Full field observation of crack formation and propagation mechanisms in NiTi alloys

Near equiatomic Ni-Ti alloys have seen increasing use in recent years in many fields of engineering and medicine, due to their unique functional properties, namely Shape Memory Effect (SME) and Pseudoelastic Effect (PE). In particular, these alloys exhibit large strain recovery capabilities thanks to thermally and/or stress-induced reversible microstructural transformations. However, it is widely accepted by the scientific community that these transformation phenomena significantly affect the crack formation and propagation mechanisms with respect to common engineering metallic alloys and, consequently, they exhibit unusual fracture and fatigue responses. As a consequence, several studies have been carried out recently, in order to better understand the fatigue and fracture behavior of NiTi alloys but many aspects related to the role of phase transition mechanisms are still unknown._x000D_ In this work the crack formation and propagation mechanisms in a commercial pseudoelastic alloy have been investigated by full field displacement techniques and the role of stress-induced phase transformation mechanisms, occurring in the crack tip region as a consequence of the highly localized stresses, is analyzed by a systematic study involving both experimental observations, numerical and analytical results.