S. Marie

An Analytical Method to Evaluate J Values in Elbows Submitted to Mechanical Loading

Regarding of structures behaviour which can be found in nuclear reactor, most of assessment method are based on the valuation of parameter J. This paper deals with the J calculation in cracked elbows submitted to internal pressure and various bending moments. Recent works on elbows including a semi-elliptical defect performed in the frome of the A16 guideline improvements are presented. The A16 methodology is based on the reference stress concept (named CEP method): It consists in an amplification of the elastic value of J by a plastic correction deduced from the reference stress. The elastic value of J is deduced from the influence factors of the pipe and from analytical relations for elastic nominal stresses. The reference stress estimation is performed using two equivalent stresses corresponding to the elastic and plastic behaviours of the cracked component. The analytical relations for the elastic value J and the reference stress determinations are presented and validated using 3D FE calculations.Copyright

Analytical Method for the Calculation of J Parameter in Cracked Components: Presentation of the RSE-M/RCC-MR Procedure and Benchmark Proposal

RSE-M and RCC-MR codes provide flaw assessment methodologies and related tools for Nuclear Power Plant. For these two codes, AREVA, CEA and EDF developed a large set of compendia for the calculation of the parameter J for various components (plates, pipes, elbows, [[ellipsis]]) and various defect geometries. The last step of these developments deals with the weld joints: since 2004, a methodology have been developed to calculate the J parameter for a defect located in a weld, less conservative than usual methods. This methodology is based on the definition of an equivalent material that leads to the same J value (with same loading conditions and defect geometry) than the bi-material component. The stress-strain curve of this equivalent material is deduced from a combination of the tensile curves of the base metal and of the weld metal. The weigth coefficients applied are specifically defined for the J calculation and generalized to deal with any weld joint geometry.© 2010 ASME

Analytical Method for the Calculation of J Parameter for Surface Cracks in Piping Welds

RSE-M and RCC-MR codes provide flaw assessment methodologies and related tools for Nuclear Power Plant. AREVA, CEA and EDF developed in particular a large set of compendia for the calculation of the parameter J for various components (plates, pipes, elbows, ...) and various defect geometries. The last step of these developments deals with the weld joints : since 2004, the partners are developing a methodology to calculate the J parameter for a defect located in a weld, less conservative than usual methods. This methodology is based on the definition of an equivalent material that leads to the same J value (with same loading conditions and defect geometry) than the bi-material component. The stress-strain curve of this equivalent material is deduced from a combination of the tensile curves of the base metal and of the weld metal. The weigth coefficients applied are specifically defined for the J calculation and generalized to deal with any weld joint geometry.Copyright

Benchmark on Residual Stress Modeling in Fracture Mechanics Assessment

In the frame of development in analytical defect assessment methods for the RSE-M and RCC-MRx codes, new work on the consideration of residual stresses is initiated by AREVA, CEA and EDF. The first step of this work is the realization of a database of F.E. reference cases.To validate assumptions and develop a good practice guideline for the consideration of residual stresses in finite element calculations, a benchmark between AREVA, CEA and EDF is going-on. A first application presented in this paper focuses on the analysis of the crack initiation of aged duplex stainless steel pipes submitted to an increasing pressure loading. Residual stresses are related to pipe fabrication process and act as shell bending condition. Two tests were performed: the first with an internal longitudinal semi-elliptical crack and the second with an external crack.The analysis first focuses on the ability to accurately estimate the measured pressure at the crack initiation of the two tests. For that purpose, the comparison of results obtained with different methods of taking into account the residual stresses (i.e. thermal fields or initial strain field). It then validates post-treatment procedures for J or G determination, and finally compares of the results obtained by the different partners.It is then shown that the numerical models can integrate properly the impact of residual stresses on the crack initiation pressure. Then, an excellent agreement is obtained between the different numerical evaluations of G provided by the participants to the benchmark so that best practice and reference F.E. solutions for residual stresses consideration can be provided based on that work.Copyright

Development of a J-Estimation Scheme for Surface Cracks in Piping Welds

The RSE-M Code provides rules and requirements for in-service inspection of French Pressurized Water Reactor power plants. The Code gives non mandatory guidance for analytical evaluation of flaws. Flaw assessment procedures rely on fracture mechanics analyses based on simplified methods (i.e. analytical). Analytical methods were developed under a cooperative program between EDF, CEA and AREVA NP to calculate the J integral in various cracked piping components (straight pipe, tapered transition, elbow and pipe-to-elbow junction). These methods are available for mechanical loading (in-plane bending moment, pressure, torsion moment), thermal loading as well as for combined loading. Moreover, they can be used either for materials with Ramberg-Osgood stress-strain curves or for real materials (stainless steels and carbon manganese steels, including those with yield plateaus). However, for the analysis of cracks in welds, they use the tensile properties of the weakest material between the base material and the weld material. This induces some conservatism on the estimated J values. A cooperative program was launched in 2004 to develop a J estimation scheme which takes into account the strength mismatch effects. The scheme relies on the definition of an ‘equivalent’ stress-plastic strain curve, as proposed in the R6 rule (section III.8: allowance for strength mismatch effects). This curve is then used with the analytical methods for homogeneous cracked components. In a first step, the method is developed for circumferential surface cracks in straight buttwelded pipes submitted to mechanical loading. It takes into account the geometry of the weld joint (V-shaped), as well as the location of the crack within the weld. This paper presents the current state of development of this J estimation-scheme.© 2008 ASME