Recent developments in the R5 procedures for assessing the high temperature response of structures

摘要

The R5 procedures have been developed within the UK power generation industry to assess the integrity of nuclear and conventional plant operating at high temperatures. Within R5, there are specific procedures for assessing creep-fatigue crack initiation in initially defect-free components (Volume 2/3) and for assessing components containing defects (Volume 4/5). The current R5 Volume 2/3 procedure for assessing creep-fatigue initiation in weldments involves analysis assuming homogeneous parent material properties with a single Fatigue Strength Reduction Factor (FSRF) used to account for both the reduction in fatigue endurance and enhancements in strain due to material mismatch and local geometry effects. A new development of this approach involves splitting the FSRF into a Weldment Endurance Reduction (WER), which accounts for reduced fatigue endurance due to weld imperfections, and a Weldment Strain Enhancement Factor (WSEF), which accounts for material mismatch and local geometry effects. The new approach results in less conservative predictions of creep damage than are obtained using the existing approach as it is only the WSEF that affects the stress at the start of the creep dwell rather than the larger FSRF in the existing approach. However, the new approach has little effect on the calculated fatigue damage as the combined effects of the WER and the WSEF are essentially equivalent to the previous FSRF. In addition to methods for predicting creep and creep-fatigue crack growth for load-controlled situations, R5 Volume 4/5 gives advice on the prediction of crack growth for situations involving combined primary and secondary loading. This advice has recently been extended to cover cases involving significant amounts of crack growth, C(t) estimation methods for elastic-plastic-creep behaviour and explicit advice on the treatment of combined loading for situations involving significant welding residual stresses. This extended advice can be used to predict the growth of defects in non-stress relieved austenitic weldments operating at elevated temperatures. This paper briefly outlines the current R5 Volume 2/3 and Volume 4/5 procedures and then focuses on these two significant recent developments in R5, both of which are highly relevant to real instances of creep and creep-fatigue cracking that have been observed in high temperature austenitic plant components.