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Phased electromagnetic acoustic transducer arrays for enhanced ultrasonic inspection
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Xiang, Lunci (2021) Phased electromagnetic acoustic transducer arrays for enhanced ultrasonic inspection. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3781788~S15
Abstract
Material defects such as surface breaking defects exist in engineering systems in all major industries, including railway, aerospace and nuclear power industries. Non-destructive testing (NDT) plays an important role for routine inspection of structures to identify issues. This work focuses on development of novel electromagnetic acoustic transducers (EMATs) for use in the field of NDT for enhanced surface crack characterisation. EMATs are an established non-contact ultrasonic technique [11], consisting typically of coil and magnet. They are safe for use in many environments and relatively inexpensive, and are gaining more attention nowadays in practical use.
In this work, firstly, a phased EMAT array system has been developed for enhanced characterisation of surface-breaking defects. Rayleigh surface waves were used as the tool for surface crack detection and characterisation. A new ultrasonic model of EMATs generating Rayleigh waves was described in detail, from considering a single linear coil used as the ultrasonic source element to the full model of the array consisting of multiple linear coils. The coil ultrasonic behaviour with/without lift-off (distance between the EMAT and the sample surface) is discussed. At zero lift-off (EMAT in contact with the sample), the single coil induced source spatial shape was studied, which was standardised to a rectangular function and extended to form the linear coil array. At a finite lift-off, the coil-induced current can exhibit a spatial distortion, and the generated ultrasound will suffer from a subsequent frequency shift. A single 1.5 mm wide linear coil showed a decrease in peak frequency for the wideband Rayleigh wave of the order of 100 kHz/mm with lift-off. Formation of the 1-D linear coil array is then presented. The array model was studied comprehensively for its Rayleigh wave frequency/wavelength behaviour without lift-off to give instruction for the phased operation, where a particular frequency is desired. When some lift-off was present, a similar phenomenon of frequency reduction to the case of the single linear coil was found and understood when using array EMATs.
An in-house built four-channel EMAT phased driver was used to control the array EMAT. An array of four linear coils that was individually controlled were developed for either narrowband or broadband Rayleigh wave generation. Controlling the delay between when the channels are activated makes it possible to change the Rayleigh wave wavelength without requiring the physical separation of the coils to be changed. The experimental results show that the four-coil phased system can generate a wavelength range from 3 to 11.7 mm, with a significantly improved signalto- noise ratio (SNR) compared to conventional single coil EMATs. The detection range of the surface crack has thereby been widened, showing proven sensitivity to surface cracks that were deeper than 1 mm. The lift-o_ behaviour comparison between physically spaced coils and those pulsed with time delays is given, showing them to be equivalent for all but the smallest separations which are required to give a high frequency/small wavelength signal.
Secondly, a dual-mode (Rayleigh wave and shear-vertical bulk wave) pitchcatch EMAT system was developed for faster inspection for surface cracks on both near- and opposite sides of a bar sample. The EMAT transmitter and receiver used the racetrack design, giving good SNRs for an incident Rayleigh wave propagating on the sample near-side surface for near-side surface crack characterisation, and an angled shear wave propagating into the bulk for far-side surface crack detection. Defects were indicated by a reduction in the transmitted Rayleigh wave amplitude, and by blocking of the shear wave. Used together, a full picture of both surfaces can be obtained, with the ability to identify the spurious crack signal that may be found when using only a single wavemode, and the ability to accurately measure the complex geometric information of the near-side surface crack.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QC Physics T Technology > TA Engineering (General). Civil engineering (General) T Technology > TK Electrical engineering. Electronics Nuclear engineering |
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Library of Congress Subject Headings (LCSH): | Electromagnetic devices, Electroacoustic transducers, Nondestructive testing, Ultrasonic testing, Ultrasonic waves, Ultrasonic transducers | ||||
Official Date: | July 2021 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Department of Physics | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Edwards, Rachel ; Dixon,Steve | ||||
Extent: | xxix, 224 leaves : illustrations, charts | ||||
Language: | eng |
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