Types+OF+Materials+..+By+waleed+wafi

=** Shape Memory Alloys ** = = =

A **shape-memory alloy** (**SMA**, **smart metal**, **memory metal**, **memory alloy**, **muscle wire**, **smart alloy**) is an [|alloy] that "remembers" its original, cold-forged [|shape]: returning the pre-deformed shape by heating. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems. Shape-memory alloys have applications in industries including medical and aerospace.

Overview
The three main types of shape-memory alloys are the [|copper]-[|zinc]-[|aluminium]-[|nickel], copper-aluminium-nickel, and nickel-[|titanium] ([|NiTi]) alloys but SMAs can also be created by alloying zinc, copper, [|gold] and [|iron]. NiTi alloys are generally more expensive and change from [|austenite] to [|martensite] upon cooling; //Mf// is the temperature at which the transition to martensite completes upon cooling. Accordingly, during heating //As// and //Af// are the temperatures at which the transformation from martensite to austenite starts and finishes. Repeated use of the shape-memory effect may lead to a shift of the characteristic transformation temperatures (this effect is known as functional fatigue, as it is closely related with a change of microstructural and functional properties of the material).[|[][|1][|]] The transition from the martensite phase to the austenite phase is only dependent on temperature and stress, not time, as most phase changes are, as there is no diffusion involved. Similarly, the austenite structure receives its name from steel alloys of a similar structure. It is the reversible diffusionless transition between these two phases that results in special properties. While martensite can be formed from austenite by rapidly cooling carbon-[|steel], this process is not reversible, so steel does not have shape-memory properties.



In this figure, ξ(T) represents the martensite fraction. The difference between the heating transition and the cooling transition gives rise to [|hysteresis] where some of the mechanical energy is lost in the process. The shape of the curve depends on the material properties of the shape-memory alloy, such as the [|alloying].[|[][|2][|]] and [|work hardening].[|[][|3][|]]

One-way vs. two-way shape memory
_



The procedures are very similar: starting from martensite (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 (c) and cooling it again (d).

One-way memory effect
_ When a shape-memory alloy is in its cold state (below //As//), the metal can be bent or stretched and will hold those shapes until heated above the transition temperature. Upon heating, the shape changes to its original. When the metal cools again it will remain in the hot shape, until deformed again. With the one-way effect, cooling from high temperatures does not cause a macroscopic shape change. A deformation is necessary to create the low-temperature shape. On heating, transformation starts at //As// and is completed at //Af// (typically 2 to 20 °C or hotter, depending on the alloy or the loading conditions). //As// is determined by the alloy type and composition and can vary between −150 °C and 200 °C.

Two-way memory effect
_ 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. A material that shows a shape-memory effect during both heating and cooling is called two-way shape memory. This can also be obtained without the application of an external force (intrinsic two-way effect). The reason the material behaves so differently in these situations lies in training. Training implies that a shape memory can "learn" to behave in a certain way. Under normal circumstances, a shape-memory alloy "remembers" its high-temperature shape, but upon heating to recover the high-temperature shape, immediately "forgets" the low-temperature shape. However, it can be "trained" to "remember" to leave some reminders of the deformed low-temperature condition in the high-temperature phases. There are several ways of doing this.[|[][|4][|]] A shaped, trained object heated beyond a certain point will lose the two-way memory effect, this is known as "amnesia".

Pseudo-elasticity
_ One of the commercial uses of shape-memory alloy exploits the pseudo-elastic properties of the metal during the high-temperature (austenitic) phase. The frames of reading glasses have been made of shape-memory alloy as they can undergo large deformations in their high-temperature state and then instantly revert back to their original shape when the stress is removed. This is the result of [|pseudoelasticity]; the [|martensitic] phase is generated by stressing the metal in the [|austenitic] state and this martensite phase is capable of large strains. With the removal of the load, the martensite transforms back into the austenite phase and resumes its original shape. This allows the metal to be bent, twisted and pulled, before reforming its shape when released. This means the frames of shape-memory alloy glasses are claimed to be "nearly indestructible" because it appears no amount of bending results in permanent [|plastic] deformation. The martensite temperature of shape-memory alloys is dependent on a number of factors including alloy chemistry. Shape-memory alloys with transformation temperatures in the range of 60–1450 K have been made.

