Piezoelectric wafer active sensors PWAS

PWAS are small, lightweight, and relatively low-cost transducers based on the piezoelectric principle that couples the electrical and mechanical variables in the material (mechanical strain, Sij, mechanical stress, T i, electrical field, E, and electrical displacement Dj) in the form 

PWAS transducers can be used in either traveling wave mode or standing wave mode. Details are given in the following section. 

3 Phased PWAS arrays and embedded ultrasonic structural radar 

A remarkable property of PWAS transducers is their frequency and size-dependent tuning with various guided Lamb waves travehng in the structure, as predicted by the following formula [14]  

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Figure 16.6 Piezoelectric wafer active sensors (PWAS) installation, (a) 7 mm PWAS on a metallic structure (b) 7 mm round and square PWAS on a composite structure.

Figure 16.7 Use of piezoelectric wafer active sensors (PWAS) for damage detection with ]MDpagating and standing guided waves in thin-waU structures, (a) pitch-catch (b) pulse-echo (c) thickness mode (d) impact and acoustic emission (AE) detection (e) electromechanical (E/M) impedance (f) PWAS phased array.

For thin shell structures, the most promising methods are those based in the analysis of the propagation of elastic waves. The wave propagation methods have often used piezoelectric wafer active sensors (PWAS) as transmitters to generate waves and simultaneously as receivers to measure the echo signals due to the defects. A time-frequency analysis allows an estimation of crack size on the basis of the relationship between new and baseline response. The sensitivity of Lamb waves to defects depends largely on the frequency, and for complex structures the dispersive Lamb waves interact with reinforcements with partial reflections and refractions. These systems have not reached the level of maturity required for industrial applications. A full discussion with alternatives is presented in the book by Giurgiutiu (2008). 

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