smartflex® niti wires for shape memory actuators

7/27/2019 SmartFlex NiTi Wires for Shape Memory Actuators

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SmartFlex NiTi Wires for Shape Memory ActuatorsL. Fumagalli, F. Butera, and A. Coda

(Submitted September 15, 2008; in revised form January 29, 2009)

Shape memory alloys (SMAs) are active metallic materials classified nowadays as smart or intelligentmaterials. One of their main areas of interest is that of actuators. The use of SMAs in actuators offersthe opportunity to develop robust, simple, and lightweight elements that can represent an alternative toelectro-magnetic actuators commonly used in several fields of industrial applications, such as automotive,appliances, etc. SAES Getters S.p.A. thanks to its vertically integrated process and to the scientific andquality approach, developed a NiTi-based wires family which can represent a solution for shape memoryactuators. In this paper, the mechanical, thermal, and electrical response of these shape memory wires, withdiameters ranging from 20 to 500 lm, will be examined and discussed, with particular focus on the designof the actuator. The thermo-mechanical properties have been investigated and measured by severalmethods. The most common and useful tests for these commercially available wires will be also described.

Keywords advanced characterization, automotive, mechanicaltesting, nondestructive testing, titanium

1. Introduction

For simple actuations involving, for example, linear move-ments of a few millimeters or angular movements of just a fewdegrees, the usage of a motor rotating at thousands of rpm

coupled with gears, clutches, and reducers is certainly not thebest solution. The use of smart materials for actuationrepresents an excellent technological opportunity for the

development of electro-mechanical components. In particular,the typical characteristic of smart materials promotes the

development of simple, very compact, reliable actuators thatcan be integrated in the components or in the structures thustransforming these from static to dynamic or, in some cases,adaptive as they can react directly to environmental stimuli.

Component designers can therefore use smart materials tosimplify products, to add new functions, to upgrade perfor-mance, to improve reliability, and to reduce component costaccompanied by a significant reduction in mechanical com-plexity and size (see Fig. 1).

Shape memory alloys (SMAs) are functional materials with

a multitude of different dependent properties. Among them, theequiatomic system NiTi has been established over the years as astandard alloy covering a wide range of application require-

ments (Ref 1). NiTi shows the best combination of properties,especially in terms of the amount of work output per materialvolume and the large amount of recoverable strain compared to

any other actuator principles (Fig. 1). The obvious simplicity of

the mechanical design and the small number of moved parts isamazing for an actuator and make SMAs particularly attractivefor microsystems applications.

SMA actuation is based on the one-way effect. In principle,the effect can be used for generating motion and/or force. Therequired heating power is supplied either indirectly by variation

of ambient temperature or directly by electrical heating of theSMA component (Joule effect). Electrically driven actuatorscarry out specific functions at ambient temperatures below theMs temperature, which can be controlled via heating power.Further functions of SMA components may be structural orreset functions.

The principle of work production with an external reset

force is applied in most SMA actuators (Ref 2). Depending onthe choice of the reset mechanism, different trajectories can beobtained in the stress-strain space. Figure 2 shows various

reset mechanisms using a SMA tension wire. In case (a), aweight produces a constant force level. Another widely usedvariant is passive bias spring (b). The case of decreasing resetforce for increasing deflection is described in (c). Moreover,the use of a second SMA tension wire with opposite actuationdirection leads to an antagonistic mechanism (d). As the reset

mechanism determines the stress profile in the SMA compo-nent, the typical hysteretic behavior of the phase transforma-tion can be adjusted or modulated by selecting an appropriatemechanism, according to specific requirements of the final

application. Definitely, the reverse bias design concept can

open new applications for SMAs actuators. The correct usageof a straight wire shape memory actuator is based on thesensible design of the surrounding mechanical structure thathas to provide the resetting force for the actuator. Under

optimal design condition, the transformation temperatures canbe significantly increased and the maximum actuator strokecan be optimized.

Under pure tension or compression load, high forces butrelatively small displacements can be reached. Larger displace-

ments are achieved under torsion or bending loads. Wires andsprings are the most important shapes of NiTi that can be usedinto the design of an actuator. Depending on the kind of

This article is an invited paper selected from presentations at ShapeMemory and Superelastic Technologies 2008, held September 21-25,2008, in Stresa, Italy, and has been expanded from the original

presentation.

L. Fumagalli, F. Butera and A. Coda, Saes Getters S.p.A, Lainate,MI, Italy. Contact e-mail: [email protected].

JMEPEG (2009) 18:691695 ASM InternationalDOI: 10.1007/s11665-009-9407-9 1059-9495/$19.00

Journal of Materials Engineering and Performance Volume 18(56) August 2009691


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application, typical distinctions can be made which should beunderstood as guidelines (Ref 3), as they are not firm rules:

Straight wire for resistance heated actuators with smalldisplacement (up to 5%),

Compression or tensile springs for indirect heated actua-tors with large displacement (more than 5%).

The mechanically stabilized SAES SmartFlex* NiTi (49-51at.%) actuators show a very sophisticated profile of properties.In this paper, the mechanical, thermal, and electrical response

of these shape memory wires, at diameters ranging from 20 to500 lm, under different working conditions, simulating the

actual operating condition in real actuators, will be examined indepth and discussed, in order to direct the design of the actuatorso that the functional properties of the material can becompletely exploited. The thermo-mechanical properties havebeen investigated and measured by several methods. The most

common and useful tests for these commercially available wireswill be also described.

