shape memory alloys full report
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SHAPE MEMORY ALLOYS
Introduction
Shape memory alloys (SMA's) are metals, which exhibit two very unique properties, pseudo-elasticity, and the shape memory effect. Arne Olander first observed these unusual properties in 1938 (Oksuta and Wayman 1998), but not until the 1960's were any serious research advances made in the field of shape memory alloys. The most effective and widely used alloys include NiTi (Nickel - Titanium), CuZnAl, and CuAlNi.
The two unique properties described above are made possible through a solid state phase change, that is a molecular rearrangement, which occurs in the shape memory alloy. Typically when one thinks of a phase change a solid to liquid or liquid to gas change is the first idea that comes to mind. A solid state phase change is similar in that a molecular rearrangement is occurring, but the molecules remain closely packed so that the substance remains a solid. In most shape memory alloys, a temperature change of only about 10°C is necessary to initiate this phase change. The two phases, which occur in shape memory alloys, are Martensite, and Austenite.
After alloying and basic processing, SMAs can be formed into a shape(eg , a coil spring) and then set to that shape by a high heat treatment. When cooled , they may be bent, stretched or deformed(with in limits) and than with subsequent moderate heating (well below the heat setting temperature), they can recover some or all of the deformation.
Shape memory alloys have found use in everything from space missions (pathfinder and many more) to floral arrangement (animated butterflies, dragon flies and fairies), from bio_medical applications, to actuators for miniature robots and cellphone antennas and even eyeglasses use SMA wires for their extreme flexibility.
General principles
Shape memory metal alloy can exist in two different temperature dependent crystal structures (phases) called martensite (lower temperature ) and austenite ( higher temperature or parent phase ). Several properties of austenite and martensite are notably different
Martensite, is the relatively soft and easily deformed phase of shape memory alloys, which exists at lower temperatures. The molecular structure in this phase is twinned which is the configuration shown in the middle of Figure 2. Upon deformation this phase takes on the second form shown in Figure 2, on the right. Austenite, the stronger phase of shape memory alloys, occurs at higher temperatures. The shape of the Austenite structure is cubic, the structure shown on the left side of Figure 2. The un-deformed Martensite phase is the same size and shape as the cubic Austenite phase on a macroscopic scale, so that no change in size or shape is visible in shape memory alloys until the Martensite is deformed.
Shape Memory Effect
The shape memory effect is observed when the temperature of a piece of shape memory alloy is cooled to below the temperature Mf. At this stage the alloy is completely composed of Martensite which can be easily deformed. After distorting the SMA the original shape can be recovered simply by heating the wire above the temperature Af. The heat transferred to the wire is the power driving the molecular rearrangement of the alloy, similar to heat melting ice into water, but the alloy remains solid. The deformed Martensite is now transformed to the cubic Austenite phase, which is configured in the original shape of the wire.
The Shape memory effect is currently being implemented in:
¢ Coffepots
¢ The space shuttle
¢ Thermostats
¢ Vascular Stents
¢ Hydraulic Fittings (for Airplanes)
Pseudo-elasticity
Pseudo-elasticity occurs in shape memory alloys when the alloy is completely composed of Austenite (temperature is greater than Af). Unlike the shape memory effect, pseudo-elasticity occurs without a change in temperature. The load on the shape memory alloy is increased until the Austenite becomes transformed into Martensite simply due to the loading; this process is shown in Figure 5. The loading is absorbed by the softer Martensite, but as soon as the loading is decreased the Martensite begins to transform back to Austenite since the temperature of the wire is still above Af, and the wire springs back to its original shape.
Some examples of applications in which pseudo-elasticity is used are:
¢ Eyeglass Frames
¢ Medical Tools
¢ Cellular Phone Antennae
Alloy Types
Since the discovery of Ni-Ti, at least fifteen different binary, ternary and quaternary alloy types have been discovered that exhibit shape changes and unusual elastic properties consequent to deformation. Some of these alloy types and variants are shown in table 1.
Table 1. Shape memory alloy types.
¢ Titanium-palladium-nickel
¢ Nickel-titanium-copper
¢ Gold-cadmium
¢ Iron-zinc-copper-aluminium
¢ Titanium-niobium-aluminium
¢ Uranium-niobium
¢ Hafnium-titanium-nickel ¢ Iron-manganese-silicon
¢ Nickel-titanium
¢ Nickel-iron-zinc-aluminium
¢ Copper-aluminium-iron
¢ Titanium-niobium
¢ Zirconium-copper-zinc
¢ Nickel-zirconium-titanium
The original nickel-titanium alloy has some of the most useful characteristics in terms of its active temperature range, cyclic performance, recoverable strain energy and relatively simple thermal processing. Ni-Ti and other alloys have two generic properties thermally induced shape recovery and super- or pseudo-elasticity. The latter means that an SMA in its elastic form can undergo a deformation approximately ten times greater than that of a spring-steel equivalent, and full elastic recovery to the original geometry may be expected. This may be possible through several million cycles. The energy density of the alloy can be used to good effect to make high-force actuators - a modern DC brushless electric motor has a mass of 5-10 times that of a thermally activated Ni-Ti alloy, to do the same work.
The super elastic Ni-Ti alloys are stressed by simply working the alloy. These stresses can be removed, just as with many other alloys, by an annealing process. The stressed condition is termed stress-induced martensite, which is the equivalent of being cold/hot worked.
SMAs, particularly nickel-titanium, are commercially available from several sources. However, world production is small compared to other metal commodities (about 200 tonnes were produced 1998) owing to difficulties in the melt/forging production process, and so the cost of the material high US$0.30-US$1.50 (UK£0.20-£1.00) per gram for wire forms 1999 prices). Fortunately, most current applications require only small amount of the material. As world production increases (as it has done quite dramatically in the 1990s) so prices should decrease. Wires, strip, rod, bar and sheet are all readily available and alloy foams, sintering powders and sputtering targets of high purity are also produced.
PROGRAMMING
The use of the one way shape memory or super elastic property of NiTi for a specific application requires a piece of SMA to be molded into the desired shape . the characteristic heat treatment is then done to set the specimen to its final shape . The heat treatment methods used to set shapes in both the shape memory and the super elastic forms of NiTi are similar. Adequate heat treatment parameters are needed to set the shape and properties of the item
The two way memory training procedure can be made by SME training or SIM training . In SME training the specimen is cooled below Mf and bent the desired shape . It is then heated to a temperature above Af and allowed freely to take its austenite shape . The procedure is repeated 20 “ 30 times which completes the training . The sample now assumes its programmed shape upon cooling under Mf and to another shape when heated above Af