History
_ The first reported steps towards the discovery of the shape-memory effect were taken in the 1930s. According to Otsuka and Wayman, A. Ölander discovered the pseudoelastic behavior of the Au-Cd alloy in 1932. Greninger and Mooradian (1938) observed the formation and disappearance of a martensitic phase by decreasing and increasing the temperature of a Cu-Zn alloy. The basic phenomenon of the memory effect governed by the thermoelastic behavior of the martensite phase was widely reported a decade later by Kurdjumov and Khandros (1949) and also by Chang and Read (1951).[|[][|1][|]] The nickel-titanium alloys were first developed in 1962–1963 by the [|United States] [|Naval Ordnance Laboratory] and commercialized under the trade name [|Nitinol] (an acronym for Nickel Titanium Naval Ordnance Laboratories). Their remarkable properties were discovered by accident. A sample that was bent out of shape many times was presented at a laboratory management meeting. One of the associate technical directors, Dr. David S. Muzzey, decided to see what would happen if the sample was subjected to heat and held his pipe lighter underneath it. To everyone's amazement the sample stretched back to its original shape.[|[][|5][|]][|[][|6][|]] There is another type of SMA, called a [|ferromagnetic shape-memory alloy] (FSMA), that changes shape under strong magnetic fields. These materials are of particular interest as the magnetic response tends to be faster and more efficient than temperature-induced responses. Metal alloys are not the only thermally-responsive materials; [|shape-memory polymers] have also been developed, and became commercially available in the late 1990s.

Crystal structures
_ Many metals have several different crystal structures at the same composition, but most metals do not show this shape-memory effect. The special property that allows shape-memory alloys to revert to their original shape after heating is that their crystal transformation is fully reversible. In most crystal transformations, the atoms in the structure will travel through the metal by diffusion, changing the composition locally, even though the metal as a whole is made of the same atoms. A reversible transformation does not involve this diffusion of atoms, instead all the atoms shift at the same time to form a new structure, much in the way a parallelogram can be made out of a square by pushing on two opposing sides. At different temperatures, different structures are preferred and when the structure is cooled through the transition temperature, the martensitic structure forms from the austenitic phase.

Manufacture
_ Shape-memory alloys are typically made by casting, using vacuum arc melting or induction melting. These are specialist techniques used to keep impurities in the alloy to a minimum and ensure the metals are well mixed. The [|ingot] is then [|hot rolled] into longer sections and then [|drawn] to turn it into wire. The way in which the alloys are "trained" depends on the properties wanted. The "training" dictates the shape that the alloy will remember when it is heated. This occurs by heating the alloy so that the [|dislocations] re-order into stable positions, but not so hot that the material [|recrystallizes]. They are heated to between 400 °C and 500 °C for 30 minutes. Typical variables for some alloys are 500 °C and for more than 5 minutes. They are then shaped while hot and are cooled rapidly by quenching in water or by cooling with air.

Properties
__ The copper-based and NiTi-based shape-memory alloys are considered to be engineering materials. These compositions can be manufactured to almost any shape and size. The yield strength of shape-memory alloys is lower than that of conventional steel, but some compositions have a higher yield strength than plastic or aluminum. The yield stress for Ni Ti can reach 500 [|MPa]. The high cost of the metal itself and the processing requirements make it difficult and expensive to implement SMAs into a design. As a result, these materials are used in applications where the super elastic properties or the shape-memory effect can be exploited. The most common application is in actuation. One of the advantages to using shape-memory alloys is the high level of recoverable plastic strain that can be induced. The maximum recoverable strain these materials can hold without permanent damage is up to 8% for some alloys. This compares with a maximum strain 0.5% for conventional steels.

ACTUATOR DESIGN
__

The shape memory effect in Ni· Ti alloys is not limited 10 the linear contraction of wires. Even larger shape changes can be achieved in the bending or IOrsional deformation mode. Accordingly, there are many possibilities regarding the shape of the actuator. Preferred configurations are : The design of shape memory elements for thermal actuators is based on the different stress/strain curves of the austenite and the martensite. As an example. Figure 7 shows the force/deflection curves of a helical compression spring at high and low temperatures. The high temperature shape of the spring with no load is 1.0 (A). If the spring is loaded with a conSlant load W in the austenitic condi tion (at temperatures above Af) (he spring is compressed along A - B with the dispiacement.1l (B). Upon cooling below Mf lhe spring transfonns into martensite. Now the load W compresses the spring 10 point C on the martensite curve with the d isplacement dL. Repeated healing/cooling cycles between points Band C. If, instead of a constant load, a steel biasing spring is used, the force/deflection curve for th is spring has to be superimposed to the austenitic and manensitic spring characteristics of the Ni-Ti spring. Under optimum conditions and no load the shape memory strain can be as high as 8%. However, for cyclic applications the usable strain is much less. The same applies for the stresS; for a one-time actuation the austenitic yield strength may be used as maximum stress. Much lower values have to be expected for cyclic applications. The following numbers may be used as guidelines:
 * straight tensile wires (high force, small motion)
 * helical compression springs (large motion, less force)
 * helical extension spri ngs (large motion. less force)
 * cantilever springs (bending)
 * "BellevilIe"-type disc springs (high force. small motion)

