Guided wave (GW) has been increasingly being studied, improved and applied for use in fault diagnosis, particularly in identifying defects. The interest of GW technique to monitor the structure health of pipes, cables and plates has been increasing.

Pipes are crucial infrastructural components as they are commonly used to provide natural gas and water to every households of a building. External piping systems are erected on the outside wall of any tall building and then connected to the internal pipes of kitchens and bathrooms through the building’s concrete walls. The section of the pipe that is covered by the wall is prone to corrosion because of water leaks thorough the wall. However, seldom any serious research has been done on investigating any effective means in detecting pipe corrosion occurred inside the wall-covered section. Continuously ignoring the leaks may cause pipe rupture that finally may trigger a gas leak and in worst case, cause a gas explosion that kill people. Similarly, defects can be a major source of concern in safety critical plate structures as well, such as the fuselages of aircraft and the bodies of trains, because such imperfections can lead to serious damage or fractures. Over the last decade, the technique employing laser generation of GW has gained increasing interest as a way of providing a non-contacted method of flaws detection. Thus, ensuring the safety of pipes and plates are prime concern in both civilian and industry infrastructures.

Recently, GW has been a popular research topic as it enables long distance inspection and covers the entire cross-section of the inspected targets. The GW based inspection methods were employed for structure health monitoring, including pipes and plates. A quality inspection must include the determination of the location of corrosion and it extent inside the wall-covered section of the pipe. A number of researchers, including the author of this thesis, have successfully detected corrosion occurred in a pipe opened to air. Detecting corrosion inside the wall-covered section of the pipe remained as a big research challenge. It is because the received GW signals reflected by the concrete wall and buried corrosion often generating disturbances, making the reveal of the true fault information very difficult. These disturbances are the conversion of different GW modes during the wave propagating inside the pipe and noise generated by the complex geometries and GW transmitters. The received GW signals that contain the fault information are often overwhelmed by these disturbances. Hence, the design of GW transmitters must be improved to minimize the disturbance. Moreover, the received signals need further analysis by use an effective signal processing method to remove the undesired modes and noise. Consequently, the true fault information can be extracted from the blurred GW signals. Furthermore, some conventional contacted inspection methods, which can be difficult to apply in some situations, such as measurements at a high temperature or on moving structures, key structural parts that possess complex geometries with impossibility to reach locations. This fact is limitation of the inspected target where environments are often complex and dangerous, may put staff live at safety risk. Therefore, it is necessary to use a non-contacted laser-based GW inspection to take preventive measures to detect the macroscopic anomalies that might occur on the metallic plate. Nevertheless, one main weakness associated with laser ultrasonic is its poor signal generated wave propagates in all directions and has infinitive modes with broad bandwidth that cause complications in the detected wave signal.

Researchers have been using different types of transmitters to excite GWs. These transmitters include the contacted ones, such as the piezoelectric transducers (PZT) and the magnetosrictive sensors (MsS), and non-contacted ones, like the laser-based system. In this thesis, a newly designed transmitter based on the use of the non-contacted laser system is reported for improving the signal-to-noise ratio (SNR) of the received GW signals. The laser-based transmitter is made by an integrated optical Mach-Zehnder (IOMZ) interferometer that has never been reported before. Moreover, a novel signal processing method, which combines the tailor-made Butterworth band pass filter (TBBP) with a special type of wavelet transform that has innovatively used the excited tone-burst of GW signal as the mother wavelet, is presented in the thesis.

With the help of the effective signal processing method and the laser-based IOMZ system, the location and the extent any defect can be successfully determined in wall-covered pipes and metal plates. Hence, the likelihood of gas explosion due to pipe ruptures and the occurrence of catastrophes caused by the fatigue of plates can be avoided.