16-02-2010, 09:34 PM
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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 knownâ€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 developmentâ€either alloy production or device design and manufactureâ€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
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 knownâ€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 developmentâ€either alloy production or device design and manufactureâ€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
