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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