NON-DESTRUCTIVE EVALUATION SERIES

Transcription

1 Industrial Radiology

2 NON-DESTRUCTIVE EVALUATION SERIES Non-destructive evaluation now has a central place in modern 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 N.Ida Industrial Radiology R. Halmshaw

3 Industrial Radiology Theory and practice 2nd edition R. Halmshaw MBE, Ph.D., ARCS, C. Phys., F. Inst. P., Hon. F. Brit. Inst. NDT, Hon. F. Indian Soc. NDT Consultant; formerly Senior Principal Scientific Officer, Procurement Executive - Ministry of Defence, Royal Armament Research and Development Establishment, Sevenoaks, UK SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.

4 First edition 1982 Second edition R. Halmshaw Originally published by Chapman & Hall in 1995 Softcover reprint of the hardcover 2nd edition 1995 Typeset in 10 on 12 pt Palatino by Pure Tech India Ltd., Pondicherry, India 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: (ob) Printed on permanent acid-free text paper, manufactured in accordance with ANSI/NISO Z and ANSI/NISO Z (Permanence of Paper).

5 Contents Preface to the second edition Preface to the first edition Introduction: capabilities and limitations of radiographic inspection 1 Principles of radiology 2 Basic properties of ionizing radiations 2.1 Nature of X-rays and gamma-rays 2.2 Units 2.3 Atomic structure 2.4 Generation of X-rays 2.5 Gamma-rays 2.6 Absorption: Attenuation 2.7 Attenuation coefficient 2.8 Attenuation curves 2.9 Scattered radiation and radiographic sensitivity References 3 X-ray sources 3.1 Introduction 3.2 X-ray spectrum 3.3 X-ray tubes and generators 3.4 X-ray generator circuits 3.5 High energy X-ray equipment 3.6 X-ray tube mountings 3.7 Portable tank-type X-ray sets 3.8 Controls for X-ray equipment ix xi

6 vi Contents 3.9 Comparison of X-ray generators 50 References 50 4 Gamma-ray sources and equipment Radioactivity Definitions Production of gamma-ray sources Specific isotopes for radiography Radioactive source-handling equipment The use of gamma-ray sources 70 References 73 5 Recording of radiation Introduction Photographic effect Ionization Scintillation counters Semiconductor devices Fluorescence 107 References Radiographic techniques: principles Introduction Equipment data Image parameters Choice of radiation energy Film and intensifying screens Filtration Masking Scattered radiation Summary of procedure for specifying a technique Techniques to cover a range of specimen thickness (thickness latitude) Specimen positioning Marking: identification Other practical considerations Special techniques 142 References 144 Appendix: measurement of focal spot size Radiographic techniques: sensitivity Introduction Definition of terms Image quality indicators 148

7 Contents vii 7.4 Film viewing conditions Codes of recommended good practice Special techniques 169 References Sensitivity performance Introduction Attainable IQI values Calculation of detail sensitivity Flaw sensitivity Defect depth determination Radiographic imaging considered in terms of spatial frequencies Noise limitations Information theory 203 References Interpretation of radiographs Introduction General aspects Weld radiographs Radiographs of castings Non-metallic materials Reference radiographs Acceptance standards for defects Assemblies Reporting results Image digitization from films Probability and reliability Copying radiographs 234 References Safety problems in radiography: units of radiation Introduction Radiation sources Radiation units Permissible dose limits Radiation monitoring equipment Protection data Calculation of protective barrier thicknesses Gamma-ray source containers General safety requirements for radiographic laboratories 251 References 254

8 viii Contents 11 Fluoroscopy, image intensifiers, television systems and tomography Introduction Image intensifier tubes Radioscopic equipment Image storage and image processing Performance of television-fluoroscopic systems (Radioscopy) Image quality measurement Practical procedures Image processing Applications Automated image interpretation Standardization in radioscopy Future developments Computed tomography Backscatter methods Laminography 281 References Special methods Introduction Neutron radiography Proton radiography Electron radiography Microradiography Autoradiography Radiometric methods 297 References 297 Index 299

9 Preface to the second edition Industrial radiography is a well-established non-destructive testing (NDT) method in which the basic principles were established many years ago. However, during the European Standards Organisation (CEN) commenced drafting many new standards on NDT including radiographic methods, and when completed these will replace national standards in all the EC member countries. In some cases these standards vary significantly from those in use in the UK at present. These CEN standards are accepted by majority, not unanimous voting, so they will become mandatory even in countries which vote against them. As most are likely to be legal by the time this second edition is published, they are described in the appropriate places in the text. The most important new technical development is the greater use of computers in radiology. In the first edition, computerized tomography was only briefly mentioned at the end of Chapter 11, as it was then largely a medical method with only a few equipments having found a place in industrial use. The method depends on a complex computer program and a large data store. Industrial equipments are now being built, although their spread into industry has been slow. Computer data storage is also being used for radiographic data. Small computers can now store all the data produced by scanning a radiographic film with a small light-spot, and various programs can be applied to these data. At present, the method is used largely for storage purposes (archiving) or for producing an enhanced image, but programs are being developed to analyse and interpret the image with the aim of providing a computer print-out of the defects shown in the image. X-ray television systems (radioscopy), also widely known as 'realtime radiography' (RTR) was covered briefly in the first edition, but it has become increasingly important as television systems, X-ray detectors and computers have developed. For some applications the television image obtainable is now as good as, or better than, the film image.

