Fatigue failure mechanism of gradient nanostructured materials produced by turning

Most safety-critical components and load-bearing structures continue to be manufactured using hard turning, a process that induces gradient nanostructures (GNS) in the surface layer. To investigate the effect of GNS layer on fatigue properties, crystal plasticity finite element model (CPFEM) and ± 0.8 % strain fatigue test were used in this study. The objectives were to investigate the correlation between turning parameters and surface GNS layer of 316 L stainless steel, and to reveal the fatigue failure mechanism of GNS layer from multiple scales. The results show that the turning parameters significantly influence the thickness of the GNS layer, with turning depth having the greatest impact, followed by cutting speed. CPFEM simulations predict stress distribution within the GNS layer across regions with varying grain sizes. stresses in fine-grained regions are primarily concentrated at grain boundaries, whereas stresses in coarse-grained regions are distributed within the grains. The model predictions of fatigue crack locations closely align with stress concentration distributions. Fatigue testing reveals that cracks in the GNS layer primarily propagate intergranular boundaries, while cracks in the coarse-grained (CG) layer exhibit both intergranular and transgranular extensions. This behavior mirrors the damage patterns predicted by simulation, demonstrating the model's high accuracy.