13885697 shape memory alloys

8/3/2019 13885697 Shape Memory Alloys

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INTRODUCTIONINTRODUCTION

TheThe developmentdevelopment ofof newnew materialsmaterials isis ofof centralcentral

importanceimportance inin everyevery technologicaltechnological advancementadvancement..

OurOur expectationexpectation ofof higherhigher functionalityfunctionality alongalong withwith

higherhigher reliabilityreliability fromfrom ourour technologytechnology hashas mademade thethe

useuse ofof advancedadvanced materialsmaterials inevitableinevitable..

TheThe currentcurrent trendtrend isis toto replacereplace conventionalconventional

materialsmaterials byby whatwhat maymay bebe calledcalled functionalfunctional

materialsmaterials..


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NEEDNEED

WithWith thethe increaseincrease inin thethe complexitycomplexity ofof thethe physicalphysicalsystems,systems, therethere isis aa needneed toto incorpo-rateincorpo-rate biologicalbiological

capabilitiescapabilities likelike selfself adaptability,adaptability, selfself sensing,sensing, memorymemory

andand feedbackfeedback intointo thethe systemssystems..

ShapeShape memorymemory alloysalloys areare functionalfunctional materialsmaterials

exhibitingexhibiting manymany uniqueunique propertiesproperties.. ByBy ex-ploitationex-ploitation ofof

thesethese uniqueunique propertiesproperties itit isis possiblepossible toto designdesignsystemssystems thatthat areare moremore compact,compact, moremore automaticautomatic andand

possesspossess previouslypreviously unthinkableunthinkable capabilitiescapabilities..


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DEFINITIONDEFINITION

Shape Memory Alloys (SMAs) is applied to a groupShape Memory Alloys (SMAs) is applied to a group

of metallic materials that when subjected toof metallic materials that when subjected to

appropriate thermal procedure demonstrate theappropriate thermal procedure demonstrate the

ability to return to some 'previously rememberedability to return to some 'previously rememberedshape.shape.

This means that it is possible to imprint someThis means that it is possible to imprint someshape in the memory of these materials.shape in the memory of these materials.


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ThisThis abilityability ofof 'memorising''memorising' aa particularparticular externalexternal shapeshape

isis aa directdirect consequenceconsequence ofof aa thermodynamicallythermodynamicallyreversiblereversible transformationtransformation ofof thethe alloy'salloy's crystalcrystal

structurestructure..

InIn general,general, therethere areare twotwo crystalcrystal structuresstructures oror phasesphases

associatedassociated withwith aa shapeshape memorymemory alloyalloy.. TheThe phasephase

correspondingcorresponding toto higherhigher temperaturetemperature isis calledcalled thethe

'austenite'austenite phase'phase' andand thethe oneone correspondingcorresponding toto lowerlower

temperaturetemperature isis calledcalled thethe 'martensite'martensite phase'phase'..


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InIn additionaddition toto thethe temperaturetemperature inducedinduced shapeshape

memorymemory effect,effect, SMAsSMAs alsoalso showshow 'superelastic'superelastic

effect'effect'.. ThisThis meansmeans thatthat ifif thethe materialmaterial isis keptkept atat

constantconstant temperaturetemperature inin thethe austeniteaustenite phasephase andand

mechanicallymechanically loaded,loaded, itit showsshows capabilitycapability ofof

recoveringrecovering largelarge strainsstrains.. TheThe yieldyield strainstrain inin

superelasticsuperelastic effecteffect isis nearlynearly 3030 timestimes thatthat ofof

normalnormal steelsteel..


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MATERIALS SHOWING SHAPEMATERIALS SHOWING SHAPE

MEMORYMEMORY

MostMost commoncommon classclass ofof shapeshape memorymemory alloysalloys isis

NitinolNitinol (Ni(NiTiTi alloys)alloys).. OtherOther alloysalloys showingshowing thisthis effecteffect

includeinclude CuZn,CuZn, NiAl,NiAl, NiMn,NiMn, CuZnAl,CuZnAl, CuZnSi,CuZnSi, CuZnGa,CuZnGa,

NiMnAl,NiMnAl, NiMnCr,NiMnCr, NiMnTi,NiMnTi, NiTiFe,NiTiFe, MnFeSi,MnFeSi, AuCdAuCd


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HISTORYHISTORY

TheThe earliestearliest recordedrecorded observationobservation ofof thethe shapeshapememorymemory effecteffect waswas byby ChangChang andand ReadRead inin 19321932..

