NDT of welds: state of the art

NDT of welds: state of the art

PII: SO963-8695(96)00010-2 NDT of welds: R. J. Ditchburn, state NDT&E International, Vol. 29, No. 2, pp. 11 l-l 17, 1996 Commonwealth of Australia ...

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PII: SO963-8695(96)00010-2

NDT of welds: R. J. Ditchburn,

state

NDT&E International, Vol. 29, No. 2, pp. 11 l-l 17, 1996 Commonwealth of Australia copyright 0 1996 Published by Elsevier Science Ltd. Printed in Great Britain 0963-8695/96 $15.00 + 0.00

of the art

S. K. Burke and C. M. Scala

Aeronautical and Maritime Research Laboratory, Defence Science and Technology Organisation, PO Box 4331, Melbourne, Victoria 3001, Australia Received 9 January

7996

Major advances have been made in recent years in nondestructive testing of welded steel structures. This paper reviews the latest developments in: (i) radiographic inspection; (ii) ultrasonics; (iii) alternating current potential drop technique; (iv) measurement of residual stress, and (v) in-production weld inspection. Areas where further useful progress can be made are also identified. Commonwealth of Australia copyright <> 1996. Keywords:

inondestructive

testing, weld inspection,

The whole area of nondestructive testing (NDT) of welded structures is currently undergoing a period of rapid change brought about by a combination of technological, regulatory and economic factors worldwide. The driving forces; for changes in NDT practices include the introduction of new materials and welding processes, advances in the inspection technologies themselves, new approaches in the philosophy of weld acceptance codes and inspector certification[‘], increasing pressures for (cost-effectiveness and quality in production, and the need to extend the life of ageing infrastructure.

defect sizing

magnetic article and dye penetrant which are covered elsewhere P21. Firstly, developments in the two major inspection techniques, radiography and ultrasonics, are considered; then nondestructive measurement of surfacecrack depth using AC potential difference is described. Advances in nondestructive measurement of residual stress, which is often neglected in discussions of weld inspection, is then presented. Finally, the use of nondestructive techniques for on-line monitoring of the welding process itself is reviewed.

Radiographic

In this paper, we present a review of the state of the art for NDT of welded structures. The present review was motivated, at least in part, by related developments in ship and submarine construction in Australia, where the use of higher strength steels allows ships to be constructed using thinner steel plate leading to reduced welding costs, and, in the case of submarine construction, high-strength steels are required for enhanced shock resistance. The use of such steels in welded structures can result in fsmaller critical defect sizes and an increased tendency towards hydrogen cracking so that NDT is required both (i) to detect defects with high reliability and (ii) to provide accurate defect size information. The decision was made to limit the scope of this review to inspection of steels, and to concentrate on the NDT technologlles themselves rather than the issues of acceptance codes, standards and certification, which in themselves would merit a separate, lengthy review. The review focmes on new innovations to meet the changing requiremems of NDT for weld inspection rather than giving details of the well-established inspection techniques such as visual, eddy current,

inspection

X-ray or gamma ray radiography, together with ultrasonics and magnetic particle inspection, are the mainstays of weld inspection. Until the advent of ultrasonic inspection, radiography was the only available method for finding buried defects in welds and current acceptance codes for welds have evolved principally from a knowledge of the inherent advantages and limitations of radiographic testingi3]. The underlying physical principles of radiographic inspection have been known for nearly a century and, not surprisingly, radiography has evolved during this time into a mature technology. Radiography relies on detecting a change in transmitted intensity of a gamma-ray or X-ray beam arising from differences in the absorption coefficient of a defect and the surrounding metal. As the X-ray absorption coefficient depends strongly on material density, radiography is particularly effective at detecting volumetric defects which contain either extra mass or missing mass (such as slag inclusions or porosity). Radiography is less

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in using radioscopy in applications requiring a high flaw sensitivity. The more advanced digital radiographic technqiues, such as computerized tomography (CT) and Compton backscatter radiography[“], have had little or no impact in weld inspection. Both these techniques involve greater inspection times and there are geometric limitations in the use of CT.

