A Material Model for the Cyclic Behavior of Nitinol

Abstract

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. There have been some indications 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. The Abaqus Nitinol material model has been extended to capture some of the phenomena observed and is described in this article. Namely, a preload beyond 6% strain alters the transformation plateaus; if the cyclic load amplitude is large enough, permanent deformations (residual martensite) are observed; the lower plateau increases; and the upper plateau changes. 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. Since quite often the geometry of a device at the point at which it is subjected to cyclic loading is very much dependent on the manufacturing, deployment, and preloading sequence, it is important that analyses be conducted with the original material behavior up to that point, and then with the cyclic behavior thereafter.

Fatigue and fracture of wires and cables for biomedical applications

“…diamond, folded strut, dog bone, etc.) 36,[187][188][189][190][191][192] processing techniques, 162,193 laser machining, [194][195][196][197] surface conditions, [198][199][200] biocompatibility 123,201 and corrosion 72,123,166,[202][203][204][205] also provide valuable background information for these alloys. Effects of composition, processing, surface condition, joining techniques, wire diameter, test environment, rotational speed/frequency, test configuration, applied mean strain and material cleanliness will be discussed.…”

Fatigue-Life Computational Analysis for the Self-Expanding Endovascular Nitinol Stents

“…To shorten the stent development cycle and reduce the associated cost, computer engineering methods and tools are being increasingly utilized. There are several examples in the open literature demonstrating the benefits offered by the computer engineering methods and tools within the vascular stent (Ref [27][28][29][30].…”

A three-dimensional phenomenological model describing cyclic behavior of shape memory alloys

“…Moreover, large deformations and hence high stresses may induce material yielding, i.e. macroscopic plasticity associated with slip (Yan et al, 2003;Rebelo et al, 2011).…”