Comparison of Rotating-Bending and Axial Fatigue Behaviors of LB-PBF 316L Stainless Steel

Additive manufactured (AM) materials are prone to internal defects such as entrapped gas pores and lack of fusions along with having a rough surface. There are different types of fatigue tests that are used to characterize the effects of such defects on the structural integrity of AM parts. The present study aims to investigate the effect of stress gradient on the fatigue behavior of 316L stainless steel (SS), fabricated using a laser beam powder bed fusion (LB-PBF) process. Axial fatigue tests are performed on as-built (non-machined) LB-PBF 316LSS round specimens with uniform gage section, while rotating bending fatigue tests are conducted on hourglass specimens (i.e. reduced gage section). Fatigue tests revealed that the specimens subjected to the axial loading exhibited lower fatigue resistance compared to the specimens failed under rotating bending test. Such differences in the fatigue life was attributed to the variation in the stress distribution resulting from different loading types and its effect on the fatigue crack propagation. Fractography analysis conducted to determine the failure mechanism showed that all of the cracks initiated from the surface of the specimen irrespective to the loading conditions. Furthermore, fracture surface observed for LB-PBF 316L SS specimens resembled a typical fracture surface of notched specimens, which supports the fact that for the as-built specimens cracks initiates from the micro-notches as a result of layer wise fabrication in AM process.