Adaptive structures, also called Intelligent or smart materials, refers to the various materials systems which automatically or remotely alter their dynamic characteristics or their geometry to meet their Intended performance. By integrating the sensors and actuators Into the structural system, typically a composite materials, control of shape, vibration and acoustic behavior an be effected. In addition to active control, passive control of system damping can be achieved in these structures. The sensors employed include piezoelectric ceramics, piezoelectric polymer films, ferroelectrics, and fiber optics. For producing the stress induced changes in dynamic characteristics of a composite the actuators are either embedded within the composite or are surface mounted. In general, the piezoelectric type actuator Is used where small strains at high frequencies are appropriate, while shape memory actuators are used when high forces and strains at lower frequencies are required. Static damping, modulus shift effect on acoustic radiation, and strain energy shift of modal response and acoustic radiation of composite materials with embedded shape memory actuators will be discussed. The constitutive equations for shape memory alloys will be described and how these are used in the design of adaptive composite structures

The term smart materials seems to have become a part of the engineering vocabulary with variants such as Intelligent materials, and their application in adaptive structures. Smart materials consist of a structural component such as a composite such as fiber reenforced resin, with distributed sensors and actuators and a microprocessor. In response to changing external or Internal conditions these materials can change their properties to more effectively perform their function. The external conditions may be environment such as light or heat, loads, vibration or the need to change the geometry or shape of the structure to cope with changing service conditions. Internal conditions may be delamination in a composite, fatigue cracks in a metallic or nonmetallic structure, or other forms of Incipient failure.

In reviewing papers presented in the past several years at conferences on smart/adaptive structures one would see a dominant number on various aspects of space structures such as mirrors. antennas, robotics booms and satellite docking. In these areas the control of vibration or the precise control of motion are most often the specific subject addressed. Much of the ongoing research is on control theory and the design of algorithms to define the sensor-actuator-microprocessor Integration. Of concern in this paper Is the actuator itself which, in response to commands from the microprocessor, produces strains and forces in the structure to modify Its acoustic or vibratory response or alter Its shape. These actuators are broadly of two types: low to medium force, low strain, high frequency systems, typically a piezoceramic such as PZT, or a high force, high strain, low frequency actuator which is most likely to be a shape memory alloy element.