Functional fatigue of NiTi Shape Memory wires for a range of end loadings and constraints

Abstract

The availability of engineering strength data on shape memory alloys (SMAs) under cyclic thermal activation (functional fatigue) is central to the rational design of smart actuators based on these materials. Test results on SMAs under functional fatigue are scarce in the technical literature and the few data available are mainly limited to constant-stress loading. Since the SMA elements used within actuators are normally biased by elastic springs or by another SMA element, their stress state is far from constant in operation. The mismatch between actual working conditions and laboratory arrangements leads to suboptimal designs and undermines the prediction of the actuator lifetime. This paper aims at bridging the gap between experiment and reality. Four test procedures are planned, covering most of the typical situations occurring in practice: constant-stress, constant-strain, constant-stress with limited maximum strain and linear stress-strain variation with limited maximum strain. The paper describes the experimental apparatus specifically designed to implement the four loading conditions and presents fatigue results obtained from commercial NiTi wires tested under all those protocols.

Fatigue and fracture of wires and cables for biomedical applications

“…8a-c) fatigue test techniques 22,24,[26][27][28][29]31,32,38,39,41,[45][46][47][48][49][50][51][52][53][54][55]57,59,61,[63][64][65][66][67][68][69][70] with a few additional investigations on alternate fatigue methods including alternating bending (Fig. 8a), 29,39,62,63 axial, 25,33,36,37,42,56,72 pulsatile (Fig. 10), 29,33,36,37,39,…”

“…A variety of test techniques have been developed to characterise the fatigue properties of wires and cables. Wire fatigue tests reported in the literature for use in evaluating biomedical materials such as alternating bending testing, 29,30,39,62,63 axial fatigue testing, 25,33,36,37,42,44,56,72 flex bending fatigue testing, 4,10,11,34 pulsatile fatigue testing, 29,39,63,67 rotating bending testing 2,7,8,12,14,[22][23][24][26][27][28][29][30][31][32]35,[38][39][40][41]43,[45][46][47][48][49][50][51]…”

“…Selected materials need to exhibit high biocompatibility, resistance to corrosion and provide the necessary balance of strength, ductility and fatigue resistance for a particular application. Some common materials used in biomedical wires and cables include stainless steels, 2,7,8,11,14,27,34,40,43,44,58,60 metal composites 2,10,31,58,60,71 and Nitinol (binary 4,22,[24][25][26][27][28][29]32,33,[36][37][38][39]41,42,[44][45][46][47][48][49][50][51][52][53][54][55][56][57]59,[61]…”

The effect of temperature on low-cycle fatigue of shape memory alloy

“…Residual martensite plates increase with the number of cycles and are considered to be one of the causes of the formation of a residual strain during cyclic loading [15]. Lifetime of SMA can be predicted by stress- [16,10], strain- [17][18][19] and energy based criteria [10] of fatigue failure. An overview of the structural fatigue of SMA under mechanical and thermomechanical loading is presented, for instance in the paper [20].…”

Actuator lifetime predictions for Ni60Ti40shape memory alloy plate actuators

“…While the effects of pseudoelastic or mechanical fatigue of SMAs are relatively well understood, comparatively few studies have been conducted on actuation or thermomechanical fatigue [9][10][11][12][13][14][15][16][17][18][19][20][21]. Additionally, studies on actuation fatigue have been predominantly on wire actuators while many actuator applications require bulk actuators.…”