In SIM (stress induced martensite ) training the specimen is bent just above Ms to produce the preferred variants of SIM and then cooled below Mf temperature. Upon subsequent heating above the Af temperature the specimen takes its original austenitic shape . This procedure is repeated 20-30 times
Applications
Shape Memory Alloys Find a wide variety of uses in Aeronautical as well as Medical fields
Aircraft Maneuverability
Aircraft maneuverability depends heavily on the movement of flaps found at the rear or trailing edge of the wings. The efficiency and reliability of operating these flaps is of critical importance.
Most aircraft in the air today operate these flaps using extensive hydraulic systems. These hydraulic systems utilize large centralized pumps to maintain pressure, and hydraulic lines to distribute the pressure to the flap actuators. In order to maintain reliability of operation, multiple hydraulic lines must be run to each set of flaps. This complex system of pumps and lines is often relatively difficult and costly to maintain.
Many alternatives to the hydraulic systems are being explored by the aerospace industry. Among the most promising alternatives are piezoelectric fibers, electrostrictive ceramics, and shape memory alloys.
The flaps on a wing generally have the same layout shown on the left, with a large hydraulic system attached to it at the point of the actuator connection. "Smart" wings, which incorporate shape memory alloys, are typically like the wing this system is much more compact and efficient, in that the shape memory wires only require an electric current for movement.
Hinge less shape memory alloy Flap

The shape memory wire is used to manipulate a flexible wing surface. The wire on the bottom of the wing is shortened through the shape memory effect, while the top wire is stretched bending the edge downwards, the opposite occurs when the wing must be bent upwards. The shape memory effect is induced in the wires simply by heating them with an electric current, which is easily supplied through electrical wiring, eliminating the need for large hydraulic lines. By removing the hydraulic system, aircraft weight, maintenance costs, and repair time are all reduced.
Medical Applications
The variety of forms and the properties of SMAs make them extremely useful for a range of medical applications. For example, a wire that in its deformed shape has a small cross-section can be introduced into a body cavity or an artery with reduced chance of causing trauma. Once in place and after it is released from a constraining catheter the device is triggered by heat from the body and will return to its original memorised shape.
Increasing a deviceâ„¢s volume by direct contact or remote heat input has allowed the development of new techniques for keyhole or minimally invasive surgery. This includes instruments that have dynamic properties, such as miniature forceps, clamps and manipulators. SMA-based devices that can dilate, constrict, pull together, push apart and so on have enabled difficult or problematic tasks in surgery to become quite feasible
1. Stents
The property of thermally induced elastic recovery can be used to change a small volume to a larger one. An example of a device using this is a stent. A stent, either in conjunction with a dilation balloon or simply by self-expansion, can dilate or support a blocked conduit in the human body. Coronary artery disease, which is a major cause of death around the world, is caused by a plaque in-growth developing on and within an arteryâ„¢s inner wall. This reduces the cross-section of the artery and consequently reduces blood flow to the heart muscle. A stent can be introduced in a deformed shape, in other words with a smaller diameter. This is achieved by travelling through the arteries with the stent contained in a catheter. When deployed, the stent expands to the appropriate diameter with sufficient force to open the vessel lumen and reinstate blood flow.
2. Vena-cava Filters
Vena-cava filters have a relatively long record of successful in-vivo application. The filters are constructed from Ni-Ti wires and are used in one of the outer heart chambers to trap blood clots, which might be the cause of a fatality if allowed to travel freely around the blood circulation system. The specially designed filters trap these small clots, preventing them from entering the pulmonary system and causing a pulmonary embolism. The vena-cava filter is introduced in a compact cylindrical form about 2.0-2.5mm in diameter. When released it forms an umbrella shape. The construction is designed with a wire mesh spacing sufficiently small to trap clots. This is an example of the use of superelastic properties, although there are also some thermally actuated vena cava filters on the market.
3. Dental and Orthodontic Applications
Another commercially important application is the use of superelastic and thermal shape recovery alloys for orthodontic applications. Archwires made of stainless steel have been employed as a corrective measure for misaligned teeth for many years. Owing to the limited stretch and tensile properties of these wires, considerable forces are applied to teeth, which can cause a great deal of discomfort. When the teeth succumb to the corrective forces applied, the stainless steel wire has to be re-tensioned. Visits may be needed to the orthodontist for re-tensioning every three to four weeks in the initial stages of treatment.
Superelastic wires are now used for these corrective measures. Owing to their elastic properties and extendibility, the level of discomfort can be reduced significantly as the SMA applies a continuous, gentle pressure over a longer period. Visits to the orthodontist are reduced to perhaps three or four per year.
Bone Plates
Bone plates are surgical tools, which are used to assist in the healing of broken and fractured bones. The breaks are first set and then held in place using bone plates in situations where casts cannot be applied to the injured area. Bone plates are often applied to fractures occurring to facial areas such the nose, jaw or eye sockets. Repairs like this fall into an area of medicine known as osteosynthesis.
Currently osteotemy equipment is made primarily of titanium and stainless steel. The broken bones are first surgically reset into their proper position. Then a plate is screwed onto the broken bones to hold them in place, while the bone heals back together. This method has been proven both successful and useful in treating all manner of breaks, however there are still some drawbacks. After initially placing the plate on the break or fracture the bones are compressed together and held under some slight pressure, which helps to speed up the healing process of the bone. Unfortunately, after only a couple of days the tension provided by the steel plate is lost and the break or fracture is no longer under compression, slowing the healing process.

