Tailoring the Response Time of Shape Memory Alloy Wires ...ytt110030/data/SMA.pdf · low frequency ranges (

Tailoring the Response Time of Shape Memory Alloy Wiresthrough Active Cooling and Pre-stress

YONAS TADESSE,* NICHOLAS THAYER AND SHASHANK PRIYA

Department of Mechanical Engineering, Center for Intelligent Material and Systems (CIMSS)Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA

ABSTRACT: Application of shape memory alloy (SMA) actuators is limited to low frequen-cies due to slow cooling time especially in the embedded conditions where heat transfer rate isthe controlling factor. In this study, we investigate various active cooling techniques and effectof pre-stress to improve the response time of two commercially available SMAs: Flexinol fromDynalloy Inc. and Biometal fiber from Toki Corporation. Flexinol and Biometal fiber of equallength and diameter were found to exhibit different actuation behavior under pre-stress. Timedomain force response of SMA actuators was found to be dependent upon the applied pre-stress, heating rate, and amplitude of applied electrical stimulus. Compared to Biometal fibers,time domain response of Flexinol was found to decrease significantly with increasing pre-stressindicating the difference in transformation behavior. Fluid flow and heat sinking were foundto be suitable methods for improving the response time by reducing the cooling cycle from1.6 s to 0.30�0.45 s. This is a significant improvement in the actuation capability of SMAs.

Key Words: actuator, shape memory alloy, active cooling, pre-stress, response time.

INTRODUCTION

SHAPE memory effect is generally referred to therecovery of original shape from deformed shape

with residual strain through thermal cycling. The super-elastic behavior found in shape memory alloys (SMAs)is defined as the recovery of large strain by mechanicalcycling under constant temperature (Otsuka andWayman, 1999; Yan and Nei, 2003). Using the 3D dia-gram shown in Figure 1 the correlation between thesetwo phenomenon can be explained. In a SMA wire,under normal atmospheric conditions, a pseudo-elasticdeformation can be induced by applying stress, which isrecoverable by thermal heat. The superelastic behaviorin SMAs occurs at high temperature where the strainexhibits hysteresis with load (Shaw, 2000). SMAs havevarious advantages as compared to that of conventionalactuators, namely (i) overall profile is low, (ii) high forceto weight ratio, (iii) deploy simple current drive, and (iv)operation is silent. In spite of these advantages, SMAshave limited application due to their low operationalfrequency and narrow bandwidth. The bandwidth is lim-ited due to the time required for heating and cooling ofthe actuators. The response time of SMA actuator isalso dependent on preloading stress, loading conditionand amplitude of activation potential. Table 1 comparesSMA actuator with other low profile actuator

technologies in terms of important performance indices.We have demonstrated working prototypes of robotichead utilizing various actuation technologies for bothactuation and sensing (Tadesse et al., 2006) that includesservo motor-based actuation with piezoelectric sensing(Tadesse and Priya, 2008) and SMA actuation. In addi-tion to humanoid faces, our interest in SMAs is fordeveloping biomimetic unmanned undersea vehicles.Both of these applications require dynamic response inlow frequency ranges (<10Hz) with high force and lowpower consumption. Further, it is important to havehigh degree of linearity over large number of cycles.Comparing the actuation technologies listed inTable 1, SMAs have high force and strain in low fre-quency regime but low efficiency or high power con-sumption. Further, the time domain response of SMAsis slow. In order to overcome these problems we inves-tigated the effect of pre-stress on the time domainresponse of SMAs and identified the role of pre-stresscondition in enhancing the response time.

SMA actuators based on nickel and titanium (NiTi)are commercially available in wire forms. Two widelyused SMA wires known as Flexinol (Dynalloy Inc.,USA) and Biometal fiber (Toki Corporation, Japan)have been demonstrated in variety of applications.Biometal fiber is being experimented for applicationsin robotics, micro-gripper, artificial arm, and wingmorphing. Implantable artificial myocardium hasbeen proposed using Biometal fiber to mimic the natu-ral ventricular contraction (Shiraishi et al., 2005).

*Author to whom correspondence should be addressed. E-mail: [email protected] 1�17 appears in color online: http://jim.sagepub.com

JOURNAL OF INTELLIGENT MATERIAL SYSTEMS AND STRUCTURES, Vol. 0— 2009 1

1045-389X/09/00 0001�22 $10.00/0 DOI: 10.1177/1045389X09352814� The Author(s), 2009. Reprints and permissions:http://www.sagepub.co.uk/journalsPermissions.nav

Journal of Intelligent Material Systems and Structures OnlineFirst, published on November 5, 2009 as doi:10.1177/1045389X09352814

Biometal fiber has also been demonstrated as a variabledamping isolator to enhance precision of optical equip-ments on-board satellites (Oh et al., 2005) and Jellyfishunmanned underwater vehicle (Villanueva et al., 2009).Flexinol has been demonstrated in various applicationsincluding robot-eye system (Choi et al., 2008), prosthetichands (Loh et al., 2005a,b), and robotic manipulator(Ashrafiuon et al., 2006). An anti-glare rear-viewmirror based on SMA actuation has been demonstratedby Luchetti et al. (2009) for automobile applicationwhere silicone elastomer coating was used to reducethe cooling time as well as thermal stress in SMA.Thus, both of these commercial SMAs are importantactuator systems with potential in practical applications.Prior studies on SMA motors or other applications

indicate that SMA actuation has challenges in terms ofachievable velocity (Pons et al., 1997). In order to over-come the problem of response time, Reynaerts and VanBrussel (1991) proposed the use of SMA coupled withfluid cooling system to realize torsional actuator.Bergamasco et al. (1989) have proposed SMA springscoupled with fluid cooling system for similar purposeand obtained a cutoff frequency of 2.0Hz using a

0.5mm diameter coiled wire. Russell and Gorbert(1995) proposed the use of a mobile sink that uses afriction clutch mechanism to cool a rotating lever arm.They determined that temperature feedback could helpthe cooling time by preventing the wire from overheat-ing. Luo et al. (2000) implemented a series of Peltiermodules to characterize the improvement in responsetime in SMA. Their results yielded a decrease inresponse time by 10 times, but it was still too slow touse in most robotic systems. Loh et al. (2005a,b) ran aSMA wire through a small metal tube filled with siliconegrease that had a high thermal conductivityand reported a frequency response of 0.2Hz but thepower consumption was too high. Hunter et al. (1991)improved the response time of SMA by providing thewire a brief but large pulse of current. This improved therecovery time of the wire but displacement was less thana millimeter for each cycle, which may not be suitablefor some robotic systems. Howe et al. (1995) have sug-gested pneumatic cooling for controlling a tactile dis-play. They managed to reach a working frequency of6�7Hz through air cooling. All of these prior reportedmethods yielded some improvement but are not directly

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T

T T

e

e

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CM

MfMf MsMs AfAf AsAs

CA

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Figure 1. Characteristics curves for SMAs: (a) 3D characteristic behavior (T-e-r), (b) superelastic behavior, (c) shape memory effect, (d) fractionof martensite as a function of temperature, and (e) stress vs transition temperature showing the four characteristic temperature, Ms, M f, As, andAf. The characteristic temperatures may be different for Flexinol and Biometal fiber.

