In biological systems, a damage of an organ or a cell triggers an autonomic natural healing process. A synthetic material has been developed which can heal itself when cracked. This has been achieved in a structural composite matrix embedded with a micro capsulated healing agent and a special catalyst. Very often, the detection of a crack on its onset becomes difficult when it occurs deep within the structure. When the material develops a crack, the micro capsules rupture and release the healing agent into the damaged region through capillary action. As the healing agent contacts the embedded catalyst, polymerization is initiated which closes the crack (Fig. 2). This prevents the structure from environmentally triggered degradation.
Smart structures possess a highly distributed control system involving distribution and integration of not only the sensing and control elements but also of the electronic components involved in signal conditioning, computing and power regulation (Fig. 1) The most widely used smart materials are Piezo electrics, Ferroelectrics, Shape Memory Alloys (SMA), Electro/ Magneto Rheological fluids and Electro active polymers. Various applications have been already tried out in Aerospace Engineering, Medical sciences, Civil Engineering, etc.
Piezoelectric actuators are being designed for micro positioning and nano positioning applications. They are also being used for semiconductor wafer handling, optical scanning and for servo valve control. Piezo actuator can produce smooth continuous motion with a resolution of the order of sub nanometer level. Its very fast response times, wide operating band width and high specific force make it the right candidate for fluid valve control, optical scanning, vibration isolation and precision machining. Piezo ceramic actuators can be used to provide active shape control of mirrors in large telescopes to compensate for atmospheric disturbances. This is achieved by positioning multi layered piezo ceramic blocks with integrated electrode patterns under the mirror skin.
Experiments have been carried out to demonstrate the feasibility of using SMA (NiTi, CuZnAl, CuAlNi) actuators for large shape change in active flexible structures. A flexible Silicone elastomer rod is embedded with two SMA actuators (made of Nitinol wires). SMA transformation occurs when it is subjected to resistive heating. This induces bending strain in the rod. Cooling of the actuator relaxes the rod to its initial position. If the rod is embedded with two opposing actuators, it can actuate in two opposing directions. Other possible applications of SMA are smart skin and de-icing system for rotor craft. Flexible and active endoscope is also being developed for checks on stomach and colon. This is designed with a robot at the end, which can slither through the path, giving very little discomfort. SMAs are also being used in eye glass frames, cell phone antennae and in orthodontic arches. SMAs are corrosion resistant, bio compatible and have good mechanical properties.
Magneto strictive materials (Cobalt, Iron, Nickel, Ferrite, Terbium, Metglass) give the best response when it is under compression .These actuators have a long life time and can be used as load carrying elements. They can work at temperatures higher than those permitted on piezo electric actuators. Magneto strictive materials are used for collision avoidance in automobiles and for active cabin noise control.
A type of polymers known as Hydro gels can absorb large amount of water and can swell up to 1000 percent times its original volume. This process can be triggered by change of temperature, salinity, or pH value. This interesting phenomenon can be used for various applications such as a wet suit that provides additional insulation in colder waters or a shaft sealing system for ships. The properties of Hydro gel can be tailored to meet specific requirements.
In another interesting research, there had been an effort to develop a micro mechanical flying insect of 10 to 25 mm wing span, eventually capable of sustained autonomous flight by acquiring energy through solar cells. Piezo electric actuator and flexible Thorax structures provide the power density and wing stroke. The aim of the project is to capture some of the exceptional flight modes of natural flies.
Scientists are also working on a class of pillbox size computers, equipped with sensors and linked together by radios which can form perceptive networks to monitor a factory or even an eco system.
Buffet, a turbulent air flow phenomenon that occurs due to the separated flow shed behind aerodynamic lifting surfaces of an aircraft can cause complications in flight. An instability in this flow leads to random fluctuations of pressure that excite vibration of the tail surfaces of the aircraft, eventually leading to fatigue damage. A smart actuator system has been devised, that consists of an array of pzt actuators installed within a rudder or any control surface. This can be used to deform the control surface for controlling the vibration modes. The frequencies and mode shapes are selected in such a way that the dynamic effects of buffet are suppressed.
At the University of Missouri-Rolla, a smart bridge was designed using composite tubes embedded with Fibre optic sensors for on line health monitoring by measuring strains and temperatures (Fig. 3).
Researchers are in the process of designing a catheter or shunt that can open or close in response to pressure levels in the brain. A sensor installed in the shunt would sense the rise in pressure and trigger a valve which will open and drain the cerebrospinal fluids till the pressure comes down to normal, hence preventing brain damage. This can be effectively used in treating infant hydrocephalus and Alzheimers disease in aged people.
Major applications for smart materials are in the field of Space Technology. NASA specialists are trying to design spacecrafts of very small size, as small as a basket ball, where functionally active smart materials play a significant role. Smart materials can also be used to damp spacecraft vibrations, that occur in the case of a light weight structure. When the spacecraft masts spring out with a solar panel or antenna, vibration sets in, which can't be quelled for a long time. Work is going on to design a system of tendons driven by piezo ceramics tied to the masts that will damp the vibrations by pulling on the mast's tendons. This can damp the vibrations in a tenth of the time normally taken. A microwave power driven smart actuator was developed to avoid the complexity and weight associated with conventional power control systems.
Another group of researchers are working on robotic surgeries which enable a surgeon to work on a patient from a remote place. A similar system helps astronauts to carry out maintenance works without going through the strenuous and dangerous space walks.
Smart materials containing tiny channels, a thousandth of the thickness of a human hair could carry drug molecules directly to the target in the body and release them when the material reaches body temperature. In this regard, certain molecules can self assemble into more complex chemical structures without human intervention. Hence scientists can design a building block that will then build itself into the desired material.
Smart textile material has been designed to cater for various environmental conditions. For example, a chemical warfare suit which would trap any toxins in their active carbon absorbent layer, allows inert gases to pass through. Yet another type of very active textiles adapt their functionality to the change in environment, which is achieved by introducing sensors (Fig. 4) into the textile structure. This type of textiles can be used for technical applications such as in bio-processing or in robotics where the textile material has to change properties with time or environment stimuli
Attempts have been made to integrate micro dynamical components into systems, that can perform mechanical tasks on macroscopic scales. Studies are carried out to design an actively deformable surface at University of Texas at Austin.
Magneto Rheological fluid is a suspension of micrometer sized magnetic particles in a carrier fluid, usually a type of oil. On application of a magnetic field, the liquid becomes a visco-elastic solid, the yield stress of which is a function of the intensity of magnetic field. Hence this fluid can be used for control based applications. These fluids are being used inside large dampers in the buildings to stabilize them during earthquakes. The magnetic field associated with the earthquake does the job by changing the viscosity of the fluid (Fig. 5). Other applications are, Active clutch mechanisms, Artificial limbs, Tele robotic surgery, wind mitigation devices, etc.
Smart houses are also being designed to respond to various environmental changes. Smart materials make it possible to create spacious and sunny living spaces responding to environmental stimuli. Its glass walls get adapted to different weather conditions.
As we have seen now, the smart materials and structures revolutionize the way we work and live, since their amazing properties can be manipulated by Scientists and Engineers.
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