Development of a PZT phased array and FBG network for structural health monitoring based on guided Lamb waves

Abstract

The development of a structural health monitoring (SHM) strategy based on a PZT phased array system is proposed. The objective is to increase the low signal-to-noise ratio (SNR) compared to PZT networks for Lamb wave-based SHM systems. This is achieved by constructive interference—beamforming—of the different waves generated by the different transducers in the array. By carefully selecting and changing the delays in between the actuation of consecutive transducers in the array, a generated wave front can be steered to different selected directions in the plane of the plate component being scanned. By increasing the amplitude of generated waves, through beamforming, potential damage reflected waves present also an increased amplitude and higher SNR, facilitating their assessment in sensor signals and consequently damage detection. The developed automatic system was designed based on the use of the fast propagating fundamental symmetric Lamb wave mode (S0). The accuracy of the method is strongly dependant on a precise multiple actuation system and particularly in the accuracy at which the diminutive time delays are introduced in between the actuation of the different array elements. This problem was addressed by developing a dedicated multiple actuation system. The developed system merits were explored particularly and more importantly considering the application of the fast propagating and dispersive fundamental Lamb wave mode, requiring alternative actuation strategies with respect to conventional NDI systems based on non-dispersive body waves. This is done also to considerably decrease the threshold for minimum detectable damage dimensions which can be verified by conventional NDI systems and by similar state-of-the-art SHM systems, in an attempt to benefit a subsequent damage repair and/or prognosis procedure. Tests using the developed system were performed with the successful and repeatable detection of the 1 mm damages applied cumulatively into both aluminum and composite plates, subjected to different boundary conditions. Damages were simulated by surface and through thickness holes and cuts with different orientations. Finally, a network and phased array were also applied to a more complex composite panel with embedded fiber Bragg grating (FBG) optical sensors. An FBG interrogation technique, based in a tunable laser and photo-detector, was developed. The laser is tuned just before test execution and it is not changed during scans. With no moving components, this technique does not impose intrinsically a maximum sampling frequency. At the same time, the influence of temperature and operational induced strains (both static and due to low frequency vibration) are eliminated. This technique proved to be robust and of great potential for a future aircraft implementation.