SSM perspective
Background
Present report is a continuation of earlier work reported in SSM2009:27 regarding an analysis strategy for fracture assessment of defects in ductile material and SSM2011:19 on the influence from residual stresses on crack initiation and ductile crack growth at high primary loads. The Swedish procedure for safety assessment of components with defects is documented in a handbook, SSM2018:18, and the earlier referred work have been incorporated in the handbook, section 7.1.12.
The Swedish procedure is based on the British R6-method where the failure mechanisms fracture (Kr) and plastic collapse (Lr) are considered for cracked components of metallic materials by evaluating the stress intensity factors and the plastic limit load. The stresses in the assessment have to be categorised as primary or secondary. An example of the latter is weld residual stresses. In cases where the secondary stresses are dominant and the primary stresses are low (Lr<0.8), the R6 method has been shown to give overly conservative results.
There are other engineering methods than the R6 when performing safety assessments of defects, one such is the American ASME XI. There is a ASME XI Code Case N-749 that for reactor pressure vessel steels suggests a lowering of the safety factors in the ductile so called “upper shelf region”, and that the residual stresses may be omitted from the assessment.
The present report investigates the inbuilt margins in the R6-method with respect to the weld residual stresses and its influence on stable crack growth, and will give a recommendation with regard to ASME XI Code Case N-749.
Results
The main results of the project show that
- secondary stresses have a significant impact on crack initiation at low primary loads (Lr<0.8),
- the R6-method, for cases with residual stresses, gives conservative estimates of J irrespective of primary load level at crack initiation, and
- it is recommended not to adapt ASME XI Code Case N-749.
The work has increased the understanding of the R6-method and its margins and has improved the knowledge on the influence of secondary stresses on ductile fracture at low primary loads.
Relevance
The results from this project provides a solid scientific base on which SSM rely when stating that ASME XI Code Case N-749 is not in line with the Swedish procedure for safety assessment of components with defects and that.