Microscopic Observation and Analysis of Resolved Shear Stresses on Fatigue Crack Propagation Threshold of Fe Ferritic Alloy

This study aimed to gain a comprehensive understanding of the microscopic phenomena in the non-propagating region of fatigue cracks. We conducted tensile tests on pure Fe and an Fe–3%Si alloy with single-phase ferrite using scanning electron microscopy (SEM) and observed the opening and closure behavior of cracks and variations in strain distribution. The Fe–3%Si alloy is available in two types: normal grains with a mean diameter of approximately 100 µm and coarse grains with a mean diameter exceeding 1 mm. Based on the observation results of the crack morphology and crystal orientation, we calculated the resolved shear stresses (RSSs) and discussed the mechanism of crack propagation immediately before non-propagation. In the in-situ SEM observation and digital image correlation in the tensile test of the Fe–3%Si alloy with normal grains, strain concentration regions appeared in two directions from the crack tip. This corresponds to the calculation results of the high RSSs in the two directions. In pure Fe and the Fe–3%Si alloy with coarse grains, the crack tip remained closed, and a strain concentration region appeared in one direction. This corresponds to the calculated results for a high RSS in one direction. Based on observations of the dislocation structures near the crack tips in the Fe–3%Si alloy using the electron channeling contrast imaging method, screw dislocations with Burgers vector b in the same direction as the high-RSS slip directions existed with high frequency. Accordingly, slip systems with high RSSs were activated immediately before non-propagation. Therefore, fatigue cracks immediately before non-propagation could be classified into two crack propagation mechanisms: first, two-direction slip systems are activated alternately, and cracks propagate with the crack tip opening and closing; and second, only one slip system is activated, and cracks propagate with the crack tip remaining closed.