2 Y. TADESSE ET AL.

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Response Time of Shape Memory Alloy Wires 3

applicable towards using SMA in compact areasdemanding large strain and force with minimum powerconsumption. There are several studies that describemethods for rapid heating of SMAs (Qiu et al., 2000),but our objective is to achieve low overall response time.These requirements are imposed by the application ofSMAs in expressive humanoid face that is capable ofdisplaying an array of emotions in real-time. Currentdesign of humanoid head has an enclosed pocket of airin the skull, which becomes heated during the operation,causing the thermal response time to increase. Since thehuman face is capable of exhibiting motion up to 8Hz,it is very important to decrease the response time ofSMA by active cooling or pre-stressing to maintain theperformance.The relevant parameters for SMA actuators selected

in this study are listed in Table 2. One of the clear phys-ical difference between the two SMA actuators (Flexinoland Biometal) is their flexibility; Biometal fiber beingmore flexible than Flexinol. Other differences can benoticed in terms of maximum stress and transformationtemperature. Two important parameters that should becompared for these actuators are blocking force and freedisplacement or strain. The blocking force is maximumforce generated at zero stress whereas the free strain ismaximum strain under no load condition. The operatingpoint of the actuator is dependent on the loading expe-rienced during application. As the loading is increased,response time of actuator will decrease. Thus an impor-tant parameter missing in Table 2 is variation ofblocking force and strain with pre-stress. In roboticapplications of SMAs, this is an extremely importantdata as the load changes can easily occur due to varyingsurrounding conditions. For example � SMAs used asartificial muscles in facial robotics may be exposed tovarying stress magnitudes due to real-time interactionwith user.

CONSTITUTIVE MODEL OF SMA

The actuation properties of SMAs can be explainedby considering the thermomechanical constitutivetheory. Several approaches have been proposed in liter-ature and we mention the models relevant to our resultsin this manuscript. A 3D thermodynamic constitutivemodel has been developed by Boyd and Lagoudas(1994) based on thermodynamics. Tanaka et al. (1994)have presented phenomenological approach based onfour-phase transformation. Patoor and Berveiller(1997) and Lubliner and Auricchio (1996) have modeledthe behavior based on plasticity theory. All these modelspresent general correlation between the parameters ofSMA, namely stress, strain, and fraction of martensiteas a function of temperature. Considering a 1D actua-tion of SMA, the thermomechanical response can bepredicted by simultaneously solving the heat transferequation and Hooke’s law. Neglecting thermal expan-sion, the 1D governing equation for SMA actuator hasbeen derived by Leo (2007) as:

� � �0 ¼ Eð"� "0Þ þ�ð� � �0Þ, ð1Þ

where r and r0 represent the stress and the pre-stress inthe SMA actuator; E is the Young’s modulus; e and e0are the current and initial strain; n and n0 are the currentand initial value of fraction of martensite with rangebetween 0 and 1 (n¼ 1 for martensitic phase and n¼ 0for austenitic phase); X is the transformation coefficientof material constant which can be determined from hys-teresis SMA loop (as shown in Figure 1). The coefficientcan be obtained from the residual strain and modulus ofelasticity as: X¼Eel.

The fraction martensite equation as a function oftemperature can be obtained from kinetic law.The characteristic curve for SMA actuators is shownin Figure 1(d) showing four transition temperatures;

Table 2. Dimensions and actuation parameters of SMA wires as provided by commercial vendors.

Commercially available SMAactuators parameters

Flexinol (Dynalloy, Inc. Available online athttp://www.toki.co.jp.) Biometal fiber BMF100

(Toki Corp. Available online athttp://www.dynalloy.com.)Sample 1 Sample 2

Diameter D¼ 100mm D¼ 127mm D¼ 100mmLength L¼ 100 mm L¼ 100 mm L¼100 mmMax. force F¼150 gm F¼230 gm F¼ 70 gma

Recommended power P¼ 4.86 W/m P¼ 4.4 W/m P¼ 5.4 W/mCurrent I¼ 180 mA I¼ 250 mA I¼ 200 mAVoltage V¼ 27 V/m V¼17.6 V/m V¼ 27 V/mResistance R¼ 150 V/m R¼ 70 V/m R¼135 V/mTransformation temperature 90�C 70�CWeight/Density W¼6.45 gm/cm3 W¼6.37 gm/cm3

Strain 3�5% 4%Specific heat 6�8 Cal/mol �C 6.1�8 Cal/mol �CLife � >106 cyclesCategory Ni�Ti alloy Ni�Ti alloy

aPractical force produced.

4 Y. TADESSE ET AL.

Ms and Mf are the martensite start and finish tempera-ture, and As and Af are the austenite start and finishtemperatures. Considering the austenite�martensiteand martensite�austenite transformations as harmonicfunctions of cosines, the fraction of martensite can beexpressed as (Leo, 2007):

�M�A ¼ 0:5 cos aAðT� AsÞ½ � þ 1� �

, ð2aÞ

where aA¼ p/Af�As

�A�M ¼ 0:5 cos aMðT�Mf Þ� �

þ 1� �

, ð2bÞ

where aM¼p/Ms�Mf.Above expressions are suitable for the case of zero

applied stress. If pre-stress is applied on the SMA actua-tor, the transformation temperature changes and a mod-ified form of Equation (2) should be utilized as follows:

�M�A ¼ 0:5 cos aAðT� AsÞ �aACA

�

� �þ 1

� , ð3aÞ

�A�M ¼ 0:5 cos aMðT�Mf Þ �aMCM

�

� �þ 1

� : ð3bÞ

Figure 1(e) illustrates the relationship between the trans-formation temperature and stress. The slope of stressand transformation temperature provides the value ofCA and CM, which corresponds to the austenite andmartensite, respectively. The rate form of Equations(1) and (3) is given as:

_� ¼ _Eð"� "0Þ þ Eð _"Þ þ _�ð� � �0Þ þ�ð _�Þ, ð4Þ

_�M�A ¼ �0:5 sin aAðT� AsÞ �aACA

�

� �� aAð _TÞ �

aACA

_�

� ,

ð5aÞ

_�A�M¼�0:5 sin aMðT�Mf Þ�aMCM

�

� �� aMð _TÞ�

aMCM

_�

� :

ð5bÞ

The term _� is the force generated by SMA actuator,which is highly non-linear due to the presence of pre-stress term within the harmonic function for martensitefraction rate. Due to the coupled nature of these equa-tions, it is expected that the relationship between forcegeneration and pre-stress load are highly non-linear.