2. Discussion

2.1 Hysteresis Test

The knowledge about the transformation and the hysteresis

related properties is of importance for the design of shapememory actuators. In this test, the wire is subjected to aconstant load and its deformation is measured during acontrolled temperature profile in an environmental cell.

In Fig. 3, the test output for Smartflex 76 under constantstress of 150 MPa is displayed. As it can be seen from the

graph, some important information can be gathered, such as themaximum stroke (difference in wire length between the coldshape and the hot shape) and the transition temperatures

(beginning and end of the direct and reverse transformation) ata given load. Most often, the transition temperatures aredetermined using the tangent method (Ref 4). In the followingcurve, the maximum stroke of the wire is around 5%, Mf at65 C and As at 85 C.

As it is well known, the applied load is an importantparameter that can affect the test: one can run tests withdifferent loads to analyze their influence on the intrinsiccharacteristics of the wire, such as transformation temperatures

F

s

F

s

F

s

F

s

F

ss ss

F

ss

F

FSMA

FWeight

FSMAFSpring

FSMAFWeight

FSMA2

FSMA1

Fig. 2 Some possible reset mechanism for tensile shape memory wire

Fig. 1 Comparison between traditional and shape memory actuators in terms of size and power/weight ratio

*SmartFlex is a registered trademark of SAES Getters S.p.A., Italy.

692Volume 18(56) August 2009 Journal of Materials Engineering and Performance


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and stroke; otherwise a standard load, near to that of usualactuators applications, can be chosen to make a comparisonbetween different wires. In Fig. 4, the transition temperaturesmeasured by hysteresis test on Smartflex 76 wire under

different applied loads are reported.As it can be seen, the Martensite (M)-Austenite (A)

transformation temperatures increase by increasing the level ofstress. The increase is due to the fact that an applied load duringM-A transformation generally supports the transformation

itself as one can derive from a modified Clausius-Clapeyronequation (Ref5):

drc

dT

DHA!M

T0 e

Typically, in actuators, the wire deformation is kept below themaximum stroke, since the smaller the difference (maximumworking strain), the shorter the wire lifetime is; so the largestrecoverable strain is a very important parameter for a SMA wire.

Wire transformation temperatures are of course fundamental

parameters to measure, know, and control (Ref6): in the case ofwire actuated by Joule effect, reverse transition temperaturesconcern the needed actuation current and time. Also, the A-Mtransition temperatures are very useful to determine the systemspeed for recovering the cold shape, once the electrical feeding

is removed. A limitation for usage of NiTi wire is the relativelylow transformation temperature, which makes applications withenvironmental temperatures above 80 C fairly critical: if theenvironmental temperature increases above this value, the SMAwire does not fully recover the initial shape, so that a certain

strain deficit accumulates over the number of cycles. Of coursethis can be healed by just one cycle at lower temperatures, but itcertainly can lead to functional problems during application.

The main problem related with the hysteresis test is theduration (a single cycle between 15 and 150 C at a rate of

1 C/min lasts more than 4 h). Another problem is the maximumusable length: in a typical hysteresis system only samples ofabout 100-150 mm can be analyzed. For this reason, Saes

Getters S.p.A. has developed and patented (Ref 7) a n ewcharacterization method in which the quality control on the totallength of the produced wire is possible. This equipment willenable an on-line 100% product quality control to measure andguarantee NiTi wire thermo-mechanical properties.

2.2 Functional Characterization

By using the equipment shown in Fig. 5(a), it is possible tocarry out what we call one-shot cycles. The wire is hangedwith a weight clamped at the bottom end and heated up byJoule effect under a constant load. A position laser sensor is

located at the bottom side of the weight. This means that studiesof current, applied load or fixed stroke effects on actuationtimes can be assessed, in order to optimize the workingconditions of shape memory wires.

0

1

2

3

4

5

6

0 20 40 60 80 100 120 140 160

Temperature (C)

Stroke(%)

MMff== 6565CC

AAss

== 8585CC

Fig. 3 Constant load test (150 MPa) on Smartflex shape memory

wire

0

100

200

300

400

500

40 60 80 100 120 140

Temperature [C]

Stress[MPa]

As

Af

Ms

Mf

=

C

MPa

dT

d2.8

Fig. 4 Transition temperatures behavior of Smartflex shape mem-

ory wire under different loading conditions

0

0,5

1

1,5

2

2,5

3

0 0,5 1 1,5 2 2,5 3

Time (s)

Stroke(%

)

(a) (b)

120 mA

130 mA

140 mA

150 mA

160 mA

170 mA

180 mA

190 mA

200 mA

210 mA

220 mA

230 mA

Fig. 5 Experimental set-up for one-shot test (a). Actuation times and strokes of Smartflex 76 using different heating currents (b)



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For example, in Fig. 5(b) the actuation time and stroke ofSmartflex 76 have been measured as a function of the feedingcurrent, by controlling the stroke at 2% during all tests. Whatcan be observed is that for currents


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than 106 cycles, assuring constant stroke and a drift 100k

Fig. 9 Fatigue lifetime for Smartflex 76 under different stress-strain

conditions


smartflex® niti wires for shape memory actuators

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