Number of Cycles Max. Strain Max Stress 100 4% 275 MPa/43 Ksi 10000 2% 140 MPa/20 Ksi 100000 1% 70 MPa/10 ksi

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= ( تخضهـــــا تخـــــاف ،، تخوفهـــــــا تتخض )=
 * Piezoelectric materials **

**Introduction: the piezoelectric effect** The piezoelectric effect describes the relation between a mechanical stress and an electrical voltage in solids. It is reversbile: an applied mechanical stress will generate a voltage and an applied voltage will change the shape of the solid by a small amount (up to a 4% change in volume). In physics, the piezoelectric effect can be described as the the link between electrostatics and mechanics.



= **History** =

The piezoelectric effect was discovered in 1880 by the Jacques and Pierre Curie brothers. They found out that when a mechanical stress was applied on crystals such as tourmaline, tourmaline, topaz, quartz, Rochelle salt and cane sugar, electrical charges appeared, and this voltage was proportional to the stress. First applications were piezoelectric ultrasonic transducers and soon swinging quartz for standards of frequency (quartz clocks). An everyday life application example is your car's airbag sensor. The material detects the intensity of the shock and sends an electricla signal which triggers the airbag.

=**Piezoelectric materials**=

The piezoelectric effect occurs only in non conductive materials. Piezoelectric materials can be divided in 2 main groups: crystals and cermaics. The most well-known piezoelectric material is quartz (SiO2).


 * [[image:http://designinsite.dk/gifs/m1306.gif align="center" link="http://designinsite.dk/htmsider/mb1306.htm"]] || They produce an electric field when exposed to a change in dimension caused by an imposed mechanical force (piezoelectric or generator effect). Conversely, an applied electric field will produce a mechanical stress (electrostrictive or motor effect).
 * [[image:http://designinsite.dk/gifs/m1306.gif align="center" link="http://designinsite.dk/htmsider/mb1306.htm"]] || They produce an electric field when exposed to a change in dimension caused by an imposed mechanical force (piezoelectric or generator effect). Conversely, an applied electric field will produce a mechanical stress (electrostrictive or motor effect).

They transform energy from mechanical to electrical and vice-versa. The stress is very small, 0.1-0.3%. They are used for sensing purposes (e.g. microphone, transducer), and for actuating applications.

Similar to piezoelectric materials are electrostrictive and magnetostrictive materials used in high prescision actuation. They are ferromagnetic materials which experience an elastic strain when subjected to an electric or magnetic field respectively. ||

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=** Magneto-rheological fluids materials ** =

A **magnetorheological fluid** (MR fluid) is a type of [|smart fluid] in a carrier fluid, usually a type of oil. When subjected to a [|magnetic field], the fluid greatly increases its [|apparent viscosity], to the point of becoming a [|viscoelastic] solid. Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity. The upshot of this is that the fluid's ability to transmit force can be controlled with an [|electromagnet], which gives rise to its many possible control-based applications. MR fluid is different from a [|ferrofluid] which has smaller particles. MR fluid particles are primarily on the [|micrometre]-scale and are too [|dense] for [|Brownian Motion] to keep them suspended (in the lower density carrier fluid). [|Ferrofluid] particles are primarily [|nanoparticles] that are suspended by [|Brownian Motion] and generally will not settle under normal conditions. As a result, these two fluids have very different applications.

The magnetic particles, which are typically [|micrometer] or [|nanometer] scale spheres or ellipsoids, are suspended within the carrier oil are distributed randomly and in suspension under normal circumstances, as below.



