New Results in the Use of Piezoelectric Wafer Active Sensors for Structural Health Monitoring

Abstract

Piezoelectric-wafer active sensors (PWAS) are small, inexpensive, non-invasive, elastic wave generators/receptors that can be easily affixed to a structure. Piezoelectric-wafer active sensors are wideband non-resonant devices. They can be wired into sensor arrays and connected to data concentrators and wireless communicators. However, the development of PWAS technology is not yet complete, and a number of issues have still to be resolved. This paper presents new results obtained in using the PWAS for structural health monitoring. The new results to be presented were obtained in the following directions: (i) tuning the PWAS of asymmetric geometries (rectangular plan forms) to excite and detect different Lamb wave modes in selective directions; (ii) PWAS durability and survivability; (iii) steering beam large-area coverage (embedded ultrasonic structural radar -EUSR); and probability of detection (POD) with PWAS probes (an initial investigation). The paper presents analytical methods and experimental results that can be successfully used in the design of active SHM systems to be installed on operational structures. The durability and survivability of the PWAS transducers has been tested under various environmental exposures and the importance of adequate adhesive and protective coating design has been shown. These initial results are sufficient to gain confidence in this new technology and start installation on realistic structures. (For longer-term durability and survivability results, further tests are planned in subsequent studies.) The EUSR steering beam method based on PWAS phased-arrays has been refined to increase it power of detection for various structural defects. It has been also proved that EUSR performance is not affected by structural curvature typical of aircraft installation. The initial investigation of PWAS POD has indicated that crack as small as 1.57 mm can be detected directly and imaged relatively easily. Overall, these results confirm that PWAS transducers are a technology with good potential for implementation in active SHM systems. The paper will finish with a summary of the presented work and a perspective for further research and industrial implementation.