SMART+MATERIALS+BY+ASHRAFKASEM

__//**SMART MATERIALS**//__

__**1) SHAPE MEMORY ALLOYS**__ The three main types of shape-memory alloys are the copper -zinc -aluminium -nickel, copper-aluminum-nickel, and nickel-titanium (Ni Ti) alloys but Sm As can also be created by alloying zinc, copper, gold and iron . Ni Ti 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.

One-way vs. two-way shape memory
Shape-memory alloys have different shape-memory effects. Two common effects are one-way and two-way shape memory. A schematic of the effects is shown below. 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. A shaped, trained object heated beyond a certain point will lose the two-way memory effect, this is known as "amnesia"

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2)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.

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3)piezo Electric 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 iezoelectric material is quartz (SiO2

 4) Magnetorheologuical fluids

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.

How it works
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 media type="youtube" key="7LArS6tlVNg" width="425" height="350" REFERANCES : . 1)wikipedia.COM 2)http://www.youtube.com 3)www.**piezomaterials**.com 4)www.bolton.ac.uk/auxnet/background/index.html