THERMAL MODEL OF SMA

Determination of a heat transfer coefficient providesinsight into the phenomena associated with differenttypes of active cooling techniques. The heat transfercoefficient of SMAwires can be obtained mathematically

by assuming a lumped capacitance model. Lumpedparameter model works on the assumption that the hotwire is suddenly submerged in an infinite medium withconstant temperature. The wire is assumed to be thinenough that it has no internal thermal gradient. Forthese assumptions to be valid, the Biot number definedby Equation (6) has to be much less than 1.

Bi ¼hLc

k� 1, ð6Þ

where Bi is the Biot number, h is the average heat trans-fer coefficient (W/m2K), Lc is the characteristic length(m), and k is the thermal conductivity of SMA (W/mK).If Equation (6) is satisfied then the transient tempera-ture can be obtained from lumped capacitance heattransfer equation (Incropera et al., 2007) and given byEquation (7) as:

_T ¼ �hA

�VcT� T1ð Þ: ð7Þ

For constant convective heat transfer the solution isgiven by Equation (8):

T ¼ T1 þ Ti � T1ð Þe��t, ð8Þ

where �¼Ash/qvc, q is the density, c is the heat capacity,v is the volume, As is the surface area, t is the time, T isthe temperature of the wire, T1 is the ambient tempera-ture, and Ti is the initial temperature of the wire. Notethat Equation (8) is a simplistic equation, which can beused to study the heat transfer coefficient for active fluidcooling.

EXPERIMENTAL

Experimental Setup for Active Cooling

In this section, we describe the method used forunderstanding of the heat dissipation from SMA wiresand implementation of suitable active cooling mechan-isms with focus on robotic systems and unmannedundersea vehicles. We investigated various suitableactive cooling techniques such as free and forced con-vection in air, fluid media and heat sink. Each of thesemethods were tested and compared with natural convec-tive cooling in ambient air. Figure 2(a) and (b) show theexperimental setup used to study the effect of activecooling techniques. The electrical power for driving theSMAs was provided from Agilent Dual output E3648A,DC power supply at the recommended operating values,180mA for Flexinol and 200mA for Biometal.Novotechnik T-Series position sensor with a resolutionof 0.002mm and stroke-length of 50mm was used to


measure the displacement. The SMA wire was mountedto a rigid surface and fixed to the position sensor at oneend. The other end of position sensor was connected toforce sensor with a bias spring to assist the SMA returnto its original position during cooling. Input current wasmeasured by inserting a known resistance in the currentloop and measuring the voltage across it. NI 9215 DAQsystem with a 4-input voltage module was used to collectthe data. A LabVIEW program was created to read thevoltages from DAQ at a frequency of 1000Hz. Thesevoltages were then converted into current, force and dis-placement using the calibration curves of transducersused in the test. The experiment was conducted on avibration isolated table to prevent any interferencefrom external sources. All the experiments were underrepeated identical conditions to ensure the consistencyof measured data.

ACTIVE COOLING EXPERIMENT PROCEDUREThis section describes the procedure for collecting and

processing the data on heat dissipation from bothFlexinol and Biometal SMA wires. Figure 3(a) showsthe setup for characterizing the response time of SMAwire with forced convection. The first low-speed forcedair convection at 0.3m/s was achieved by using a 12Vcomputer fan and confining the flow within a rect-angular channel of dimensions 40mm width� 89mmheight� 245mm length. The second high-speed forcedair convection was achieved by using a compressor.The experimental setup for high-speed forced air con-vection is shown in Figure 3(b). The SMA wire was runthrough an 8.9mm diameter carbon fiber tube. Air froma compressor was blown through the tube at 4.6m/s to

provide active cooling to the SMA wire. The carbonfiber tube covered almost the entire length of SMAwire, which was in a suspended state inside the tube.Since the velocity of air flow was quite high and thethermal conductivity of carbon is low, we can neglectthe heat transfer from SMA wire to the tube. Thus theonly heat transfer we need to consider takes placebetween the SMA wire and the flowing air. The thirdactive cooling method tested was fluid quenching usingthe experimental setup shown in Figure 3(c). Using thecarbon fiber tube, water was injected onto the wire via asyringe at velocity of 2.7m/s. The heat transfer is pro-portional to the velocity of quenching. The fourthmethod implemented was heat sink using the experimen-tal setup shown in Figure 3(d). For this experiment, analuminum tube was used as a heat sink. As soon as theapplied voltage to SMA wire was cut off the tube wasallowed to make contact with wire. Careful experimen-tation was conducted to push the aluminum tube imme-diately in contact with SMA wire while voltage wasbeing shut down. The aluminum tube was as long asthe contracted length of SMA wire (�185mm) andhad a diameter of 26mm and thickness of 2mm. Thelast active cooling method tested was using thermal geland the test setup is shown in Figure 3(e). This method issimilar to that reported by Loh et al. (2000) where theSMA wire was run through a small copper tube that wasfilled with thermal grease (Arctic Silver) which had athermal conductivity of 8.9W/mK. This provides SMAwire large surface area to dissipate heat rapidly. Allthese experiments shown in Figure 3 were first con-ducted on Flexinol wire and then the active coolingmethods that showed highest decrease in response time

Power supply

Power supply

Resistor

SMA

–

–

+

+

Force sensor

Biasedspring

Positionsensor

Labview

DAQSignal

conditioner

Figure 2. Schematic diagram of the experimental setup to test active cooling techniques.

6 Y. TADESSE ET AL.

were experimented on Biometal fiber. After completionof static tests, Flexinol and Biometal fiber were sub-jected to dynamic current pulses at frequencies of0.1 or 0.05Hz to test the effect of multiple actuationson thermal cycle.

