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Title: Torsional guided wave EMAT for pipeline monitoring
Author: Herdovics, Balint
ISNI:       0000 0004 7659 0967
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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Non-destructive tests are performed regularly to asses a component's fitness for service. Guided wave ultrasound inspection is a well-established method for non-destructive testing of structures. As the propagation of guided waves can cover larger areas, fewer measurements are required to inspect large structures. While the sensitivity is lower, this enables rapid screening of structures for large defects. Piezoelectric guided wave transducers are widely available, but they require surface preparation and bonding. There is a concern, that adhesive property changes, especially when exposed to repeated temperature changes, can result in output signal variability. The highest possible stability is required for improved sensitivity, therefore the aim of this project is to investigate EMATs, which are non-contact and might be more stable. The EMAT's excitation mechanism is contactless, therefore it can potentially offer a good alternative for the bonded piezoelectric transducers. The thesis presents the design, simulation and testing of a torsional guided wave EMAT, which excites the fundamental torsional guided wave mode. A high temperature version of the transducer was designed, and signals were recorded at temperatures above 200°C. The benefit of the permanently installed transducers is that they can deliver more repeatable measurement data, as uncertainties associated with the installation process are more likely to remain constant. However, temperature variations affect the recorded signal, and they need to be compensated for. Previous work has focused on the compensation of propagation speed in ultrasonic signals. This thesis investigates the benefit of additionally compensating the transducer induced waveform changes. By adding the extra compensation term, significant improvement is reached at the location of pipe features. Structural Health Monitoring (SHM) systems rely on stable transducers to guarantee the required sensitivity for damage detection. The stability of the EMAT prototype was evaluated during a 1.5-year long test where the transducer and the pipe was exposed to repeated temperature variations. The results show good performance (the changes at the coherent noise level are less, than 0.8% of the signal amplitude), further improvements to transducer design that potentially increase the stability near reflectors are suggested. These can form the basis for future work.
Supervisor: Cegla, Frederic Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral