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Structural integrity methods, where next? (October 2002)

   
R E Dolby and C S Wiesner

Paper presented at FESI Conference, 8-9 October 2002, Manchester UK

Abstract

This paper examines issues related to the future development of flaw acceptance procedures. In the first instance the paper discusses relevant procedures, codes and standards and attempts to answer the question 'Where we will be inten years time?' The second element of this paper deals with enhancements which could and should be produced for current standards and codes such as BS 7910 and R6. Finally the paper discusses user and customer competence and, inparticular, examines the need for better training and greater awareness of procedures which are, by their nature, complex.

1. Procedures, codes and standards

It is important to have some sense of history concerning the development of the key flaw acceptance procedures publicly available, and Tables 1 and 2 show the developments and timescales for the production of the two most internationally used documents, BS 7910 [1] and R6. [2]

Table 1 BS 7910 Development - Guide on methods for flaw acceptance in metallic structures

1968 Concept paper - J D Harrison, F M Burdekin and J G Young
1970 BSI/WEE/37 Committee formed
1980 Published document PD 6493 issued
1991 PD6493 revision, some harmonisation with R6
1999/2000 BS7910 issued, more harmonisation with R6, some SINTAP input

Both started with important concept papers which then led to the first issues in the early 1970s of draft published documents. R6 was developed to suit the needs of the nuclear industry, whereas BS 7910 arose from a background offailures in structures and vessels for the shipping, oil and gas, construction and offshore sectors.

Table 2 R6 Development - Assessment of the Integrity of Structures containing defects

1975 Concept paper - A R Dowling & H Townley
1976 First issue of R6 with Failure Assessment Diagram
1977/2000 Revisions 1-3, incorporating new methods
2001 Revision 4 issued, incorporating SINTAP output

Early on, the two procedures were distinctly different in approach but after 20 years of experience there has been a welcome convergence of methods and principles. The latest version of BS 7910 which was issued in 1999 (and amendedin 2000), shows considerable harmonisation with R6 and has also received substantial new input from the recent SINTAP European Project on fracture and plastic collapse assessment procedures. [3] The latest issue of R6 was introduced in 2001 and again incorporates significant material from the SINTAP Project.

It is very surprising that the United Kingdom has developed not one, but two, procedural flaw assessment documents which have been widely accepted internationally and the UK should be proud of the development of both. BS 7910 is controlled and updated by the British Standards Institution committee, WEE/37, and to date BSI has sold around 700 copies worldwide.

R6 on the other hand is controlled and updated by a panel which comprises experts from the UK nuclear industry with specialism in structural integrity and receives peer review from the UK Technical Advisory Group on Structural Integrity (TAGSI), particularly for novel approaches. R6 currently has over 200 registered users worldwide.

Both of these high profile procedures have been fed by advances in fracture mechanics over the last decade and have benefited particularly from new standards such as the revised British fracture toughness testing standard BS 7448, [4] but also from European programmes such as SINTAP mentioned above.

There are other approaches around the world including API 579 [5] for relevant pipework, pressure vessels and tanks and the development of specialised ASME methods. In Germany a recent document [6] has brought together different approaches from SINTAP for fracture and plastic collapse assessment and from BS 7910 for fatigue evaluation. In Japan, the latest standard on flaw acceptance [7] has borrowed extensively from PD 6493, the predecessor document of BS 7910. New procedures continue to be developed under European collaborative projects similar to SINTAP. For example, FITNET involving many European organisations is currently looking at the development of enhanced procedures for fracture, fatigue, collapse and creep assessment and corrosion damage and is coordinated by GKSS in Germany. [8]

The vision that the authors hold is that there will be a European standard on the assessment of flaws in metallic structures within a decade or thereabouts. A number of elements must work to achieve this. First we need national champions in Europe to promote and develop the procedures. We must have a Europe-wide peer review process to ensure wide acceptance and we need an appropriate response from CEN to set up a standardization project. Finally, we must have continuation of FITNET or similar projects over the next 10 years to provide the required technical input.

There is a widely held view, at least in the UK, that the scope of a future European Standard should initially deal with procedures which will help with design, fabrication support, fitness-for-service and failure analysis and then cover application-specific procedures appropriate to different industry sectors, for example nuclear, offshore and pressure vessels. BS 7910 and R6 would eventually be subsumed by this CEN document.