Bone plates can also be fabricated using shape memory alloys, in particular nickel titanium. Using a bone plate made out of NiTi, which has a transformation temperature of around Af much greater than 15 °C surgeons follow the same procedure as is used with conventional bone plates. The NiTi plates are first cooled to well below their transformation temperature, then they are placed on the set break just like titanium plates. However, when the body heats the plate up to body temperature the NiTi attempts to contract applying sustained pressure on the break or fracture for far longer than stainless steel or titanium. This steady pressure assists the healing process and reduces recovery time. There are still some problems to consider before NiTi bone plates will become commonplace. Designing plates to apply the appropriate amount of pressure to breaks and fractures is the most important difficulty, which must be overcome.
Example of how even a badly fractured face can be reconstructed using bone plates
Robotic Muscles
There have been many attempts made to re-create human anatomy through mechanical means. The human body however, is so complex that it is very difficult to duplicate even simple functions. Robotics and electronics are making great strides in this field, of particular interest are limbs such hands, arms, and legs.
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Shape memory alloys mimic human muscles and tendons very well. SMA's are strong and compact so that large groups of them can be used for robotic applications, and the motion with which they contract and expand are very smooth creating a life-like movement unavailable in other systems.
Creating human motion using SMA wires is a complex task but a simple explanation is detailed here. For example to create a single direction of movement (like the middle knuckle of your fingers) the setup shown in Figure 1 could be used. The bias spring shown in the upper portion of the finger would hold the finger straight, stretching the SMA wire, then the SMA wire on the bottom portion of the finger can be heated which will cause it to shorten bending the joint downwards (as in Figure 1). The heating takes place by running an electric current through the wire; the timing and magnitude of this current can be controlled through a computer interface used to manipulate the joint.
There are still some challenges that must be overcome before robotic hands can become more commonplace. The first is generating the computer software used to control the artificial muscle systems within the robotic limbs. The second is creating large enough movements to emulate human flexibility (i.e. being able to bend the joints as far as humans can). The third problem is reproducing the speed and accuracy of human reflexes.
Advantages and Disadvantages
Some of the main advantages of shape memory alloys include:
¢ Bio-compatibility
¢ Diverse Fields of Application
¢ Good Mechanical Properties (strong, corrosion resistant)
The use of NiTi as a biomaterial has severable possible advantages.Its shape memory property and super elasticity are unique characteristics and totally new in the medical field. The possibility to make self-locking, self expanding and self- compressing thermally activated implants is fascinating. As far as special properties and good bio compatibility are concerned, it is evident that NiTi has a potential to be a clinical success in several applications in future.
There are still some difficulties with shape memory alloys that must be overcome before they can live up to their full potential. These alloys are still relatively expensive to manufacture and machine compared to other materials such as steel and aluminum. Most SMA's have poor fatigue properties; this means that while under the same loading conditions (i.e. twisting, bending, compressing) a steel component may survive for more than one hundred times more cycles than an SMA element.
Current examples of applications of shape memory alloys.
¢ Aids for disabled
¢ Aircraft flap/slat adjusters
¢ Anti-scald devices
¢ Arterial clips
¢ Automotive thermostats
¢ Braille print punch
¢ Catheter guide wires
¢ Cold start vehicle actuators
¢ Contraceptive devices
¢ Electrical circuit breakers
¢ Fibre-optic coupling
¢ Filter struts
¢ Fire dampers
¢ Fire sprinklers
¢ Gas discharge
¢ Graft stents
¢ Intraocular lens mount
¢ Kettle switches
¢ Keyhole instruments
¢ Key-hole surgery instruments ¢ Micro-actuators
¢ Mobile phone antennas
¢ Orthodontic archwires
¢ Penile implant
¢ Pipe couplings
¢ Robot actuators
¢ Rock splitting
¢ Root canal drills
¢ Satellite antenna deployment
¢ Scoliosis correction
¢ Solar actuators
¢ Spectacle frames
¢ Steam valves
¢ Stents
¢ Switch vibration damper
¢ Thermostats
¢ Underwired bras
¢ Vibration dampers
¢
Conclusion
The many uses and applications of shape memory alloys ensure a bright future for these metals. Research is currently carried out at many robotics departments and materials science departments. With the innovative ideas for applications of SMAs and the number of products on the market using SMAs continually growing, advances in the field of shape memory alloys for use in many different fields of study seem very promising. Future Applications
There are many possible applications for SMAs. Future applications are envisioned to include engines in cars and airplanes and electrical generators utilizing the mechanical energy resulting from the shape transformations. Nitinol with its shape memory property is also envisioned for use as car frames. (Kauffman and Mayo, 7) Other possible automotive applications using SMA springs include engine cooling, carburetor and engine lubrication controls.
REFERNCES
http://sun.vmi.edu/hall/afpics.htm
http://herkules.oulu.fi/sma
madsci.org
afrlhorizons.org
http://smart.tamu.edu/
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SHAPE MEMORY ALLOYS
1. INTRODUCTION
A shape memory alloy (SMA) (also known as memory metal or smart wire) is a metal that "remembers" its geometry. After a sample of SMA has been deformed from its "original" conformation, it regains its original geometry by itself during heating (one-way effect) or, at higher ambient temperatures, simply during unloading (pseudo-elasticity or superelasticity). These extraordinary properties are due to a temperature-dependent martensitic phase transformation from a low-symmetry to a highly symmetric crystallographic structure. Those crystal structures are known as martensite and austenite
The two unique properties described above are made possible through a solid state phase change, that is a molecular rearrangement, which occurs in the shape memory alloy. Typically when one thinks of a phase change a solid to liquid or liquid to gas change is the first idea that comes to mind. A solid state phase change is similar in that a molecular rearrangement is occurring, but the molecules remain closely packed so that the substance remains a solid. In most shape memory alloys, a temperature change of only about 10°C is necessary to initiate this phase change.The two phases, which occur in shape memory alloys, are Martensite, and Austenite.
