Fatigue behavior of complex geometries produced via additive manufacturing
Advances in additive manufacturing are enabling the design and production of part geometries that have previously been unrealistic and/or economically infeasible. Direct metal laser sintering, in particular, is an additive manufacturing method that is capable of producing parts from high strength and temperature resistant materials including the nickel-based superalloy, Inconel 718, which is the material used in this thesis. This present work focuses on two geometries. The first is a test sample with one or two spherical (three-dimensional) voids in the test section; of particular interest was the behavior of the void(s) under fatigue loading, especially, the behavior of two voids as they coalesced. For the geometries tested, fracture surface analysis showed that cracks grew radially from the voids and coalescence occurred secondary to the radial crack growth. Perhaps more interestingly, despite the stress concentration due to the voids, half of the samples failed at locations away from the void(s) indicating that the additively manufacture samples had defects with stress concentrations of similar magnitude to the void(s) elsewhere in the material. The second geometry is a cellular microlattice or microtruss type structure. The microtrusses were tested under fatigue loading and with a range of stress amplitudes. The failure of the microtrusses was observed and fracture surface analysis carried out on some of the failed ligaments. More work is required to determine the factors that make a ligament susceptible to failure, through preliminary results suggest that surface defects play a significant role. The testing also suggested that the notch sensitivity of the microtruss ligaments decreases as the load is increased.
Sangid, Purdue University.
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