effective for detecting arbitrarily oriented planar defects (such as tight cracks or lack of fusion) in thick metal sections unless the likely location and orientation of cracking is known in advance. The thickness of steel which can be inspected depends on the incident X-ray energy: a commercial 150 kV X-ray set is capable of inspecting thickness up to some 20 mm while thicknesses up to 500 mm can be inspected using higher energy X-ray beams (25 MeV) generated by linear accelerators or betatrons. The principles of gamma-ray radiography are the same as X-ray radiography except that a radioisotope is used as the radiation source rather than an X-ray tube. Most gamma radiography is performed using either Co6’ (for thicknesses in the range 50-l 50 mm) and Ir’92 (for thicknesses lo-75 mm). One of the newer sources, Yb169, can be used for thicknesses in the range 2- 10 rnrni4]. The main advantage of radioisotope sources is that the associated equipment tends to be cheaper and more portable than X-ray sets, and is less reliant on the availability of electricity and water supply. However, due to the larger effective source size, gamma-radiographs are generally of lower quality than the best X-radiographs of the same componentL2].

The main limitations of radiography lie in the area of crack detection and crack sizing. As discussed below, radiographic techniques are less reliable than automated ultrasonic techniques for crack detection, particularly in thick-section welds. Furthermore, unlike ultrasonics, conventional radiography cannot provide accurate measurements of the through-thickness crack dimension (ie the crack depth) i2].For in-service applications, a knowledge of crack depth is essential if accept/reject decisions are made using the principles of fracture mechanics. Needless to say, the other significant limitation of radiography lies not with its technical performance but with the safety hazards associated with the use of X-ray and gamma radiation. A significant effort is required to ensure a safe working environment, particularly when inspections are carried out in the field or when radioisotopes are used.

The benchmark for radiographic testing of welds is still high-quality film radiography and good radiographic practice is now enshrined by a series of national standards, covering factors such as choice of voltage, film-source distances, intensifiers, image quality indicators, film density, film processing etc. There has been a number of advances in film radiography over the past lo-15 years including more reliable microfocus tubeslsl and the application of image processing techniques to film radiographsL6]. Here, the photographic image is digitized and computer techniques are used to sharpen the image and increase image contrast.

Ultrasonics Ultrasonics was introduced as an NDT technique for weld inspection in the 1960’s. Since then, the technique has undergone extensive development and gained increasing acceptance. Consequently, ultrasonics is now the major technique used for validation of welded structures in many in-service inspection applications, eg in off-shore structuresl’21, in nuclear and pressure vessel industries[13”41and in a range of naval applications1151.

The most significant recent development in radiography has been in radioscopy (or real-time radiography) which is essentially filmless radiography. There are several useful methods for recording transmitted X-ray intensity without using film[71. For weld inspection, the most commonly used techniques rely on the conversion of Xrays to visible light using a fluorescent screen or X-ray image intensifier and coupling the light output to a TV or video camera. In future applications, discrete solid-state linear or area detector arrays offer potential improvements due to decreased size and improved spatial resolution. The principal advantages of radioscopy are that it is well suited to automation and the images of the component under inspection are available directly without time delays due to film exposure and processing. Furthermore, as the images are provided in digital form, image processing and automatic defect interpretation software can be readily incorporated into the inspection system [7-91. High quality images can be obtained using radioscopy if a microfocus source and large geometric magnifications are used. However, the technique is not yet considered to be a replacement for all film radiography, and Halmshawllol recommends caution

The emergence of ultrasonics as a preferred technique over X-radiography in these in-service inspections is due both to inherent limitations in radiography and to actual benefits in applying ultrasonics. As described above, radiography is excellent for identifying volumetric defects but is limited in its ability to detect or size planar defects, such as cracks, which are likely to be the more serious defect type. Ultrasonic waves are scattered by planar and volumetric defects, making the ultrasonic technique useful for detecting and sizing both types of defects. Even closed cracks are detectable by ultrasonics, provided that appropriate procedures are used[16’171. Ultrasonics also readily gives depth information concerning a defect, whereas for X-rays, specialized and expensive techniques such as computer tomography are needed to obtain such information. Ultrasonics also offers benefits over radiography in terms of cost savings through increased productivity. Finally, in the 1990’s the increasing concerns about radiation safety are a severe disincentive to the continued use of X-radiography. In the last few decades, ultrasonics has developed from a purely manual technique, to a manual technique with

