Smart+Materials+-+Mahmoud+Fouad

**Smart Materials**

 **Smart materials** are materials that have one or more properties that can be significantly changed in a controlled fashion by externa l stimuli, suchas stress, temperature, moisture,pH,,electric or magnetic fields. There are a number of types of smart material, some of which are already common. Some examples are as following :-  **I-Shape Memory Alloys (SMA) :** = I. What are Shape Memory Alloys? = Shape Memory Alloy's are metals, which exhibit two very unique properties,, and the. 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.

=II. Applications of Shape Memory Alloys:= The unusual properties mentioned above are being applied to a wide variety of applications in a number of different fields. The buttons below are links to pages about some of the most promising applications of SMAs. Each page contains information about the application as well as videos and interactive applets which allow you to become more familiar with the behavior of SMAs.

=III. How Shape Memory Alloys 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 10C is necessary to initiate this phase change. The two phases, which occur in shape memory alloys, are, and.

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.



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**II. Piezoelectric Materials** Piezoelectric materials have two unique properties which are interrelated. When a piezoelectric material is deformed, it gives off a small electrical discharge. Alternately, when an electrical current is passed through a piezoelectric material it experiences a significant increase in size (up to a 4% change in volume)

Piezoelectric materials are most widely used as sensors in different environments. They are often used to measure fluid compositions, fluid density, fluid viscosity, or the force of an impact. An example of a piezoelectric material in everyday life is the airbag sensor in your car. The material senses the force of an impact on the car and sends and electric charge deploying the airbag. http://en.wikipedia.org/wiki/Nickel_titanium Here is a list of other piezoelectric materials:
 * Lithium tantalate
 * Polyvinylidene fluoride
 * Lanthanum gallium silicate
 * Potassium sodium tartrate

Applications :- The most known application in Piezoelectric alloys is Sensors. Piezoelectric sensor are devices using the piezoelectric effect to measure acceleration, pressure, strain or force and converting them to an electrical signal. media type="youtube" key="K3G2QM5a-9U" height="315" width="560" align="center"

**III. 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.





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:
 * [[image:http://upload.wikimedia.org/wikipedia/en/math/f/b/a/fba4e76ea65842c0de065c37aa750f39.png caption="tau =tau_y(H) + etafrac{dv}{dz}, tau>tau_y"]] ||
 * tau =tau_y(H) + etafrac{dv}{dz}, tau>tau_y ||

media type="youtube" key="SBXQ-6uI8GY" height="315" width="560" align="center" 4.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.



**References :**
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