SMART MATERIALS AND SENSORS

Now days, smart materials have found an important place in the modern engineering applications. Smart materials or intelligent materials system include integration of sensors, actuators and control with a material or structural component possesses intelligent and life features .The development of smart material is inspired by the biological structure systems and their basic characteristics of functionality, efficiency, precision, self - repair and durability. Smart materials are not only singular materials but also Hybrid composites or integrated systems of materials.

Shape Memory Alloys are one of the major categories of smart materials which after being strained at certain temperature revert back to the original shape because of unique properties such as Shape Memory effect, Pseudo elasticity and high damping capacity.  These properties in smart hybrid composites provide them the tremendous potential for creating new paradigms for material-structural interactions and demonstrate various successes in engineering applications like Aeronautical engineering, in medical fields like Vascular stents and Osteosynthesis etc., and in commercial fields also. 

The main advantages of shape memory alloys are, they are Bio-compatible, strong and good corrosion resistant.  They generally have high power to weight ratio and can withstand large amount of recoverable strain and when heated above transition temperature, they can exert high recovery stresses of 700MPa which can be used to perform work.In this paper, we are presenting one of the major categories of smart materials, SMAs, their properties, different types of SMAs and their applications in various fields

INTRODUCTION:

Shape memory alloys are metals that, after being strained, at a certain temperature revert back to their original shape. A change in their crystal structure above their transformatations temperature causes them to return to their original shape.

SMAs enable large forces (generated when encountering any resistance during their transformation) and large movements’ actuation, as they can recover large strains.

SMAs exhibit two very unique properties pseudo-elasticity and the shape memory effect. Typical Alloys which exhibit these properties are Ni-Ti alloy, Iron base SMA alloy, Copper base SMA  alloy, Super Elastic glasses etc.,

SHAPE MEMORY EFFECT:

The ability of SMAs to return to their original shape after heating to their transformation temperature after having been deformed is termed as “shape memory effect”. This is due to the change in the crystalline structure during the transition from martensitic phase to austenitic phase. 

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 as shown in the middle of figure. Upon deformation this phase takes on the second form shown in figure2, on the right. Austenite, the stronger phase of shape memory alloys, occurs at higher temperatures. The shape of the Austenite structure is cubic, shown on the left side of figure2.The un-deformed Martensite phase is the same size and shape as the cubic Austenite phase on a macroscopic scale, so that no change is visible in shape memory alloys until the Martensite is deformed.

The Shape Memory effect is observed when the temperature of SMA is cooled to below the temperature Mf. At this stage the alloy is completely composed of Martensite which can be easily deformed. The original shape of SMA can be recovered simply by heating the SMA above the temperature Af. 

The shape memory effect is currently being implemented in:

Coffeepots

The Space Shuttle

Thermostats       

Vascular Stents

Hydraulic Fitting (for Airplanes)

CONCLUSION:

Thus, the shape memory Alloys are extensively used in Aeronautics, Osteosynthesis and in some commercial products.  Still they have got some drawbacks(Less efficient, expensive etc,).  We can hope that the ongoing researches in SMAs will rectify these drawbacks so that they will emerge as a universally accessible one.