[March 2007]  Procedures for assessing the integrity of components containing crack-like defects and for assessing the lifetime of high temperature components were developed within the former Central Electricity Generating Board (CEGB). These enabled assessments of the integrity of both fossil-fired and nuclear power generating plant. Although there were design codes for high temperature plant at that time, more accurate life assessment methods were needed to address, for example, weldments and defect tolerance, not covered in the codes. The procedures developed within the CEGB became known as the R6 and R5 Procedures and are now maintained by British Energy.

The defect assessment methods in R6 were first produced in 1976 and have since been subject to extensive development and to world-wide review. This has led to the underlying methods being adopted in many codes and standards. The current version of R6 is Revision 4. This covers basic fracture assessment methods to define limiting loads or crack sizes and also advanced methods such as leak-before-break assessment, probabilistic methods and micro-mechanistic approaches.

The R5 high temperature assessment procedures address both creep-fatigue crack initiation in initially defect-free components and the growth of flaws by creep and creep-fatigue mechanisms. The R5 procedures include a number of novel features such as: the shakedown reference stress approach for structural assessment; the ductility exhaustion method for estimating creep damage; the inclusion of size effects in fatigue damage calculations to enable assessments of thin section components; and the treatment of the effects of crack closure on creep-fatigue crack growth rates.

Both R6 and R5 continue to be developed by British Energy in collaboration with Rolls-Royce and Serco Assurance. Recently TWI have joined the collaboration to develop R6. Further information on the procedures may be found in the review articles listed below or by contacting Graham Marshall at graham.marshall@british-energy.com

References For Further Reading

Budden, P.J., Sharples, J.K., and Dowling, A.R., “The R6 procedure: recent
developments and comparison with alternative approaches”, International
Journal of Pressure Vessels and Piping, 77, 2000, 895-903.
Ainsworth, R.A., “R5 procedures for assessing structural integrity of components under creep and creep-fatigue conditions”, International Materials Reviews, 51, 2006, 107-126.