eprintid: 10123839
rev_number: 10
eprint_status: archive
userid: 620
dir: disk0/10/12/38/39
datestamp: 2021-03-11 11:28:17
lastmod: 2021-03-11 11:28:17
status_changed: 2021-03-11 11:28:17
type: thesis
metadata_visibility: show
creators_name: Lewis, AM
title: The modelling of electromagnetic methods for the nondestructive testing of fatigue cracks
ispublished: unpub
abstract: This thesis describes a theoretical and experimental investigation of electromagnetic methods for the detection and measurement of metal fatigue cracks. The available methods are reviewed, with particular attention being paid to mathematical models, and a new model of the electromagnetic field near a metal fatigue crack for small skin-depths is presented which uses a surface impedance boundary condition with the addition of a line source to represent the crack. This leads to a coupled system of two magnetic scalar potentials, one on the crack face which obeys the two-dimensional Laplace equation and one outside the test-piece which obeys the three-dimensional Laplace equation. The behaviour of the field is governed by a parameter m =1l(µ,8), where 1 is the size of the field perturbation, la, is the relative permeability and 8 is the skin-depth. When m is small, almost all the flux is concentrated inside the metal and the exterior potential also obeys the two-dimensional Laplace equation, on the test-piece surface. When m is large, the perturbation part of the exterior field has a negligible effect on the field inside the crack so that the crack-face potential may be found by the Born approximation. The general m problem is solved for rectangular and semi-elliptical cracks in flat plates, interrogated by uniform fields, and the solution is verified experimentally. A method for calculating the crack depth from the magnetic field is given, with descriptions of industrial applications. The theory is further developed to find the impedance change in an air-cored circular coil caused by a crack, to find the field near overlapping cracks and to find the field near a crack in an interior corner. Finally, a semi-empirical analysis is presented for a ferrite-cored measuring coil.
date: 1991
oa_status: green
full_text_type: other
thesis_class: doctoral_open
thesis_award: Ph.D
language: eng
thesis_view: UCL_Thesis
primo: open
primo_central: open_green
verified: verified_manual
full_text_status: public
pages: 185
institution: UCL (University College London)
department: Mechanical Engineering
thesis_type: Doctoral
citation:        Lewis, AM;      (1991)    The modelling of electromagnetic methods for the nondestructive testing of fatigue cracks.                   Doctoral thesis  (Ph.D), UCL (University College London).     Green open access   
 
document_url: https://discovery.ucl.ac.uk/id/eprint/10123839/1/Lewis_10123839_thesis.pdf