TheyThey notednoted thethe reversiblereversible changechange inin thethe crystalcrystal

structurestructure ofof AuCdAuCd..

TheThe realreal breakthroughbreakthrough camecame inin 19621962 whenwhen thetheeffecteffect waswas foundfound inin equiatomicequiatomic NiTiNiTi.. NickelNickel TitaniumTitanium

alloysalloys..

AA genericgeneric namename ofof thisthis groupgroup ofof alloysalloys waswas coinedcoined asas

NitinolNitinol.. NitinolNitinol standsstands forfor NickelNickel Ti-taniumTi-tanium NavalNavalOrdinanceOrdinance LaboratoryLaboratory.. InIn 19801980,, itit waswas usedused byby NASANASA

inin anan EarthEarth orbitingorbiting spacespace stationstation..


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THE SHAPE MEMORY EFFECT:THE SHAPE MEMORY EFFECT:

MECHANISMMECHANISM

TheThe martensiticmartensitic transformationstransformations involveinvolve shearingshearing

deformationdeformation resultingresulting inin cooperativecooperative diffusionlessdiffusionless

atomicatomic movementmovement.. ThisThis meansmeans thatthat thethe atomsatoms inin thethe

austeniteaustenite phasephase areare notnot shiftedshifted independentlyindependently butbut

undergoundergo shearingshearing deformationdeformation asas aa singlesingle unitunit whilewhile

maintainingmaintaining relativerelative neighborhoodneighborhood..

AA oneonetotooneone latticelattice correspondencecorrespondence isis maintainedmaintained

be-tweenbe-tween thethe atomsatoms inin thethe parentparent phasephase andand thethe

transformedtransformed phasephase..


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HYSTERESIS LOOPHYSTERESIS LOOP

The phase transformation from martensite toaustenite and back again, are described by a wide

hysteresis loop, shown in Fig.

The phase transitions are characterised by four

transformation temperatures:

(i) As, the austenite start temperature; (ii) Af, the

austenite finish temperature;

(iii) Ms, the martensite start temperature; and (iv) Mf,the martensite finish temperature.


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HYSTERESIS LOOP


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The two phases of NiTi and their transformations are

depicted by the 2dimensional matchbox model in

Figure. The stronger austenite phase, also known as the

parent phase,has a cubic atomic structure and is

represented by squares in Fig.

As the alloy cools to the martensite phase in a

process called twinning, the crystal structure

becomes rhomboidal and is represented by collapsed

matchboxes.

When heated again, it returns to its original cubic

form in the parent phase.


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SHAPE MEMORY EFFECT :SHAPE MEMORY EFFECT :

CHARACTERISTICSCHARACTERISTICS

OneOne wayway andand twotwo wayway shapeshape memorymemory effecteffect

Two wayOne way


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(a)(a) AddingAdding aa reversiblereversible deformationdeformation forfor thethe oneonewayway

effecteffect oror severesevere deformationdeformation withwith anan irreversibleirreversible

amountamount forfor thethe twotwowayway..

(b)(b) heatingheating thethe samplesample

(c)(c) andand coolingcooling itit againagain

(d)(d) WithWith thethe oneone wayway effect,effect, coolingcooling fromfrom highhightemperaturestemperatures doesdoes notnot causecause aa macroscopicmacroscopic shapeshape

changechange..

TheThe twotwowayway shapeshape memorymemory effecteffect isis thethe effecteffect thatthat

thethe materialmaterial remembersremembers twotwo differentdifferent shapesshapes:: oneone atatlowlow temperatures,temperatures, andand oneone atat thethe highhigh temperaturetemperature

shapeshape..


