Fatigue Behavior of Stainless Steel 316L Microstruts Fabricated by Laser Powder Bed Fusion

A better understanding of mechanical behavior under cyclic loading is needed for thin struts fabricated by laser powder bed fusion, whose mechanical responses are significantly influenced by their as-built surface morphologies. The current work presents an attempt to characterize tensile-tensile cyclic stresses behavior of 250μm nomoinal diameter LPBF SS316L microstruts at R∼0.04−0.06. The resultant lower fatigue performance of the microstruts was investigated via fracture surface imaging and X-ray microtomography. In particular, microtomography combined with a back-tracking approach was used to characterize the location, shape, and size of the defect responsible for fatigue crack initiation. The identified defect, located at the root of a notch-like surface feature, was analyzed using a modified Murakami Area parameter model. The modified Area parameter of the crack initiating defect estimated the fatigue strength as 130±19MPa, explaining the role of microstrut surface morphological towards the depreciation of fatigue performance compared to as-built macro-scale specimens. The propagation of the initiated crack was characterized using the modified model via interrupted cyclic testing and corresponding tomography at various stress cycle intervals. The obtained crack growth rate vs. the crack's stress intensity range was obtained and compared with crack propagation in macroscale specimens that follow the Paris-law curve.