10 x Preface to the second edition Some uses have been found for the measurement of Compton backscatter and there are successful industrial applications which are described in Chapter 11. Microfocus X-ray tubes are now produced commercially with a reliable specification and performance, and are in more widespread use. Some other developments in equipment which appear to be near the marketable stage are briefly described, and the safety aspects of ionizing radiations are updated, based on the latest regulations. An attempt has been made to examine all the available literature on industrial radiology published since the first edition of the book, particularly the NOT periodicals; new references, where relevant, are incorporated into appropriate chapters. Ron Halmshaw 1995

11 Preface to the first edition Few manufacturing processes are so standardized, automated and rigidly controlled that the product can be guaranteed perfect over large-scale mass production. If structures are to be constructed to meet design requirements and materials are to be used economically and efficiently, some form of testing of the finished product will almost certainly be necessary. Whenever production depends on human skills, human errors creep in and faulty products occasionally occur. With some small products, samples of production can be extracted and physically tested to destruction without great cost losses; proof tests can be done on a pressure vessel, or vibration testing can be carried out to simulate service conditions, but on many large structures such sampling or proof testing is virtually impossible. Also, if one postulates occasional human errors, sampling will not eliminate the defective items and on many critical components and structures 100% inspection is often desirable. NDT or non-destructive inspection (NDI) are the terms used to describe a wide range of testing techniques designed to produce information about the condition of a specimen without doing any damage to it, i.e. after the testing the fitness of the specimen for use in service is unchanged. In practice only a few NDT methods directly test a product for fitness for service or directly measure its mechanical or physical properties. Most NDT methods are flaw detection techniques. Thus, one finds a crack at a particular point in a welded structure and determines its length and height, but then it is necessary for someone to decide whether or not that particular crack will prejudice the performance of the structure, i.e. cause premature failure. The term 'non-destructive evaluation' (NDE) is sometimes used for this interpretation stage of the inspection process, but equally it can be argued that NDT ends at the stage where the flaws are quantitatively described, and the final stage is for the designer. NDT is not limited to the finished/assembled stage of

12 xii Preface to the first edition production; if materials are to be used to the limits of their strength to satisfy the demands of sophisticated equipment, NDT must be considered for incorporation at every stage of the design-product-servicemaintenance cycle. Materials must be examined prior to construction; the design must be such as to enable efficient inspection during manufacture and in-service monitoring should be considered as a possible requirement. In recent years a new term has developed: 'condition monitoring' or 'machinery health monitoring'; this also uses NOT methods. The well-established 'big five' NOT methods are magnetic particle crack detection, penetrant crack detection, radiography, ultrasonic testing and eddy current testing; there are a dozen or more other methods which have more limited applications to special conditions or materials, and new techniques are constantly being developed. Magnetic and penetrant inspection are essentially surface crack detection methods to increase the efficiency of visual inspection of surfaces; eddy current testing can detect internal flaws in thin material, but only radiography and ultrasonic testing can be applied to the detection of flaws anywhere through the volume of large masses of material. Each has its own advantages and disadvantages, and each requires considerable operator skill to use the method to its best advantage. NOT is a multidisciplinary subject and as yet there are relatively few professionals adequately trained in the subject; the need for information dissemination, particularly scientifically based data, is especially acute because of the relatively small number of well-trained experts in each particular field. Industrial radiography has been developing steadily for about 40 years, and as the manufacturers of equipment have broadened the range of their products, a large variety of possible techniques has become available, with a wide range of attainable flaw sensitivities. Generally, good radiographic inspection involves a careful compromise between a number of technique parameters and it is comparatively easy, through lack of knowledge, to use an unsuitable or insensitive technique. This book has been written to establish the physical basis of industrial radiology, to help users to understand the techniques, to make the correct choice of technique for a particular application easier, and to enable users to get the best results out of radiological inspection. An attempt has been made to provide as much basic data on techniques and performance as possible; although new forms of equipment could possibly require some of these data to be modified slightly, it seems unlikely that major changes in equipment performance will occur which will render present techniques obsolete, if only for physical reasons. The 'Glossary of Terms used in Radiology' (British Standard BS 2597, 1955) has been used throughout this book as far as possible. According

13 Preface to the first edition xiii to this glossary, radiography is concerned with the production of radiographs, that is, the production of an image on photographic film by means of ionizing radiation (X-rays, gamma-rays). Other means of imaging are also possible: fluoroscopy is the name given to the production of a visual image on a fluorescent screen by means of X-rays and by extension, television-fluoroscopy is commonly used when a closedcircuit television camera is used to view the fluoroscopic screen image and present it on a conventional television display monitor. Although neutrons are atomic particles and not ionizing radiation, they penetrate many materials with progressive attenuation and so can be used with suitable conversion screens to produce 'radiographs' analogous to X-ray images. Neutron radiography is now a well-established term and the appropriate techniques are outlined in Chapter 12. Ionizing radiation has biological effects and the safety aspects of industrial radiology must not be neglected. In this field both medical and industrial radiologists follow the recommendations and terminology in the 'Recommendations of the International Commission for Radiological Units' (ICRU); new units (gray, becquerel, sievert) have officially replaced the better-known older units (roentgen, curie, rad, rem). As the new units are not yet widely used conversions to the older units have been included. RHALMSHAW 1981