Microvoid Characterization in Structural Steels Under Ultralow Cycle Fatigue: Fractographic Insights and Modeling

ABSTRACT The present study quantifies microvoid characteristics formed at fracture in cylindrical notched specimens made of ASTM A36, A572, and A992 steels subjected to ultralow cycle fatigue (ULCF) loading. Microvoid size and circularity were evaluated at different locations across the fracture surface using SEM micrographs. The microvoid size distributions under ULCF were significantly positively skewed, prompting evaluation of multiple statistical descriptors such as mean, median, 80th percentile, and 90th percentile sizes. The mean and median microvoid sizes under ULCF loading were 1.8 to 3.7 times smaller than those under monotonic tension for ASTM A992 steel. Contrary to the existing notion, microvoids on ULCF fracture surfaces have higher circularity when compared to their monotonic counterparts, and the largest microvoids are concentrated at the periphery of the fracture surfaces. Finally, the experimentally evaluated microvoid sizes were used for calibrating a micromechanical ULCF fracture model, which yielded conservative estimates of ULCF cycles to failure.