Martensite, is the relatively soft and easily deformed phase of shape memory alloys, which exists at lower temperatures. The molecular structure in this phase is twinned which is the configuration shown in the middle of Figure 2. Upon deformation this phase takes on the second form shown in Figure 2, on the right. Austenite, the stronger phase of shape memory alloys, occurs at higher temperatures. The shape of the Austenite structure is cubic, the structure shown on the left side of Figure 2. The un-deformed Martensite phase is the same size and shape as the cubic Austenite phase on a macroscopic scale, so that no change in size or shape is visible in shape memory alloys until the Martensite is deformed.
The temperatures at which each of these phases begin and finish forming are represented by the following variables: Ms, Mf, As, Af. The amount of loading placed on a piece of shape memory alloy increases the values of these four variables as shown in Figure 3. The initial values of these four variables are also dramatically affected by the composition of the wire (i.e. what amounts of each element are present).
3.1 One way effect
Alloys that exhibit this effect only when heated are said to have a "one-way shape memory". This transformation sequence is illustrated below. Free recovery refers to applications in which the single function of the memory element is to cause motions or strains up to 8%
The shape memory effect is observed when the temperature of a piece of shape memory alloy is cooled to below the temperature Mf. At this stage the alloy is completely composed of Martensite which can be easily deformed. After distorting the SMA the original shape can be recovered simply by heating the wire above the temperature Af. The heat transferred to the wire is the power driving the molecular rearrangement of the alloy, similar to heat melting ice into water, but the alloy remains solid. The deformed Martensite is now transformed to the cubic Austenite phase, which is configured in the original shape of the wire.
Figure 4 : MICROSCOPIC DIAGRAM OF SHAPE MEMORY EFFECT
The transformation temperature is a function of the alloy type, composition and also of the thermomechanical treatments applied. Alloys with transformation temperatures below -100C and above 150C are known. The heating and cooling transformations do not overlap and the transformation is said to exhibit hysteresis. The magnitude of the hysteresis also varies with the alloy type and is typically in the range 10-50C. A typical transformation is illustrated below where a physical property of the alloy, e.g. electrical resistance, is monitored to follow the phase transformations.
Figure 5: TEMPERATURE Vs. ELECTRICAL RESISTANCE
Ms-Temperature at which martensite starts to form
Mf-Temperature at which conversion to martensite is complete
As-Temperature at which austenite starts to form
Af-Temperature at which conversion to austenite is complete
T -Transformation hysteresis
3.2 Two way effect
Some materials also show a memory effect on subsequent cooling and these are said to have a "two-way shape memory" but for these materials the recoverable strain is limited to about 3%.
Although two-way memory offers advantages, it is not without its drawbacks. For instance, the alloy must be "taught" the two shapes, which requires any of several different methods. One is shape-memory cycling, whereby the alloy is cooled, deformed, and then heated repeatedly until the shapes are drummed into memory. Overheating can result in a loss of memory. Also, long-term fatigue and stability characteristics are not well understood. As a result, device manufacturers may prefer one-way memory using a biasing force acting against the "remembered" shape to produce an alternate form upon cooling. These devices have demonstrated excellent response and long-term stability up to million times.
Figure 7. THE TWO WAY MEMORY EFFECT.A SPONATNEOUS SHAPE CHANGE OCCURS DURING COOLING TO A TEMPERTURE BELOW Mf (A to B). THIS SHAPE CHANGE IS RECOVERED DURING SUBSEQUENT HEATING TO A TEMPERTUARE ABOVE Af (B to C).
The Shape memory effect is currently being implemented in:
¢ Coffepots
¢ The space shuttle
¢ Thermostats
¢ Vascular Stents
¢ Hydraulic Fittings (for Airplanes)
4. Superelasticity
Figure 8 : LOAD DIAGRAM OF THE PSEUDO-ELASTIC EFFECT OCCURRING
Pseudo-elasticity occurs in shape memory alloys when the alloy is completely composed of Austenite (temperature is greater than Af). Unlike the shape memory effect, pseudo-elasticity occurs without a change in temperature. The load on the shape memory alloy is increased until the Austenite becomes transformed into Martensite simply due to the loading; this process is shown in Figure . The loading is absorbed by the softer Martensite, but as soon as the loading is decreased the Martensite begins to transform back to Austenite since the temperature of the wire is still above Af, and the wire springs back to its original shape.
Superelastic or pseudoelastic applications are isothermal in nature and involve the storage of potential energy.
Some examples of applications in which pseudo-elasticity is used are:
¢ Eyeglass Frames
¢ Cellular Phone Antennae
5. OTHER FUNCTIONAL PROPERTIES
5.1 Constrained recovery includes applications in which the memory element is prevented from changing shape and thereby generates stresses up to 800 MPa (see fig.below).
Figure10. THE GENERATION OF SHAPE RECOVERY STRESSES. THE SAMPLE IS DEFORMED (A TO B) AND UNLOADED (B TO C) AT A TEMPERTUARE BELOW Mf. RECOVERY STRESSES ARE GENERATED DURING HEATING (D TO E) STARTING FROM THE CONTACT TEMPERTUREE Tc (D), SITUAED BETWEEN As AND Af.
5.2 Actuator or work production applications are those in which there is a motion against a bias force and thus work, up to 5 J/g, is done by the shape memory element.
Figure11. THE WORK OUTPUT. THE SAMPLE IS DEFORMED AT A TEMPERATURE BELOW Mf (A TO B), FOLLOWED BY UNLOADING (B TO C) AND AGAIN LOADING WITH A WEIGHT W (C TO D).SHAPE RECOVERY OCCURS AT AN OPPOSING FORCE W DURING HEATING TO A TEMPERTUARE ABOVE Af (D TO E), SO WORK IS DONE.
5.3 High damping capacity-these alloys show in the martensitic state a strong amplitude dependent internal friction. For impact loads, the specific damping capacity can be as high as 90 %.
7.APPLICATIONS
The unusual properties mentioned above are being applied to a wide variety of applications in a number of different fields. Other potential applications of SMAs are
almost too numerous to mention. They range from changing the of shape an aerofoil to make more efficient wings with reduced drag, to using the ability of SMAs to deform to absorb energy in a collision - a feature which is the subject of work at the National Science Foundation in the US, where researchers think SMAs could help buildings to absorb the forces generated in an earthquake.
Some of the most promising applications of SMAs are.
7.1 Aircraft maneuverability
Aircraft maneuverability depends heavily on the movement of flaps found at the rear or trailing edge of the wings. The efficiency and reliability of operating these flaps is of critical importance.
Most aircraft in the air today operate these flaps using extensive hydraulic systems. These hydraulic systems utilize large centralized pumps to maintain pressure, and hydraulic lines to distribute the pressure to the flap actuators. In order to maintain reliability of operation, multiple hydraulic lines must be run to each set of flaps. This complex system of pumps and lines is often relatively difficult and costly to maintain.
Many alternatives to the hydraulic systems are being explored by the aerospace industry. Among the most promising alternatives are piezoelectric fibers, electrostrictive ceramics, and shape memory alloys.
