INTRODUCTIONINTRODUCTION

The development of new materials is of The development of new materials is of central importance in every central importance in every technological advancement. technological advancement.

Our expectation of higher functionality Our expectation of higher functionality along with higher reliability from our along with higher reliability from our technology has made the use of advanced technology has made the use of advanced materials inevitable. materials inevitable.

The current trend is to replace The current trend is to replace conventional materials by what may be conventional materials by what may be called “functional materials”.called “functional materials”.

NEEDNEED With the increase in the complexity of the With the increase in the complexity of the

physical systems, there is a need to incorpo physical systems, there is a need to incorpo rate biological capabilities like self rate biological capabilities like self adaptability, self sensing, memory and adaptability, self sensing, memory and feedback into the systems. feedback into the systems.

Shape memory alloys are functional Shape memory alloys are functional materials exhibiting many unique properties. materials exhibiting many unique properties. By ex ploitation of these unique properties it By ex ploitation of these unique properties it is possible to design systems that are more is possible to design systems that are more compact, more automatic and possess compact, more automatic and possess previously unthinkable capabilities.previously unthinkable capabilities.

DEFINITIONDEFINITION

Shape Memory Alloys (SMAs) is applied to Shape Memory Alloys (SMAs) is applied to a group of metallic materials that when a group of metallic materials that when subjected to appropriate thermal subjected to appropriate thermal procedure demonstrate the ability to procedure demonstrate the ability to return to some 'previously remembered return to some 'previously remembered shape.shape.

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

This ability of 'memorising' a particular This ability of 'memorising' a particular external shape is a direct consequence of a external shape is a direct consequence of a thermodynamically reversible transformation thermodynamically reversible transformation of the alloy's crystal structure.of the alloy's crystal structure.

In general, there are two crystal structures or In general, there are two crystal structures or

phases associated with a shape memory alloy. phases associated with a shape memory alloy. The phase corresponding to higher The phase corresponding to higher temperature is called the 'austenite phase' temperature is called the 'austenite phase' and the one corresponding to lower and the one corresponding to lower temperature is called the 'martensite phase'.temperature is called the 'martensite phase'.

In addition to the temperature induced In addition to the temperature induced shape memory effect, SMAs also show shape memory effect, SMAs also show 'superelastic effect'. This means that if the 'superelastic effect'. This means that if the material is kept at constant temperature in material is kept at constant temperature in the austenite phase and mechanically the austenite phase and mechanically loaded, it shows capability of recovering loaded, it shows capability of recovering large strains. The yield strain in large strains. The yield strain in superelastic effect is nearly 30 times that superelastic effect is nearly 30 times that of normal steel.of normal steel.

MATERIALS SHOWING MATERIALS SHOWING SHAPE MEMORYSHAPE MEMORY

Most common class of shape memory Most common class of shape memory alloys is Nitinol (Ni-Ti alloys). Other alloys alloys is Nitinol (Ni-Ti alloys). Other alloys showing this effect include CuZn, NiAl, showing this effect include CuZn, NiAl, NiMn, CuZnAl, CuZnSi, CuZnGa, NiMnAl, NiMn, CuZnAl, CuZnSi, CuZnGa, NiMnAl, NiMnCr, NiMnTi, NiTiFe, MnFeSi, AuCd NiMnCr, NiMnTi, NiTiFe, MnFeSi, AuCd

HISTORYHISTORY The earliest recorded observation of the The earliest recorded observation of the

shape memory effect was by Chang and shape memory effect was by Chang and Read in 1932. They noted the reversible Read in 1932. They noted the reversible change in the crystal structure of AuCd. change in the crystal structure of AuCd.

The real breakthrough came in 1962 when The real breakthrough came in 1962 when the effect was found in equiatomic NiTi. the effect was found in equiatomic NiTi. Nickel Titanium alloys.Nickel Titanium alloys.

A generic name of this group of alloys was A generic name of this group of alloys was coined as Nitinol. Nitinol stands for Nickel Ti coined as Nitinol. Nitinol stands for Nickel Ti tanium Naval Ordinance Laboratory. In tanium Naval Ordinance Laboratory. In 1980, it was used by NASA in an Earth 1980, it was used by NASA in an Earth orbiting space station.orbiting space station.

THE SHAPE MEMORY THE SHAPE MEMORY EFFECT: MECHANISMEFFECT: MECHANISM

The martensitic transformations involve The martensitic transformations involve shearing deformation resulting in shearing deformation resulting in cooperative diffusionless atomic movement. cooperative diffusionless atomic movement. This means that the atoms in the austenite This means that the atoms in the austenite phase are not shifted independently but phase are not shifted independently but undergo shearing deformation as a single undergo shearing deformation as a single unit while maintaining relative neighborhood. unit while maintaining relative neighborhood.

A one-to-one lattice correspondence is A one-to-one lattice correspondence is maintained be tween the atoms in the parent maintained be tween the atoms in the parent phase and the transformed phase. phase and the transformed phase.

HYSTERESIS LOOPHYSTERESIS LOOP The phase transformation from martensite to

austenite 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.

HYSTERESIS LOOP

The two phases of NiTi and their transformations are depicted by the 2-dimensional 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.

SHAPE MEMORY EFFECT : SHAPE MEMORY EFFECT : CHARACTERISTICSCHARACTERISTICS

One way and two way shape memory One way and two way shape memory effecteffect

Two way One way

(a) Adding a reversible deformation for the (a) Adding a reversible deformation for the one-way effect or severe deformation with one-way effect or severe deformation with an irreversible amount for the two-way.an irreversible amount for the two-way.

(b) heating the sample (b) heating the sample

(c) and cooling it again (c) and cooling it again

(d) With the one way effect, cooling from high (d) With the one way effect, cooling from high temperatures does not cause a macroscopic temperatures does not cause a macroscopic shape change. shape change.