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STRESS STRAIN CURVESTRESS STRAIN CURVE


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When an external stress is applied to the alloy when itis fully martensitic, the alloy deforms elastically

(curve 1).

If the stress exceeds the martensite yield strength,detwinning occurs and a large nonelastic deformation

will result until the structure is fully detwinned(curve 2).

The martensite is strain recoverable up to this stage.However, further increase in stress causes thedetwinned structure to deform (curve 3 ) until theexternal stress begins to break the atomic bondsbetween the martensite layers, resulting in permanentplastic deformation


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For the austenite phase however, it has a higher yieldstrength compared to martensite. Initially, the alloy

will behave elastically (curve 1 )until the stress

exceeds its yield strength.

From that point onwards, plastic deformation will

ensue causing unrecoverable stretching uponunloading (curves 2 and 3)


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EFFECTS OF ADDITIVES ANDEFFECTS OF ADDITIVES AND

IMPURITIESIMPURITIES

FeFe substitutionsubstitution inin NitinolNitinol lowerslowers thethe transformationtransformation

temperaturestemperatures substantiallysubstantially.. CuCu doesdoes notnot changechange thethe

shapeshape memorymemory properties,properties, butbut itit causescauses aa reductionreduction inin

hysteresishysteresis (As(As -- Ms)Ms).. Also,Also, itit improvesimproves thethe tensiletensilestrengthstrength andand otherother mechanicalmechanical characteristicscharacteristics ..

TheThe introductionintroduction ofof carboncarbon inin NitinolNitinol affectsaffects thethe MsMs

temperaturetemperature.. TiCTiC precipitateprecipitate formsforms andand causecause slightslightdegradationdegradation inin tensiletensile propertiesproperties butbut improvesimproves fracturefracture

propertiesproperties byby ren-deringren-dering increaseincrease inin fracturefracture stressstress andand

strainstrain


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ExcessExcess additionsadditions ofof NiNi (upto(upto 11%%)) inin NitinolNitinol stronglystrongly

depressesdepresses thethe transformationtransformation tem-peraturetem-perature andandincreaseincrease thethe yieldyield strengthstrength inin thethe austeniteaustenite..

Oxygen,Oxygen, whenwhen higherhigher thanthan 00..6161%%,, maymay causecause anan

intermediateintermediate phasephase inin NitinolNitinol..

NitrogenNitrogen implantationimplantation improvesimproves thethe corrosioncorrosion

resistanceresistance ofof TiNiTiNi butbut doesdoes notnot affectsaffects thetheshapeshape memorymemory behaviourbehaviour ..


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APPLICATIONAPPLICATION

TheThe ShapeShape memorymemory effecteffect isis currentlycurrently beingbeing

implementedimplemented inin::

CoffeepotsCoffeepots

TheThe spacespace shuttleshuttle

ThermostatsThermostats

VascularVascular StentsStents

HydraulicHydraulic FittingsFittings (for(for Airplanes)Airplanes)


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Some examples of applications in which pseudoSome examples of applications in which pseudo

elasticity is used are:elasticity is used are: Eyeglass FramesEyeglass Frames

UndergarmentUndergarment

Medical ToolsMedical Tools Cellular Phone AntennaeCellular Phone Antennae

Orthodontic ArchesOrthodontic Arches


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EXAMPLESEXAMPLES

Aerospace ApplicationsAerospace Applications

TransportationTransportation ofof largelarge sophisticatedsophisticated apparatusapparatus suchsuch

asas aa radioradio antennaantenna toto spacespace ..

SMASMA wirewire tendonstendons cancan bebe usedused asas embeddedembedded

actuatoractuator elementselements toto controlcontrol thethe shapesshapes ofof partsparts

suchsuch asas elevatorselevators ..

WithWith thethe useuse ofof quickquick connectconnectdisconnectdisconnect connectors,connectors,itit isis possiblepossible toto havehave nonnonexplosiveexplosive triggeringtriggering ofof

auxiliaryauxiliary fuelfuel tanktank andand satellitesatellite releaserelease..


