R6 and R5 procedures: The way forward

摘要

This paper first briefly summarises the existing methods in the low-temperature fracture assessment procedure, R6, and the high-temperature procedure, R5, for treating the effects of secondary stresses on structural integrity. Recently, there have been a number of developments, which identify the way forward for these procedures. A modified J-integral definition has been derived, which is path independent for cases of proportional and non-proportional loading and is ideal for evaluating the crack driving force for defects in secondary and residual stress fields. Results of finite element analysis are presented that show that the use of the modified J-integral can lead to a lower crack driving force for secondary stresses than current simplified R6 methods. More detailed calculations have assessed the effects on fracture of a slowly growing crack and constraint effects associated with secondary stresses. Preliminary results are presented, showing the long-term potential of more advanced methods in providing significant benefits in structural integrity assessments. For high-temperature applications, the paper presents methods for calculating the relaxation of secondary stresses due to both creep strain and creep crack growth, extending current methods in R5 that only allow for relaxation due to creep strain. Related work addressing the combined effects of plasticity and creep on relaxation of the crack tip fields is also presented and the results are illustrated for a typical geometry and loading.