Development and experimental validation of a constitutive model for NiTi medical devices subjected to fatigue and plasticity

Nickel-Titanium alloys (NiTi), are commonly adopted for producing minimally invasive devices as self-expandable stents, aortic valves and stent-grafts (exploiting pseudo-elasticity) and microactuators, micro-pumps and surgical instrumentation (exploiting shape memory effect). These devices are subjected to cyclic loadings (due to blood pulsatility or leg movements or repeating procedures), that can induce fatigue fracture, and also may be subjected tovery large deformations (due to crimping procedure, or physiological tortuous path, or overloads), that can induce material yield. Accordingly, for a correct numerical prediction of their performance, models including progressivestrain accumulation during repeated thermo-mechanical cycles and plasticity effects on phase transformation are necessary. In this work, a new constitutive model, able to consider both plasticity and fatigue and their interaction isformulated.