Finite Element Analysis on Micromechanical Models of Particle/Void Assemblies in Nitinol Transcatheter Endovascular Devices

Transcatheter endovascular devices are subjected to a complex thermo-mechanical load history prior to millions of cycles of multi-axial in vivo loading. The load history may include heat treatment during shape setting, loading into a delivery system after cooling, e.g. in ice water, and deployment into a complex anatomy at body temperature. Recent investigations of the influence of pre-strained Nitinol wire on the fatigue life showed that large compressive strains accelerate the crack initiation even at low strain amplitudes [1]. However, the underlying fatigue failure micromechanism does not change with the amount of pre-strain [1]; the crack still initiates at the boundary between a particle and a void in a typical processing-induced Particle/Void Assembly (PVA) in Nitinol, which is also well reported in literature for an unstrained material.

In this work, micromechanical finite-element models of PVA’s are investigated under a complex thermo-mechanical load history. This includes large macroscopic compression (pre-strain), unloading and cyclic loading according to the experiments reported in reference [1]. Local stress and strain fields are analyzed and the results are correlated to these experimental findings [1]. The material model described in reference [2] including plasticity is used in the finite-element simulations.

[1] Gupta, S. et al., High compressive pre-strains reduce the bending fatigue life of nitinol wire, Journal of the Mechanical Behavior of Biomedical Materials (44), 96-108 (2015)

[2] Junker, P., An accurate, fast and stable material model for shape memory alloys, Smart Materials and Structures 23 (11): 115010 (2014).