TABLE OF CONTENTS Piezoelectric wafer active sensors (PWAS) are inexpensive transducers that operate on the piezoelectric principle. Initially, PWAS were used for vibrations control, as pioneered by Crawley et al. (1987, 1990) and Fuller et al. (1990). Tzou and Tseng (1990) and Lester and Lefebvre (1993) modeled the piezoelectric sensor/actuator design for dynamic measurement/control. For damage detection, Banks et al. (1996) used PWAS to excite a structure and then sense the free decay response. The use of PWAS for structural health monitoring has followed three main paths: (a) modal analysis and transfer function; (b) electromechanical E/M impedance; (c) wave propagation. The use of PWAS for damage detection with Lamb-wave propagation was pioneered by Chang and his coworkers (Chang, 1995, 1998, 2001; Wang and Chang, 1999; Ihn and Chang, 2002). They have studied the use of PWAS for generation and reception of elastic waves in composite materials. Passive reception of elastic waves was used for impact detection. Pitch-catch transmission reception of low-frequency Lamb waves was used for damage detection. PWAS wave propagation was also studied by Lakshmanan and Pines (1997), Culshaw et al. (1998), Lin and Yuan (2001), Dupont et al. (2000), Osmont et al. (2000), Diamanti, Hodgkinson, and Soutis (2002). The use of PWAS for high-frequency local modal sensing with the E/M impedance method was pursued by Liang et al. (1994), Sun et al. (1994), Chaudhry et al. (1994), Ayres et al. (1996), Park et al. (2001), Giurgiutiu et al. (1997 2002), and others.
PWAS couple the electrical and mechanical effects (mechanical strain