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computer-assisted processing, to the use of automatic scanners and more rece:ntly to the development of fully automated systems incorporating multiple piezoelectric transducers for weld assessment (*5~18-201.Reliability studies on the use of this range of increasingly sophisticated systems for defect detection have formed a major factor in establishing the credibility of ultrasonics for weld inspection. Studies such as the Programme for Inspection of Steel Components (PISC) I, II and III have aimed to achieve best practice in ultrasonic ins ection in the nuclear and pressure vessel P Outside the PISC studies, useful work industries[13,14. has been carried out to determine reliable procedures for inspecting specific weld geometries including single-V and double-V weldsI’81and also butt-welds”‘]. Several of these studies have also compared the reliability of radiography vs ultrasonic inspection 118,‘91. Overall, the results of the reliability studies indicate that the probability of detecting a defect with ultrasonics increases with the degree of sophistication of the system. According to Lebowitz and DeNale’ the results also indicate that manual ultrasonics, which involves the least sophisticated ultrasonic procedures, ‘can be expected to reject an equal or greater percentage of the discontinuities present than will radiography’.

Peak Response

(4

20 dB Drop Technique

Transducer B n

Transducer A / r

/

I

I

I

I

\

Waves

(b)

TOFD Technique

Figure 1 Ultrasonic defect sizing. (a) The 20dB intensity drop technique: where the transducer is positioned at points either side of the peak response corresponding to a decrease in received intensity of 20dB. The distance between these points is used along with probe calibration characteristics to estimate the defect length. (b) Time-of-flinht-diffraction (TOFD) techniaue: A two orobe technique used to determine‘ crack’size and location itilising the diffracted waves from the tips of the defect

Ultrasonic validation of welded structures requires not only reliable defect detection but also sufficiently accurate defect location and sizing to allow acceptance/ rejection criteria to be correctly implemented. Manual ultrasonic systems usually rely on the use of amplitude dependent techniques flor defect sizing. Techniques commonly used are the 20dB drop (shown in Figure la), the 6dB drop, or comparison with the amplitude from a drilled hole. However, these techniques are known to be inaccurate. The inaccuracies are caused not only by the effects of defect shape, orientation and location, but also by attenuation, coupling, resolution The 1. incorporation and equipment characteri sties (231332’ of computer-assisted processing into ultrasonic systems has allowed the easy implementation of potentially better methods for defect detecuon and sizing such as time-offlight-diffraction (TOFD)[21 (see Figure lb), eg in PISC II, the addition of TOFD to standard procedures gave nearly perfect results in terms of required rejection rate for defects[22]. Important advances in defect sizing have also been made possible by the incorporation in automated ultrasonic systems of ultrasonics imaging based on synthetic aperture focusing (SAFT) and variants such as SUPERSAFT[20,231.

is relatively uncomplicated, the microstructures of austenitic welds have caused special concerns. These materials strongly attenuate ultrasonic waves, cause high background noise due to scattering from the large grains present, and result in skewing of the ultrasonic beam unless the propagation is along principal crystallographic axes121.Thus, much recent research has been directed toward the development of specialized ultrasonic techniques to deal with these complications. Considerable progress has already been made113’241,especially under PISC II and III where detailed modelling of wave propagation in austenitic materials has been undertaken[251. In the future, these models should allow more accurate estimation of location and sizing errors for specific defects, and provide the basis for improved codes for inspection of austenitic steels and weld steels.

