|NDT.net June 2002, Vol. 7 No.06|
Perhaps the sign that any new innovation has reached maturity is when it becomes the subject of standards which define how it should be applied. This process has started for TOFD with the issue of a British Standard and the launch of a draft European Standard as described in Chapter 10 of the book. The difficulties in gaining acceptance of the latter indicate that this process has still some way to go. Another related area is that of schemes which verify and certificate the competence of those who apply the method and, here again, there is considerable scope for further innovation.
Fortunately, the difficulty of issuing standards no longer provides an insuperable obstacle to the use of new methods such as TOFD. This is due to the widespread adoption of qualification of entire inspections as an alternative way to demonstrate that an inspection is capable of meeting the requirements placed on it. This process, also referred to as performance demonstration, means that inspections do not have to he specified in detail by those requiring it (though they must still be defined in inspection procedures by those implementing the chosen inspection to ensure they are applied in a uniform way). Instead, their performance is assessed by an independent body through the use of theoretical arguments and practical application to test pieces. Inspections are acceptable so long as they meet the stipulated requirements for defect detection, location and size measurement. TOFD has been subjected to qualification of this type on a number of occasions now and has proved equal to the challenge.
This second edition of 'Engineering Applications of Ultrasonic Time-of-Flight Diffraction' therefore provides a welcome update of the subject and again sets out the principles of the method together with a range of recent applications. It continues to he an essential reference for those with a responsibility for the wellbeing of engineering plant and for those who wish to apply the method.
M. J. Whittle
Nuclear reactors of the PWR type have thick steel walls withstanding considerable internal pressure. It is therefore necessary to establish with a very high level of confidence that there are no cracks bigger than the critical size, in the parent metal, or in the welds. At about the time that the decision to build one of these reactors in the UK was taken, results were published which suggested that conventional ultrasonic inspection techniques could not size planar cracks bigger than the critical size as accurately as would be necessary to achieve the confidence level required.
This led the nuclear industry in both the United Kingdom and Europe, to invest heavily in a research and development programmes aimed at improving ultrasonic inspection of thick-section steel. The programme in the UK covered conventional ultrasonic inspection techniques but also devoted considerable effort to ultrasonic Time-of-Flight Diffraction because it had already shown great promise as a tool capable of accurately sizing planar, through-wall cracks -exactly what was required.
The first edition of our book, published in 1989, came at a time when much of the development work had been completed and several test-block trials had also been undertaken. The technique had proved itself and was being adopted as one of the essential tools, alongside enhanced pulse-echo inspection, for nuclear reactor inspection. Out hope then was that the technique would spread into other industrial sectors. In the intervening years, this has taken place and the technique is now a mature one.
As we enter a new millennium, it seems the right time to bring our exposition of the technique up-to-date. To do this we have kept much the same form as the previous edition, starting with the theoretical background. One of the strengths of ultrasonic Time-of-Flight Diffraction is that theoretical understanding was developed at an early stage and this has been used consistently to develop the inspection techniques used in real applications. The technique, if used correctly, is capable of yielding very accurate measurements of crack size but, to achieve this, it is necessary to have a good understanding of potential sources of error. We have therefore considerably extended the section on errors and how to minimise them.
Since the technique now has more data to back it up, both from more complex test-block trials and more realistic field applications, we have extended the sections covering both these aspects. As a mature technique it has begun to be specified in codes and standards and we have described the current status in this area.
No other industry has been pressing for such a thorough understanding as the safety case for a PWR required, so only a small amount of additional development work has been done since the first edition of the book. Somewhat surprisingly, some of the signal processing techniques that were covered in the first edition are still not regularly applied, despite computer processing power having increased a thousandfold since then. There is room for further work in this area to demonstrate what could be achieved with modern technology.
This book aims to provide a thorough background to the theory and practice of the technique and we hope that it will encourage an even wider range of applications and further advances in capability.
Components are designed with more than adequate strength to resist the stresses arising in normal service and even to tolerate certain levels of abnormal conditions. When failure occurs, it is often because the component contained a defect, normally of a crack-like nature, sufficiently large to cause a major reduction of strength. Such defects may arise from faulty manufacture or the effects of service in a corrosive environment and may be enlarged by fatigue. To ensure their absence after manufacture or to detect them in service, a variety of non-destructive testing (NDT) techniques may be used. Of these, ultrasonic testing is the most widely applicable, being capable of detecting and sizing cracks in a wide variety of locations and orientations, in many materials used in engineering and even for considerable thickness of material (greater than 300mm in steel, for example). A particular type of ultrasonic testing technique is the subject of this book.
