Smart materials and structures.

A smart structure is a system containing multifunctional parts that can perform sensing, control, and actuation; it is a primitive analogue of a biological body. Smart materials are used to construct these smart structures, which can perform both sensing and actuation functions. The ‘‘I.Q.’’ of smart materials is measured in terms of their ‘‘responsiveness’’ to environmental stimuli and their ‘‘agility.’’ The first criterion requires a large amplitude change, whereas the second assigns faster response materials with higher ‘‘I.Q.’’ Commonly encountered smart materials and structures can be categorized into three different levels: (i) single-phase materials, (ii) composite materials, and (iii) smart structures. Many ferroic materials and those with one or more large anomalies associated with phase-transition phenomena belong to the first category. Functional composites are generally designed to use nonfunctional materials to enhance functional materials or to combine several functional materials to make a multifunctional composite. The third category is an integration of sensors, actuators, and a control system that mimics the biological body in performing many desirable functions, such as synchronization with environmental changes, self-repair of damages, etc. These three levels cover the general definition of smart materials and structures. In this short summary, we will use a few examples to illustrate such systems and to provide some general guidelines for designing ‘‘smart’’ systems. The difference between an ordinary and a ‘‘smart’’ material can be demonstrated through the following positive temperature coefficient (PTC)-resistance materials. A large group of temperature sensors is based on the temperature dependence of the electrical resistivity of conductors. Platinum, for example, is a widely used metal for PTC sensors. The resistance rises constantly with increasing temperature over a wide range from about 20 to 1,500 K. Temperature sensors based on this material show the advantage to be chemically and mechanically robust and to cover a large temperature range with an almost linear characteristic. The change, however, is less than .03 mVzcmyK. Therefore, the material cannot be used for self-regulated heating purposes. An example of smart PTC materials is donor-doped barium titanate ceramics. In this case, there is a temperature range (from '350 to 450 K) in which the resistivity rises by almost six orders of magnitude, as shown in Fig. 1. Resistive heating elements are usually built with materials of intermediate resistivity level. Applying an electrical voltage to these elements causes a current to flow, which generates Joule’s heat in the resistor. If there is a surge of current or a blockage of heat circulation, the resistor frequently overheats and may even cause a fire. When a smart PTC resistor material is used, it can form a self-protection circuit. The principle can be understood as follows. At the beginning, the PTC heater is at room temperature with low resistance. Closing the switch in the circuit will produce a large current, which causes a fast temperature increase. Because of this rise in temperature, the resistance increases drastically (see Fig. 1); hence, the current will be reduced under a constant voltage source. Finally, this self-regulation leads to a temperature stabilization at the steepest part of the characteristic curve. This established temperature is quite independent of the ambient temperature and the amount of heat extracted from the heating element. Therefore, a smart self-regulating heating circuit is formed. Donor-doped BaTiO3 ceramics can be regarded as a typical smart material in which the sensed temperature signal is inherently fed back to the heat generation. A temperature control is achieved with no additional electronics. Magnetic probe is a good example of a multifunctional composite (1) in which a magnetostrictive material is integrated with a piezoelectric material to produce a large magnetoelectric effect. The magnetostrictive material will produce shape deformation under a magnetic field, and this shape deformation produces a stress on the piezoelectric material which generates electric charge. The so obtained magnetoelectric effect could be two orders of magnitude larger than that of Cr2O3. Smart structures are an integration of sensors, actuators, and a control system. Apart from the use of better functional materials as sensors and actuators, an important part of a ‘‘smarter’’ structure is to develop an optimized control algorithm that could guide the actuators to perform required functions after sensing changes. Active damping is one of the most studied areas using smart structures. By using collocated actuator and sensors (i.e.,