Article ID: ISIJINT-2025-214
The strain distribution developed during high-cycle fatigue bending tests exceeding 106 cycles was visualized in 780 MPa high-strength steels with various microstructures using the digital image correlation (DIC) method for the secondary electron images of the specimen surface. The tensile stress was approximately identical among the steels with ferrite (F), bainite (B), and ferrite + pearlite (FP) microstructures. Microcracks were detected in the B and FP steels after the fatigue bending test, whereas no cracks were present in the F steel even after 106 cycles. The strain distribution developed in the high-cycle fatigue bending test was successfully visualized using the DIC method for the first time. The strain was inhomogeneously distributed for up to 102 cycles even under an applied stress less than the yield stress. The average strain along the loading direction was approximately zero regardless of the number of cycles for all specimens. The standard deviation calculated from the strain histogram continuously increased with an increasing number of cycles for all steels. This suggests that strain gradually accumulated during the fatigue test. Microcracks tended to nucleate in high-strain regions. The plane of the microcrack was the slip plane, and its Schmid factor was high. Therefore, the microcracks were generated through intrusion and extrusion mechanisms with local slip deformation at the specimen surface.
