Very low cycle fatigue behavior of SA508 Gr. 1a LAS and SA312 TP316 SS in displacement-control mode cyclic loads

This study investigates the deformation and failure behavior of two nuclear structural materials—SA508 Gr.1a low-alloy steel (LAS) and SA312 TP316 stainless steel (SS)—in the very low cycle fatigue (VLCF) regime. Displacement-controlled cyclic tests were performed on notched round-bar specimens with two notch radii (R1.5 and R6.0), spanning both low cycle fatigue (LCF) and VLCF regimes. All tests were conducted at room temperature (RT) under quasi-static loading. In the VLCF regime, SA312 TP316 SS exhibited pronounced cyclic hardening that intensified with smaller notch radius and larger displacement amplitude, whereas SA508 Gr.1a LAS showed negligible hardening with little sensitivity to notch radius. Even in the VLCF regime, no significant reduction in fatigue life was observed when the fatigue life exceeded 10 cycles for either material; the fatigue life could be appropriately described by the Coffin–Manson relationship established from the LCF regime. Regardless of the material type or notch radius, all specimens in the VLCF regime with more than 10 cycles failed due to surface cracking, which is a typical failure mode in the LCF regime.