?Smart Materials and Intelligent Systems
A step further in mimicking the natural composites is to to integrate the engineering
composite materials with engineering sensors and actuators to imitate the biological
systems. Mateirals that allow structures to adapt to their environment are known as
actuators. They can change shape, stiffness, positionm natural frequency and other
mechanical characteristics in response to temperature or electromagnetic fields.
Similarly, materials that respond to the environmental impact significantly can be used as sensors. Some common actuator materials and sensor materials being used today are given below. A. Shape Memory Alloys (SMA) – Shape memory alloys are metals that revert back to their original shape at a certain elevated temperature. In the process of returning to their remembered shape, the alloys can generate a large force useful for actuating. For example, Nitinol experiences shape change due to phase transformation from lowtemperature martensite to high-temperature austenite when subjected to temperature increase. The high-temperature austenitic phase holds the parent shape while the lowtemperature martensitic phase is organized by unstable twinned structures. Nitinol exhibits substantial resistance to corrosion and fatigue. In addition, it sustains large elastic deformations up to 8% strain of the alloy’s length in martensite, hence is also called superelastic material. The main drawback of shape memory alloys is their slow rate of change because actuating depends on heating and cooling which respond only as fast as the temperature can shift.
B. Piezoelectric Materials – Piezoelectric materials undergo voltage change when
subjected to expansion and contraction, and vice versa. Lead zirconate titanate (PZT) is the most widely used type. However, the best of piezoelectric materials recovers only about 1% of strain though it acts very quickly, within thousandths of a second. Hence,they are useful for precise, high-speed actuating.
C. magnetostrictive Materials – This group of materials is similar to piezoelectrics
except that it responds to magnetic, rather than electric fields. The magnetic domains in the substance rotate until they line up with an external field. In this way, the domains can expand the material. Terfenol-D, which contains the rare earth element terbium, expands by more than 0.1%. D. Electrorheological (ER) and magnetorheological (MR) Fluids – These substances contain micro-size particles that form chains when placed in an electric or magnetic field,resulting in an increase in apparent viscosity of up to several orders of magnitude in milliseconds. When combining with composite structures, the change of viscosity in ER or MR fluids can be used to improve the structural stiffness. However, several problems plague these fluids, such as abrasiveness and chemical instability.
E. Optic Fibers – Glass and silica fibers form a basis for a broad range of sensors. As stress, strain and temperature change, the refractive indicies of these material change.This well known phenomenon is called photoelastic effect. Optic-fibers can be embeddedin composite materials as a nerve system in the body to detect deformation, damage andthermal history of curing cycle.
F. Micro-electro-mechanical Systems (MEMS) – Based on techniques used in semiconductor processing, sensors and actuators can be miniaturized down to
microscopic levels. They can be integrated with composite materials to form intelligent systems.
A step further in mimicking the natural composites is to to integrate the engineering
composite materials with engineering sensors and actuators to imitate the biological
systems. Mateirals that allow structures to adapt to their environment are known as
actuators. They can change shape, stiffness, positionm natural frequency and other
mechanical characteristics in response to temperature or electromagnetic fields.
Similarly, materials that respond to the environmental impact significantly can be used as sensors. Some common actuator materials and sensor materials being used today are given below. A. Shape Memory Alloys (SMA) – Shape memory alloys are metals that revert back to their original shape at a certain elevated temperature. In the process of returning to their remembered shape, the alloys can generate a large force useful for actuating. For example, Nitinol experiences shape change due to phase transformation from lowtemperature martensite to high-temperature austenite when subjected to temperature increase. The high-temperature austenitic phase holds the parent shape while the lowtemperature martensitic phase is organized by unstable twinned structures. Nitinol exhibits substantial resistance to corrosion and fatigue. In addition, it sustains large elastic deformations up to 8% strain of the alloy’s length in martensite, hence is also called superelastic material. The main drawback of shape memory alloys is their slow rate of change because actuating depends on heating and cooling which respond only as fast as the temperature can shift.
B. Piezoelectric Materials – Piezoelectric materials undergo voltage change when
subjected to expansion and contraction, and vice versa. Lead zirconate titanate (PZT) is the most widely used type. However, the best of piezoelectric materials recovers only about 1% of strain though it acts very quickly, within thousandths of a second. Hence,they are useful for precise, high-speed actuating.
C. magnetostrictive Materials – This group of materials is similar to piezoelectrics
except that it responds to magnetic, rather than electric fields. The magnetic domains in the substance rotate until they line up with an external field. In this way, the domains can expand the material. Terfenol-D, which contains the rare earth element terbium, expands by more than 0.1%. D. Electrorheological (ER) and magnetorheological (MR) Fluids – These substances contain micro-size particles that form chains when placed in an electric or magnetic field,resulting in an increase in apparent viscosity of up to several orders of magnitude in milliseconds. When combining with composite structures, the change of viscosity in ER or MR fluids can be used to improve the structural stiffness. However, several problems plague these fluids, such as abrasiveness and chemical instability.
E. Optic Fibers – Glass and silica fibers form a basis for a broad range of sensors. As stress, strain and temperature change, the refractive indicies of these material change.This well known phenomenon is called photoelastic effect. Optic-fibers can be embeddedin composite materials as a nerve system in the body to detect deformation, damage andthermal history of curing cycle.
F. Micro-electro-mechanical Systems (MEMS) – Based on techniques used in semiconductor processing, sensors and actuators can be miniaturized down to
microscopic levels. They can be integrated with composite materials to form intelligent systems.