2. Enhancements needed now

We can divide enhancements into three areas; stress, material properties and flaws. Tables 3 and 4 illustrate the key areas which need further work. These include more research into high strain and high strain rate performance, varying constraint situations and improved methods for handling residual stresses. In the material properties context, there is a lack of relevant fracture toughness data. Current correlations with say, Charpy V tests, can often lead to lower bound selection of fracture toughness data which can be quite pessimistic in terms of predicted critical flaw sizes. In the flaw context, both detection and sizing issues require various assumptions at present and sizing is a function of operator competence.

Table 3 Stress related enhancements needed now

1 More K and limit load solutions
2 High strain and high strain rate situations
3 Thin sheet performance
4 Varying constraint situations
5 Interaction of secondary and primary stresses (residual and thermal stress issues)

Table 4 Material property and flaw related enhancements needed now

1 More fracture toughness data (correlations usually lead to lower bound selection)
2 Difficulty in extrapolating miniature and small specimens
3 Need for a validated data bank
4 Detection and sizing issues (function of operator competence)

3. User and customer competence

TWI deals on a regular basis with customers seeking advice on flaw acceptance and, in the authors' experience, most customers think that the current flaw assessment frameworks are sufficient for decision-making and have brought considerable savings in repair and consequential costs. However, for some industrial sectors, the process is quite complicated and occasionally conservative against practical experience.

Material properties are inherently variable and probabilistic approaches are required to make quantified likelihood of failure predictions. Probabilistic approaches are not always popular because of their inherent complexity. There is also the continuing misconception by some that manufacturing standards could be lowered by the widespread application of flaw assessment approaches.

It is clear therefore that those who are in the business of working with the flaw acceptance codes and standards will need to demonstrate adequate competence to customers. Some pertinent questions are: should there be approved training centres for such practitioners and should examinations be taken with appropriate certification for successful candidates to ensure competence can be demonstrated?

Also, it is clear that most practitioners would benefit by sharing their experience with others and a more formal user group activity for R6 and BS 7910 would be an important step forward. There is wide use of software to help carrying out the many calculations needed, but the correct use of software and the judgement needed for treatment of input data remains complicated and improved user-friendly approaches would be valuable.

For customers or users of the information provided by experts, some form of training touching on the basics of fracture mechanics, for example, seems to be important. Appreciation courses for such people are required. Equally, the outputs from the process need to be readily understood and many industries have similar initial questions. For example, can post weld heat treatment be avoided or can a hydrotest be waived? Improved communication between customers/users of the assessment results and the experts of the procedures should make it possible to identify outputs required to answer such common questions in a user-friendly fashion.

4. Conclusions

  1. A European Standard on flaw acceptance procedures is needed as soon as possible, certainly within a decade.
  2. Continued European-wide funding is needed for co-ordinating appropriate research to improve current procedures.
  3. Better training and user-group activity for increased competence in both practitioners and customers is needed and the idea of a certification scheme to demonstrate practitioner competence is proposed.

5. Acknowledgements

The authors thank Dr I Hadley and Dr H Pisarski for their helpful discussions.

6. References

  1. BS 7910: 1999 (incorporating Amendment No.1): ' Guide on methods of assessing the acceptability of flaws in metallic structures'. British Standards Institution, London, 2000.
  2. R6 Revision 4: ' Assessment of the Integrity of Structures containing Defects'. British Energy Generation LH, Barnwood, UK, 2001
  3. BRITE EURAM Project: ' Structural Integrity Assessment Procedure for European Industries - SINTAP'. Procedure Document, British Steel (now Corus) Swindon Technology Centre, Rotherham, UK, 1999.
  4. BS 7448: 1991: ' Fracture Mechanics Toughness Tests'. British Standards Institution, London, 1991.
  5. API RP579: ' Fitness-for service'. American Petroleum Institute, Washington DC, 2000.
  6. FKM Guide: ' Fracture mechanics assessment of mechanical components'. (FKM Richtlinie: 'Bruchmechanischer Festigkeitsnachweis für Maschinenbauteile'). Forschungskuratorium Maschinenbau (FKM), Germany, 2001 (in German).
  7. WES 2805-1997: ' Method of assessment for defects in fusion welded joints with respect to brittle and fatigue crack growth'. Japanese Welding Engineering Socity, Tokyo, Japan, 1997.
  8. Fitnet: ' European Fitness-for-service network', project part-funded by the EU Competitive and Sustainable Growth Programme, February 2002 to 2004, see www.eurofitnet.org