7.1 Introduction to piezoelectric wafer active sensors

Piezoelectric wafer active sensors (PWAS) are inexpensive low profile transducers that operate on the piezoelectric principle. Initially, PWAS were used for vibrations control, as pioneered by ^{[[CRA 87]]} and ^{[[FUL 90]]}. The use of PWAS for structural health monitoring and non-destructive evaluation has followed three main paths: (a) modal analysis and transfer function, (b) wave propagation and (c) electromechanical impedance. The use of PWAS for damage detection with Lamb wave propagation was pioneered by Chang et al. ^{[[CHA 95]]}, ^{[[WAN 00]]}, ^{[[IHN 02]]}. They have studied the use of PWAS for the 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 methods were also studied in ^{[[CUL 98]]}, ^{[[LIN 01]]}, ^{[[DUP 00]]}, ^{[[OSM 00]]}, ^{[[DIA 02]]}. The use of PWAS for high-frequency local modal sensing with the electromechanical impedance method was pursued by ^{[[LIA 94]]}, [PAR 01], and others. The author and his collaborators have pursued the PWAS technology along both the electromechanical impedance ^{[[GIU 02c]]} and the wave propagation paths ^{[[GIU 02a]]}, ^{[[GIU 03a]]}, ^[[b]].

PWAS couple the electrical and mechanical effects (mechanical strain, S _ij mechanical stress, T _kl, electrical field, E _k and electrical displacement D _j) through the tensorial piezoelectric constitutive equations ^{[[IEE 87]]}