Fatigue Performance Investigations for 40Cr Steel via Traditional Frequency Loading, Ultrasonic Frequency Loading, and Infrared Thermography Method

This study compares the fatigue behavior of 40Cr steel under traditional (80 Hz) and ultrasonic (20 kHz) loading frequencies, indicating a 29% increase in fatigue limit under ultrasonic loading (876 MPa vs. 677 MPa). Infrared thermography revealed abrupt temperature rises at critical stresses, yielding fatigue limits of 662 and 675 MPa under 80 Hz, contrasting to 858 and 861 MPa under 20 kHz, closely matching with S‐N curve data. Intrinsic dissipation analysis demonstrated that under stress loading of 700 MPa, the intrinsic dissipation per cycle at 80 Hz was 5000 (J/m 3 )/cycle, while at 20 kHz, it was about only 250 (J/m 3 )/cycle, indicating that more fatigue damage would be generated at traditional frequency than that at ultrasonic frequency. Thus, it is adopted to explain much higher fatigue strength under ultrasonic frequency. A dislocation gliding model attributes this improvement to increased pinning defect resistance during ultrasonic excitation. The combined thermographic and energy dissipation approaches validate ultrasonic frequency's significant strengthening effect, providing mechanistic insights for fatigue‐resistant material design.