Experimental Setup for Pre-stress Characterization

In this section, we describe the experimental setup,measurement procedure and data processing techniqueused to determine the behavior of two types of SMAactuators, Flexinol and Biometal fiber under pre-stress.The experimental setup consists of Polytec laser vibrom-eter (Model OFV3001), linear stage, biasing spring,force transducer, and National Instruments data acqui-sition system (NI 9215) with a computer interface and

LabVIEW program. Schematic diagram of experimentalsetup is shown in Figure 4(a). A SMA wire with ringterminals was attached to biasing spring that was con-nected to force sensor mounted on a linear stage todetect the force during preloading as well as SMA actua-tion. Power was supplied to SMA actuator terminalusing Agilent Dual output E3648A, power supply. Areflecting tape was attached to the moving point P(as shown in Figure 4(a)). In order to ensure accuratemeasurement the tape was supported with a verticalplate bent from the terminal of the SMA. The statevector output and input, namely voltage V, force F,and displacement D, were connected to the Nationalinstrument data acquisition module interfaced with theLabVIEW program. The time domain response of thedisplacement and force for various input voltages was

(a)+

+

+

+ +

+

–

–

–

––

–

(c)

(e)

SMA

SMAContactbetweenSMA andcylinder

Aluminumcylinder

Tubing for SMA

Direction of air flow

Air jet

SMA

SMA

SMA

Arctic5 cooling gelOuter cupper tube

Wood block

Directino of air flow

Direction of water flow

Wood block

Wood jet

Tubing for SMA

Computerfan

(b)

(d)

Figure 3. Illustration of various active cooling: (a) top view of low-speed air active cooling method, (b) high-speed forced air active coolingmethod, (c) fluid quenching, (d) heat sink cooling method, (e) thermal gel cooling method.


measured using the force transducer and laser vibrom-eter respectively. Pre-stress was varied by traslating thelinear stage in discrete steps and a step DC voltage wasapplied for 10 s duration across the SMA wire. Theexperimental data was measured by varying the pre-stress amplitude. The force trasducer was calibratedbeforehand and a force conversion factor (Fs¼ 972.06Vsenþ 146.78) was used to convert the output voltagefrom trasduction (Vsen) to the force (Fs) being measured.The displacement conversion factor for the laser vibrom-eter used was ds¼ 1.280 (mm/V). Figure 4(a) and 4(b,c)show the details of experiment and picture of the labo-ratory setup. It should be noted here that the character-ization uses voltage and current stimulus rather thantemperature.

RESULTS AND DISCUSSION

Effect of Pre-stress on Response Time

Previously, experimental investigation of deforma-tion-recovery behavior for Ni�Ti�Nb SMA has beenreported with emphasis on test temperature dependence(Kusagawa et al., 2001). Monotonic tension and defor-mation-recovery tests were conducted in temperaturerange of 253�473K. It was determined that the domi-nant mechanism of inelastic deformation was plasticabove 323K and pseudo-elastic below 323K. It wasalso shown that strain recovery strongly depends onthe magnitude of pre-strain and pre-straining tempera-ture. Larger strain recovery was obtained for larger

Laservibrometer

Powersupply

Powersupply

Fixed end ofSMA

Reflectingtape

Biasing spring

Force sensor

Forcesensor

SMA actuator

Vibrometer

PC DAQ

(c)

Translatingstage

(b)

(a) Spring

SMA wire P

D

VF

Lab VIEW

DAQ

Forcesensor

Linearstage

Signalocnditioner

Figure 4. (a) Schematic diagram of the experimental setup for characterization of the effect of pre-stress; picture of the laboratory experimentalsetup for pre-stress characterization, (b) isometric view and (c) top view.

8 Y. TADESSE ET AL.

pre-strain at lower pre-straining temperatures. Wadaand Liu (2005) have investigated thermally inducedshape recovery behavior of Ni�Ti SMAs under bothconstant pre-strain with variable constrained stress andconstant constrained stress with variable pre-strain.It was shown that the development of maximumstrain is independent of deformation procedures and ismainly determined by the magnitude of martensiticstrain generated during either pre-deformation or ther-mal cycling.A study conducted on phase transformation behavior

of pre-strained Ni�Ti SMA in reinforced metal matrixindicated that the recovery stress is dependent upon theheating cycle. A higher recovery stress was obtainedin the first heating cycle than the subsequent ones.The mechanism for this phenomenon was identified interms of changes in the microstructure in relation to theparent phase (Li et al., 2001). Pre-straining increases thetransformation temperatures of Ni�Ti SMAs and boththeoretical and experimental studies have shown theshift in transformation temperature. A large increase

in the transformation temperature during the heatingcycle with just 0.5% pre-strain has been reported inNi�Ti SMA (Huang and Wong, 1999).

Figure 5(a) and (b) show the typical variation of statevariables, output displacement and force for specificinput voltage. The data is shown for both Flexinol andBiometal fiber actuators for a tyical magnitude of pre-load force of 130 g. It can be observed in this figure thatthe response time of force and diplacement of Flexinol isslower than the Biometal fiber. Generally at zero pre-stress, SMA actuators produce highest force with thefastest rise time which decreases as the pre-stress isincreased. This can be quantified by the change inslope of output force and displacement. Figure 6(a)shows the force and corresponding displacement ofBiometal fiber under varying pre-loading force. It canbe noticed from this figure that for wide range of pre-loading force: (i) amplitude of generated force wasnearly constant, (ii) slope change was small, and (iii)displacement range was 4�4.7mm (4% strain). Figure6(b) and (c) shows the variation of displacement andforce output for the Flexinol actuator with two differentdiameters of 100 mm (0.004 in.) and 127 mm (0.005 in.).The results in these figures show that: (i) amplitudeof output force decreases as the pre-loading increases,(ii) slope of force generation with respect to time axiswas much higher than that of Biometal fiber, and (iii)displacement amplitude decreases as the preloading wasincreased with higher slope compared to Biometal fiber.The results of Figure 6 show that under identical drivingand pre-loading conditions, Biometal fiber providesbetter response time with smaller degradation in magni-tude of displacement and force. It is to be noted that theinitial elongation of the actuator due to pre-stress wasdeducted and the displacement due to actuation is pre-sented in each displacement diagram.

Magnified views of both rising and falling edge ofdisplacement response for Biometal fiber is shown inFigure 7(a), indicating that the rise time is in range of0.8�2 s and fall time between 1 and 1.6 s for pre-stress inthe range of 0�245 gf. The rise time is defined here as thetime required for attaining a steady state output andthe fall time is the time required to cool and reach orig-inal position before application of step input voltage.Figure 7(b) and (c) demonstrate the rise and fall timeof displacement for the Flexinol actuator with two dif-ferent diameters of 100 and 127mm. The results for127 mm diameter indicate that the displacement slopeincreases significantly as pre-stress increases. The risetime in this case is in range of 4�6 s and fall timebetween 1.5 and 2 s for pre-stress between 0 and220 gf. In the case of 100 mm diameter, the rise timeranges between 3 and 10 s and the fall time between 1and 1.5 s depending upon the pre-stress.