The flaps on a wing generally have the same layout shown on the left, with a large hydraulic system like the one shown in Figure 2 attached to it at the point of the actuator connection. "Smart" wings, which incorporate shape memory alloys, are typically like the wing shown in Figure 3, this system is much more compact and efficient, in that the shape memory wires only require an electric current for movement.
Figure 14: ELECTROMECHANICAL ACTUATOR
Figure 15: HINGE LESS SHAPE MEMORY ALLOY FLAP
The shape memory wire is used to manipulate a flexible wing surface. The wire on the bottom of the wing is shortened through the shape memory effect, while the top wire is stretched bending the edge downwards, the opposite occurs when the wing must be bent upwards. The shape memory effect is induced in the wires simply by heating them with an electric current, which is easily supplied through electrical wiring, eliminating the need for large hydraulic lines. By removing the hydraulic system, aircraft weight, maintenance costs, and repair time are all reduced. The smart wing system is currently being developed cooperatively through the Defense Advanced Researched Project Agency (DARPA, a branch of the United States Department of Defense), and Boeing.
7.2 Bone plate
Bone plates are surgical tools, which are used to assist in the healing of broken and fractured bones. The breaks are first set and then held in place using bone plates in situations where casts cannot be applied to the injured area. Bone plates are often applied to fractures occurring to facial areas such the nose, jaw or eye sockets. Repairs like this fall into an area of medicine known as osteosynthesis.
Currently osteotemy equipment is made primarily of titanium and stainless steel. The broken bones are first surgically reset into their proper position. Then a plate is screwed onto the broken bones to hold them in place, while the bone heals back together. This method has been proven both successful and useful in treating all manner of breaks, however there are still some drawbacks. After initially placing the plate on the break or fracture the bones are compressed together and held under some slight pressure, which helps to speed up the healing process of the bone. Unfortunately, after only a couple of days the tension provided by the steel plate is lost and the break or fracture is no longer under compression, slowing the healing process.
Figure 17: TYPICAL OSTEOSYNTHESIS TOOLS
Bone plates can also be fabricated using shape memory alloys, in particular nickel titanium. Using a bone plate made out of NiTi, which has a transformation temperature of around Af much greater than 15 °C surgeons follow the same procedure as is used with conventional bone plates. The NiTi plates are first cooled to well below their transformation temperature, then they are placed on the set break just like titanium plates. However, when the body heats the plate up to body temperature the NiTi attempts to contract applying sustained pressure on the break or fracture for far longer than stainless steel or titanium. This steady pressure assists the healing process and reduces recovery time. There are still some problems to consider before NiTi bone plates will become commonplace. Designing plates to apply the appropriate amount of pressure to breaks and fractures is the most important difficulty, which must be overcome.
7.3 Robotic muscles
There have been many attempts made to re-create human anatomy through mechanical means. The human body however, is so complex that it is very difficult to duplicate even simple functions. Robotics and electronics are making great strides in this field, of particular interest are limbs such hands, arms, and legs.
In order to reproduce human extremities there are a number of aspects that must be considered:
¢ The gripping force required to manipulate different objects (eggs, pens, tools)
¢ The motion capabilities of each joint of the hand
¢ The ability to feel or touch objects (tactile senses)
¢ The method of controlling movement within the limb
¢ Emulating real human movement (smoothness, and speed of response).
Many different solutions have been proposed for this problem, some include using "muscles" controlled by air pressure, piezoelectric materials, or shape memory alloys.
Shape memory alloys mimic human muscles and tendons very well. SMA's are strong and compact so that large groups of them can be used for robotic applications, and the motion with which they contract and expand are very smooth creating a life-like movement unavailable in other systems.
Creating human motion using SMA wires is a complex task but a simple explanation is detailed here. For example to create a single direction of movement (like the middle knuckle of your fingers) the setup shown in Figure 1 could be used. The bias spring shown in the upper portion of the finger would hold the finger straight, stretching the SMA wire, then the SMA wire on the bottom portion of the finger can be heated which will cause it to shorten bending the joint downwards (as in Figure 1). The heating takes place by running an electric current through the wire; the timing and magnitude of this current can be controlled through a computer interface used to manipulate the joint.
There are still some challenges that must be overcome before robotic hands can become more commonplace. The first is generating the computer software used to control the artificial muscle systems within the robotic limbs. The second is creating large enough movements to emulate human flexibility (i.e. being able to bend the joints as far as humans can). The third problem is reproducing the speed and accuracy of human reflexes
The wires in such robotic hands are modeled through simple experiments. The first page link takes you to a video clip showing one of these simple experiments in action. The next page link is demonstration of how the interactive applet modeling this experiment works, while the third page link goes to the applet itself. Finally, the fourth page link is to a game involving an SMA wire in the context of the experiment you have just seen.
7.4 Microactuator
NiTi is also being used in combination with other materials. One recent application is its deposition onto thin-film silicon. The materials are being fashioned into the tiniest medical devices known”microelectromechanical systems (MEMS). Applying heat by means of a low electrical current coaxes the alloy into a preset shape, which can be used to drive a miniature pump, for example, or to compress a syringe. Cutting off the electrical current causes cooling and transformation to the alternate shape.
"Per unit volume, nitinol is the most powerful actuator available today," says Jacques Matteau, CEO of TiNi Alloy Co. (San Leandro, CA). Actuators made from such thin films and NiTi might be used to infuse drugs, or they might be placed in strategic locations in the body to assist circulation. Such actuators might also be machined into a gripper that samples tissue for biopsy or grabs an implant, such as a coil, for retrieval. Long-range research by engineers at NiTi Alloy Co. is focused on developing a microstent, a stent that could be used in extremely small blood vessels. It, like MEMS, would be fabricated with both nitinol and thin-film silicon wafers. "This is very much a cutting-edge research effort," notes Matteau.
