔK th /2| R=0 ; and the propagation rate, as d a/d n = A(1− R) γm [( ΔK/2) m − {(1 − R) γ | ΔK th /2| R=0} m ] for −1≦ R≦0.33 or d a/d n = A(1 − 0.33) − γm [( ΔK/2) m {(1 − R) γ | ΔK th /2| R=0} m ] for 0.33 < ≦ 0.8.