When a magnetic field is applied, however, the microscopic particles (usually in the 0.1–10 µm range) align themselves along the lines of [|magnetic flux], see below. When the fluid is contained between two poles (typically of separation 0.5–2 mm in the majority of devices), the resulting chains of particles restrict the movement of the fluid, perpendicular to the direction of flux, effectively increasing its viscosity. Importantly, mechanical properties of the fluid in its “on” state are [|anisotropic]. Thus in designing a magnetorheological (or MR) device, it is crucial to ensure that the lines of flux are perpendicular to the direction of the motion to be restricted.



Material behavior
To understand and predict the behavior of the MR fluid it is necessary to model the fluid mathematically, a task slightly complicated by the varying material properties (such as [|yield stress]). As mentioned above, smart fluids are such that they have a low viscosity in the absence of an applied magnetic field, but become quasi-solid with the application of such a field. In the case of MR fluids (and [|ER]), the fluid actually assumes properties comparable to a solid when in the activated ("on") state, up until a point of yield (the [|shear stress] above which shearing occurs). This yield stress (commonly referred to as apparent yield stress) is dependent on the magnetic field applied to the fluid, but will reach a maximum point after which increases in [|magnetic flux density] have no further effect, as the fluid is then magnetically saturated. The behavior of a MR fluid can thus be considered similar to a [|Bingham plastic], a material model which has been well-investigated. However, a MR fluid does not exactly follow the characteristics of a Bingham plastic. For example, below the yield stress (in the activated or "on" state), the fluid behaves as a [|viscoelastic] material, with a [|complex modulus] that is also known to be dependent on the magnetic field intensity. MR fluids are also known to be subject to [|shear thinning], whereby the viscosity above yield decreases with increased shear rate. Furthermore, the behavior of MR fluids when in the "off" state is also [|non-Newtonian] and temperature dependent, however it deviates little enough for the fluid to be ultimately considered as a Bingham plastic for a simple analysis. Thus our model of MR fluid behavior becomes: Where τ = shear stress; τ//y// = yield stress; //H// = Magnetic field intensity η = Newtonian viscosity; is the velocity gradient in the z-direction.

Shear strength
Low [|shear strength] has been the primary reason for limited range of applications. In the absence of external pressure the maximum shear strength is about 100 kPa. If the fluid is compressed in the magnetic field direction and the compressive stress is 2 MPa, the shear strength is raised to 1100 kPa.[|[][|1][|]] If the standard magnetic particles are replaced with elongated magnetic particles, the shear strength is also improved.[|[][|2][|]]

Particle sedimentation
Ferroparticles settle out of the suspension over time due to the inherent density difference between the particles and their carrier fluid. The rate and degree to which this occurs is one of the primary attributes considered in industry when implementing or designing an MR device. [|Surfactants] are typically used to offset this effect, but at a cost of the fluid's magnetic saturation, and thus the maximum yield stress exhibited in its activated state.

Common MR fluid surfactants
MR fluids often contain [|surfactants] including, but not limited to: These surfactants serve to decrease the rate of ferroparticle settling, of which a high rate is an unfavorable characteristic of MR fluids. The ideal MR fluid would never settle, but developing this ideal fluid is as highly improbable as developing a [|perpetual motion machine] according to our current understanding of the laws of physics. Surfactant-aided prolonged settling is typically achieved in one of two ways: by addition of surfactants, and by addition of spherical ferromagnetic nanoparticles. Addition of the nanoparticles results in the larger particles staying suspended longer since to the non-settling nanoparticles interfere with the settling of the larger micrometre-scale particles due to [|Brownian motion]. Addition of a surfactant allows [|micelles] to form around the ferroparticles. A surfactant has a [|polar] head and non-polar tail (or vice versa), one of which [|adsorbs] to a nanoparticle, while the non-polar tail (or polar head) sticks out into the carrier medium, forming an inverse or regular [|micelle],respectively, around the particle. This increases the effective particle diameter. [|Steric] repulsion then prevents heavy agglomeration of the particles in their settled state, which makes fluid remixing (particle redispersion) occur far faster and with less effort. For example, [|magnetorheological dampers] will remix within one cycle with a surfactant additive, but are nearly impossible to remix without them. While surfactants are useful in prolonging the settling rate in MR fluids, they also prove detrimental to the fluid's magnetic properties (specifically, the magnetic saturation), which is commonly a parameter which users wish to maximize in order to increase the maximum apparent yield stress. Whether the anti-settling additive is nanosphere-based or surfactant-based, their addition decreases the packing density of the ferroparticles while in its activated state, thus decreasing the fluids on-state/activated viscosity, resulting in a "softer" activated fluid with a lower maximum apparent yield stress. While the on-state viscosity (the "hardness" of the activated fluid) is also a primary concern for many MR fluid applications, it is a primary fluid property for the majority of their commercial and industrial applications and therefore a compromise must be met when considering on-state viscosity, maximum apparent yields stress, and settling rate of an MR fluid.
 * [|oleic acid]
 * [|tetramethylammonium hydroxide]
 * [|citric acid]
 * [|soy lecithin]

