Characterization of the Uniaxial Cyclic Behavior of Nitinol Wires

The uniaxial behavior of Nitinol in different forms and at different temperatures has been well documented in the literature. Mathematical models for the three dimensional behavior of this class of materials, covering superelasticity, plasticity and shape memory effects have been previously developed. Phenomenological models embedded in FEA analysis are part of common practice today in the development of devices made out of Nitinol. In-vivo loading of medical devices has cyclic characteristics. It is now known in the literature that cyclic loading of Nitinol modifies substantially its behavior. A consortium of several stent manufacturers, Safe Technology and Dassault Systèmes Simulia Corp., dedicated to the development of fatigue laws suitable for life prediction of Nitinol devices has conducted an extensive experimental study of the modifications in uniaxial behavior of both Nitinol wire and tubing due to cyclic loading. Additional data in Nitinol wire has now become available. This data has been characterized in terms of the Abaqus Nitinol material model which has been extended to capture cyclic behavior. Namely, characterization of changes in transformation plateaus with pre-straining, of permanent deformations with cycling, and of changes to both upper and lower plateaus. 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. Characterization of cyclic changes due to small amplitude cycling also depends on whether the cycling is made from the upper plateau, or from the lower plateau. The resulting model reproduces the experimental behavior quite well.