The development of reliable procedures for the application of ultrasonics to weld inspection has required an understanding of the interaction of ultrasonic waves with the various types of weld defects, of wave propagation in complicated geometries, of the particular problems caused by inspecting for defects close to the surface of a structure, of the effects of cladding and other microstructural influences on wave propagation12~13~221. While wave propagation in ferritic and light-alloy welds

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In today’s world, there is an increasing need to minimize the cost of weld inspection. The advent of automated scanners, the use of multiple probes and computerassisted processing in modern ultrasonic systems have reduced costs by increasing both the speed and reliability of inspection[26]. On the negative side, equipment and calibration costs are higher with automated equipment. Also, the costs in actually interpreting ultrasonic data could rise due to the recent advances in ultrasonic systems, since all types of defects and even very small defects can be detected, whether or not the defects are critical. The solution to this problem would be improvements in the

R. J. Ditchburn

et al.

(ACPD) methods are the only two established nondestructive techniques used for measuring crack depth in welds. Unlike ultrasonic inspection, which is used both for detection and sizing, ACPD is used almost exclusively for crack sizing. The ACPD method is only applicable to surface breaking cracks and requires electrical contact with the specimen.

automated application of acceptance/rejection criteria on defect criticality [27]. Hence, considerable effort is now being directed towards the development of neural networks to be used in ultrasonic systems to classify defect type, size and location, and resulting conformance with a particular inspection code. Very promising results have already been obtained in several laboratories in studies both on simulated weld defects, where a 100% correct classification rate was achieved in defect type1271,and on real weld defects where success rates of the order of 90% were achieved using a variety of methods[281. Preliminary work has also been made on the automated application of acceptance/rejection codes via neural networks[‘51.

The ACPD technique involves applying an alternating current to a conducting specimen using contacting probes to establish a uniform thin-skin electric field on the surface of the specimen as shown in Figure 2(a). A surface breaking crack disturbs the current flow, resulting in a potential drop across the crack. This potential drop can be measured using point contact techniques and the crack depth can then be calculated from the ratio of the surface potential difference measured (i) across the crack and (ii) on an unflawed region adjacent to the crack, as depicted in Figure 2(b). In cases where the material is homogeneous and the electric field remains uniform on the specimen surface and the crack faces, the crack depth can be calculated using a simple one-dimensional calculation. Nonuniformity of the electric field can lead to errors if the one-dimensional formula is used outside its range of validity and recent improvements in both probe design and theoretical interpretation have been made which allow for correction of field non-uniformity P9,301

Clearly, neural networks will only prove successful if they can be trialed on representative data. However, representative data can prove expensive to acquire. For example, the PISC programme is currently costed at $200M[261,and it seems unlikely that this type of effort will be duplicated in other industries in the near future. An alternative approach would be to trial the networks using data generated from robust mathematical models of the interaction of ultrasonic waves with weld defects. The development of such models is a continuing PISC objective under PISC III. Until these mathematical models are more complete, an emphasis on the use of scientifically and technically qualified staff seems necessary for ultrasonic weld validation[**] .

Unlike a direct current which is transmitted through the entire volume of a specimen, an alternating current is carried only in a thin layer at the surface by virtue of the electromagnetic ‘skin effect’ so that the AC potential drop technique is more sensitive to surface breaking cracks than its DC counterpart. The depth of penetration

For the future, many challenges remain in optimizing ultrasonic inspection of welds. Substantial improvements are possible in the application of advanced methods such as TOFD. Also, the implementation of neural networks has only just started. Various options exist for the improved generation and detection of ultrasound in welding applications, eg by the use of phased arrays, laser techniques (as described below) and other specialist probes. A number of additional factors need to be considered in the ultrasonics reliability area, eg residual stress, the effect of higher frequencies, more extensive consideration of real rather than simulated defects[221.

Uniform Surface

Greater consideration also needs to be given to the overall cost effectiveness of inspection iz6j. One of the elements in maximizing cost-effectiveness is the selection of the most appropriate technique for a given inspection, including the possible use of more than one technique to validate different parts of a welded structure. For example, magnetic particle testing is already used in conjunction with ultrasonics for rapid and cost-effective detection of surface cracks in welds. New electromagnetic methods could also have a role to play here (as discussed in the following section). Finally, there are clearly challenges in implementing advances in ultrasonic inspection technology into the codes for weld validation.