Ultrasonic testing makes use of high frequency, but very low amplitude, sound waves to detect, characterise and size defects in components. The sources and receivers of these ultrasonic waves are transducers, usually, but not always, made from a piezoelectric material which deforms under the application of a voltage. Applying a voltage generates a mechanical distortion which propagates into und travels through the component as a wave. When such a wave arrives at the receiver, the piezoelectric material converts this into a voltage which depends on the orientation and magnitude of the distortion.
Other methods of creating and detecting ultrasonic waves are possible, such as electromagnetic acoustic transducers (EMATs) which essentially use (electro)magnetostriction as the method of translating a distortion into a voltage and vice versa, or the use of lasers to ablate part of die surface to generate an ultrasonic pulse coupled with an interferometer to read the surface ripples on the component when signals arrive back. While most of what we discuss in this book is independent of the mode of generation or reception of the ultrasonic waves, we usually have in mind ceramic piezoelectric transducers.
The physical method of sending and receiving signals may be unimportant bot the characteristics of the signals generated and received can be important. As we shall see later, the pulse length, the angular spread of the ultrasonic beam, the polarisation of the waves in the signal and their phase are all important.
Pulse-echo ultrasonic inspection techniques rely on the arnplitude and range of a signal returned from the defect to the interrogating equipment in order for the defect to be detected, sized and, possibly, characterised. The process governing the amplitude is usually specular reflection, in which any crack acts like a mirror for die ultrasound. For a given arrangement of ultrasonic transducers on the component undergoing inspection, this process of specular reflection can only occur for a limited range of orientations of the defect. In the absence of a specular reflection, the signals retumed will be those arising from diffuse scattering from the surfaces of the crack and by diffraction from the edges of the crack. These diffracted signals are of particular interest, since, being associated with the extremities of the defect, they may be used to determine the size of the defect accurately and thus assess the integrity of the component. The ultrasonic Time-of-Flight Diffraction technique is based on the exploitation of the exploitation of these signals diffracted from the defect edges.
|Fig1: The two probe basis of the Time-of-Flight Diffraction technique|
In this chapter we present predictions, based on just such a mathematical model, of the magnitude of Time-of-Flight Diffraction signals, compared with those from a reference reflector. We use a model for two reasons. First, it is much easier to vary the parameters of defects within a model than it is experimentally with test-blocks. Secondly, it is possible to isolate the different factors influencing the outcome in a more straightforward way. The theoretical predictions are compared with experimental data as appropriate.
The results of these calculations illustrate how the signals from a variety of cracklike defects are expected to vary with the shape, size and orientation of the defect and highlight one of the strengths of the technique: its relative insensitivity to the orientation of cracks.
The first results we shall present are for the case in which the centre of a planar crack and the centres of both transmitter and receiver lie in the same plane, normal to the inspection surface. This is not a severe restriction, since in almost all cases the probes will be scanned over the defective region and this configuration will be passed through during the scan. Sometimes the shape of the component may preclude reaching such a position and so we later present results applicable to less restricted geometry.
In this chapter we shall confine ourselves to analysis techniques which are generally applicable and sufficient for a fall analysis in simple geometries like butt welds in flat plates or girth welds in cylindrical vessels. When the geometry is more complicated, the analyst needs some geometrical assistance from the system to help locate the sources of defect signals and this will be described in Chapter 6.
At the time of the Defect Detection Trials, it was the common practice to collect TOM data on systems which had few or no facilities for data analysis, the data being transferred, for analysis, to other computer systems containing what were then very expensive image display systems. As the cost and size of computers and image display equipment decreased, the analysis functions for TOM tended to be more and more integrated into the data collection system, so that, now, it is usual for the whole process of collection and analysis to be carried out on one portable instrument.
Where two cylinders intersect, for instance, two cylindrical components of an offshore structure or a nozzle attached to a pressure vessel, the weld forms a three dimensional saddle shape. Probes with a given, fixed, beam angle placed on any one of the surfaces cannot always cover the entire weld volume which needs to be inspected. Thus, design of scanners for such geometries necessitates even more care than is taken with scanners for the simpler geometries of flat plates.
