Patrick Le Delliou

Influence Coefficients to Calculate Stress Intensity Factors for an Elliptical Crack in a Plate

Crack assessment in engineering structures relies first on accurate evaluation of the stress intensity factors. In recent years, a large work has been conducted in France by the Atomic Energy Commission to develop influence coefficients for surface cracks in pipes. However, the problem of embedded cracks in plates (and pipes) which is also of practical importance has not received so much attention. Presently, solutions for elliptical cracks are available either in infinite solid with a polynomial distribution of normal loading or in plate, but restricted to constant or linearly varying tension. This paper presents the work conducted at EDF R&D to obtain influence coefficients for plates containing an elliptical crack with a wide range of the parameters: relative size (2a/t ratio), shape (a/c ratio) and crack eccentricity (2e/t ratio where e is the distance from the center of the ellipse to the plate mid plane). These coefficients were developed through extensive 3D finite element calculations: 200 geometrical configurations were modeled, each containing from 18000 to 26000 nodes. The limiting case of the tunnel crack (a/c = 0) was also analyzed with 2D finite element calculation (50 geometrical configurations). The accuracy of the results was checked by comparison with analytical solutions for infinite solids and, when possible, with solutions for finite-thickness plates (generally loaded in constant tension). These solutions will be introduced in the RSE-M Code that provides rules and requirements for in-service inspection of French PWR components.Copyright

Progress in the Development of J Estimation Schemes for the RSE-M Code

The RSE-M Code provides rules and requirements for in-service inspection of French Pressurized Water Reactor power plant components. The Code gives non mandatory guidance for analytical evaluation of flaws. To calculate the stress intensity factors in pipes and shells containing semi-elliptical surface defects, influence coefficients are given for a wide range of geometrical parameters. To calculate the J integral for a circumferential surface crack in a straight pipe, simplified methods are available in the present version of the Code (2000 Addenda) for mechanical loads (in-plane bending and torsion moments, pressure), thermal loads as well as for the combination of these loads. This paper presents the recent advances in the development of J-estimation schemes for two configurations: • a longitudinal surface crack in a straight pipe, • a longitudinal surface crack in the mid-section of an elbow.© 2002 ASME

STYLE Project: Assessment of Transferability of Fracture Material Properties From Specimens to Large Components by Local Approach to Fracture

Within the framework of the FP7 European project STYLE, a large scale experiment was performed at EDF R&D on a cladded ferritic pipe (Mock-Up 3). The objective of this experiment was to investigate the transferability of material properties from small specimens to large scale components.The large scale experiment involved applying 4-point bending under displacement control at room temperature to a clad ferritic steel pipe with an inner surface crack. The goal of the experiment was to initiate ductile crack growth and track the resulting stable crack growth until the surface flaw breaks through the wall.The pipe was representative of a surge line, consisting of a clad ferritic pipe with an outer diameter of 424 mm, and base metal wall thickness of 31 mm, with an austenitic stainless steel cladding layer 5 mm thick on the inner surface. The base metal is a low alloy 20 MnMoNi 5 5 steel (corresponding to the specifications of an SA 508 Grade 3, Class 1 steel).The pipe test was conducted in 2012 in the EDF R&D 4-point bending frame. Following the experiment, various specimens were taken from the mock-up to identify the material behavior and provide data to investigate the transferability of the material fracture properties.This paper recalls briefly the large scale experiment results and presents the main experimental results from the specimens. Then the results of the local approach finite element computations with the Rousselier model are presented and compared with the experimental results.Copyright

Recent Advances for J Simplified Assessment in RSE-M Code

The RSE-M Code [1] provides rules and requirements for in-service inspection of French Pressurized Water Reactor power plant components. Non mandatory guidance is given in the Code for analytical flaw evaluation in a wide range of situations. In Appendix 5.4 of the Code, influence coefficients are provided to calculate stress intensity factors in pipes and shells containing semi-elliptical surface defects. The J assessment method is based on the reference stress concept with two options for reference loads evaluation: “CEP elastic plastic stress” and “CLC modified limit load”. In this paper recent advances concerning J assessment under mechanical loading for a crack located in a pipe-elbow junction are presented. Reference stress evaluation with “CLC” option is developed and mechanical foundations of the equation of large scale yielding under complex loading (pressure, in-plane and out-of-plane bending) are presented.Copyright