The key to making such miniature devices successfully, Matteau adds, is finding ways to manufacture them economically in large quantities. The same is true of all nitinol-based products. Rather than developing new shape-memory alloys for medical applications, the challenge is to come up with ways to use more effectively the one that has been at hand for the better part of 40 years.
8. MANUFACTURE
There are various ways to manufacture Nitinol. Current techniques of producing nickel-titanium alloys include vacuum melting techniques such as electron-beam melting, vacuum arc melting or vacuum induction melting. "The cast ingot is press-forged and/or rotary forged prior to rod and wire rolling. Hot working to this point is done at temperatures between 700 ° C and 900 ° C" .
There is also a process of cold working of Ni-Ti alloys. The procedure is similar to titanium wire fabrication. Carbide and diamond dies are used in the process to produce wires ranging from 0.075mm to 1.25mm in diameter. Cold working of Nitinol causes "marked changes in the mechanical and physical properties of the alloy"
8.1 MANUFACTURING CHALLENGES
Biocompatibility aside, NiTi still presents some serious drawbacks. Foremost, the alloy is a manufacturing nightmare. NiTi is very sensitive to changes in composition. Ideally it is composed of half nickel and half titanium, on the basis of atomic number. Changing this balance markedly affects the material properties, with excess nickel strongly affecting the transformation temperature. "A change of one tenth of one percent in the nickel composition will move you about ten degrees centigrade from the transformation temperature," Hodgson says. "So you have to control the composition of these ingots to a couple hundredths of a percent to get the transformation temperature you want."
Moreover, common contaminants such as oxygen, carbon, and nitrogen can cause a shift in transformation temperature while degrading the mechanical properties of the alloy. As a result, a major challenge in dealing with NiTi is controlling its manufacture so as to produce the desired properties. Because of the hyperreactivity of the titanium in the alloy, all melting must be done in either a vacuum or an inert atmosphere to eliminate or at least markedly reduce the risk of contamination by oxygen or nitrogen. Manufacturers get around this problem through a variety of methods, including plasma-arm melting, electron-beam melting, and vacuum induction melting. Because contamination is less of a problem after ingots of the alloy have been made, forging, bar rolling, and extrusion can be done successfully in air. The ingots can be worked cold, but the alloy tends to harden quickly, which means annealing must be done frequently. Special tools are required to turn and mill the material, and welding, brazing, and soldering are also difficult.
Component design poses additional challenges. Because the mechanical and physical properties of SMAs change, no single set of property values can be used in a design. And even these properties, which depend on the temperature of the material either above or below the transformation point, are affected by composition. Contaminants and differences in the ratio of nickel and titanium can also vastly change the behavior of the alloy.
Companies venturing into this area of development”either alloy production or device design and manufacture”must have a thorough knowledge of shape- memory behavior and might be advised to bolster that knowledge with specialized computer programs. Nitinol Devices and Components has a group dedicated to providing finite element analysis in the design process. With computerized assistance, knowledgeable engineers, and a little common sense, the design of nitinol devices is manageable. "You don't need a super expert," Pelton says. "The main jump is you don't treat it like stainless steel, and you don't treat it like a rubber band. It's somewhere in between."
Cost presents a further barrier to development. Nitinol is one of the most expensive materials used in medical devices. Suppliers may charge about $20 per foot for tubing, which might be cut into 1-in. pieces. These pieces are then laser sliced, for example, into stents and sold for $100. The price jumps to several hundred dollars when the stent is combined with a catheter-delivery system, which then may be sold to the hospital for $1000 or more. Cost per unit may go down, however, as use of the material rises. Today, only about 5% of stents are made from NiTi. "There is a tremendous opportunity in stents and other vessel strengtheners," says Hodgson, who predicts that within five years the percentage of stents based on NiTi could jump to 25% of the market.
10.ADVANTAGES AND DISADVANTAGES
Some of the main advantages of shape memory alloys include:
¢ Bio-compatibility
¢ Diverse Fields of Application
¢ Good Mechanical Properties (strong, corrosion resistant)
There are still some difficulties with shape memory alloys that must be overcome before they can live up to their full potential. These alloys are still relatively expensive to manufacture and machine compared to other materials such as steel and aluminum. Most SMA's have poor fatigue properties; this means that while under the same loading conditions (i.e. twisting, bending, compressing) a steel component may survive for more than one hundred times more cycles than an SMA element.
11. CONCLUSION
There are many possible applications for SMAs. Future applications are envisioned to include engines in cars and airplanes and electrical generators utilizing the mechanical energy resulting from the shape transformations. Nitinol with its shape memory property is also envisioned for use as car frames.Other possible automotive applications using SMA springs include engine cooling, carburetor and engine lubrication controls, and the control of a radiator blind ("to reduce the flow of air through the radiator at start-up when the engine is cold and hence to reduce fuel usage and exhaust emissions").
Research is currently carried out at many robotics departments and materials science departments. With the innovative ideas for applications of SMAs and the number of products on the market using SMAs continually growing, advances in the field of shape memory alloys for use in many different fields of study seem very promising.
SMAs are "ideally suited for use as fasteners, seals, connectors, and clamps" in a variety of applications. Tighter connections and easier and more efficient installations result from the use of shape memory alloys.
11. REFRENCES
1. cs.ualberta.ca/~database/MEMS/sma_mems/sma.html
2. en.wikipediawiki/Shape_memory_alloy
3. edufiveseminarstopics.html
4. Rajput R.K., Material science and engineering, 7th edition, S.K. Kartaria & sons, Delhi, 2003.
5. Khanna O.P., A textbook of material science and metallurgy, 13th edition, Dhanpat rai publication (P) LTD., New Delhi, 2002
6. Higgins R.A.,Engineering metallurgy,6th edition,viva books private limited, New delhi,2003
7. Timoshenko S.P.;Goodier J.N.,Theory of elasticity,3rd edition, McGraw-hill international editions,Singapore,2004
8. George E. Dieter, Mechanical metallurgy,4th edition, McGraw-hill book company,Singapore,2002
9. SEMINAR TOPIC FROM :: edufiveseminarstopics.html
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#3
[attachment=3522]


