Scale and Shape Effects on the Fatigue Behaviour of Additively Manufactured SS316L Structures: A Preliminary Study

The freedom in geometry given by additive manufacturing allows to produce cellular materials, also called lattice structures, with unit cells and mesoscale features that are impossible to obtain with traditional manufacturing techniques. The geometric modeling of lattice structures still presents issues such as robustness and automation, but, with a novel modeling approach based on subdivision surface algorithm, these troubles were limited. Furthermore, the subdivision method smooths surfaces, avoiding sharp edges at nodal points and increasing performances in fatigue properties. The aim of this work is twofold; a. The subdivision surface method is validated through fatigue tests on specimen additively manufactured by selective laser melting technology in SS316L stainless steel; dynamic tests were carried out on two types of lattice structures based on cubic cell: one obtained with a traditional modeling method, one obtained with a subdivision surface approach. b. Additional tests on bulk cylindrical samples, allowed to propose a preliminary model that describes the fatigue behaviour of additively manufactured lattices as a function of the bulk material properties, considering the shape and scale effects coming from stress concentration factor, increased area, surface roughness and porosity of the part. Results show that the subdivision surface approach improves the fatigue life of lattice structures, as expected. More, the lattices have a worse fatigue life compared to the bulk samples due to the scale and shape effects, that results in a higher sensibility to surface and internal defects related to the manufacturing process.