Life Management of Reactor Coolant Piping — Probabilistic Calibration of Partial Safety Factors in Flaw Acceptance Criteria: Application to Cast Duplex Stainless Steel Components

The RSE-M Code provides rules and requirements for in-service inspection of the components of the French PWR power plant. The Code gives non mandatory guidance for analytical evaluation of flaws, comprising fracture mechanics analyses based on engineering methods, flaw acceptance criteria and codification of material characteristics. Based on a probabilistic calibration methodology, partial safety factors on the main random variables involved in flaw assessments (loading, crack size, yield strength and material toughness) are given in Appendix 5.5 of the Code for each category of operating conditions (A, C or D) and for the possible failure modes (ductile tearing or brittle fracture). These partial safety factors should be used with the material characteristics specified in Appendix 5.6 of the Code, to insure the consistency of the methodology. The criteria of the RSE-M Code have been implemented for the acceptance of generic flaws in cast duplex stainless steel elbows of the Reactor Coolant System. Statistical analyses of the mechanical properties of the base-metal have been carried out to get their characteristic values consistent with the Code criteria: • tensile properties comprising yield strength, ultimate tensile strength and non dimensional reference true stress - true strain curves taking into account thermal ageing, • value of the J-integral in the ductile regime at the onset of crack extension (J0.2 after 0.2 mm of crack extension), J-Δa curves in the ductile regime taking into account thermal ageing in the hot leg conditions, • fatigue crack growth rates. The results show that the aged cast duplex stainless steel elbows satisfy the Code criteria for each category of operating conditions.Copyright

Large EDF Tests on Aged Cast Duplex Stainless Steel Components: Part I — Reduced Scale Tests

Thermal ageing of cast duplex stainless steel primary loops components (elbows, pump casings and branch connections) is a concern for long-term operation of EDF nuclear power plants. The thermal ageing embrittlement results from the micro-structural evolution of the ferrite phase (spinodal decomposition), and can reduce the fracture toughness properties of the steel. In addition, it is necessary to consider manufacturing quality and the possible occurrence of casting defects such as shrinkage cavities. In a context of life extension, it is important to assess the safety margins to crack initiation and crack propagation instability.This paper presents several tests conducted by EDF on aged cast duplex stainless steel NPP components, respectively on two-third scale elbows and welded mock-ups. The main characteristics of the tests are recalled, the results are presented, and finally, the lessons drawn are summarized.These tests and their detailed analyses contribute to validate and justify the methodology used by EDF in the integrity assessment of in-service cast duplex stainless steel components.Copyright

Integrity and Life Assessment of Cast Duplex Stainless Steel Elbows of EDF PWR Main Coolant Piping

Thermal ageing of cast duplex stainless steel elbows is a concern for long-term operation of EDF nuclear power plants. The thermal ageing embrittlement results from the micro-structural evolution of the ferrite phase (spinodal decomposition), and can reduce the fracture toughness properties of the steel. In addition, it is necessary to consider manufacturing quality and the possible occurrence of casting defects such as shrinkage cavities. In a context of life extension, it is important to assess the safety margins to crack initiation and crack propagation instability.This paper reports the present integrity and life assessment methodologies as carried out by EDF. The approach is based on the in-service inspection and surveillance RSE-M Code and on French regulation requirements for NPPs in operation. This work is supported by an extensive R&D programme on one hand and on field experience analysis on the other hand.The paper details the three main topics of the life assessment methodology:- estimation of the fracture toughness of the steel with predictive formulae using the chemical composition and ageing conditions,- definition of a reference crack size based on an inventory of the manufacturing quality of the elbows,- fracture mechanics evaluation based on the J parameter, computed either by an engineering estimation method or by a finite element analysis. The calculated J parameter is then compared with the estimated fracture toughness of the material. Partial safety coefficients are included in the calculation process as required by the RSE-M Code.Copyright

