Analysis of High Cycle Fatigue Behavior of Metallic Materials Fabricated by Additive Manufacturing

High-cycle fatigue testing was performed on Inconel 625 made by selective laser melting and Ti-6Al-4V made by electron beam melting. Samples were built in the vertical direction and tests were run at stress ratios of R = -1.0, 0.1 and 0.5. Two approaches were used to analyze the data. In a fatigue life approximation, data for tests run at stress ratios of 0.1 and 0.5 showed good agreement with a Goodman curve derived from data collected at R = -1. These results show that preliminary design guidelines for AM alloys in dynamic loading could be derived from testing at R = -1. In a fatigue notch approach, S-N curves were compared to established design guidelines for these alloys in MIL-HDBK-5. The S-N curves for 625 alloy lay between corresponding curves at each stress ratio for annealed bar and annealed bar notched to a K_t of 3. The S-N curves of the Ti-6Al-4V fell below notched annealed bar or extrusions with K_t of 2.43 to 2.8. While the notch is a macroscopic feature of the test bar, an understanding of the contribution of the surface roughness of the AM materials to reduced fatigue endurance can be approximated by conceiving of the surface roughness as a notch. Empirically based models of average stress at a notch root such as Neuber’s that relate K_t with the notch factor k_f were used in combination with fatigue specimen fractography to estimate the stress concentration factor associated with AM materials. These results provide an empirical basis for estimating the fatigue behavior of AM-built parts subjected to dynamic loading conditions.