Presented By:
VIVEKANANDA.S.HIREMATH


SHAPE MEMORY ALLOYS

The development of new materials is of central importance in every technological advancement.
Our expectation of higher functionality along with higher reliability from our technology has made the use of advanced materials inevitable.
The current trend is to replace conventional materials by what may be called functional materials.



NEED

With the increase in the complexity of the physical systems, there is a need to incorpo¬rate biological capabilities like self adaptability, self sensing, memory and feedback into the systems.
Shape memory alloys are functional materials exhibiting many unique properties. By ex-ploitation of these unique properties it is possible to design systems that are more compact, more automatic and possess previously unthinkable capabilities.


DEFINITION

Shape Memory Alloys (SMAs) is applied to a group of metallic materials that when subjected to appropriate thermal procedure demonstrate the ability to return to some 'previously remembered shape.
This means that it is possible to imprint some shape in the memory of these materials.
This ability of 'memorising' a particular external shape is a direct consequence of a thermodynamically reversible transformation of the alloy's crystal structure.

In general, there are two crystal structures or phases associated with a shape memory alloy. The phase corresponding to higher temperature is called the 'austenite phase' and the one corresponding to lower temperature is called the 'martensite phase'.
In addition to the temperature induced shape memory effect, SMAs also show 'superelastic effect'. This means that if the material is kept at constant temperature in the austenite phase and mechanically loaded, it shows capability of recovering large strains. The yield strain in superelastic effect is nearly 30 times that of normal steel.



MATERIALS SHOWING SHAPE MEMORY

Most common class of shape memory alloys is Nitinol (Ni-Ti alloys). Other alloys showing this effect include CuZn, NiAl, NiMn, CuZnAl, CuZnSi, CuZnGa, NiMnAl, NiMnCr, NiMnTi, NiTiFe, MnFeSi, AuCd
HISTORY
The earliest recorded observation of the shape memory effect was by Chang and Read in 1932. They noted the reversible change in the crystal structure of AuCd.
The real breakthrough came in 1962 when the effect was found in equiatomic NiTi. Nickel Titanium alloys.
A generic name of this group of alloys was coined as Nitinol. Nitinol stands for Nickel Ti-tanium Naval Ordinance Laboratory. In 1980, it was used by NASA in an Earth orbiting space station.



THE SHAPE MEMORY EFFECT: MECHANISM

The martensitic transformations involve shearing deformation resulting in cooperative diffusionless atomic movement. This means that the atoms in the austenite phase are not shifted independently but undergo shearing deformation as a single unit while maintaining relative neighborhood.
A one-to-one lattice correspondence is maintained be¬tween the atoms in the parent phase and the transformed phase.



HYSTERESIS 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.
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 : CHARACTERISTICS

One way and two way shape memory effect
(a) Adding a reversible deformation for the one-way effect or severe deformation with an irreversible amount for the two-way.
(b) heating the sample
© and cooling it again
(d) With the one way effect, cooling from high temperatures does not cause a macroscopic shape change.
The two-way shape memory effect is the effect that the material remembers two different shapes: one at low temperatures, and one at the high temperature shape.


STRESS 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 AND IMPURITIES

Fe substitution in Nitinol lowers the transformation temperatures substantially. Cu does not change the shape memory properties, but it causes a reduction in hysteresis (As - Ms). Also, it improves the tensile strength and other mechanical characteristics .
The introduction of carbon in Nitinol affects the Ms temperature. TiC precipitate forms and cause slight degradation in tensile properties but improves fracture properties by ren-dering increase in fracture stress and strain
Excess additions of Ni (upto 1%) in Nitinol strongly depresses the transformation tem-perature and increase the yield strength in the austenite.
Oxygen, when higher than 0.61%, may cause an intermediate phase in Nitinol.
Nitrogen implantation improves the corrosion resistance of TiNi but does not affects the
shape memory behaviour .
Pseudo-elasticity occurs in shape memory alloys when the alloy is completely composed of Austenite (temperature is greater than AF). Unlike the shape memory effect, pseudo-elasticity occurs without a change in temperature. The load on the shape memory alloy is increased until the Austenite becomes transformed into Martensite simply due to the loading; this process is shown in Figure


APPLICATION

The Shape memory effect is currently being implemented in:
Coffeepots
The space shuttle
Thermostats
Vascular Stents
Hydraulic Fittings (for Airplanes)
Some examples of applications in which pseudo elasticity is used are:
Eyeglass Frames
Undergarment
Medical Tools
Cellular Phone Antennae
Orthodontic Arches
EXAMPLES


Aerospace Applications

Transportation of large sophisticated apparatus such as a radio antenna to space .
SMA wire tendons can be used as embedded actuator elements to control the shapes of parts such as elevators .
With the use of quick connect-disconnect connectors, it is possible to have non-explosive triggering of auxiliary fuel tank and satellite release.
Industrial Applications

Connectors and Fasteners
Monolithic Microgripper
robotics actuators and micromanupulators
Actuator for flow “Control gas valve


BIOMEDICAL APPLICATIONS

Orthodontic Archwires: These use the superelasticity property of SMAs. When deflect¬ed, these superelastic archwires will return gradually to their original shape exerting a small and nearly constant force on the misaligned teeth.
A prime application of the free recovery property of SMAs is the blood clot filter [21]. The TiNi wire is first cooled and introduced into the vein. As it warms up to the blood temperature, it forms a filter inside the vein and catches the passing clots.


ADVANTAGES

mechanical simplicity .
high power to weight ratio.
small size.
clean, silent, spark free operation.