Comparison of the Stress Intensity Factor Influence Coefficients for Axial ID Surface Cracks in Cylinders of RSE-M and API 579-1

Analytical evaluation procedures for determining the acceptability of flaw detected during in-service inspection of nuclear power plant components are provided in Appendix 5.4 of the French RSE-M Code. Linear elastic fracture mechanics based evaluation procedures require calculation of the stress intensity factor (SIF). In Appendix 5.4 of the RSE-M Code, influence coefficients needed to compute the SIF are provided for a wide range of surface axial or circumferential flaws in cylinders, the through-wall stress field being represented by a cubic equation. On the other hand, Appendix C of API 579-1 FFS procedure provides also a very complete set of influence coefficients.The paper presents the comparison of the influence coefficients from both documents, focused on axial ID semi-elliptical surface flaws in cylinders. The cylinder and crack geometries are represented by three ratios: Ri/t, a/t, and a/c, where Ri, t, a, and c are respectively the inner radius, the wall thickness, the crack depth and one-half of the crack length. The solutions for the coefficients G0 and G1 at the deepest point and at the surface point are investigated.At the deepest point, the agreement between the solutions is good, the relative difference being lower than 2 %, except for the plate (Ri/t = ∞) at a/c = 0.125 and 0.0625 and a/t = 0.8 (around 5 %). At the surface point, the agreement between both solutions is not so good. At this point, the relative differences depend strongly on the a/c ratio, being larger for elongated cracks (with low a/c ratios). However, it must be recalled that the absolute values of the coefficients are low at the surface point for elongated cracks, and that for these cracks the critical point regarding the stress intensity factor is the deepest point.Copyright

STYLE Project: A Large Scale Ductile Tearing Experiment on a Cladded Ferritic Pipe

Within the framework of the FP7 European project STYLE, a large scale experiment has been performed at EDF on a cladded ferritic pipe. The objective of such an experiment was to investigate transferability of material properties from small specimens to large scale components.The large scale experiment involves applying 4-point bending under displacement control at room temperature to a clad ferritic steel pipe with an internal surface crack. The goal of the experiment is to initiate ductile crack growth and track the resulting stable crack growth until the surface flaw fails by producing a through-wall crack.The test specimen is representative from a surge line consisting of a clad ferritic pipe with an outer diameter of 420 mm, length of 520 mm, and base metal wall thickness of 31 mm, with an internal austenitic stainless steel cladding layer of thickness 5 mm. The base metal is a low alloy 20MnMoNi55 steel (corresponding to the specifications of an SA 508 Grade 3, Class 1 steel), and the necessary extensions are made of a high strength ferritic steel.A wide range of instrumentation was implemented to provide data for mock-up behavior understanding and detect the ductile tearing initiation during the test.The test has been conducted with full success on the EDF 4 point bending test facility. After the experiment, samples have been taken from the mock-up for full SEM fractographic examinations of the fracture surface for identification of failure modes.The present paper describes the large scale experiment and presents the main experimental results and data. A synthesis of SEM fractographic examinations is also presented, to better understand the rupture behavior during the test.© 2014 ASME

Project STYLE: Post-Test Analysis of Mock-Up3

The safety and reliability of all systems has to be maintained throughout the lifetime of a nuclear power plant. Continuous R&D work is needed in targeted areas to meet the challenges of long term operation of existing and new plants designs. The European project STYLE aims to develop and validate advanced methods of structural integrity assessment applicable in the ageing and lifetime management of primary circuit components.There are three large scale mock-up tests in STYLE each of them dedicated to investigate specific effects. This paper presents the work related to Mock-up3, which is dedicated to investigate influence of cladding on the crack initiation and propagation as well as the transferability of material properties from small scale specimens to a large scale component.The performed post-test analyses focus on both the further understanding and interpretation of the Mock-up3 test and on the effect of cladding on structural integrity and LBB behavior of reactor coolant pressure boundary components.Copyright