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. …