If the rising edge takes Tr second and falling edgetakes Tf second, then the maximum working frequency

0 10 20 30012

3

V (

V)

0 10 20 30130

165

200

F (

gm)

0 10 20 300246

D (

mm

)

0 10 20 300

1

2

3

V (

v)

0 10 20 30

130

150

F (

gm)

0 10 20 30

0

1

2

3

Time (s)

D (

mm

)

(a)

(b)

Figure 5. Typical state vector (force, displacement, voltage, time)showing force and displacement at certain step input voltage and130 gf pre-stress condition: (a) 100�m diameter Biometal fiber and(b) 100�m diameter Flexinol wire.


of SMA actuator can be computed from: fmax¼ 1/(TrþTf). The frequency of operation without losingthe force generation capability would be 0.28�0.55Hzfor Biometal fiber, 0.13�0.18Hz for 127 mm Flexinolwire, and 0.08�0.25Hz for 100 mm Flexinol wire depend-ing upon the preloading stress. Further analysis of theforce response of SMA wire leads to the force generationrate as a function of pre-stress load. It was demonstratedin Figures 6 and 7 that the output force for Flexinoldecreases appreciably with pre-stress while that forBiometal remains nearly constant. The relationshipbetween pre-stress amplitude and force generation rateis illustrated in Figure 8. Flexinol force generation ratewas of the order of 1�7MPa/s for a pre-stress range of

0�160MPa whereas for Biometal fiber the force gener-ation rate was 10�80MPa/sec in preload range of0�320MPa. Figure 8 also shows the resulting non-linear relationship between response time and preloadcondition.

The response time of both Flexinol and Biometal fiberbecomes faster as the voltage is increased under a spe-cific pre-stress. To illustrate this phenomenon, for aBiometal fiber actuator under a pre-load of 60 gm, therecommended driving voltage results increase in voltagelevel did not provide any significant improvement in risetime except slight increase in the amplitude in both dis-placement and force. Figure 9(a) (i�iii) demonstrates therelationship between response time achieved by voltage

0 10 20 30

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Time (s)

For

ce (

gm)

10 15 20 25

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Time (s)

Dis

plac

emen

t (m

m)

F1

F2

F3

F4

F5

F6

F7

F8

F9

F1=0; F2=41; F3=59; F4=94; F5=131; F6=148.5; F7=166.5; F8=205; F9=245 gf

0 10 20 300

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ce (

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0.5

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plac

emen

t (m

m)

F1F2

F3F4F5

F6F7

F8F9F10F11

F12F13

F1=10; F2=26; F3=40; F4=55; F5=71; F6=87; F7=101; F8=116; F9=131; F10=145;F11=158; F12=170; F13=182 gf

(a)

(b)

Figure 6. Force and displacement characteristics of (a) 100�m diameter Biometal fiber, (b) 100�m diameter Flexinol, and (c) 127�m diameterFlexinol actuator.

10 Y. TADESSE ET AL.

compensation in terms of stress�strain diagram throughthree dimensional relationships of stress, strain, andtime for applied voltage levels of 2.86, 3.79, and 4.72V.Similar measurements were conducted for Flexinol actu-ator as shown in Figure 9(b) (i�iii); however, in thiscase the recommended voltage resulted in delay, whichcould be compensated by increasing the driving voltagelevel.Comparing the results from Figures 6�9, it can be

seen that Flexinol exhibits higher rise time with pre-stress and takes longer duration to reach steady state.On the other hand Biometal fiber shows almost constantrise time with pre-stress. At the applied pre-stress ofgreater than 175 gf, the force�time response is signifi-cantly reduced for Flexinol while Biometal fiber canexhibit constant magnitude of force generation until250 gf. The displacement for both Biometal fiber andFlexinol increases with stress and then decreases.However, in case of Flexinol the displacement decreasesby more than 57% as compared to 20% for Biometalfiber with increase in 200 gf pre-stress (total length ofSMA¼ 100mm). Further, at these higher pre-stressmagnitudes, Biometal fiber reaches steady state fasterthan Flexinol. It was found that under load of 50 gf,Flexinol requires additional driving voltage to reachthe steady state response. All the force generation and

displacement results can be explained in terms ofEquation (5), which shows that the fraction transforma-tion from martensite to austenite (and vice versa) isdependent upon the transformation temperatures,slope of the stress�temperature diagram, and superelas-tic behavior. It is evident that the material compositionsand processing conditions of these commercial alloys isdifferent leading to changes in the magnitude of aA, CA,aM, and CM. Since Equation (5b) is nonlinear, a smalldifference in the magnitude of these parameters can leadto large difference in the rate of fraction of austenite.Further, the magnitude is typically influenced by thedifference between the rate of change of temperatureand stress states along with the associated material con-stants. The magnitude of cosine function will remainbetween 0 and 1. As the pre-stress increases, the differ-ence between the rate of change of temperature andstress becomes close to zero leading to no phase trans-formation. These results show that Biometal fiber canprovide consistent force�displacement relationship inapplications where there is presence of non-zero pre-stress. Other behaviors such as cyclic actuation and fati-gue under various pre-stress conditions of Biometal fiberand Flexinol need to be further investigated. Certainrepresentative cyclic behavior will be discussed in thesection ‘Cyclic Actuation.’

0 10 20 30

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250

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ce (

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0.5

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Time (s)

Dis

plac

emen

t (m

m)

F1

F2

F3

F6

F7

F8

F9

F10

F11

F12

F13

F1=0; F2=21; F3=59.5; F6=60.5; F7=104; F8=124; F9=146; F10=167; F11=187; F12=201;F13=221.5 gf

(c)

Figure 6. Continued.


Effect of Active Cooling Techniques on Response Time

In this section, the response time of SMAs is com-pared utilizing the active cooling methods described ear-lier as well as variable initial conditions. Figure 10 showsthe comparison between various methods of active

cooling and the changes in profile of thermal responseof a 195mm long, (i) 127 mm diameter and (ii) 100 mmdiameter Flexinol wire while being cooled from maxi-mum strain state. The black line shows the response ofSMA wire with no active cooling (ambient cooling) andas is evident from the figure that it takes the longest time

9 10 11 12 13–1

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t ris

e(m

m)

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–1

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5

6

Time (s)

Dis

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emen

t fal

l (m

m)

F1

F2

F3

F4

F5

F6

F7

F8

F9

10 15 20

0

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1

1.5

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t (m

m)

18 20 22 24

0

0.5

1

1.5

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4

Time (s)

Dis

plac

emen

t (m

m)

F1

F2

F3

F4

F5

F6

F7

F8

F9

F10

F11

F12

F13

F1=10; F2=26; F3=40; F4=55; F5=71; F6=87; F7=101; F8=116; F9=131; F10=145;F11=158; F12=170; F13=182 gf

(a)

(b)

F1=0; F2=41; F3=59; F4=94; F5=131; F6=148.5; F7=166.5; F8=205; F9=245gf

Figure 7. Magnified view of the rise and fall time of (a) 100�m diameter Biometal fiber, (b) 100�m diameter Flexinol actuator, (c) 127�mdiameter Flexinol wire for step input voltage under varying pre-stress.


to reach the undeformed state (�1.6 s to reach zero per-cent strain). The low-speed forced air cooling had asmall effect on the response time, reducing it by 0.5 sas compared to ambient cooling. This may be due tothe fact that the fan needs some time itself to achievemaximum velocity. It may also be related to the fact that

air flow takes some time to reach the end of channelwhere SMA is mounted.

