Influence of crack driving force on correlating stress ratio effects in fatigue crack growth rate of a nickel base super alloy IN720

Damage tolerance evaluation under service loads requires a unified constant amplitude fatigue crack growth law for the material with nullified 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 various materials. In the present investigation, an effort was made to use four different crack driving force parameters to eliminate stress ratio effects in a nickel base super alloy IN720. Constant amplitude fatigue crack growth rate of Inconel 720 (IN720) was experimentally determined at various stress ratios, R ranging from R = 0.1 to 0.7. All the tests were performed at room temperature and in laboratory air condition in a 100-KN servo-hydraulic test machine using compact tension specimen and following ASTM test standard procedures. 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 versus stress intensity factor range, ΔK data was modified and re-plotted as a function of four different crack driving force parameters, viz., (a) Kujawski’s K*, (b) two parameter ΔK*, (c) Walker’s ΔKw and (d) Foreman’s ΔKf. It was observed that Kujawski’s crack driving force parameter, K*, was correlating stress ratio effects better than all other models in all the three regimes of crack growth rates in this material. Further, all these models were employed to predict fatigue crack growth behaviour under a truncated FALSTAFF spectrum load sequence. It was observed that the total fatigue crack growth life estimated varied from about 25 blocks to about 75 blocks. Thus, the use of appropriate crack growth model appears to influence accuracy of life prediction under spectrum loads