The Time-of-Flight Diffraction technique opens up new alternatives for inspection of complex geometries compared with pulse-echo techniques because of its insensitivity to the relative orientation of probes and defect. With pulse-echo techniques relying on specular reflection it often proves very difficult to arrange for probe beams to illuminate areas of concern, such as welds, at near normal incidence. Tandem techniques also prove difficult because the back wall of the specimen is very often not parallel to the inspection surface in nozzle to shell welds or offshore nodes, for example.
Most metallic crystals show anisotropic elastic behaviour but, in fine-grained bulk samples with no preferred grain orientation, the macroscopic properties are isotropic. If, however, the grains approach in size the wavelength of the ultrasound, or are preferentially aligned, the resultant anisotropy and scattering affects ultrasonic inspection. This problem is particularly relevant to austenitic steels, both in bulk and in the form of cladding layers on ferritic steel. Section 7.1 discusses the problem of applying the Time-of-Flight Diffraction technique to such material.
Another problem arises from the differences in temperature and stress levels during service and those occurring when inspection is carried out. Examples of this are: aircraft, where in flight at 33,000ft the temperature is -25°C and the pressure is ~ 0.3 bar, which contrasts with typical inspection conditions of a temperature of 20°C and a pressure of ~ 1 bar; nuclear reactor coolant circuits, where inspection is almost always carried out at temperatures and pressures well below their normal operating point; offshore structures, when inspection is carried out in calm weather when the wind and sea loadings are very different from those during severe weather. As a result of these changes in ambient conditions between normal service operation and those during inspection, cracks which were under tensile stress sufficient to cause growth during some conditions of service could be under compressive stress when inspected. The effect of compressive stress on the amplitude of Time-of-Flight Diffraction signals is discussed in Section 7.2.
Finally, in cylindrical geometries, the speed of the lateral wave, which is used as a timing reference, is found to vary from its value on a flat plate. It is necessary to know what this variation is, if Time-of-Flight Diffraction is to be applied confidently to curved geometries and this problem is discussed in Section 7.3.
Both the laboratory tests and the field experience can be embodied in a standard procedure for applying the inspection technique once it has reached a certain maturity. The Time-of-Flight Diffraction technique is now at this stage, having been encapsulated in both British and European Standards. This follows several demonstrations of capability in a wide range of large-scale test-block exercises which are reviewed in this chapter. Results from several significant test-block trials are presented in some detail to highlight the capability of Time-of-Flight Diffraction for accurate determination of the through-wall extent of cracks. The test-block exercises test technique capability, rather than reliability or repeatability in practice.
In this chapter, we confine our discussion to the principal test block exercises. A number of other, generally smaller and more specialised, exercises are covered in Chapter 9.
Capability is not sufficient in itself., reliability in practice is also required. The level of reliability required of an inspection is that which, when combined with a knowledge of the severity of defects, will lead to the desired level of structural integrity under normal operation or possible accident loading conditions. Even good techniques applied reliably will exhibit some spread of errors which have implications for the structural integrity of the component under test. This concept is reviewed with special reference to the pressure vessel of a pressurised water reactor; the approach is, however, universally applicable.
Before embarking on a discussion of the results obtained in test-block trials, we bring out some of the limitations of such tests.
It is in the nature of routine applications that they rarely give rise to published papers. Much of the work cited in this chapter on applications could more accurately be described as studies of capability, undertaken before embarking on routine deployment of the technique. Some of the applications have been referred to in earlier chapters but are presented here again to make this survey as comprehensive as possible. The content of the cited papers is described only briefly and the reader is referred to the original references for greater detail.
Wedgwood [19951 reviews the advantages and disadvantages of TOFD vis-a-vis other ultrasonic inspection methods, citing better reliability as one of the major reasons for choosing TOFD. He describes some applications of TOFD to offshore and nuclear plant but does not attempt a comprehensive list of all applications to date. In the following sections we present applications grouped into specific areas.
The ultrasonic inspection is designed to detect, size and possibly characterise defects in the component. The next stage of assessment is to classify those defects as acceptable or unacceptable. Whether a defect is acceptable or not depends on the component in which it is found and the stresses to which it will be subjected. Defects which are sufficient to cause the component to fail under applied loads which might occur in practice will be classed as unacceptable. In order to ensure that the assessment process is properly carried out, procedures are laid down in codes and standards.
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