The data for thermal gel was neglected because theSMA wire failed to contract. This can be explained asfollowing: (i) the thermal gel is dissipating heat rapidlyduring contraction and thus the wire cannot retainenough heat to contract completely, and (ii) the SMAcould not produce enough force to overcome the viscos-ity of thermal gel. Contraction was eventually producedby increasing the magnitude of current running throughthe wire to 2A. However, this is practically not usefuland thus the active cooling using thermal gel will not bediscussed further in this manuscript. The other threemethods of active cooling, namely, high-speed forcedair, heat sink, and high-speed fluid quenching werefound to be the most effective ones. High- speed fluidquenching reduces the cooling time from 1.6 to 0.2 swhile the heat sink and forced air reduced the coolingtime to 0.4 s. The smaller wire leads to faster coolingtimes due to less energy being contained within thewire. It is expected that Biometal fiber will follow a sim-ilar trend as a function of diameter.

Figure 11(a)�(c) compares the thermal response timesfor Flexinol and Biometal fiber using active coolingbased on forced air, heat sink, and fluid quenching.

10 15 20

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0.5

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2

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4

Time (s)

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t (m

m)

20 22 24

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0.5

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1.5

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4

Time (s)

Dis

plac

emen

t (m

m)

F1

F2

F3

F6

F7

F8

F9

F10

F11

F12

F13

F1=0; F2=21; F3=59.5; F6=60.5; F7=104; F8=124; F9=146; F10=167;F11=187; F12=201; F13= 221.5 gf

(c)


1200

Fle

xino

l rat

e (M

Pa/

s)

2

4

6

8

10

12FLexinol d=100µmFlexinol d=127µmBiometal d=100µm

Pre-stress (MPa)

10

Bio

met

al r

ate

(MP

a/s)

20

30

40

50

60

70

80

3200 40 80 280240200160

Figure 8. Force generation rate for pre-stress applied on SMAactuators. Note that the left axis is for Flexinol actuator and theright axis is for the Biometal fiber.


The Biometal fiber did not reach the same level of dis-placement as Flexinol. This may be related to slight dif-ference in pretension of the wire and can be determinedusing the results presented in Figure 6. Since the trans-formation temperature of Biometal wire (70�C) is 20�Cless than that of Flexinol (as can be seen in Table 2),it shows faster response time in all of these three activecooling methods. These results are quite interesting andin conjunction with the results presented in previoussection, it can be summarized that response time ofBiometal fiber can be increased (reducing the delay) byactive cooling and over a wide range of pre-stresscondition without significantly affecting the generatedforce�displacement.

ENERGY DISSIPATION DURING HEATINGSMA wires contract due to phase change from mar-

tensite to austenite, which is dependent upon the tem-perature. Therefore, if a wire is dissipating heat while itis being heated, the phase change will not occur unless

the heating method is generating enough energy to over-come the dissipation. Since SMAs are traditionallyheated by passing current through the wire, the effectsof heat dissipation can be overcome by passing morecurrent through the wire. Figure 12 demonstrates theenhancement of strain by increasing the current withcontinuous dissipation. This figure shows the strain inFlexinol wire at different currents under low-speedforced air convection. In this case, an increase in currentby 44mA results in increase of the strain from 1% to3.3%, while the computer fan was on during the heatingcycle. Since our goal is to find the best method forincreasing the response time while keeping the electricalpower consumption small, we can conclude that anycooling during the heating cycle will negatively affectthe system performance.

MARTENSITE FRACTIONThe fraction of martensite can be determined indir-

ectly from temperature versus time behavior during

For

ce (

gm)

Dis

plac

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t (m

m)

Time (s) Time (s)

10

60

0 05075

100125

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6

1 2Strain (%)

Strain (MPa) Time (s)

Str

ain

(%)

3 4 4.7

80

10 20 30 40 50 60

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Str

ess

(MP

a)

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3.5

4

4.5

(a)-(i)

(a)-(ii) (a)-(iii)

70

80

90

100

110

15 20 25 10 15 20 25

V0=2.86V

V0=2.86V

V0=2.86V

V1=3.79V

V1=3.79V

V2=4.72V

V2=4.72V

V1=3.79V V2=4.72V

Figure 9. Response characteristics under 50 g preload force and variable amplitude of input voltage: (a) Biometal fiber actuator and (b)Flexinol. The italic number for each case represents (i) time domain force and displacement for step input, (ii) stress�strain diagram, and(iii) the three dimensional relationship between strain, stress, and time.


heating and cooling in combination with Equation (3).The required temperature versus time curve can beobtained by creating a relationship between two sets ofexperimental data corresponding to strain versus timeand strain vs. temperature.The strain versus temperature behavior of

SMA during heating and cooling cycle is shown inFigure 13. In this case, the strain data was recorded bya position sensor while the SMA was being heated in afurnace. The furnace was heated from room temperature(26�C) to 150�C and then cooled again to room tempera-ture. A standard K-Type thermocouple was used formeasuring the temperature. The temperature andstrain data were collected multiple times using thesame DAQ system as in the previous experiment to con-firm the behavior. The graphs shown in Figure 13(a)and (b) are average results from multiple repetitionsfor both Flexinol and Biometal fiber, respectively. Byfitting a polynomial curve to the cooling cycles, a rela-tionship between strain and temperature can beobtained for both types of SMAs. Equations (8a) and

(9b) are given for Biometal fiber and Flexinol, respec-tively. This equation can then be used to transform thestrain versus time data, which was measured for variousactive cooling methods separately into temperatureversus time data.

T ¼ 0:078"5 � 0:86"4 þ 4:7"3 � 15"2 þ 28"þ 36, ð9aÞ

T ¼ �0:057"4 þ 0:68"3 � 1:6"2 þ 2"þ 50: ð9bÞ

Once the temperature profile as a function of time,T(t), is known it can be inserted into Equation (3).The characteristic temperatures, Ms and Mf can befound from Figure 13 (Biometal: Ms¼ 85�C,Mf¼ 45�C and Flexinol: Ms¼ 68�C, Mf¼ 49�C) whilea commonly used average value of CM¼ 8MPa/�C forFlexinol (Nascimento and Araujo 2006; Zhou andYoon, 2006) and CM¼ 5.88MPa/�C was taken fromthe slope of stress versus transformation temperaturefor Biometal fiber. The stress was calculated from the

50

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120

Strain (%)

Strain (MPa) Time (s)

Str

ess

(MP

a)

Str

ess

(MP

a)

For

ce (

gm)

Dis

plac

emen

t (m

m)

Time (s) Time (s)

100

0 1 2 3 4

60

0

1

2

3

4

5V0=1.72V

V0=1.72V

V0=1.72VV1=2.27V

V1=2.27VV2=2.71V

V2=2.71V

V1=2.27VV2=2.71V

(b)-(i)

70

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4020 30 100 20 3025

010

2030

5075

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(b)-(ii) (b)-(iii)

12345



deformation length of biasing spring (172MPa).Figure 14(a) and (b) shows the variation of martensitefraction as a function of time calculate usingEquation (3b)As can be observed from Figure 14(a-i) and (a-ii)

that both Biometal and Flexinol exhibited fastesttransformation from austenite to martensite duringfluid quenching. Using the data in this figure, strainversus. martensite fraction plot can be constructed asshown in Figure 14(b) for both Flexinol and Biometalfiber. Figure 14(b) is characteristic curve for variousmethods of cooling investigated in this studybecause the martensite fraction is directly relatedto the percent strain generated. It can also beobserved from this figure that Biometal fiber exhibitszero martensite fraction at lower strain magnitudewhich implies that Flexinol has slightly highercontraction.

