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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
The development of durable and cost effective high performance construction materials and systems is important for the economic well being of a country mainly because the cost of civil infrastructure constitutes a major portion of the national wealth. To address the problems of deteriorating civil infrastructure, research is very essential on smart materials. This paper highlights the use of smart materials for the optimal performance and safe design of buildings and other infrastructures particularly those under the threat of earthquake and other natural hazards. The peculiar properties of the shape memory alloys for smart structures render a promising area of research in this field.
TYPES OF SMART MATERIAL:
- Shape Memory Alloys (SMA)
- Piezo-electric materials
- Carbon Fiber Reinforced Concrete (CFRC)
- Smart Bricks
- Smart Fluids
MATERIALS AND APPLICATION:
SHAPE MEMORY ALLOYS (SMA):
The term shape memory refers to the ability of certain alloys (Ni – Ti, Cu – Al – Zn etc.) to undergo large strains, while recovering their initial configuration at the end of the deformation process spontaneously or by heating without any residual deformation .The particular properties of SMA’s are strictly associated to a solid-solid phase transformation which can be thermal or stress induced. Currently, SMAs are mainly applied in medical sciences, electrical, aerospace and mechanical engineering and also can open new applications in civil engineering specifically in seismic protection of buildings.
Its properties which enable them for civil engineering application are
- Repeated absorption of large amounts of strain energy under loading without permanent deformation. Possibility to obtain a wide range of cyclic behaviour –from supplemental and fully recentering to highly dissipating-by simply varying the number and/or the characteristics of SMA components.
- Usable strain range of 70%
- Extraordinary fatigue resistance under large strain cycles
- Their great durability and reliability in the long run.
1) ACTIVE CONTROL OF STRUCTURES:
The concept of adaptive behavior has been an underlying theme of active control of structures which are subjected to earthquake and other environmental type of loads. The structure adapts its dynamic characteristics to meet the performance objectives at any instant. A futuristic smart bridge system (An artist rendition) (Courtesy: USA Today date. 03.03.97). Sun and Sun used a thermo mechanical approach to develop a constitutive relation for bending of a composite beam with continuous SMA fibers embedded eccentric to neutral axis. The authors concluded that SMA’s can be successfully used for the active structural vibration control. Thompson et al also conducted an analytical investigation on the use of SMA wires to dampen the dynamic response of a cantilever beam constrained by SMA wires
2) PASSIVE CONTROL OF STRUCTURES:
Two families of passive seismic control devices exploiting the peculiar properties of SMA kernel components have been implemented and tested within the MANSIDE project (Memory Alloys for New Seismic Isolation and Energy Dissipation Devices). They are special braces for framed structures and isolation devices for buildings and bridges. Fig.2.shows the arrangement of SMA brace in the scaled frame model and the reduced scale isolation system.
3) SMART MATERIAL TAG:
These smart material tags can be used in composite structures. These tags can be monitored externally through out the life of the structure to relate the internal material condition. Such measurements as stress, moisture, voids, cracks and discontinuities may be interpreted via a remote sensor
SMAs can be used as self-stressing fibers and thus they can be applied for retrofitting. Self-stressing fibers are the ones in which reinforcement is placed into the composite in a non-stressed state. A prestressing force is introduced into the system without the use of large mechanical actuators, by providing SMAs. These materials do not need specialized electric equipments nor do they create safety problems in the field. Treatment can be applied at any time after hardening of the matrix instead of during its curing and hardening. Long or short term pre-stressing is introduced by triggering the change in SMAs shape using temperature or electricity.
Experimentally proved self-healing behavior which can be applied at a material micro level widens their spectrum of use. Here significant deformation beyond the first crack can be fully recovered and cracks can be fully closed.
6) SELF-STRESSING FOR ACTIVE CONTROL:
Can be used with cementitious fiber-composites with some pre-stress, which impart self-stressing thus avoiding difficulties due to the provision of large actuators in active control which require continuous maintenance of mechanical parts and rapid movement which in turn created additional inertia forces. In addition to SMA’s some other materials such as polymers can also be temporarily frozen in a prestrained state that have a potential to be used for manufacturing of self-stressing cementitious composites.
7) STRUCTURAL HEALTH MONITORING:
Use of piezo-transducers, surface bonded to the structure or embedded in the walls of the structure can be used for structural health monitoring and local damage detection. Problems of vibration and UPV testing can be avoided here. Jones et. al., applied neural networks to find the magnitude and location of an impact on isotropic plates and experimented using an array of piezo-transducers surface bonded to the plate.
SUBSTITUTE FOR STEEL?
It is reported that the fatigue behaviour of Cu Zn Al-SMA’s is comparable with steel. If larger diameter rods can be manufactured. It has a potential for use in civil engineering applications. Use of fiber reinforced plastics with SMA reinforcements requires future experimental investigations.
Piezo-electric materials are the materials that generate a change in response to a mechanical deformation, or alternatively they provide mechanical strain when an electric filed is applied across them. This capability permits these materials to be employed either as actuators or sensors in development of smart structures. Typically these sensors are used for tactile sensing, temperature sensing and strain sensing.
CARBON FIBRE REINFORCED CONCRETE (CFRC):
Its ability to conduct electricity and most importantly, capacity to change its conductivity with mechanical stress makes a promising material for smart structures .It is evolved as a part of DRC technology (Densified Reinforced Composites).The high density coupled with a choice of fibers ranging from stainless steel to chopped carbon and kelvar, applied under high pressure gives the product with outstanding qualities as per DRC technology. This technology makes it possible to produce surfaces with strength and durability superior to metals and plastics.
