Micromechanical simulations for fatigue damage incubation mechanisms of LENSTM processed steel

A Laser Engineered Net Shaping (LENSTM) processed AISI 316L-grade stainless steel possesses unique microstructural features that affect its fatigue damage incubation mechanisms and fatigue life. As observed experimentally, fatigue damage was incubated almost exclusively at a relatively large pore located at or near the specimen surface, or at an incompletely melted powder particle on the surface in some rare cases. Micromechanical simulations were conducted on a series of representative volume elements to probe the micromechanism of fatigue damage incubation. A non-local plastic shear strain range-based fatigue manage parameter was introduced to represent the micronotch plasticity accumulation state under a large deformation gradient region. This microplasticity parameter was found to correlate to the fatigue endurance limit and transition between high cycle and low cycle fatigue regimes quite well. In the high strain amplitude regime (>0.3%), the spatial distribution extends no or little effect on fatigue damage incubation. The scatter in fatigue life is primarily dominated by small crack growth. In the low strain amplitude regime, the void on the edge of the specimen induced two times higher microplasticity compared to void inside the specimen. Due to the high population of the voids inside the alloy, the fatigue strength is limited by the voids on the surface. The microplasticity threshold is approximately 0.15% which corresponds to the fatigue endurance limit of 30.8 MPa.Micromechanical simulation demonstrated that the microplasticity induced by the unmelted powder particle is significantly lower than that generated by the void in the microplasticity threshold regime. Therefore, there must be an interactive effect between unmelted powder particles and the surround porosity in the alloy that incubated the fatigue damage.