ABSTRACT
Piezoelectric wafer active sensors (PWAS) are inexpensive transducers that operate on the piezoelectric principle. Initially, PWAS were used for vibration control as pioneered by Crawley and deLuis (1987) and Fuller et al. (1990). Tzou and Tseng (1990) and Lester and Lefebvre (1993)modeled the piezoelectric sensor=actuator design for dynamicmeasurement= control. For damage detection, Banks et al. (1996) used PZT wafers to excite a structure and then sense the free decay response. The use of PWAS for structural health monitoring has followed threemain paths: (1) modal analysis and transfer function, (2) electromechanical impedance, and (3) 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, 2000; Ihn and Chang, 2002). They have studied the use of PWAS for generation and reception of elastic waves in compositematerials. 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 Culshaw et al. (1998), Lin and Yuan (2001), Osmont et al. (2000), and Diamanti et al. (2002). The use of PWAS for high-frequency local modal sensing with the electromechanical impedance method was pursed by Liang et al. (1994), Sun et al. (1994), Chaudhry et al. (1995), Park and Inman (2001), Giurgiutiu et al. (1998-2002), and others. PWAS couple the electrical and mechanical effects (mechanical strain, Sij, mechanical
stress, Tkl, electrical field, Ek, and electrical displacement Dj) through the tensorial piezoelectric constitutive equations
Sij ¼ sEijklTkl þ dkijEk Dj ¼ djklTkl þ eTjkEk
(11:1)
where sEijkl is the mechanical compliance of the material measured at zero electric field
(E¼ 0), eTjk is the dielectric permittivity measured at zero mechanical stress (T¼ 0), and dkij represents the piezoelectric coupling effect. As apparent in Figure 11.1, PWAS are small and unobtrusive. PWAS utilize the d31 coupling between in-plane strain and transverse electric field. A 7 mm diameter PWAS, 0.2 mm thin, weighs a bare 78 mg. At less than $10 each, PWAS are nomore expensive than conventional high-quality resistance strain gauges. However, the PWAS performance exceeds by far that of conventional resistance strain gauges. This is especially apparent in high-frequency applications at hundreds of kilohertz and beyond. There are several ways in which PWAS can be used, as shown next.
