Advanced high-strength steels (AHSSs) are essential for achieving the required strength-to-weight ratio, surpassing conventional high-strength low-alloy (HSLA) steels, for automotive applications. However, in heavy-duty vehicles, AHSSs encounter challenges in outperforming HSLA steels, particularly concerning fatigue properties after sheet shearing processes crucial for chassis production.

In this work, we conduct a related study on the relationship between microstructure and fatigue crack initiation and propagation mechanisms in three commercial AHSSs (800CP, 700MC, and 700MCPlus) and compare them with a conventional HSLA (500MC) steel. Tensile testing, high cycle fatigue (HCF) testing, and fatigue crack growth rate (FCGR) assessments are conducted. The mechanisms controlling the performance of these four steel alloys are examined through comprehensive microstructural characterization pre- and post-fatigue testing in both as-received hot-rolled and punched conditions.

The results of HCF testing on punched specimens reveal improved fatigue strength at $10^5$ cycles in 700MCPlus as compared to the other studied steels. This enhanced fatigue strength is associated with increased fatigue crack growth resistance in 700MCPlus, resulting from its unique texture, which restricts slip activity, and the presence of martensite at grain boundaries, contributing to fatigue crack deflection.

However, these martensite at grain boundaries are detected as preferential sites for strain localization during cyclic loading due to the large strength difference between martensite and matrix, thus facilitating fatigue crack initiation and subsequent steeper slope in the S-N curve. The tendency for strain localization is lower in the other steel alloys due to reduced strength differences between their microconstituents. In this case, strain localization promotes dislocation rearrangement and subsequent formation of sub-grains near the punched edge. Formation of these sub-grains is also observed near the fatigue crack initiation site of the as-received samples and is believed to facilitate fatigue crack initiation by reducing the local threshold stress intensity factor range ($\Delta K_{th}$). Finally, the results obtained in this work supports the understanding of mechanisms for punching-induced fatigue performance degradation and could unlock the potential of AHSS for utilization in heavy-duty truck chassis components.