Rosli, M. H. (2013) Surface defect characterisation using non-contact ultrasound. PhD thesis, University of Warwick.

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Abstract

Electromagnetic acoustic transducers (EMATs) have been used as a non-contact ultrasound
approach for detecting and characterising surface defects in aluminium bars and
billet. The characterisation was made from understanding the interaction of broadband
Rayleigh surface waves with surface crack growing normal or inclined to the sample surface,
based on rolling contact fatigue (RCF) cracks in rail tracks.
The interaction with normal cracks have been previously reported. For inclined
cracks, mode conversion of Rayleigh waves to Lamb-like waves occur in the wedge section
formed by the crack, resulting in strong and prominent enhancement in the signal
detected. This is confirmed by finite element analysis (FEA) models and Lamb waves
arrival times calculation. Signal enhancement from the interaction creates features in
B-scan images, and they have been used for initial crack classifications. Then, a number
of analyses were performed to estimate the crack inclination, and accurately determine
the crack vertical depth. A feature extraction and image classification program based
on genetic programming have been developed (through a collaboration work) to perform
automated classification on the B-scans. The program produces more than 90%
accuracy using the experimental data set.
The viability of EMATs to detect and fully characterise narrow cracks have been investigated
through experiments using laser interferometer and comparison with EMATs
measurements. The results confirmed that narrow cracks can be detected with EMATs,
with initial classification (in B-scans) to normal/inclined. However, the depth sizing
may not be accurate, and suggestion for better designs of EMATs have been made. FEA
models have been used to study the interaction of the Rayleigh waves with branched
cracks. Interesting results are observed in terms of Rayleigh waves reflections, which
helps to determine the presence of a branch on RCF-like cracks. A method has been
proposed for calculating the length of the branch, following a number of analyses.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics T Technology > T Technology (General)
Library of Congress Subject Headings (LCSH):	Ultrasonic testing, Nondestructive testing, Surfaces (Technology) -- Defects
Official Date:	December 2013
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Edwards, R. S. (Rachel S.); Dixon, Steve
Sponsors:	European Research Council (ERC); University of Warwick
Extent:	xviii,191 leaves : illustrations.
Language:	eng

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