Ultrasonic transducers for nondestructive testing

Ultrasonic transducers for nondestructive testing

Research techniques in non-destructive testing, Volume VII Edited by R.S. Sharpe Academic Press, Orlando, FL, USA, 1984 (380 pp, $80) The Editor, Mr ...

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Research techniques in non-destructive testing, Volume VII Edited by R.S. Sharpe

Academic Press, Orlando, FL, USA, 1984 (380 pp, $80) The Editor, Mr Roy Sharpe, must be congratulated for maintaining such a consistently high standard throughout the seven volumes of this series. 1 have personally benefited from earlier volumes and have returned many times to chapters to revise my understanding of topics covered. The present volume is no exception and provides valuable reviews by active researchers on a diverse range of NDT techniques. It is important to note, however, that several of the chapters contain information of value to practitioners of NDT as well as to the research worker. Whilst impossible in a short review to do justice to all eight chapters of the book, I have tried below to give a few comments on each.

W.G. Gebhardt describes in chapter 4 the practical realization of an electronically controlled system of ultrasonic beam forming and steering Further development of the system shows that it is possible from a series of scans to reconstruct a two-dimensional display of a defect. The chapter contains many practical results from which to judge the capability of the system and is therefore of benefit to both practitioners and researchers.

We are all familiar with the range of visual "tricks" presented on our television screens these days which are possible because of the advances in high-speed digital converters and whole-frame video stores. Chapter 1 reviews progress in applying with the aid of powerful modern computers, such techniques to enhance images of low contrast or poor signal-to-noise ratio pictures generated by many NDT methods. This chapter is recommended to anyone who is generating results in two-dimensional pictorial format and has not considered the enhancement possibilities of digital processing Chapters 2 and 3 give detailed expositions on the analysis of electromagnetic fields as potentially used in eddy-current and AC magnetic field techniques. Whilst chapter 2 concentrates on analytical solutions, chapter 3 uses primarily finite difference methods (perhaps we may see a chapter in future on the growing application of finite element methods in this field of NDT). Whilst these are chapters largely for the researcher, both authors turn their attention to the practical problems of equipment design and the more difficult area of signal interpretation.


Mode conversion effects have been used for many years in ultrasonic NDT to generate different types of waves. More recently, techniques to size and characterize defects have evolved that rely on recognizing mode-converted waves occurring at flaws or component surfaces. Bond and Saffari review in chapter 5 the application of such techniques and how, with suitable data processing equipment, more use can be made of the awfilable information to characterize defects. Vast improvements in fibre-optic technology have not only been a major benefit to the communications industry. Culshaw in chapter 6 considers how they may also be used to make sensors for acoustic surface waves, temperature

and strain measurements. The principal advantages of fibre-optic sensors are the lack of interference with the measurement, their immunity to electromagnetic interference and intrinsic safety due to electrical isolation from the test specimen. This is a chapter well worth reading by people with difficult environments in which to make NDT measurements. Reflection acoustic microscopy is explained in chapter 7 together with many examples of its application to studying the surfaces of metallic, crystalline and ceramic materials. Where normal optical microscopy may miss small surface defects or discontinuities, acoustic microscopy may offer distinct advantages. With resolution of less than 1 ttm achievable, the technique should take its place alongside optical, X-ray and electron microscopy techniques in the materials investigation laboratory. Finally, chapter 8 reviews the extremely broad field of the potential application of lasers in NDE. However, the author suggests that industry has been slow to take up the possibilities, perhaps, it is suggested, because of the expense and safety considerations. Many possible applications to QA in the microelectronic industry are illustrated. In summary, this volume contains something of interest to anyone in any field of NDE. Whilst not a book to aid the everyday application of NDE, it should be available to consider those impossible problems we all meet in NDT every other day!

N.F. Haines

Ultrasonic transducers for nondestructive testing M.G. Silk

Adam Hilger, Bristol, UK, 1984 (176 pp, £24, $39) This is a valuable book restrained in presentation and length. The reader is taken through the principles of transducer action and design without being blinded with unnecessary theory. Equally importank he will not be bankrupted by buying it ! The author's restraint extends to limiting the range of transducers discussed to those of proven use for non-destructive testing: these

include piezoelectric and electromagnetic acoustic transducers, with laser generation of ultrasound as a longshot Sections particularly well done include a brief account of the physics of piezoelectric materials, including composites and plastic films, though, regrettably, in a book aimed at practice and claiming to see future trends, lead metaniobate gets only a passing mention.


