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High power phased EMAT arrays for non-destructive testing of as-cast steel
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Tkocz, Jozef (2019) High power phased EMAT arrays for non-destructive testing of as-cast steel. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3492769~S1
Abstract
There is industrial motivation for the development of an online, non-destructive means of detecting internal defects during the continuous casting of steel, but to date, no methods have been reported, largely due to the hostile nature of the casting environment and the difficulty of performing volumetric inspections of thick steel slabs. Electromagnetic acoustic transducers (EMATs) have previously been identified as a means of generating and detecting ultrasound waves online during the continuous casting process for the purpose of surface quality inspection and solidification front detection, but their relatively poor transduction efficiency has thus far precluded the development of measurements for internal defects. This thesis outlines the development of a phased EMAT array for internal inspection of as-cast industrial steel samples, that would be suitable for adoption at high temperatures during the casting process.
An analytical model of the transduction mechanism of a coil-only Lorentz force EMAT device is presented and used to calculate the surface pressure magnitude for two-dimensional excitation current distributions corresponding to linear and racetrack geometry EMAT coils. These calculations are then used to support finite element models of the acoustic wavefields generated by coil-only EMAT devices. The motivation for the development of a four-element phased EMAT array is introduced, and finite element and experimental data are presented, which demonstrate a signal enhancement by a factor of 3.5 (compared to a single EMAT coil) when using the phased array system to transmit a longitudinal ultrasound pulse through a 225 mm thick as-cast steel slab sample. Defect detection experiments are then presented, demonstrating the possibility to perform image reconstruction using mode-converted shear wave indication signals, and to detect vertical cracking defects in as-cast industrial steel slab samples by exploiting the beam-steering properties of the array.
A model of surface wave generation from a finite-width linear EMAT coil is also derived and used to determine the optimal coil width parameter for the case of sinusoidal and Gaussian current excitations. Optimal generation is shown to occur when the coil width, a, is equal to half the wavelength for the sinusoidal excitation case, and when a=ꝺc = 4:0 for the Gaussian case, where ꝺ is the Gaussian's width parameter and c is the Rayleigh wave speed. This result is verified using finite element and experimental data, and a means of improving the generated signal amplitude further, using phase delays to coherently add to a propagating Rayleigh wavefront, is demonstrated.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QC Physics T Technology > TK Electrical engineering. Electronics Nuclear engineering |
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Library of Congress Subject Headings (LCSH): | Electroacoustic transducers, Steel, Cast -- Nondestructive testing | ||||
Official Date: | 2019 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Department of Physics | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Dixon, Steve M. | ||||
Sponsors: | Tata Iron and Steel Company ; Engineering and Physical Sciences Research Council | ||||
Format of File: | |||||
Extent: | xx, 176 leaves : illustrations, charts | ||||
Language: | eng |
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