HEAT TRANSFER ANALYSISIt is interesting to compute the heat dissipation from

SMA wire as a function of time. In order to use thelumped capacitance parameter model given byEquation (6), it is important to satisfy the criterionused in its derivation. For a given characteristic length,LC¼ 2.54� 10�5 (given by wire volume over surfacearea) and thermal conductivity of the wire, K¼ 18, theBiot number (Bi) must be less than 0.1. Substitutingthese parameters in Equation (6), the inequality forheat transfer coefficient can be obtained ash< 70867W/m2K. Since this h value is much higherthan that can be achieved in the experiment, thelumped capacitance model can be assumed to be valid.Using Equation (8), the heat transfer constant (ho) wasobtained by fitting the experimental data for tempera-ture as a function of time for each active coolingmethod and the values are listed in Table 3. In this cal-culation the magnitude of density was taken to be6450 kg/m3, heat capacity was taken to be 322 J/kg K,and the ambient temperature was taken as the marten-site finish transformation temperature of 50�C. To put itin perspective, a human hand has an average heat trans-fer coefficient of 6W/m2K and a steam turbine has anaverage heat transfer coefficient of 1200W/m2K. In lit-erature (Luo et al., 2000; Nascimento et al., 2006, Leo,2007), heat transfer coefficient for thermomechanicalresponse of SMA is considered to be constant. Forvariable heat transfer coefficient which depends onenvironmental conditions and temperature of wire,a second order polynomial function can be used(Jayender et al., 2005; Moallem and Tabrizi, 2009) asgiven by Equation (10):

h ¼ h0 þ �T2, ð10Þ

where h0 is the constant value for heat transfer coeffi-cient, � is the temperature coefficient and T is the tem-perature of the SMA wire. The magnitude of h shows asignificant variation in the range of 185�2800W/m2K asshown in Table 3. Figure 15 shows the measured tem-perature�time response and the fitted temperature pro-file by using different magnitudes of constant andvariable heat transfer coefficients. As shown in eachcase, using a constant h value in Equation (8) providesa temperature profile with reasonable accuracy. Variableheat transfer coefficients leads to a better estimate of thetemperature profile mimicking the realistic condition.The coefficients of the heat transfer were also obtainedby solving Equations (7) and (10) in Simulink andMatlab and are compared with experimental values.It can be inferred from Figure 15 that the values oftemperature coefficient are negative and small inmagnitude. We can also see that variable heattransfer coefficient closely matches with experimentaldata.

0

5

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(a)

(b)

Low-speed airHigh-speed airHigh-sinkFluid quenchingFree convection

Low-speed airHigh-speed airHigh-sinkFluid quenchingFree convection

Time (s)

0.0 0.2 0.4 0.6 0.8 1.0Time (s)

Str

ain

(%)

Str

ain

(%)

Figure 10. The thermal response times of Flexinol SMA wire for fouractive cooling methods: (a) 127�m diameter, and (b) 100�mdiameter.


CYCLIC ACTUATIONThe cyclic actuation behavior of SMA wires for vari-

ous active cooling methods was investigated becausemany systems require operation over extended periodof time. The SMA wires were subjected to square wave

of 0.1Hz for forced air and heat sink testing and a squarewave of 0.05Hz for fluid quenching test. The extra timebetween the contractions for fluid quenching test wasnecessary to remove excess water from the tube beforethe next contraction cycle. The results from these tests areshown in Figure 16(a�c). This figure shows that theFlexinol wire maintains its strain characteristics overrepeated actuation cycles while the Biometal fibershows a shift in performance especially in the forced airactive cooling method. As the SMA wire is repeatedlycontracted, some energy from the previous actuation isstored in the SMA wire, which causes shifts in the strain.There is a slight decrease in the magnitude of contractionbut most of the effect was exhibited in the shifting ofcontraction. This result indicates that Flexinol wirewould be better in repeated cycle application.

The cyclic actuation lifetime of SMA under pre-stressis highly dependent on the magnitude of applied stress(Kumar et al., 2006). This limit is imposed by permanentplastic deformation resulting from fatigue. The fatiguebehavior of SMA is dependent upon the stress ampli-tude, frequency of actuation, operating environmentand mounting conditions (Tobushi et al., 2000).Experimental investigation on the fatigue behavior of

0

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6(a)

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High-speed air Fluid quenching

FlexinolBiometal

FlexinolBiometal

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ain

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ain

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Str

ain

(%)

0 1 2 3 4 5 6 5 6 7 8 9 10 11 120 1 2 43

5 6 70 1 2 43

Figure 11. Comparison of response times for Flexinol and Biometal wires with three different active cooling methods: (a) high-speed forced air,(b) fluid quenching, and (c) heat sink.

0.00.0 0.2 0.4 0.6 0.8 1.0 1.2

0.5

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0.323 A

0.279 A2.0

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Flexinol with low-speed cooling

Str

ain

(%)

Time (s)

Figure 12. Behavior of SMA wires for low-speed air coolingmethods during the heating phase with two current amplitude.


SMA actuator based on NiTiCu wires have been con-ducted in the applied stress range of 50�250MPa byLagoudas et al. (2009). It was found that the fatiguelife decreases as the stress amplitude increases. Thenumber of cycles to fail at a high stress level of

200MPa was on the order of �2000 cycles whichincreases to �30,000 cycles by reducing the stress levelto 25MPa. Bertacchini et al. (2008) have conducted fati-gue testing on ‘nickel-rich’ NiTi SMA actuator in astress range of 100�250MPa showing fatigue limit of

0.0

–0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 4.03.5 4.5 5.0 5.5

Time (s)

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0.3 0.4

High-speed air

High sink

Biometal

Flexinol

Fluid quenching

High-speed air

High sink

Fluid quenching

0.1 0.20.0

Time (s)

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Flexinol martensite fraction Biometal martensite fraction

Martensitic fraction during cooling

0.0

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0.6

0.8

Mar

tens

ite fr

actio

n

0.0

0.2

1.0

0.4

0.6

0.8 M

arte

nsite

frac

tion

Figure 14. (a) Fraction of martensite vs transition time behavior for (i) Flexinol and (ii) Biometal fiber, (b) martensite fraction vs. strain curves for0.00500 diameter Flexinol and Biometal fiber.