The longest chapters are on mathematical modelling of transducer action and on methods of altering the pulse shape by mechanical and electrical d a m p i n g This latter chapter is so well done that it seems a pity that the author has not developed the consequences of broadening the bandwidth and shortening the pulse to a consideration of the differences between the field patterns of shortpulse and continuous wave emitters: the discussion and figures of the chapter on ultrasonic beam modification refer to continuous waves while the author himself is well aware thai "'for NI)T work the

ultrasonic pulse is rarely more than a few cycles in length" - - if that. In addition, the field patterns are described in terms of a fluidpropagating medium, the extra complications arising in solids not being covered. The latest references are from around 1981 with a few tkom the author's own laboratory going up to late 1983. This is probably above par for hard-cover monographs nowadays, comparing well, lbr example, with an earlier review by Sachse and Hill published in 1979.

J.P. Weight

Automated visual inspection Edited by B.G. Batchelor, D.~ Hill and D.C. Hodgson

IFS (Pubfications), Bedford, UK, 1985 (561 pp, £35) This book reviews the current status of technology needed to automate the process of inspection of products lor their visual quality and dimensions. In his preface, the principal Editor admits to some difficulty in choice of title, understandably so because when the techniques described are completely successful, 'vision'. defined in the dictionary as the "faculty of seeing', is not involved. Most workers in this field see clearly the benefits of automated optical inspection in terms of increased productivity and improved product quality, but rarely succeed in designing a simple, cost-effective system capable of solving a wide variety of inspection problems. The 13 authors contributing towards the 19 chapters of this book explain why. No attempt is made to conceal the fact that man has not yet come even close to matching the total capability of the trained human hand-eye-brain combination when applied to the task of product inspection. Nevertheless, we read that worthwhile industrial inspection problems have been solved by careful attention to all the technical disciplines and by optimization of the various design parameters relating to a particular requirement. The industrialist planning to employ automated inspection, perhaps for the first time, should study the early chapters which define and describe the place of automated inspection in the production environment and list


criteria to be satisfied in its social and economic justification. Sub-system design and perlormance characteristics are dealt with in the lbllowing nine chapters on illumination, image transfer, scanning and photon detection systems. In view of its importance to the systems designer, it is surprising however, to see little reference to photometry - - it does not even appear in the index. A few worked examples showing why a particular inspection problem requires a particular combination of photon emitter and detector, to achieve the desired result, would have been of considerable value. The importance of the correct method of illumination of the surface to be inspected to achieve the best chance of detecting a given defect is repeatedly stressed but the reader is still left in some doubt regarding the tolerances placed on the numerous interrelated parameters and which component in the total train is likely to be restricting system performance. One even has the impression that many of the numerous examples of inspection systems described were selected more by practical than theoretical considerations. The various solutions proposed do, however, appear credible, probably because all the authors have long first-hand experience in solving real industrial problems. The next six chapters deal with the all important topic of digital video

signal processing the rapid development of which is going to be vital if we are to handle sufficiently quickly and cheaply the large quantities of information usually arising from automated inspection. No attempt is made to conceal the limitations of present systems and t'rom a quick glance at the large number of processed pictures, to illustrate the capability of image processors, one could be forgiven for concluding that they were better equipped to degrade rather than enhance picture content! Many useful problems can, however, be tackled provided adequate care is devoted to the choice of illumination and imaging systems so as not to place too great a strain on the analysis capabil~i,ties of the processor. This group of chapters is probably the most useful to the designer who needs to select an optimized interl~lce between the sensor output and a display and/or control system. It is here that most technical advances leading to the widespread adoption by industry of automated inspection are likely to arise. Mention should also be made of a chapter describing laboratory criteria to be satisfied lbr the establishment of a test facility to enable a wide range of industrial products to be inspected by determining the optimum methods of illumination and defect signal detection. Various facilities of this type are now available in the UK for the benefit of industry and their use is strongly recommended as a first step in considering the feasibility of automated optical inspection. The whole subject is brought to a conclusion in a final chapter dealing with 27 short case-studies including, for example, cylinder bore, disc brake, dinner plate, lamp, liquid crystal display, glass component and sheet product inspection. followed by four detailed casestudies which embody useful and significant operational information. The book has been well planned, is clearly written and printed and contains a large number of clear diagrams, plates and references to other work. It complements the various conference proceedings dealing with selected aspects of the subject and at this price should be read by systems designers and all those contemplating the use of automated optical inspection.

L.R. Baker