Influence of Ultrasonic Surface Rolling Process on the Correlation Between Fatigue Strength and Crack Initiation Radius Ratio in Notched Inconel 718 Specimens

This study examines the effect of ultrasonic rolling on the high-cycle fatigue strength of notched Inconel 718 superalloy. Fatigue strength and crack initiation location are analyzed from microstructural, stress-state, and fatigue-strength perspectives. Results show fatigue strength varies significantly with crack initiation location (normalized radius ratio A). Small cracks originate within the modified layer due to axial stress gradients. Peak strength is governed by stress concentration and triaxiality at crack initiation, affecting dislocation dynamics and γ″ phase strengthening. For Kt = 1.13, peak fatigue strength (700 MPa) occurs when A reaches 0.064, balancing dislocation driving force and precipitation resistance. Beyond A = 0.064, increased triaxiality reduces dislocation emission force, leading to stress concentration, cleavage cracking, and reduced fatigue strength (400 MPa). Focused ion beam (FIB) analysis confirms significantly higher dislocation density in high-stress triaxial regions compared with low-stress regions due to difficulty in dislocation pile-up around precipitates.