Alternating technique Ultrasonic

and

current

alternating

potential

current

drop

potential

Figure 2 The alternating current potential drop (ACPD) nique. (a) Shows the uniform alternating electric field incident surface breaking crack. (b) The voltages measured across the (VI) and adjacent to the crack (V,) are used to determine the depth

drop

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techon a crack crack

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of the AC fields into the metal is determined by the electromagnetic ‘skin depth’, which is a function of the AC frequency and the electrical and magnetic properties of the metal. The skin depth can be optimized for a given material by adjusting the frequency of the alternating current. This allows a more sensitive measurement of cracks in the specimen material by producing a larger potential drop for a given input current[311. Measurements can be made along the entire crack length to establish a crack profile and for most situations the known theoretical relationship removes the need for calibration. More elaborate models than the simple onedimensional model h,ave been used successfully to measure the crack depth at the interface between an austenitic and ferritic steel in a transition weld[32].Crack depth measurements w:ith a mean error better than 3% were obtained when co’mpared to results from destructive examination. In practice, this accuracy may not be achieved if there is significant crack closure or crack-face bridging.

are time consuming and suffer from the uncertainty that the component which was destructively sectioned may not be truly representative. Hole drilling techniques are also well established but cannot provide details of the stress distribution at any great depth and also introduce a degree of damage into the structure. The most widespread nondestructive technique for measurement of residual stress is based on X-ray or neutron diffraction techniques[351. These techniques rely on measuring interatomic spacing (and hence strain) by accurate measurements of high-angle Bragg diffraction peaks. The measured strain is then converted to stress by using an appropriate elastic constant. Due to the limited penetration of X-rays into steel at wavelengths suited to such diffraction measurements, the X-ray technique is limited to measurements of near surface residual stress (ie averaged over the first 100,~~rnor so of the surface). Neutron diffraction on the other hand, can provide information on the residual stress distribution to a depth of 2-3 cm in steels, with a position resolution of l-2 mm. While portable X-ray residual stress units are commercially available, neutron diffraction measurements must be carried out at specialized nuclear establishments and the overall size of components which can be tested is limited. The principal impact of the neutron diffraction technique has been in fundamental studies of residual stresses, and for validation and calibration of a new class of magnetic techniques, described below.

The surface current introduced into the specimen by the ACPD technique induces a magnetic field in free space above the specimen surface. Mapping of the perturbation of this magnetic field provides an alternative means of measuring crack depth and crack length without the requirement for a contacting probe[30,331.This technique is termed alternating current field measurement (ACFM). The large perturbation of the magnetic field that occurs at the ends of a defect also allows this method to be used for defect detection. Therefore ACFM offers the capability of both detection and sizing of surface breaking defects without the need for calibration and without the requirement for cleaning to bare metal. Recent improvements in instrumentation for both techniques have allowed phase measurement which is essential for subsurface crack sizingL301.

Measurement

of residual

The most notable recent advance in nondestructive measurement of residual stress has arisen through the refinement of two magnetic methods, stress-induced magnetic anisotropy and magnetoacoustic emission, leading to the development of a reliable, portable method for the in-situ measurement of near-surface biaxial stress in ferritic steels I34. This latter development has been described as being ‘of strategic importance to all sectors of European industry where knowledge of stresses is crucial to product quality or structural integrity’.

stress

Residual stresses are introduced into welded structures by virtue of the large temperature gradients produced during the welding process. For example, large tensile residual stresses, of the order of the yield stress of the weld metal, can be produced as colder parent metal resists the contraction of the weld during solidification and cooling. The magnitude of such residual stresses can be reduced by post-weld heat treatment or by vibratory stress-relief. In cases where stress relief cannot be adequately carried out, or in highly critical structural elements where uncertainty in the total stress (applied plus residual stress) cannot be tolerated, it is desirable to have a knowledge of the residual stress distribution.

