Shear-horizontal guided wave tomography

Abstract

The petrochemical industry’s pipe-network is increasingly suffering from corrosion-induced shutdowns and thus a quantitative measurement approach is vital for making adequate service-life predictions and to guarantee safety. At pipe-supports, access is limited and traditional point-inspection methods such as ultrasonic thickness (UT) testing are not feasible. Guided wave tomography has been proposed as a solution to this problem. In this technique ultrasonic guided waves are excited from an array around the defect and received signals capture the interaction of the defect with these waves. From these signals, velocity maps are reconstructed, and the dispersive nature of the waves utilised to invert to thickness maps. Recently, it was found, that although reconstruction algorithms such as the hybrid algorithm for robust breast ultrasound tomography (HARBUT) allow for very high-resolution reconstructions, in practice, the mismatch of traditional guided wave scattering of Lamb waves used in guided wave tomography with the acoustic scattering models on which algorithms are based lead to poor resolution. In this thesis, another type of guided wave mode is investigated instead, shear-horizontal (SH) guided waves. Traditionally, these have been harder to excite but recent advances in electromagnetic acoustic transducer (EMAT) excitation made them a practical alternative. Since the fundamental SH0 mode is non-dispersive, the aim of this thesis is to assess whether the first higher-order SH mode, SH1, is applicable to guided wave tomography. SH1 exists at a higher frequency than those used in guided wave tomography with fundamental Lamb wave modes, leading to a smaller wavelength and it offers no out-of-plane displacement, which should avoid leakage into any pipe-loading. The thesis presents a thorough numerical investigation of the SH1 mode, examining its scattering behaviour and reconstruction of various defect types using three different SH excitation sources; (i) pure SH1 point source (ii) SH surface point source and (iii) directional SH surface source. These numerical results were validated experimentally, confirming the robustness of SH1 based guided wave tomography. In addition, the thesis investigates a technique to separate out individual modes via the application of the nonuniform fast Fourier transform (NUFFT) algorithm to non-uniformly sampled guided wave data. The benefits of SH1 for guided wave tomography are shown throughout the thesis and have been demonstrated by accurate and reliable thickness mapping of the defects. The SH1 wave was able to achieve a 2.4 times better resolution than was previously achieved using Lamb waves; yielding the best numerical and experimental guided wave tomography reconstructions to date. The application of the NUFFT algorithm showed how filtering in the frequency-wavenumber domain could be used to separate modal components even without the application of time-based windowing approaches. This technique can also be used to robustly determine bulk shear velocity even from directional data. Finally, this thesis demonstrated proof of concept for SH1 guided wave tomography, suggesting that this technique should be further investigated for employment in the accurate sizing and quantification of corrosion in pipelines.