Influence of work-hardening on fatigue crack growth, effective threshold and crack opening behavior in the nickel-based superalloy Inconel 718

In this study, the influence of rotary swaging induced cold work on long fatigue crack growth, threshold and opening behavior in the nickel-based superalloy Inconel 718 was investigated. Ambient temperature experiments were carried out using arc-shaped tension specimens according to ASTM standard E399. The direct current potential drop (DCPD) method was used for crack length measurements and crack opening force evaluation. Fatigue crack growth rates (FCGR) measured at load ratios of R=0.2,R=0.5 and R=0.7 were found to increase due to work-hardening in continuous (increasing ΔK) and load shedding (decreasing ΔK) tests. Effective fatigue crack growth thresholds (ΔKth,eff) were determined according to the Kmax=const. procedure and found to decrease monotonically with increasing degree of work-hardening. Compared to the non-deformed material, crack opening stress intensities Kop were reduced in work-hardened states at three different crack lengths. To quantify the detrimental effects of cold work, the effective stress intensity factor range (ΔKeff) model was applied. SEM and TEM micrographs were taken to complement the study.