NDT research for industrial applications

TWI Bulletin, November/December 1990

Peter Mudge joined the newly formed Non-Destructive Testing Research Section at TWI in 1976 after graduating from Nottingham University in Metallurgy and Materials Science with first-class honours. Now Head of the NDT Research Department, he is responsible for technical and administrative aspects of the Department's activities, which include specialist site work, consultancy, independent reviews of procedures and results, techniques development and research programmes in the majority of commonly used NDT methods.

Since joining TWI Peter has taken part in a wide variety of field and laboratory NDT investigations, both in the UK and overseas. He has lectured widely on ultrasonic testing, has published a number of technical papers and has been involved with NDT aspects of British Standards. He is a Chartered Engineer, and a corporate Member of The Welding Institute, the Institute of Metals and the British Institute of NDT.

Earlier this year Peter was awarded the Leslie Lidstone ESAB Gold Medal for his contribution to the advancement of welding technology.

The non destructive Testing Research Department at TWI provides expertise and technical back-up for Industrial Members on questions of inspection for a range of applications. Department Head Peter Mudge describes some recent activities.

The importance to Industrial Members of NDT and inspection for ensuring the as-built quality, or fitness for service, of fabricated components is evident from the continuing high demand for the services which the NDT Research Department supplies. These may be divided into consultancy, laboratory investigations, specialist site services and research.

TWI is also a NAMAS-accredited testing laboratory; the principal NDT techniques are included in this accreditation.

Consultancy

Provides a rapid response to enquiries concerning NDT, and can take inspection expertise to members' sites.

Laboratory investigations

Range from developing techniques for examining new joint geometries or components made from novel materials to demonstration and validation of the performance of chosen techniques. The Department is also heavily involved in the inspection and QA aspects of manufacture of special test blocks containing flaws for external NDT validation programmes - see Figures 1 & 2.

Specialist site services

Considerable interest has been generated in the use of P-scan and other advanced ultrasonic techniques for on-site critical examination of important regions. The Department now has a team of highly skilled and experienced technicians, who have used these test methods for weld examination on pressure vessels, pipework, offshore risers and structural welds - see Figures 3, 4 & 5.

Considerable expertise has been built up in the interpretation of the complex computer-generated displays of flaws. This applies to both P-scan output for detection and general evaluation of weld flaws, see Figure 6 - and to time-of-flight diffraction (TOFD) for critical sizing of flaws already detected - see Figure 7.

P-scan can also be used for corrosion mapping and TWI has carried out several surveys for various clients at their job sites. An example, already reported in the Bulletin, ^[1] is the examination of ships' hulls for internal corrosion - see Figures 8 & 9. The principal advantages over manual thickness measurements are that a permanent record of the test is obtained and that the abrupt changes in thickness associated with pitting corrosion can be detected. These may not be observed with manual spot-measurements.

Major research activities

A considerable part of the Department's resources is devoted to large research programmes. These are of two types, the Industrial Member/DTI Co-operative Research Programme and the Group Sponsored Project.

Co-operative research Programme

The Six NDT projects are:

• Detection of near-surface flaws. The aim of this work is to examine the capabilities of eddy current testing for detecting sub-surface, as well as surface-breaking, flaws. Surveys carried out among Industrial Membersand published work indicate that there is a need for test methods which are less sensitive to variation in surface preparation than are magnetic particle or dye penetrant tests, and which allow subsurface flaws to be detected.

  Application to welds is predominantly for examining structural joints, such as Ts and fillets, although there is some interest in testing electrically-resistance-welded tube. Interest has also been shown in the improvement of eddycurrent testing of seamless tube during manufacture.
  

  
  
• NDT of power beam welds. This project was initiated to provide solutions to two particular inspection problems for electron beam welds: the poor detection and size assessment of tightly closed missed-joint flaws, andover-estimation of porosity in the fade-out region.

  The work has concentrated on ultrasonic testing, as the joint geometry of electron beam welds is not always conducive to radiography and the section thicknesses are often too great to obtain an adequate radiographic image usingreadily-available sources of radiation. Several test configurations have been studied, including 0-degree compression wave, angled-beam shearwave pulse-echo, tandem and obscuration, and TOFD. The P-scan instrument has been usedextensively to produce images of the ultrasonic signals. The results indicate that consistently good inspection results may be obtained by adopting carefully-designed procedures and by using a combination of the above techniques.

  Inspection of laser welds is also being addressed.
  

  
  
• NDT of joints with thin planar interfaces. The aims of this project are two-fold. The first is to develop examination techniques for thin planar joints, such as diffusion bonds and brazes. The work on brazed jointswill include metal-ceramic joints, and will make use of the Department's high-precision, high-frequency C-scan system - see Figure 10.
  

