TABLE OF CONTENTS Electroactive and magnetoactive materials are materials that modify their shape in response to electric or magnetic stimuli. Such materials permit induced-strain actuation and strain sensing which are of considerable importance in SHM. Induced-strain actuation allows us to create motion at the micro scale without pistons, gears, or other mechanisms. Induced-strain actuation relies on the direct conversion of electric or magnetic energy into mechanical energy. It is a solid-state actuation, has much fewer parts than conventional actuation, and is much more reliable. It offers the opportunity for creating SHM systems that are miniaturized, effective, and efficient. On the other hand, strain sensing with electroactive and magnetoactive materials creates direct conversion of mechanical energy into electric and magnetic energy. With piezoelectric strain sensors, strong and clear voltage signals are obtained directly from the sensor without the need for intermediate gage bridges, signal conditioners, and signal amplifiers. These direct sensing properties are especially significant in dynamics, vibration, and audio applications in which alternating effects occur in rapid succession thus preventing charge leaking. Other applications of active materials are in sonic and ultrasonic transduction, in which the transducer acts as both sensor and actuator, first transmitting a sonic or ultrasonic pulse, and then detecting the echoes received from the defect or target.
In this chapter, we will discuss several types of active materials: piezoelectric ceramics, electrostrictive ceramics, piezoelectric polymers, and magnetostrictive compounds. Various formulations of these materials are currently available commercially. The names PZT (a piezoelectric ceramic), PMN (an electrostrictive ceramic), Terfenol-D...
