Clough, A. R. (2013) A study on the near-field interactions of ultrasonic surface waves with surface-breaking defects. PhD thesis, University of Warwick.

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Abstract

This thesis is concerned with the detection of surface-breaking defects, such
as stress corrosion cracking, using an ultrasonic scanning approach in which a laser
source and detector are scanned over the near-field of a defect. Large increases
in the amplitude and frequency content of an incident ultrasonic wave are present
when either the source or the detector is very close to the defect, leading to a phenomenon
known as ultrasonic near-field enhancement. The extent of the ultrasonic
enhancement varies with defect characteristics such as defect depth and angle to the
surface.
Ultrasonic enhancement is observed in both experiment and finite element
simulations using Rayleigh waves for both scanning laser detection and scanning
laser source methods. The near-field enhancement is shown to vary as a function of
the angle of the defect to the horizontal for Rayleigh wave enhancements, allowing
the positioning and characterisation of artificial angled defects that are similar to
rolling contact fatigue defects in railtrack. The mechanisms behind the near-field
enhancement of Rayleigh waves at angled defects are identified, and this aids in the
understanding of the behaviour of ultrasound as it interacts with surface-breaking
defects.
Ultrasonic enhancements are also reported to be present in individual Lamb
wave modes for interactions with artificial open-mouthed defects in thin plates,
which are similar to the open end of stress corrosion defects. The mechanisms behind
both the scanning laser detection and scanning laser source enhancements are
identified and used to explain the variation in the enhancement as a function of
increasing defect severity. Positioning of these defects is also achieved by identification
of the enhancement location.
Finally, the scanning laser technique is applied to real stress-driven defects,
and both scanning approaches are shown to be capable of detecting partially-closed
defects in a variety of sample geometries. The position, geometric alignment and an
estimate of the defect depth are obtained for real defects in thin plates, pipework
sections and in irregularly shaped engine components.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QC Physics T Technology > T Technology (General)
Library of Congress Subject Headings (LCSH):	Rayleigh waves, Acoustic surface waves, Ultrasonic waves, Surfaces (Technology) -- Defects
Official Date:	September 2013
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Edwards, R. S. (Rachel S.)
Sponsors:	European Research Council (ERC)
Extent:	xxiii, 226 leaves : illustrations, charts.
Language:	eng

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