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Additive manufacturing (AM) techniques are under constant development and selective laser melting (SLM) is among the most promising ones. However, widespread use of AM techniques in many industries is limited by the different/unusual mechanical properties of AM metallic parts, with respect to traditionally processed ones, especially when dealing with complex fatigue loading conditions. In fact, crack formation and propagation mechanisms are mainly affected by the development of internal defects, residual stresses, and microstructural changes. This is actually one of the major issues the materials engineering community is facing today. In many applications, AM components are subjected to multiaxial fatigue loads, arising from operating conditions and/or from complex geometries, that unavoidably generate crack initiation and propagation mechanisms. The aim of this study is to investigate the multiaxial fatigue behavior of additively manufactured Ti6Al4V samples, made by SLM. Fatigue tests, combining proportional axial and torsional loads, were performed on thin-walled tubular specimens. Full-field measurement techniques, such as the infrared thermography and digital image correlation, were also used to capture temperature and strain evolutions, at both local scales and global scales. Fatigue results highlighted damage mechanisms, and failure modes are strongly related to the applied stress level.

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The whole low-to-high cycle fatigue behavior of a pseudoelastic NiTi SMA was analyzed. Global and local strains measurements were carried out by an extensometer and digital image correlation. Significant differences were observed at the two scales, due to the localized nature of stress-induced transformations. Local mechanisms play a crucial role on fatigue damage and have a direct effect on the Z-shaped strain-life curves observed in previous works. The effective nature of this unusual behavior was revealed by systematic analysis of local strain. Finally, strain decomposition was made and data were analyzed by a recent modified Coffin-Manson approach.