Structural Health Monitoring Systems Based on SH Wave Transducers Sparse Array and Phased Array

Abstract

Guided wave based structural health monitoring (SHM) had been widely used in modern industries to detect early damage in structures and thus prevent catastrophic failure. In recent years, Lamb wave based SHM had been extensively investigated but the inherent multi-mode characteristics and dispersion limited its further development. In comparison, the fundamental shear horizontal (SH) wave based SHM seems to be more promising for its non-dispersion. However, the lack of omni-directional SH wave piezoelectric transducer (OSH-PT) always restricted its development. In this paper, we firstly proposed an OSH-PT consisting of twelve elements based on thickness-poled thickness-shear mode. Both simulations and experiments indicated that it can generate single mode of SH wave with signal-to-noise ratio (SNR) up to 26 dB and receive SH wave only with SNR up to 23 dB. Moreover, the deviation of its omni-directivity was only about 6% for both SH wave generation and reception. After validating its performance, four OSH-PTs was employed to construct a sparse array system for damage detecting in plates. Results showed that the proposed system can accurately locate a through-thickness hole in a super resolution of 0.12λ (4 mm) at varied frequencies with located error less than 12 mm. Besides, it can also detect multi defects based on baseline subtraction method. Next, the twelve-elements based OSH-PT was optimized by varying its size and element’s number. Results indicated that after size optimization, even for two half-rings based OSH-PT, it can still generate SH wave omni-directionally with SNR up to 20 dB. This half-ring based OSH-PT was very suitable for practical SHM applications for its simple structure and good performances. Finally, a phased array consisting of nine half-ring based OSH-PTs was developed for defect inspection and total focusing method (TFM) was employed for the experiments. Results showed that the phased array system can detect both surface defect and throughthickness defect effectively without baseline. It can detect a through-thickness hole as small as 2 mm in diameter with located error only about 6.3 mm. Moreover, it can also detect multi defects simultaneously. Considering the easy fabrication of the OSH-PT and the low working frequency thus large monitoring area of the proposed systems, this work is expected to pave the way to SH wave based SHM.