A family of materials with an ability to change few of its original properties by the application of any external stimuli, such as stress, temperature, moisture, pH, electric and magnetic fields are called Smart Materials. Some of the materials which include in this class of materials are piezoelectric materials, magneto-rheostatic materials, electro-rheostatic materials, thermo-responsive materials, pH-sensitive polymers, halochromic materials, electro chromic materials, thermo chromic materials and photo chromic materials. Smart materials are lifeless materials that assimilate different functions such as sensing, actuation, logic and control to adaptively react to alterations in their environment to which they are exposed, in a constructive and mostly recurring way.
To quote a few illustrative examples of smart materials undergoing change in their property due to effect of any external stimuli, we will consider piezo electric materials.Piezo electric materials are those materials which generate voltage due to the application of stress. The reverse effect of production of stress when voltage is applied across the piezo electric materials also holds good. Hence, we find extensive application of piezo electric materials as sensors in different environments. They are mainly used to measure fluid compositions, fluid density, fluid viscosity, or the force of an impact. An example from our day to day life would be an airbag sensor in cars, where the piezo electric material senses the force of an impact on the car and sends an electric charge, there by triggering airbag inflation.
Another example of piezo electric material would be electro–rheostatic and magneto-rheostatic materials, which undergo change in their viscosity. These are fluids which almost change to a solid substance from a thick fluid in a matter of a millisecond, when exposed to a magnetic or electric field. Electro-rheostatic fluids undergo viscosity change when exposed to an electric field whereas magneto-rheostatic fluids undergo similar changes when exposed to a magnetic field. Some common electro–rheostatic fluids are milk chocolate or cornstarch, while magneto-rheostatic fluids are minute iron particles suspended in oil.
Thermo-responsive materials such as shape memory alloys or shape memory polymers are smart materials which change their shape with change in temperature. Magnetic shape memory alloys experience shape due to considerable changes in magnetic field. pH-sensitive polymers enlarge or collapse when they experience change in pH of the surrounding medium. Halochromic materials change their color in response to change in acidity. One of the most common application of such materials would be in paints which undergo change in their color as an indication of corrosion of the material beneath them. Chromogenic systems change their color due to the effect of electrical, optical or thermal changes. Electro chromic materials change their color or opacity as a result of the application of voltage, thermo chromic materials change in color based on changes in temperature, and photo chromic materials change their color in response to a change in light. An application of electro chromic material would be in liquid crystal displays and an application of photo chromic materials would be in sunglasses which darken on exposure to bright sunlight.
Smart materials find a wide range of application areas due to their varied response to external stimuli. The different areas of application can be in our day to day life, aerospace, civil engineering applications and mechatronics to name a few. The scope of application of smart material includes solving engineering problems with unattainable efficiency and provides an opportunity for creation of new products that generate revenue. Sensual devices which can sense their environment and produce information to make use of in health and usage monitoring systems (HUMS) find applications in aerospace for the purpose of aircraft checking. An airline requires umpteen numbers of man power which conduct routine, ramp, intermediate and most important checks in order to check the health and usage of fleet. These checks involve quite a number of tasks that demands a lot of time. Hence, an aircraft constructed from a sensual structure has an advantage of self-checking its performance to a greater level than that of current data recording, and provide ground crews with improved health and usage monitoring. This would reduce the expenses associated with HUMS and thus such aircrafts could fly for more hours without human intervention.
These sensual structures also find application in the area of civil engineering. They are used to monitor the civil engineering structures to evaluate their durability. They are also used in food packaging to keep a check on safe storage and cooking. However, smart materials and structures are not restricted to sensing but they also adapt to their surrounding environment and such materials have an ability to move, vibrate and demonstrate various other responses, in addition to the sensual aspects. Few applications of such adaptive materials include the capability to control the aero elastic form of the aircraft wing to reduce the pull and improve operational efficiency, to control the vibration of satellites' lightweight structures, etc.
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