The second is to investigate testing methods for resistance spot welds. This has shown that the external profile of the weld has an overriding influence on the ultrasonic response, and electrical contact between the welded sheets around the periphery of the fused nugget dominates electrical resistance methods. Neither is therefore capable of producing reliable results over a range of joint conditions. A considerable number of test data, collected both from the Co-operative Research Programme and from sponsored work, have now been reported to members. ^[2] The findings have initiated development of an ultrasonic probe (Figure 11) which is more tolerant of external surface profile and for which a patent has been obtained. They have also prompted modifications to the electrical resistance measurement technique to overcome bridging effects. The work on ultrasonic testing is being continued under funding from a group of sponsors from the automotive industry, with the aim of producing an effective in-plant testing system.

Automated non-contact inspection of joints. This project was set up to provide enabling technology for a number of the Department's activities, Infrared imaging has been studied for detecting flaws in materials with poor thermal conductivity, principally fibre-reinforced plastics. Flaws have been imaged using real-time X-ray detectors, particularly an uprated microfocus X-ray source. Examples of the results of microfocus X-ray tests on electrical components are shown in Figures 12 & 13.

• These studies have highlighted the importance of generating, storing and manipulating images as a means of presenting better, more readily understood results from non-destructive tests, The feasibility of capturing images of magnetic particle and dye penetrant has been examined and a considerable amount of work has been done on interpreting ultrasonic images.
  

  
  
• Rapid examination techniques. This project has centred on developing the active, long-range ultrasonic acoustic pulsing technique. An analytical technique for separating individual wave modes in multi-mode ultrasonic plane wave signals, developed in conjuction with Imperial College, has been tested. This is being patented and will provide the basis for interpreting results from further large-scale experiments to establish defect detection performance.
  
  
• NDT of welds in plastics and composites. Work has centred on a number of topics: testing butt welds in polyethylene pipes, examining lap joints in fibre-reinforced thermoplastics, and detecting flaws in glass-fibre-reinforced plastic.

  The most notable development has been the improvement gained in the performance of ultrasonic probes for testing bulk plastics by using special polymeric compounds for the probe wedge - see Figures 14 & 15. Imaging techniques using ultrasonic P-scan and C-scan systems have proved to be highly successful in revealing flaws in lap joints between sheets of carbon-fibre-reinforced thermoplastic material - see Figures 16 & 17.

This work continues with the purchase of a low kV X-ray tube for experiments on sensitometry of radiography for flaw detection; considerable improvements in results are expected.

Group Sponsored Projects

The NDT Research Department is currently running five multi-sponsored projects.

1. The second part of a programme to develop and predict optimised test conditions for magnetic particle inspection is nearing completion. This has involved major studies of the magnetic properties of steel and of the ways in which they vary around surface cracks, and finite element modelling of leakage fields from cracks.
2. During 1990 there has been considerable interest in a programme to develop acceptance criteria for welds in pressure vessels which are written in ultrasonic terms rather than the radiographic criteria usually found in quality codes, such as BS 5500. The scope of this exercise has widened somewhat and the work now forms the basis of a major submission to the European standardising body CEN for acceptance criteria to be applied to welds in the developing European standards.
3. Drawing on the work in the project above and the general expertise within the Department, a project has begun to develop an ultrasonic procedure generator and checker based on expert system technology. This will allow the designer, engineer or other 'knowledgeable non-expert' in NDT to specify and draw up appropriate ultrasonic test procedures for given joints, using a personal computer. Further, the system will allow the user to check procedures supplied by a contractor to confirm their adequacy. Such a package is restricted to ultrasonics at present, but the aim is to make the system modular so that other test methods, and indeed test method selection options, can be added.
4. Following the work on resistance spot welds described above, a programme is now under way, sponsored by automotive companies, to refine, demonstrate and implement NDT techniques for spot weld quality determination.
5. Finally, a major collaborative project with the Danish Welding institute is investigating the use of robotic manipulation to examine welds at intersections of structural tubulars for critical evaluation of offshore structures. Figure 18 shows some sample modelling of scanner movements around joints which are extremely difficult to inspect using manual techniques - see Figure 19.

Future GSPs

Projects scheduled for launch during the next 12 months include:

• NDT of welds in duplex stainless steels;
• NDT of glass-fibre-reinforced plastics;
• Acoustic pulsing applications studies;
• Detection of creep damage by non-destructive methods;
• NDT methods for adhesive joints;
• Further work on ultrasonic acceptance criteria and quality levels.

In conclusion

This article attempts to convey both the breadth of activities within NDT Research at TWI and the many ways in which the Department can help to solve Industrial Members' problems through:

• Advice on immediate queries, and consultancy;
• Laboratory investigation;
• Specialist site services;
• Participation in long-term research.

If you have any questions concerning the Department's work, access to its facilities, or participation in Group Sponsored Projects, please contact Peter Mudge.

References