Electrical and Magnetic Methods of Non-destructive Testing

Transcription

1 Electrical and Magnetic Methods of Non-destructive Testing

2 NON-DESTRUCTIVE EVALUATION SERIES Non-destructive evaluation now has a central place in modem technology both as a means of evaluating materials and products as they are manufactured and for confirmation of fitness for purpose while they are in use. This series provides in-depth coverage of the wide range of techniques that are now available for the non-destructive evaluation of materials. Each volume will contain material that is relevant to final year undergraduates in Engineering, Materials Science and Physics in addition to post graduate students, experienced research workers, and practising engineers. In some cases they will be written with taught courses in mind, while other texts will be for the qualified engineer or scientist who wishes to become familiar with a new topic at research level. Series editors Professor S. Palmer Department of Physics University of Warwick Coventry UK Professor W. Lord Department of Electrical and Computer Engineering Iowa State University Iowa USA Titles available Numerical Modeling for Electromagnetic Non-Destructive Evaluation Nathan Ida Industrial Radiology R. Halmshaw Ultrasonic Methods of Non-destructive Testing Blitz and Simpson

3 Electrical and Magnetic Methods of Non-destructive Testing Second edition Jack Blitz Formerly Senior Lecturer Department of Physics Brunel University SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.

4 First edition 1991 IOP Publishing Second edition Springer Science+Business Media Dordrecht Originally published by Chapman & Hali in 1997 Softcover reprint of the hardcover 2nd edition 1997 Typeset in Palatino by AFS Image Setters Ltd, Glasgow ISBN ISBN (ebook) DOI / Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the London address printed on this page. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. A catalogue record for this book is available from the British Library Library of Congress Catalog Card number: (c)()) Printed on permanent acid-free text paper, manufactured in accordance \vîth ANSI/NISO Z and ANSI/NISO Z (Permanence of Paper).

5 Contents Preface to the first edition Preface 1 Introduction 1.1 General considerations 1.2 Methods of non-destructive testing 1.3 Electrical and magnetic methods 1.4 Choosing a method 1.5 Automation in non-destructive testing 1.6 Conclusion 1.7 Further reading 2 Fundamental theory 2.1 General considerations 2.2 Electrical conductivity and resistivity 2.3 Dielectric materials 2.4 Electromagnetism 2.5 Alternating currents 2.6 Circuit networks 2.7 Ferromagnetic materials 2.8 Electromagnetic radiation 3 Magnetic methods 3.1 Introduction 3.2 Flux leakage methods 3.3 Magnetic particle inspection (MPI) 3.4 Magnetic tape inspection (magnetography) 3.5 Quantitative flux leakage detectors 3.6 Quantitative flux leakage applications 3.7 Magnetization and hysteresis methods 3.8 Less common magnetic techniques xi xi

6 vi Contents 4 Eddy current principles Introduction Coils encircling defect-free metal rods Coils encircling defect-free metal tubes Internal coaxial coils in defect-free metal tubes Coils scanning the surfaces of defect-free conductors Defect modelling Eddy current methods General considerations Fundamental measurements Probe design Requirements for eddy current measurements Basic eddy current tests: measurements Basic eddy current defect detection and sizing More advanced eddy current testing methods Automatic testing Multifrequency testing Remote field testing Lift-off flaw detection Pulsed eddy current testing Microwave eddy current testing Fibre-reinforced plastics Neural networks Defect imaging Microwave methods Introduction Microwave radiation Microwave instrumentation Microwave measurements Miscellaneous methods General considerations Potential drop methods Resistance strain gauges Electrified particle testing Direct measurement of resistance or capacitance 227 Appendices 232 A Notes on units 232 B Standards 234 C Bessel functions 237

7 Contents D BASIC programs for predicting impedance components 241 D.1 Coils encircling electrically conducting cylindrical rods 241 D.2 Coils encircling electrically conducting cylindrical tubes 243 D.3 Air-cored coils: scanning the surfaces of electrical conductors 248 References 251 Index 257 Vll

8 Preface to the first edition This book is intended to help satisfy an urgent requirement for up-todate comprehensive texts at graduate and senior undergraduate levels on the subjects in non-destructive testing (NDT). The subject matter here is confined to electrical and magnetic methods, with emphasis on the widely used eddy current and magnetic flux leakage methods (including particle inspection), but proper attention is paid to other techniques, such as microwave and AC field applications, which are rapidly growing in importance. Theoretical analyses relating to the various methods are discussed and the depths of presentation are often governed by whether or not the information is readily available elsewhere. Thus, for example, a considerable amount of space is devoted to eddy current theory at what the author considers to be a reasonable standard and not, as usually experienced, in either a too elementary manner or at a level appreciated only by a postgraduate theoretical physicist. The inclusion of the introductory chapter is intended to acquaint the reader with some of the philosophy of NDT and to compare, briefly, the relative performances of the more important methods of testing. Chapter 2 provides a summary of the basic electrical and magnetic theory relating to the subject. It is mainly intended for reference rather than for tutorial purposes, although some of the more important aspects of the theory are treated at greater length. The remainder of the chapters discuss the various techniques and applications. An appendix contains information on SI units, British and American Standards relevant to electrical and magnetic testing, a brief introduction to the Bessel and other functions appertaining to eddy current theory and some programs in BASIC for calculating, with a personal computer, the impedance components of eddy current coils used for testing defect-free metal samples. In the preparation of this work, the author is especially indebted to Professor Dr F. Forster and his colleagues at the Institut Dr Forster and their former UK agents Wells-Krautkramer Ltd for the valuable help and cooperation provided. He is also grateful to Atomic Energy of

9 x Preface to the first edition Canada Ltd, the British Gas Corporation, CNS Electronics Ltd, Eddy Current Technology Inc., EIcometer Instruments Ltd, Rolls-Royce pic, Technical Software Consultants Ltd, the NDE Centre at University College, London, and to Dr J. Beynon, Dr R. Collins, Professor W. D. Dover, Mr W. G. King, Mr M. Hajian, Professor C. A. Hogarth, Professor W. Lord, Dr L. Morgan, Dr S. R. Oaten, Mr D. G. Rogers, Commander G. M. B. Selous and his colleagues, Dr E. L. Short and Mr D. Topp for their kind help in different ways. Thanks are also given to the various sources of information acknowledged in the text. The author is grateful to Professor D. C. Imrie, dean of the Faculty of Science at BruneI University, for allowing him the use of the facilities of the Department of Physics. Jack Blitz Department of Physics, BruneI University, Uxbridge, UK March 1991

10 Preface Important new developments in the fields of electrical and magnetic testing have been taken into account for this second edition and the book is slightly longer. There has also been an opportunity to make some minor amendments to the text; I am grateful to Dr Thomas Rose of the University of Cologne for his efforts in this direction. I am also grateful to Professor Cyril Hogarth, Mr Karol Schlachter and Mrs Kay Lawrence at BruneI University, Mr John Bailes at CNS Electronics Ltd, Mr Michael Borden at Technical Software Consultants Ltd, Mr Pete Burrows at Staveley NDT Technologies and Mr Monty O'Connor, president of Eddy Current Technology Incorporated for their kind help. Thanks are given to the various sources of information acknowledged in the text and to Professor Derek Imrie, pro vice chancellor and head of physics at BruneI University, for allowing the use of the facilities in the department. Jack Blitz, Wembley, UK.