DISADVANTAGES

There are still some difficulties with shape memory alloys that must be overcome before they can live up to their full potential. These alloys are still relatively expensive to manufacture and machine compared to other materials such as steel and aluminum. Most SMA's have poor fatigue properties; this means that while under the same loading conditions (i.e. twisting, bending, compressing) a steel component may survive for more than one hundred times more cycles than an 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



REFERENCES

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.
http://dynalloyTechnicalData.html.
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#4
SUBMITTED BY:
ROHIT KUMAR DAS

[attachment=11844]
INTRODUCTION
Shape memory alloys: As apart of Smart materials
SHAPE MEMORY ALLOYS

Shape memory alloys (SMA's) are alloys like NiTi alloy, which exhibit two very unique properties, pseudo-elasticity, and the shape memory effect. Pseudo-elasticity is the rubber like flexibility shown by these alloys and shape memory effect is ability of the alloys to regain the original shape by heating after severe deformation. Due to these unique properties these alloys are known as Shape Memory alloys.
Arne Olander first bserved these unusual properties in 1938 (Oksuta and Wayman 1998), but not until the 1960's were any serious research advances made in the field of shape memory alloys. The most effective and widely used alloys include NiTi (Nickel - Titanium), CuZnAl, and CuAlNi.
PROPERTIES OF SHAPE MEMORY ALLOYS
General Properties

A NiTi shape memory metal alloy can exist in two different temperature-dependent crystal structures (phases) called martensite (lower temperature) and austenite (higher temperature or parent phase). Several properties of austenite NiTi and martensite NiTi are notably different. When martensite NiTi is heated, it begins to change into austenite (Fig. 1). The temperature at which this phenomenon starts is called austenite start temperature (As ). The temperature at which this phenomenon is complete is called austenite finish temperature (Af ). When austenite NiTi is cooled, it begins to change into martensite. The temperature at which this phenomenon starts is called martensite start temperature (Ms ). The temperature at which martensite is again completely reverted is called martensite finish temperature (Mf ) [BUEHLER et al., 1967].
The composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTi can have three different forms: martensite, stress-induced martensite (superelastic), and austenite. When the material is in its martensite form, it is soft and ductile and can be easy deformed. Superelastic NiTi is highly elastic (rubber-like), while austenitic NiTi is quite strong and hard (similar to titanium) (Fig.2). The NiTi material has all these properties, their specific expression depending on the temperature in which it is used. In Fig. 1 Md represents the highest temperature to straininduced martensite and the grey area represents the area of optimal superelasticity.
Hysteresis
The temperature range for the martensite-to-austenite transformation, i.e. soft-to-hard transition that takes place upon heating is somewhat higher than that for the reverse transformation upon cooling (Fig.1). The difference between the transition temperatures upon heating and cooling is called hysteresis. Hysteresis is generally defined as the difference between the temperatures at which the material is in 50% transformed to austenite upon heating and in 50% transformed to martensite upon cooling. This difference can be up to 20–30 .
Thermoelastic Martensitic Transformation
The unique behavior of NiTi is based on the temperature-dependent austenite-to-martensite phase transformation on an atomic scale, which is also called thermoelastic martensitic transformation. The thermoelastic martensitic transformation causing the shape recovery is a result of the need of the crystal lattice structure to accommodate to the minimum energy state for a given temperature [OTSUKA etal., 1998].
In NiTi, the relative symmetries between the two phases lead to a highly ordered transformation, where the displacements of individual atoms can be accurately predicted and eventually lead to a shape change on a macroscopic scale. The crystal structure of martensite is relatively less symmetric compared to that of the parent phase. If a single crystal of the parent phase is cooled below Mf , then martensite variants with a total of 24 crystallographically equivalent habit planes are generally created. There is, however, only one possible parent phase (austenite) orientation, and all martensitic configurations revert to that single defined structure and shape upon heating above Af. The mechanism by which single martensite variants deform is called twinning, and it can be described as a mirror symmetry displacement of atoms across a particular atom-plane, the twinning plane.
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#5
shape memory alloys

[attachment=17430]

“Shape memory alloys (SMA's) are metals, which exhibit two very unique properties, pseudo-elasticity, and the shape memory effect. Arne Olander first observed these unusual properties in 1938 (Oksuta and Wayman 1998), but not until the 1960's were any serious research advances made in the field of shape memory alloys. The most effective and widely used alloys include NiTi (Nickel - Titanium), CuZnAl, and CuAlNi.”


Meaning What
Shape Memory Alloys are Really Just a “smart” material that returns to it’s Normal shape and size after something like heat has manipulated it.

How does this work
The two unique properties described above are made possible through a solid state phase change, that is a molecular rearrangement, which occurs in the shape memory alloy. Typically when one thinks of a phase change a solid to liquid or liquid to gas change is the first idea that comes to mind. A solid state phase change is similar in that a molecular rearrangement is occurring, but the molecules remain closely packed so that the substance remains a solid. In most shape memory alloys, a temperature change of only about 10°C is necessary to initiate this phase change. The two phases, which occur in shape memory alloys, are Martensite, and Austenite. Martensite, is the relatively soft and easily deformed phase of shape memory alloys, which exists at lower temperatures. The molecular structure in this phase is twinned which is the configuration shown in the middle of Figure 2. Upon deformation this phase takes on the second form shown in Figure 2, on the right. Austenite, the stronger phase of shape memory alloys, occurs at higher temperatures. The shape of the Austenite structure is cubic, the structure shown on the left side of Figure 2. The un-deformed Martensite phase is the same size and shape as the cubic Austenite phase on a macroscopic scale, so that no change in size or shape is visible in shape memory alloys until the Martensite is deformed.”

A Typical use: The Smart Wing

The smart is a new technology that uses Shape memory alloys to Change the Shape of the Wing of a Plane to make it more maneuverable. This is done by simply sending a electric current throw the part of the plane to heat it to the desired temperature. This changes the shape of the wing making the Plane more maneuverable. This was previously done with a heavy Hydraulic system, thus significantly reducing the weight of the plane. This is demonstrated to the right.

Advantages:

Bio-compatibility
Diverse Fields of Application
Good Mechanical Properties (strong, corrosion resistant)

Limitations
expensive to manufacture
fatigue properties (twisting, bending, compressing)
Extreme Heat and cold
Must be a SMA
YOUR IMAGINATION




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#6
thank you for your useful informations. but i need informations about recent development of shape memory alloy in medical applications for my paper presentation, can you help me?
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#7
to get information about the topic "superelasticity of shape memory alloy" full report ppt and related topic refer the page link bellow

http://studentbank.in/report-shape-memor...e=threaded

http://studentbank.in/report-shape-memor...e=threaded

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http://studentbank.in/report-shape-memor...ort?page=2

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