Journal of Nondestructive Evaluation, VoL 9, No. 2/3, 1990

Preface

This special issue on the modeling of the ultrasonic inspection concentrates on realistic aspects of industrial problems. Industrial sectors such as aerospace, defense, transport, electricity generation, oil and gas production, and supply and civil engineering use ultrasonic inspec- tion to ensure that materials are of good quality and components are free from defects large enough to threaten their economic or safe operation. The selection of 11 papers covers some problems specific to particular in- dustries while others are of a more generic nature.

Modeling plays a part in developing an understand- ing of any physical process. Existing models, which have been successfully validated against experiment, can be used to further understanding, to aid in the design of experiments, to optimize inspections and to demonstrate their reliability, and to help in the interpretation of sig- nals observed in real inspections.

Much has already been done with analytical solu- tions to the forward scattering problem for elastic waves impinging on simple shapes and there are many good articles and textbooks on this subject. Industrial prob- lems, however, usually require numerical answers to problems which are often more complicated than those with analytical solutions. For example, one often wishes to know not just which particular wave mode will give the most reliable information about material character- istics including defects but also how many dB the signals are above the noise level and how this will change when the frequency changes from 2-5 MHz.

Components are rarely semi-infinite half-spaces, materials are not necessarily homogeneous or isotropic, and defects of concern are indeed sometimes spherical or cylindrical voids, but are often not, and they do not always occur in isolation. Each of the papers in this issue tackles a problem with some additional, realistic com- plication.

There is a detectable difference of approach be- tween the papers in this issue. Some papers model im- portant generic scattering problems, while others are tied more closely to particular industrial problems. There is also a difference between those papers which present semi-analytical solutions, in which a key element is the mathematical analysis, and the purely numerical models, for which the enabling mathematical analysis has gen- erally been carried out elsewhere. Modeling is the art of compromise: a useful model nearly always makes some

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simplifications but retains the crucial elements of the real situation. In this issue, you will find papers that do not mention defects at all because propagation is so com- plicated, for example, and one paper which mimics the whole inspection including the transducer, scan pattern, and defect characteristics. The order of the papers in this issue represents this progression toward a model of an inspection system.

The first two papers are of the semi-analytical kind. That by Chimenti and Nayfeh, looks at the reflection and propagation of guided elastic waves in fiber-rein- forced composite materials loaded with a fluid. Next, Achenbach and Zhang derive explicit expressions for the reflection and transmission of time harmonic waves by a layer of damaged material containing a distribution of similar microcracks.

The issue continues with seven papers applying a range of numerical techniques to problems of ultrasonic propagation and scattering. Harumi and Uchida illustrate the breadth of elastic-wave scattering phenomena pos- sible. Such visualizations, using either computed dis- p lacements or those obtained with photoelas t ic experiments, are an important tool in developing an un- derstanding of inspection problems.

The boundary element method for elastic-wave problems is covered by the next two papers. Goswami et al. apply the method to the acoustic scattering from an arbitrarily-shaped elastic body submerged in a fluid. Schafbuch et al. apply the boundary element method to scattering by irregular defects, giving comparisons with iheveral calculation techniques, advantage other The o f

e boundary ei~ement method is that only surfaces need to be discretized thereby reducing the computer re- sources required to model intrinsically 3-D problems.

Finite element methods and their application to ul- trasonic modeling is considered in the paper by Lord et al. The advantage of finite elements is their ability to handle complex defect, or component shapes. This ad- vantage is shared with the boundary element method. Finite elements are, however, to be preferred for prob- lems with inhomogeneous materials since elastic con- stants can vary from element to element.

Problems of inhomogeneous materials, where the variation of elastic constants takes place over a distance less than or about the ultrasonic wavelength are the sub- ject of the next two papers. Both papers compare ray

0195-9298/90/0900-0049506.00/0 © I990 Plenum Publishing Corporation

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tracing with numerical solution of the wave equation. Nouailhas et al. consider inspection of a safety critical item--the safe end weld of a nuclear reactor. Ray tracing and finite e!ement modeling techniques are compared. Harker et al. consider propagation through particular weld macrostructures and compare ray tracing with 2-D and 3-D finite difference models.

Firth and Gould also use a finite difference model but overcome one of the usual limitations of such models, ~b.at oniy the region close to the scatterer can be treated, by developing a hybrid model which uses the Helmholtz integral to project the scattered field far from the scat- terer.

The final two papers revert again to semi-analytical models but represent a good deal of the inspection sys-

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tem itself. Roberts presents a model of the acoustic mi- croscope for materials characterization. Finally, Chapman presents a system model of ultrasonic inspection of nu- clear power plant components. The model, implemented as a suite of computer programs, can be used on a mi- crocomputer by practical nondestructive testing engi- neers.

The list of topics covered in this issue is not, of course, comprehensive. It represents a snapshot of a sample of what is going on around the world at the beginning of the last decade of this century. My thanks to all the authors who have made this an interesting, exciting, and significant special issue.

Andrew Temple Harwell, September 1990