Ultrasonic determination of the internal structure of solids

Abstract

This project commenced from the concept that some of the wide variety of signal processing techniques developed in radar and sonar systems should be capable of useful application in the field of materials inspection. The particular aim was to assess the possibilities of an imaging arrangement and with this objective the author's main topic was defined as the signal processing section of an overall system. Inevitably perhaps, a multiplicity of practical difficulties associated largely with transducers and the actual handling of the signals before the imaging operations, diverted the main part of the effort away from the original topic into these other problems. The thesis itself is organised into four main sections. Section I, containing Chapters 1 and 2, is by way of an introduction to the topic, discussing in particular the pattern of development followed and surveying the work reported in acoustic holography. Section II, consisting of Chapters 3 through to 7, contains a theoretical analysis of the general problem of reconstruction of fields from measurements taken at some surface (Chapter 5) and of many details relating to specific portions of the complete imaging arrangement. Section III, (Chapters 8, 9 and 10) is a brief description of the particular solution synthesised in the present work to meet the practical requirements of an imaging system. The final section, Section IV, presents the experimental results obtained (Chapters 11 and 12) and a general summary of the whole project with suggestions for further development (Chapter 13). A number of appendices contain details of some pieces of apparatus, discuss a computer simulation of transducer behaviour and hold some lengthy mathematical expositions. Chapters 5 and 12 are the most important in the whole thesis. The theoretical development of field reconstruction from recordings of wideband, elastic wave signals as presented in Chapter 5 is based on well known mathematical techniques but as far as the author is aware these have not previously been fully applied to the situation treated here. Chapter 12 contains a record of many of the actual images obtained and discusses the correlation with theoretical predictions. Chapters 6 and 7 are important to the determination of potential performance of the scheme devised here and similarly Chapter 2 is significant as a record of previous work with which to compare the present results. During the period of this project the whole topic of ultrasonic imaging has gathered a rapidly increasing number of workers in many laboratories and a large number of results displaying images of considerably greater fidelity than any results obtained here, have been published. However, it is considered that the local effort offers much greater potential of a very high level of ultimate performance since the other methods so far reported are near the limits of their capabilities whereas the developments described in this thesis are capable of improvement in nearly every significant aspect by direct extension of the techniques already established by the work to date. The most significant of these is considered to be the extension into wideband imaging, a capability already incorporated in the special apparatus constructed but one which could not be fully exploited because of present computer limitations. As far as is known, no other work in progress offers this possibility.