Modes of operation and applications
An MR fluid is used in one of three main modes of operation, these being flow mode, shear mode and squeeze-flow mode. These modes involve, respectively, fluid flowing as a result of pressure gradient between two stationary plates; fluid between two plates moving relative to one another; and fluid between two plates moving in the direction perpendicular to their planes. In all cases the magnetic field is perpendicular to the planes of the plates, so as to restrict fluid in the direction parallel to the plates.

Squeeze-Flow Mode


The applications of these various modes are numerous. Flow mode can be used in dampers and shock absorbers, by using the movement to be controlled to force the fluid through channels, across which a magnetic field is applied. Shear mode is particularly useful in clutches and brakes - in places where rotational motion must be controlled. Squeeze-flow mode, on the other hand, is most suitable for applications controlling small, millimeter-order movements but involving large forces. This particular flow mode has seen the least investigation so far. Overall, between these three modes of operation, MR fluids can be applied successfully to a wide range of applications. However, some limitations exist which are necessary to mention here.

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= Auxetic Materials =

An auxetic material is one which has a negative Poisson’s ratio, n 1. This means that, unlike an elastic band for example, which gets thinner when stretched, an auxetic material will get fatter. Equally, if an auxetic material is compressed, it will get thinner. This interesting and counter-intuitive property is found in some natural materials such as single-crystal arsenic2, catskin3 and load-bearing cancellous bone from human shins4. However, interest in this area really began to grow in 1987 when Roderic Lakes produced an auxetic polymeric foam at Iowa University5. He achieved this by converting an ordinary foam using a relatively simple process of heating and squashing6. Since then, a whole range of synthetic auxetic materials have been produced, including carbon fibre composites7, honeycomb structures8 and microporous polymers9-11.

Auxetic materials are an unusual class of materials but, apart from their novelty value, there are a number of reasons why these materials are interesting. These all centre on the possibility of enhancements in mechanical properties due to a negative Poisson’s ratio as predicted by classical elasticity theory. Take, for example, the case of the shear modulus, G. This is given by:

(1) G = E/2(1+ n )

So, as n approaches –1, the shear modulus is predicted to become very large indeed, provided that the Young’s modulus, E, is not significantly affected. Similarly, in the Hertzian12 model of elastic indentation resistance, the hardness, H, is related to the Poisson’s ratio as:

(2) H µ (1- n 2)-2/3

The hardness has been investigated for many of the synthetic auxetic materials produced to date and enhancements have been found across the board in materials as diverse as polymeric and metallic foams13,14, carbon fibre composite laminates15 and microporous polymers16, where the auxetic form has been found to be up to three times more difficult to indent than conventionally processed polymers. Very recent investigations into low velocity impact of auxetic carbon fibre laminates have also shown enhancements in energy absorption of up to a third for the first failure point17.

A further advantage of using auxetic materials which may be of interest is their drapeability. Take, for example, a panel structure, which may be typically in the form of a honeycomb as illustrated in below. The problem with these materials is that they cannot easily be curved into a doubly curved or domed shape, rather the core forms a saddle shape on bending (a). So, to produce

a doubly curved panel, it is necessary to either to machine the required shape (thus wasting material) or to physically force the panel to dome, resulting in considerable damage. However, with an auxetic material, double curvature is readily achieved (b).

So, auxetic materials are both novel and interesting due to both their intrinsic behaviour and their properties.

__** Refrences : **__

- WikiPedia - Active Materials & Adapative structures - session 10 - KE Evans, MA Nkansah, IJ Hutchinson and SC Rogers, ‘Molecular network design’, Nature, 1991, 353, 124. - JL Williams and JL Lewis, ‘Properties and an anisotropic model of cancellous bone from the proximal tibial epiphysis’, Trans. ASME, J. Biomech. Eng., 1982, 104, 50-56.