Correlating stress ratio effects on the fatigue crack growth rate behavior of a nickel-based superalloy GTM718

Abstract

Damage tolerance evaluation under service loads requires a unified constant amplitude fatigue crack growth law for the material which takes care of stress ratio effects. Several types of crack driving force parameters have been proposed in the past to correlate stress ratio effects on fatigue crack growth rates. In the present investigation, a unified crack growth law is derived for a nickel-based superalloy by considering various crack driving force parameters to eliminate stress ratio effects. Initially, the constant amplitude fatigue crack growth rate (FCGR) behavior of GTM718 alloy was experimentally determined at various stress ratios, R ranging from R = 0.1 to 0.7. As expected, increasing the stress ratio, increased crack growth rates, and decreased threshold stress intensity factor range, ΔKth. The conventional crack growth rate, da/dN vs stress intensity factor range, ΔK data was then modified and re-plotted as a function of different crack driving force parameters viz., (a) Elber’s parameter, ΔKeff, (b) Walkers’ parameter, Kw (c) Kujawski’s parameter, K*, (d) Forman’s parameter, Kf, and (e) Two-parameter crack driving force, ΔK*. It was observed that Kf and ΔK* crack driving force parameters were correlating stress ratio effects better than other models in all three regimes of crack growth rates. Further, a unified crack growth law was derived based on the Kf and ΔK* crack driving force parameters which is useful in predicting FCG behavior under service loads.