Wave mode decomposition using array sensors

Most physical structure undergoes wear and tear as a result of usage over time. As means to ensure the safety of structures, regular inspections are a necessity to evaluate and maintain the integrity of structures. However, this process is time consuming and requires disassembling of structures which is economically inviable. Besides that, these conventional inspection techniques are restrained as some areas that are out of reach has the potential to not be examined. Fortunately, technology development has a resolution to detect deterioration which is known as Structural Health Monitoring (SHM). SHM features an important role in the field of Non-Destructive Testing (NDT) and Non-Destructive Evaluation (NDE). SHM has introduced the exploitation of piezoelectric transducers that are permanently attached to structures and Lamb wave propagation as a solution to a lot of issues encountered in this field of study. This has led to the focus of this research where the combination of ultrasonic Lamb waves with the continuous monitoring using piezoelectric sensors forms the basis of this study. The thesis is motivated by the aim of the application of piezoelectric array sensors to locate the point of origin of the wave and also its wave mode using post processing techniques. This produces a fundamental study of the application of piezoelectric array sensors and Lamb waves in a SHM system. The research utilises piezoelectric sensors that are very versatile and reliable as a tool to record information about the propagating Lamb waves. The approach that was suggested in this thesis is to arrange the piezoelectric sensors in a square matrix array so that more information can be gathered. Upon acquiring vital information about the propagating Lamb waves, post processing techniques are then introduced into the study so that a systematic approach in analysing these information can be carried out. Finite Element (FE) studies are used as an instrument to investigate the capabilities of the proposed method. This thesis aims to propose a strategy or a systematic approach in analysing Lamb waves for the identification of wave mode and its location of origin using array sensors. The analysis that is proposed involves resolving two main goals which are locating the point of origin of the wave and also revealing the modal content of the propagating Lamb wave. The post processing of the results employs the use of 2D Fast Fourier Transform (2D FFT) coupled with propagation angles to help achieve the aim of the study. This thesis compose of validation models, strategy for analysis, FE simulations for the case studies and signal processing techniques. The fundamental symmetric Lamb mode, S0 is used in the FE simulations to model the propagation of Lamb waves. The gathered data from the array sensors formed by piezoelectric transducers are post processed using 2D FFT that is carried out in multiple directions to produce meaningful information from those acquired results. As such, frequency-wavenumber plots are obtained and this is compared with the theoretical dispersion curves to reveal or identify the propagating wave mode. Finally, in order to attain the second goal which is to locate the origin of the wave source, propagation angles are introduced so that the data collected can be used to predict and redirect the user back to the origin of the wave source. It was found that the predictions of the location of the origin matches well with the actual location of the origin of the wave and thus, the accuracy of those predictions are well justified.