Load path dependency in Nitinol structures

In simulations using the commercially available Finite Element (FE) software packages ANSYS and ABAQUS, the Auricchio Nitinol material model resulted in irregularities for the simulation of implantable medical devices experiencing peak loading and partial relaxation. These irregularities, which were identified by variability in the final strain state for similar deployment configurations, were attributed to possible Nitinol load path dependency effects in which the magnitude of peak strain influenced the final stress and strain state of the model post relaxation.

Further investigations to explore this effect were carried out using a cantilever beam  subject to a simple loading regime. Investigations found that the beam was in a noticeably different final state of stress and strain depending on the magnitude of peak strain experienced by the beam. With an increase in peak strain, regions which experienced a phase transformation recorded higher values of strain once relaxation had taken place. The present work will discuss the background to this effect and the phenomenon will be clarified. The implications for FE endovascular device compaction and release simulations  will also be discussed.