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 to very 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 progressive strain 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 is formulated. A campaign of experimental tests investigating plasticity and fatigue influence on SMA behavior were conducted and the results used to verify the model prediction capability.

Lastly, the model was implemented in the commercial code ABAQUS through the UMAT user subroutine, and some numerical simulations on biomedical devices, as vascular stents and aortic valves, were performed. The simulations allowed to verify the robustness of the model as well as the strong influence of fatigue and plasticity on the device response to boundary conditions resembling the implanting procedure and in-vivo loads.