Smart materials

SMART MATERIALS

Basically, smart material is the one that reacts quickly to a stimulus in a specific manner. The change in the material can also be reversible, as a change in stimulus can bring the material back to its previous state. The smart materials used in built environment can be defined as those offering specific functional and adaptable properties in response to thermal, optical, structural, and environmental stimuli. These materials not only enhance the overall performances of new building construction, but also promise safer structures, longer durability of building elements, more building energy savings, greater environmental sustainability, and even higher indoor user comfort.

The structures which make use of smart materials are known as smart structures. The two commonly used smart materials are piezoelectric materials and electrorheological fluids.

1. PIEZOELECTRIC MATERIALS

Piezoelectric materials are the widely used smart materials. They produce voltage when surface strain is introduced as shown in Fig. 3. Also, the materials undergo surface elongation when an electric field is applied across them as shown in Fig. 4.

The materials exhibiting piezoelectric property are non-metallic materials such as quartz, rock salt and tairmalite. Polyvnylidene fluoride, which can be easily formed into thin sheets and adhere to any surface is very commonly used smart material.

2. ELECTRORHEOLOGICAL FLUIDS

Electrorheological fluids are colloidal suspensions that exhibit reversible change in viscosity when subjected to an electric field. The property of reversible transition from liquid to solid state is utilized in many engineering applications. These changes could be reversed in time intervals of the order of a few milliseconds.

3. FUNCTIONALITY OF SMART STRUCTURES

A smart structure has four major components, namely, the structure, sensor, actuator, and controller as shown in Fig. 5. Actuators and sensors are widely used in various applications and are generally integrated with main structures via surface bonding or embedding. The piezoelectric sensor is bonded to the surface of the beam using a small strip of two-sided adhesive tape. It detects the vibration and sends signal to the controller. Subsequently, the controller sends corrective signal to the piezoelectric actuator. The actuator in turn applies local strain to the beam and thereby the effect of vibration is minimized.

The above smart beam concept is used in civil structures like bridges to dampen the vibrations produced due to earthquake. The concept is also used in applications like helicopter and airplane wings, wind mill blades and space structures to dampen vibrations induced by varying wind velocities.

Sensors maybe single or multi-layered. Embedded piezoelectric material may be in the form of rods or patches. In the former case, the piezoelectric material expands/contracts along the direction of the applied electric field. In the latter case, it produces extensions in a perpendicular direction to the applied field. The smart beam concept can be used to sense and reduce vehicle vibrations acting on the bridge structures resulting in their improved life span.

4. REQUIREMENTS FOR SMART STRUCTURES

The smart structures provide the following attributes:

1. High degree of reliability, efficiency and sustainability for structures.

2. High security to the infrastructures particularly when subjected to extreme and unconventional conditions.

3. Continuous health and integrity monitoring.

4. Damage detection and self-recovery.

5. Intelligent operational management system.