In-production

weld

inspection

On-line monitoring and control of the welding process has the potential to improve weld quality and increase productivity in automated welding. Weld monitoring and control can be achieved by the integration of realtime nondestructive evaluation techniques with the welding process. In-production weld inspection can improve weld quality and may provide a significant cost reduction. The welding parameters can usually be adjusted to prevent defects from forming. Furthermore, if welding defects do occur the flaws can be found and repaired before they are covered by subsequent welding passes, leading to a decrease in the level of post-weld inspection and repair.

There are several approaches to the measurement of residual stresses in welded structures, all of which are limited in their applicability[341. Techniques based on destructive sectioning can provide detailed information on the subsurface stress distribution but these techniques

Good quality welds rely on the correct weld pool size, geometry and position relative to the weld preparation.

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spot-welding can be monitored ultrasonically and are making progress in developing an in-production monitoring system for resistance spot-welding.

Feedback Controller Information on Size and Position of Weld Pool

Using a piezoelectric transducer and couplant presents the possibility of contamination of the weld by the coupling medium, obviously impractical for production. To overcome this problem, a non-contact ultrasonic system has been developed [39]. The system developed uses a pulsed Nd:YAG laser for ultrasound generation and an electromagnetic acoustic transducer (EMAT) for ultrasound reception.

Voltage and Current Control

Figure 3 Schematic diagram depicting the production monitoring of the welding process

principle

of

Two other non-contact techniques are currently under investigation. The first technique involves the simultaneous observation of the infrared (IR) and the ultraviolet (UV) radiation from the welding process using dualwavelength fibre-optic sensors. The radiation is produced both from the hot melt pool and from the plasma produced by the beam/vapour interaction. The technique has been successfully employed to indicate disturbances encountered in laser welding[411. The second technique uses in-production processing of video images. Encouraging results have been reported that provide weld joim area and bead centreline cooling rates in GMA welding 1421 . This information is then used by a fuzzy logic controller and an artificial neural network to modify process parameters.

in-

In-production automated weld monitoring systems usually have sensors providing information on the state of the weld pool. Using this information and determining a relationship between the state of the weld pool and at least one of the critical welding parameters (eg current, voltage, torch position and travel speed) the welding process can be adjusted by a feed-back loop from the sensors (see Figure 3). The system continuously adjusts the process parameters to maintain the desired stable process state. This can be achieved with little operator intervention.

The increasing demands of high production rates and greater weld quality at lower costs will necessitate the strengthening of the bond between the technologies of welding and nondestructive inspection. In achieving these goals, in-production monitoring of the welding process will increase in importance and may well become indispensable.

The dynamic nature of welding means that data acquisition and processing must be rapid enough to extract useful information before any major- change occurs in the welding process. A two-step real-time radiographic analysis involving a fast search for defective regions followed by fine identification and location of defects has achieved this requirement137]. Real-time radiographic images have been used in the control of arc welding conditions in butt-joint welds[381.A combined approach using real-time radiographic images of the weld pool depression and images of the solidifying weld immediately behind the pool was used for weld penetration and quality control.

Conclusions

and future

work

In recent years some exciting developments have occurred in NDT techniques for weld inspection. Major advances have been made in several fields, particularly in ultrasonics, electromagnetic methods for both crack sizing and residual stress measurement, and in on-line monitoring of the welding process.

In-production ultrasonic sensing has been used to determine the quality of both gas metal arc (GMA) and gas tungsten arc (GTA) welding processes[391. This technique allows detection of weld pool geometry and weld defects in real time. The technique evaluated the quality of the molten weld pool at the electrode and the solidified weld metal behind the electrode. Two types of discontinuities were detected: incomplete sidewall penetration and porosity. These discontinuities were distinguished from sound welds using an expert-system technique with a success rate of 92%. Unfortunately the expert-system algorithm was unable to discriminate between the discontinuity types as successfully. Porosity was identified correctly 70% of the time and incomplete sidewall fusion was correctly identified 63% of the time. Bull et aLi4’] have shown that GTA and resistance

Many of the advances in NDT techniques have been driven by today’s increasing pressures for cost-effective weld inspection. Cost effectiveness is linked to factors such as reliability, sensitivity, speed and coverage of NDT techniques. The need for greater reliability, speed and coverage has resulted in the increasing use of automated ultrasonic systems for weld inspection, particularly in the nuclear and pressure vessel industries and in a range of marine applications. Rapid development has occurred in a range of non-contact NDT techniques which should improve speed of inspection in the future, as will the continuing development of neural networks for automated data processing. Worldwide, the need exists to implement these advances in NDT technology in weld validation codes. Clearly,

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there is no value in replacing an acceptable inspection method for the sake of technological sophistication alone. However, substantial improvements are possible by the incorporation of advanced concepts such as Timeof-Flight-Diffraction in weld inspection.