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Industrial ApplicationsIndustrial Applications

Connectors and FastenersConnectors and Fasteners

Monolithic MicrogripperMonolithic Microgripper

robotics actuators and micromanupulatorsrobotics actuators and micromanupulatorsActuator for flowActuator for flow Control gas valveControl gas valve


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BIOMEDICAL APPLICATIONSBIOMEDICAL APPLICATIONS OrthodonticOrthodontic ArchwiresArchwires:: TheseThese useuse thethe

superelasticitysuperelasticity propertyproperty ofof SMAsSMAs.. WhenWhen deflect-ed,deflect-ed,

thesethese superelasticsuperelastic archwiresarchwires willwill returnreturn graduallygradually toto

theirtheir originaloriginal shapeshape exertingexerting aa smallsmall andand nearlynearly

constantconstant forceforce onon thethe misalignedmisaligned teethteeth..

AA primeprime applicationapplication ofof thethe freefree recoveryrecovery propertyproperty ofof

SMAsSMAs isis thethe bloodblood clotclot filterfilter [[2121]].. TheThe TiNiTiNi wirewire isis

firstfirst cooledcooled andand introducedintroduced intointo thethe veinvein.. AsAs itit

warmswarms upup toto thethe bloodblood temperature,temperature, itit formsforms aa filterfilter

insideinside thethe veinvein andand catchescatches thethe passingpassing clotsclots..


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Super elastic glasses

Coffeepot thermostat

Hip replacement

Dental wires


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IMPROVING THE SPEED OF

SHAPE MEMORY ALLOY

ACTUATORS BY

FASTER ELECTRICAL HEATING


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Long Term Objective:

To obtain fast, accurate, controlled motions and

forces from SMA actuators, so that we can build

and experiment with low inertia robots.

This work takes us one step in that direction, with a

simple method for rapid heating of SMA.


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ADVANTAGESADVANTAGES

mechanical simplicity .

high power to weight ratio.

small size.

clean, silent, spark free operation.


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WHY FOCUS ON HEATING? The limiting factors on the speed of an actuator are

the heating and cooling rates of the SMA elements.

The cooling rate can be increased by various

means, including forced air cooling, oil or water

cooling, and using thinner SMA wires;

The heating rate can be increased simply by

passing a larger current through the element.

currents beyond a certain magnitude have thecapacity to overheat the SMA, causing permanent

damage.


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KURIBAYASHIS METHOD

Measure the temperature of the wire

If temperature is below threshold value

then allow large heating current

else set heating current to zero


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FASTER ELECTRICAL HEATINGFASTER ELECTRICAL HEATING

Measure the resistance of the wire

Calculate a maximum safe heating current as a

function of measured resistance

Set the heating current to the minimum of

1.the maximum safe heating current2.the current requested by the control system


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SELECTING THRESHOLD

RESISTANCE


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Rthresh,

That marks the boundary between safe resistancesand possibly unsafe resistances.

This quantity is defined to be the resistance of thehot SMA element, plus a safety margin that

accounts for resistance measurement errors and

strain induced resistance changes.


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Maximum Safe Heating Current

Given Rthresh, we can define a maximum safe

heating current, Imax(R), which is a function of the

measured resistance of the SMA element.


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CONCLUSION

Electrical resistance provides an indication of

SMA temperature that is sufficient for preventing

overheating.

Rapid heating via the proposed method yields a

substantial increase in speed, without changing

the cooling regime.

Next step: A better motion controller Movie clip: results


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REFERENCESREFERENCES

Y. H. Teh 2003. A Control System for Achieving Rapid

Controlled Motions From Shape Memory Alloy (SMA)

Actuator Wires. B.Eng. Honours Thesis, Dept. Engineering,

The Australian National University.

R. Featherstone & Y. H. Teh 2004. Improving the Speed of

Shape Memory Alloy Actuators by Faster Electrical

Heating. Int. Symp. Experimental Robotics.

Y. H. Teh & R. Featherstone 2004. A New Control System

for Fast Motion Control of SMA Actuator Wires. ShapeMemory And Related Technologies.

http://www.dynalloy.com/TechnicalData.html.


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THANK YOUTHANK YOU

13885697 shape memory alloys

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