A mere addition of 0.5%specially treated carbon fibers enables the increase of electrical conductivity of concrete. Putting a load on this concrete reduces the effectiveness of the contact between each fiber and the surrounding matrix and thus slightly reduces its conductivity. On removing the load the concrete regains its original conductivity. Because of this peculiar property the product is called ―Smart Concrete‖. The concrete could serve both as a structural material as well as a sensor. The smart concrete could function as a traffic-sensing recorder when used as road pavements. It has got higher potential and could be exploited to make concrete reflective to radio waves and thus suitable for use in electromagnetic shielding. The smart concrete can be used to lay smart highways to guide self steering cars which at present follow tracks of buried magnets. The strain sensitive concrete might even be used to detect earthquakes.
ACTIVE RAILWAY TRACK SUPPORT:
Active control system for sleepers is adopted to achieve speed improvements on existing bridges and to maintain the track in a straight and nondeformed configuration as the train passes with the help of optimal control methodology the train will passthe bridge with reduced track deflections and vibrations and thus velocity could be safely increased
ACTIVE STRUCTURAL CONTROL AGAINST WIND:
Aerodynamic control devices to mitigate the bi-directional wind induced vibrations in tall buildings are energy efficient, since the energy in the flow is used to produce the desired control forces. Aerodynamic flap system (AFS) is an active system driven by a feedback control algorithm based on information obtained from the vibration sensors The area of flaps and angular amplitude of rotation are the principal design parameters. Shows an active aerodynamic control device.
APPLICATIONS OF SMART CONCRETE:
- Purpose of weighing vehicle on the highway
- Determine where each vehicle was
- What its weight and
- Speeds were.
- Real time vibrations sensing of bridges or other highway structures.
- Also used in buildings to dampen vibrations to reduce earthquake damage
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BENEFITS OF SMART CONCRETE:
- Even by adding carbon fibers, the extra cost of material will increase about 30%; this expense is still significantly cheaper than attaching or embedding sensors into the structure.
- Smart concrete is stronger than conventional concrete by the use of carbon fibers.
- It takes greater force for smart concrete to bend, and it absorbs more energy before fracture.
The prototype ―smart bricks‖ has a thermistor, two axis accelerometer, multiplexer, antenna and a hidden battery. This will provide information that could be vital to firemen doing their rescuing work in a multi-storey building. The batteries are charged through the bricks by an inductive coil, similar to those used in electric toothbrushes and certain artificial heart pumps. The smart bricks are fully wireless. The other applications could include monitoring nurseries, daycare centers and creating interactive ―smart toys‖ that respond to human touch.
ELECTRO-RHELOGICAL FLUIDS (ER-FLUIDS):
Electro-Rheological fluids (ER Fluids) are suspensions of fine particles, typically polymeric or metallic, in non- conducting liquids such as silicone oils or heptanes. These suspensions can be changed reversible from a liquid like state to a solid like state upon the application of an electrical field with typical field strength of 2kN/mm2. The change in state is accomplished almost instantaneously in approximately 0.001 sec depending upon the chemical composition of suspension.
It’s a gray, oily liquid that’s about three times denser than water.
- Carbonyl Iron Particles — 20 to 40 percent of the fluid is made of these soft iron particles that are just 3 to 5 micrometers in diameter.
- A Carrier Liquid — The iron particles are suspended in a liquid, usually hydrocarbon oil.
- Proprietary Additives — These additives are put in to
- Inhibit gravitational settling of the iron particles
- Promote particle suspension
- Enhance lubricity
- Modify viscosity and
- Inhibit wear.
WORKING OF MR FLUID:
Lord Corporation is one of the largest producers of a unique substance, called Magneto Rheological fluid (MR fluid), which is being used inside large dampers to stabilized buildings during earthquakes. MR fluid is a liquid that changes to a near-solid when exposed to a magnetic force, and then back to liquid once the magnetic force is removed. During an earthquake, MR fluid inside the dampers will change from solid to liquid and back as tremors activate a magnetic force inside the damper. Using these dampers in
buildings and on bridges will create smart structures that automatically react to seismic activity. This will limit the amount of damage caused by earthquakes.
WORKING OF MR FLUID DAMPER:
Inside the MR fluid damper, an electromagnetic coil is wrapped around three sections of the piston. Approximately 5 liters of MR fluid is used to fill the damper’s main chamber. During an earthquake, sensors attached to the building will signal the computer to supply the dampers with an electrical charge. This electrical charge then magnetizes the coil, turning the MR fluid from a liquid to a near-solid. This vibration will cause the MR fluid to change from liquid to solid thousands of times per second, and may cause the temperature of the fluid to rise. This accumulator prevents a dangerous rise in pressure as the fluid expands.
- A full-scale MR fluid damper that is 1-meter long and weighs 250 kilograms.
- This one damper can exert 20 tons of force on a building.
Report and Abstract for Smart Materials
The technologies using smart materials are useful for both new and existing constructions. Even though it is uneconomical, it protects environment from natural disasters the most. Of the many emerging technologies available the few described here need further research to evolve the design guidelines of systems. Codes, standards and practices are of crucial importance for the further development. Due to the lack of technology, high investment and laws in India, we have to wait for minimum half a decade to build smart structures in our country.
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