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In conclusion, many challenges remain in NDT of welds, particularly in minimizing inspection costs without prejudicing structural integrity. These challenges are best met by close cooperation between welding engineers and NDT expertsI261so that best practice is achieved based on a knowledge of not only NDT but also weld manufacture, fracture mechanics and structural mechanics.

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Acknowledgements 20

The authors are grateful to Mr C. Pope (C. W. Pope & Assoc.) and Dr B. Dixon for a critical reading of the manuscript.

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References 22 1

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See for example, Hanseni, B. ‘Acceptance criteria for welds in relation to NDE’ in Non-Destructive Testing Proc. 4th European Conf. NDT, vol 1, eds J. M. Farley and R. W. Nichols, Pergamon Press, Oxford (1988) pp 325-337 Hahnshaw, R. Introduction to the Non-Destructive Testing of Welded Joints, The Welding Institute, Cambridge (1988) Mudge, P. J. and Jessop. T. J. ‘Size measurement and characterization of weld defects by ultrasonic testing: Findings of a collaborative programme’ in Advances in Non-Destructive Examination for Structural Integrity, ed R. W. Nichols, Applied Science Publishers, Essex (1982) pp 337 -356 Hahnshaw, R. ‘Developments in NDE methods other than ultrasonics-radiographic methods’ in Advances in Non-Destructive Examination for Structural Integrity, ed R. W. Nichols, Applied Science Publishers, Essex (1982) pp 3-8 Halmshaw, R. Brit. J. NDT 36 (1994) pp 7998 1 Halmshaw, R. ‘A review of digital radiological methods’ in Impact of Nondestructive Testing 28th British Conf. NDT. eds C. Brook and P. D. Hanstead, Pergamon Press, Oxford (199b) pp 103-l 12 Link, R., Nuding, W., Wiacker, H., Busse, H. P. and Munro, J. J. ‘Weld inspection using real-time radiography’ in International Advances in Nondestruc:ive Testing, vol 14, ed W. J. McGonnagle, Gordon and Breach, Montreux (1989) pp 1433173 Kato, R., Sugita, Y., Onda, K., Okudaira, E., Matsui, S., Itoga, K. and Sugimoto, K. ‘Development of computer aided radiographic inspection system (I) Delscription of system’ in 10th International Conference on NDE in the Nuclear and Pressure Vessel Industries, eds M. J. White, J. E. Doherty and K. Iida, ASM International, Materials Park (1990) pp 6877692 Sugita, Y., Onda, K., Iuchi, S., Itoga, K., Harada, T., Sugimoto, K. of computer aided inspection sysand Michiba, K. ‘Development tem (II): method of identification and categorization of welded defects’ ibid (1990) pp 693-699 Halmshaw, R. ‘Real-time radiography. A review of recent developments’ in Non-Destructive Testing Proc. 4th European Conf. NDT, vol 1, eds J. M. Farley and R. W. Nichols, Pergamon Press, Oxford (1988) pp 1399154 Babot, D., Berodias, G. and Peix, G. NDT & E Intern. 24 (1991) pp 247-25 1 Keld, R. Non-Destructive Testing - Australia 28 (1991) pp 73-74 Ultrasonic inspection of heavy section steel components: the PISC II final report, eds R. W. Nichols and S. Crutzen, Elsevier, London (1988) Crutzen, S., Jehenson, P., Nichols, R. and McDonald, N. ‘PISC III: Status report and future trends’ in 10th International Conference on NDE in the Nuclear and Pressure Vessel Industries, eds M. J. White,

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