N. Rebelo, Dassault Systemes Simulia Corp. Western Region, Fremont, CA; M. Schlun, A. Zipse, G. Dreher, Bard Peripheral Vascular, Karlsruhe, Germany
The widespread development and usage of implants made from NiTi is accompanied by the publication of many NiTi material characterization studies. These publications have increased significantly the knowledge about the mechanical properties of NiTi. However, this knowledge also increased the complexity of the numerical simulation of NiTi implants or devices. This study focused on the uniaxial behavior of NiTi tubing due to cyclic loading and had the goal to deliver both precise and application-oriented results. Single aspects of this study have already been published; however, there is no publication known which shows all the single effects combined in a “duty cycle”. It was of particular importance to summarize the main effects of pre-strain and subsequent small or large strain amplitudes on the material properties. The phenomena observed were captured in an extended Abaqus Nitinol material model, which is presented elsewhere in this Conference. The cyclic tensile tests were performed using a videoextensometer to get accurate strain measurement on small e.-polished dogbone specimen, which were incorporated into a stent framework so that standard manufacturing methods could be used for the fabrication. This study indicates that a preload beyond 6% strain alters the transformation plateaus and if the cyclic load amplitude is large enough, additional permanent deformations are observed, the lower plateau and most notably the upper plateau change. The modifications to the upper plateau are very interesting in the sense that it appears broken: its start stress gets lowered creating a new plateau up to the highest level of cyclic strain, followed by resuming the original plateau until full transformation. This study was conducted in the course of the work of a consortium of several stent manufacturers, SAFE Technology Limited and Dassault Systèmes Simulia Corp., dedicated to the development of fatigue laws suitable for life prediction of Nitinol devices.
Summary: This study focused on the uniaxial behavior of NiTi tubing due to cyclic loading and had the goal to deliver both precise and application-oriented results.
