A Methodology for Predicting Surface Crack Nucleation in Additively Manufactured Metallic Components

The as-built surfaces of metallic parts made by additive manufacturing (AM) processes often contain notch-like features that serve as stress concentration sites where fatigue crack nucleation may occur. These notch-like features, which result from the surface roughness inherent to AM processes, should be considered in assessing the fatigue performance of AM parts. In this investigation, a methodology for predicting surface crack nucleation in AM materials has been developed by considering (1) the stress concentration factors (k_t) associated with the surface micro-notches, (2) the cycles to surface crack nucleation, and (3) the threshold stress ranges for the growth or non-growth of the microcracks. The combination of the micro-notch and low-cycle fatigue (LCF) life model, named micro-LCF model, shows that there exists a minimum surface roughness profile, defined in terms of a ratio of valley depth (R_vi) to valley spacing (λ_i) of 0.01, below which fatigue life debit does not occur. Furthermore, fatigue crack nucleation and growth dominates at R_vi/λ_i < 0.18 and fatigue crack nucleation with non-propagating cracks prevails at R_vi/λ_i > 0.18 and k_t > 1.5. A surface roughness with an R_vi/λ_i ratio = 1 and k_t = 3 is also identified as the worst-case condition where fatigue life is entirely controlled by crack growth. This methodology has been applied to a nickel-based superalloy AM 718Plus to confirm the various ways in which surface roughness affects surface crack nucleation and growth. In addition, potential fatigue life enhancement that can be achieved by machining portions, if not all, of the as-built surfaces is assessed.