Mechanical cycling leads to changes of the mechanical behavior (decrease of the plateau stresses during loading and unloading, decrease of the mechanical hysteresis, increase of residual strain) of NiTi wires. When the strain amplitude is changed during cycling, several plateaus - which are closely related to the locations of previous strain maxima - can be observed in the stress-strain curves. In the past, this phenomenon has been identified as one of the rather “mysterious” properties of NiTi.
This paper addresses the origin of the multiple stress-plateaus associated with complex mechanical cycling. We present experimental results on cycling with increasing strain amplitude, internal loop cycling, and on the effect of short-time heating between consecutive mechanical cycles. We discuss the different processes associated with mechanical cycling which occur simultaneously at three length scales: Microstructural changes, deformation at the mesoscale (i.e., localized transformation in parts of the gage length), and macroscopic response of the tensile specimen. We demonstrate that the seemingly complex fatigue behavior of pseudo elastic NiTi can be rationalized by a simple scenario which explicitly considers localization of deformation, and thus fatigue, due to Lüders-like deformation.