Effects of Cyclic Loading on the Uniaxial Behavior of Nitinol

Abstract

The widespread development and use 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 is 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 (Wagner in Ein Beitrag zur strukturellen und funktionalen Ermü dung von Drähten und Federn aus NiTi-Formgedaechtnislegierungen, Ph.D. Thesis, 2005; Eucken and Duerig in Acta Metall 37: 2245-2252, 1989 Yawny et al. in Z Metallkd 96:608-618, 2005); however, there is no publication known that shows all the single effects combined in a ''duty cycle case.'' 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 Ò Nitinol material model, presented by Rebelo et al. (A Material Model for the Cyclic Behavior of Nitinol, SMST Extended Abstracts 2010). The cyclic tensile tests were performed using a video extensometer to obtain accurate strain measurement on small electro-polished dog-bone specimen that were incorporated into a stent framework so that standard manufacturing methods could be used for the fabrication. This study indicates that a prestrain beyond 6% strain alters the transformation plateaus and if the cyclic displacement amplitude is large enough, additional permanent deformations are observed, the lower plateau and most notably the upper plateau change. The changes to the upper plateau are very interesting in the sense that an additional stress plateau develops: its ''start stress'' is lowered thereby 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.

High compressive pre-strains reduce the bending fatigue life of nitinol wire

“…While numerous computational (Grujicic et al, 2012;Kumar et al, 2013;Rebelo et al, 2009) and some experimental studies (Pelton, 2011;Pelton et al, 2008) of device-level fatigue have incorporated crimping and deployment into the investigational protocol, few studies have clearly examined how this pre-strain cycle impacts overall fatigue properties. Schlun et al subjected pseudoelastic NiTi to an 8% tensile pre-strain and examined the evolution of the cyclic strain-stress behavior at 0.2% and 1.2% strain amplitude and 2% mean strain for 100 cycles (Schlun et al, 2011). The authors observed hysteresis and permanent set in the stress-strain curve at 1.2% strain amplitude over 100 cycles but the curve remained constant at 0.2% strain amplitude.…”

Study on the Effect of Strain Rate and Temperature on Mechanicalproperties of Nitinol

“…It was observed that annealing the alloy above the recrystallization temperature (600°C) reduces the maximum recovery stress significantly. M. Schlun et al [5] studied the uniaxial behavior of Nitinol tubing due to cyclic loading. Their study showed that a pre-straining level >6% will alter the plateau stresses.…”