Microporosity effects on cyclic plasticity and fatigue of LENS™-processed steel

Special microstructures of the newly developed Laser Engineered Net Shaping (LENS™)-processed steel induce a new variability in fatigue damage formation and evolution mechanisms. The microporosity and mechanism of fatigue damage formation and growth were invested using X-ray computed tomography and scanning electron microscopy. Systematic observations were made of the variations in the fracture surfaces according to three fatigue damage evolution stages: fatigue crack formation (incubation), microstructurally/physically small cracks, and long cracks. The fatigue crack was formed almost exclusively at the relatively large pores located at or near the specimen surface, with rare cases at incompletely melted power particles on the surface. Distributed cracks from large interior pores coalesced with each other in the microstructurally small crack regime to form the major critical crack that eventually fractured the specimen. This coalescence accelerated the fatigue crack growth, which in turn decreased the fatigue life but not significantly. In the long-crack regime, the fracture surface was rougher, but the overall surface tended to be perpendicular to the loading direction, indicating a Mode I type fracture. Cyclic strain-softening, with reduced strain-hardening, was also observed. The multistage fatigue model of McDowell et al. was used to capture the microstructure effects in the three fatigue damage evolution regimes, and the upper and lower bounds for the strain–life are predicted.