The two-way shape memory effect is the The two-way shape memory effect is the effect that the material remembers two effect that the material remembers two different shapes: one at low temperatures, different shapes: one at low temperatures, and one at the high temperature shape. and one at the high temperature shape.

STRESS STRAIN CURVESTRESS STRAIN CURVE

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

(curve 1).

If the stress exceeds the martensite yield strength, detwinning occurs and a large non-elastic 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 the detwinned structure to deform (curve 3 ) until the external stress begins to break the atomic bonds between the martensite layers, resulting in permanent plastic deformation

For the austenite phase however, it has a higher yield strength 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 upon unloading (curves 2 and 3)

EFFECTS OF ADDITIVES EFFECTS OF ADDITIVES AND IMPURITIESAND IMPURITIES

Fe substitution in Nitinol lowers the Fe substitution in Nitinol lowers the transformation temperatures substantially. Cu transformation temperatures substantially. Cu does not change the shape memory properties, does not change the shape memory properties, but it causes a reduction in hysteresis but it causes a reduction in hysteresis (As - (As - Ms). Ms). Also, it improves the tensile strength and Also, it improves the tensile strength and other mechanical characteristicsother mechanical characteristics . .

The introduction of carbon in Nitinol affects The introduction of carbon in Nitinol affects the the Ms Ms temperature. TiC precipitate forms and temperature. TiC precipitate forms and cause slight degradation in tensile properties cause slight degradation in tensile properties but improves fracture properties by ren dering but improves fracture properties by ren dering increase in fracture stress and strain increase in fracture stress and strain

Excess additions of Ni (upto 1%) in Nitinol Excess additions of Ni (upto 1%) in Nitinol strongly depresses the transformation strongly depresses the transformation tem perature and increase the yield tem perature and increase the yield strength in the austenite.strength in the austenite.

Oxygen, when higher than 0.61%, may Oxygen, when higher than 0.61%, may cause an intermediate phase in Nitinol. cause an intermediate phase in Nitinol.

Nitrogen implantation improves the Nitrogen implantation improves the corrosion resistance of TiNi but does not corrosion resistance of TiNi but does not affects theaffects theshape memory behaviour .shape memory behaviour .

APPLICATIONAPPLICATION The Shape memory effect is currently The Shape memory effect is currently

being implemented in: being implemented in: Coffeepots Coffeepots The space shuttle The space shuttle Thermostats Thermostats Vascular Stents Vascular Stents Hydraulic Fittings (for Airplanes) Hydraulic Fittings (for Airplanes)

Some examples of applications in which Some examples of applications in which pseudo elasticity is used are: pseudo elasticity is used are:

Eyeglass Frames Eyeglass Frames UndergarmentUndergarment Medical Tools Medical Tools Cellular Phone Antennae Cellular Phone Antennae Orthodontic Arches Orthodontic Arches

EXAMPLESEXAMPLESAerospace ApplicationsAerospace Applications

Transportation of large sophisticated Transportation of large sophisticated apparatus such as a radio antenna to space .apparatus such as a radio antenna to space .

SMA wire tendons can be used as embedded SMA wire tendons can be used as embedded actuator elements to control the shapes of actuator elements to control the shapes of parts such as elevators .parts such as elevators .

With the use of quick connect-disconnect With the use of quick connect-disconnect connectors, it is possible to have non-connectors, it is possible to have non-explosive triggering of auxiliary fuel tank and explosive triggering of auxiliary fuel tank and satellite release. satellite release.

Industrial ApplicationsIndustrial Applications

Connectors and FastenersConnectors and Fasteners Monolithic Microgripper Monolithic Microgripper robotics actuators and robotics actuators and

micromanupulators micromanupulators Actuator for flow –Control gas valveActuator for flow –Control gas valve

BIOMEDICAL BIOMEDICAL APPLICATIONSAPPLICATIONS Orthodontic Archwires: These use the Orthodontic Archwires: These use the

superelasticity property of SMAs. When superelasticity property of SMAs. When deflect ed, these superelastic archwires will deflect ed, these superelastic archwires will return gradually to their original shape return gradually to their original shape exerting a small and nearly constant force exerting a small and nearly constant force on the misaligned teeth. on the misaligned teeth.

A prime application of the free recovery A prime application of the free recovery property of SMAs is the blood clot filter [21]. property of SMAs is the blood clot filter [21]. The TiNi wire is first cooled and introduced The TiNi wire is first cooled and introduced into the vein. As it warms up to the blood into the vein. As it warms up to the blood temperature, it forms a filter inside the vein temperature, it forms a filter inside the vein and catches the passing clots.and catches the passing clots.

Super elastic glassesCoffeepot thermostat

Hip replacement

Dental wires

IMPROVING THE SPEED OF SHAPE MEMORY

ALLOY ACTUATORS BY FASTER ELECTRICAL

HEATING

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.

ADVANTAGESADVANTAGES

mechanical simplicity .

high power to weight ratio.

small size.

clean, silent, spark free operation.

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 the capacity to overheat the SMA, causing permanent damage.

KURIBAYASHI’S METHOD

Measure the temperature of the wireIf temperature is below threshold valuethen allow large heating currentelse set heating current to zero

FASTER ELECTRICAL FASTER ELECTRICAL HEATINGHEATING

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 current

2.the current requested by the control system

SELECTING THRESHOLD RESISTANCE

Rthresh, That marks the boundary between ‘safe’

resistances and ‘possibly unsafe’ resistances.

This quantity is defined to be the resistance of the hot SMA element, plus a safety margin that accounts for resistance measurement errors and strain induced resistance changes.

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.

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

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.  Shape Memory And Related Technologies.

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