50 40

60

80

100

120

140

60

70

80

90

100

(a) (b)

110

120

130

0 1 2 3 4 50 1 2 3 4 5

AFAF

AS

ASMS

MS

MF MF

Tem

pera

ture

(°C

)

Tem

pera

ture

(°C

)

Strain (%) Strain (%)

Figure 13. Experimental heating and cooling curves for (a) Flexinol and (b) Biometal.


5000 to 60,000 cycles. Thus, higher nickel concentrationimproves the fatigue limit of SMA. Fatigue testing onNi50.1Ti49.9 and Ni50.7Ti49.7 demonstrated that thenumber of cycles to failure were in the range of 103

and 104 when strain was in between 4% and 5%(McNichols and Brooks, 1981). From these results, itcan be concluded that for an applied stress higher than200MPa or strain more than 4%, the number of usefulcycles are in the vicinity of �1000.

FUTURE WORK/IMPLEMENTATION

The implementation of active cooling method andpre-stressing of SMA wire in applications such as

0.5 1 1.550

55

60

65

Time (s)

Tem

pera

ture

(°C

)T

empe

ratu

re (

°C)

Tem

pera

ture

(°C

)

Tem

pera

ture

(°C

)

Free convection

h=185–0.01T 2

h=185–0.02T 2

h=185–0.03T 2

h=185

Experiment

0 0.2 0.4 0.6 0.8 1

50

55

60

65

Time (s)

Tem

pera

ture

(°C

)

Low-speed air convection

h=300–0.01T 2

h=300–0.04T 2

h=300–0.05T 2

h=300Experiment

0.2 0.4 0.650

55

60

65

Time (s)

Heat sink

h=650–0.05T 2

h=650–0.09T 2

h=650–0.1T 2

h=650

Experiment

0.05 0.1 0.15 0.2 0.25 0.350

55

60

65

Time (s)

High-speed air convection

h=800–0.01T 2

h=800–0.06T 2

h=800–0.1T 2

h=800Experiment

0.1 0.2 0.350

55

60

65

Time (s)

Quenching

h=2800–0.1T 2

h=2800–0.3T 2

h=2800–0.5T 2

h=2800

Experiment

Figure 15. Constant and variable convective heat transfer coefficients for various active cooling techniques.

Table 3. Heat transfer coefficients for various activecooling methods for Flexinol wire.

Cooling method

Constant heattransfer coefficient

hVh0 (W/m2K)

Variable heattransfer coefficient

(hVh0Q bT2)

Fluid quenching 2800 �0.3High speed conv. 800 �0.1Heat sink 650 �0.09Low speed conv. 300 �0.04Free 185 �0.02


robotics systems and unmanned undersea vehicle, toenhance its performance needs to be investigated. Asshown in Figure 17, the cooling mechanism can beimplemented in humanoid face that is capable of dis-playing emotions through pulling of SMA wire-basedartificial muscles. The human face is capable of produ-cing motions at frequencies of 6�7Hz. Therefore, theenhancement of response time of SMA wires makesthis actuator suitable in this application. The mechanismshown in Figure 17 utilizes the SP 135 FZ micro vanepump to induce flow through a channel where SMAwire resides. The SP 135 FZ micro vane pump hasdimensions of 27mm length and 15mm in diameter.As the SMA wire contracts it will depress the biasspring located within the channel. Since the current isno longer applied to the wire, the SP 135 FZ will createa vacuum and allow air to flow in the channelthus resulting in cooling of the wire. The spring isplaced to help the wire return to its normal equilib-rium position as well as to maintain the tension inwire at all times.

CONCLUSIONS

Pre-stress dependence of force and displacement wasinvestigated for two commonly utilized SMA actuators,Flexinol and Biometal. The time domain response ofactuators was measured by applying recommendedstep input voltage. The results show that Biometalfiber can be used for relatively higher operating frequen-cies. Without compromising the force generation cap-ability, highest frequency of operation was found to be0.55Hz for 100 mm diameter Biometal fibers (BMF100),0.25Hz for 127 mm diameter Flexinol, and 0.18Hz for100 mm diameter Flexinol. Compared to Biometal fiber,the strain rate (slope of displacement curve with respectto time) decreases faster for Flexinol as the preloadingincreases. Further, Biometal fiber was found to generatenearly constant amplitude of force and displacementunder varying range of pre-stress whereas Flexinol actu-ator shows decrement as the pre-stress was increased.

Time (s)

0

1

2

3

4

5

0

1

2

3

4

5

0

1

2

3

4

5

FlexinolBiometal

FlexinolBiometal

FlexinolBiometal

Fluid quenching Heat sink

Heat-speed air

(a)(b)

0 604020 80 100 120 140

Time (s)

0 604020 80 100 120 140

Time (s)

0 604020 80 100 120 140

Str

ain

(%)

Str

ain

(%)

0

1

2

3

4

5

Str

ain

(%)

Str

ain

(%)

0

1

2

3

4

5

Str

ain

(%)

(c)

Figure 16. Thermal response of Biometal fiber and Flexinol under multiple heating/cooling cycles with (a) fluid quenching, (b) heat sinking, and(c) high-speed forced air cooling.


For 100 mm diameter Flexinol wire, the force generationrate was of the order of 1�7MPa/s in pre-stress range of0�160MPa whereas for 100 mm diameter Biometal fiberthe force generation rate was 10�80MPa/s in preloadrange of 0�320MPa.In order to further enhance the response time, active

cooling of SMA wires was investigated. Active coolingtechnique based on thermal gel yielded 0% wire contrac-tion due to excessive heat dissipation during the heatingcycle. The active cooling technique based on flowing airreduced the response time by 35%. Heat sinking andforced air decreased the response time by 75% whilefluid quenching decreased the response time by 87%(from 1.6 to 0.2 s). Flexinol was found to providehigher displacement and exhibited predictable behaviorduring cyclic actuations where Biometal fiber displayeda decaying performance due to stored energy within thewire. A concept has been proposed that uses forced aircooling coupled with biased springs for application ofactive cooling mechanism in a humanoid face. We hopethat these experiments will advance the application ofSMA into existing and new structures.

ACKNOWLEDGMENT

The authors gratefully acknowledge the financial sup-port from office of Naval Research (ONR) throughgrant number N000140810654 and Institute of Criticalresearch and Applied Science (ICTAS) at Virginia Tech.

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