Ductile fracture behavior in ferritic steels is investigated using two complementary experimental databases. The first database involves a wide range of cracked and uncracked specimen geometries tested at various temperatures for a A533 (18MND5) steel, enabling a detailed analysis of the effect of stress states, particularly stress triaxiality and the Lode parameter, on damage nucleation and growth. The second database, for a WB36 (15NiCuMoNb5) steel, includes both laboratory-scale specimens and full-scale structural tests on precracked pipes at various temperatures. A gradient-enhanced energy GTN (Gurson-Tvergaard-Needleman) model incorporating a Lode-parameter-dependent nucleation function is employed to simulate ductile damage across different stress states. The model is first calibrated and validated on the A533 dataset. It is then applied to the WB36 dataset to assess its transferability from specimens to structural components. The results confirm the model's ability to accurately capture damage evolution and crack propagation, demonstrating its robustness and relevance for structural integrity assessments.

The primary and secondary PWR (Pressurized Water Reactor) circuits are a complex set of pressure-welded steel piping and components. As the second barrier to the spread of fissile materials is concerned, ensuring the integrity of these circuits is a major safety objective. It must cover all possible loading situations encountered in service (normal operation as well as accidental situations) and all possible failure modes, material functions and associated conditions of use. Fracture mechanics assessment in welded joints in these structures require particular attention. Indeed, welds may present defects (ex. lack of fusion) and generally have a lower fracture resistance than the base metal. Additionally, welding process generate Welding Residual Stresses (WRS) that might be considered in the assessment. Their potential impact on the risk of failure is directly related to material behavior. These WRS have been shown to have an impact on the risk of failure in the brittle domain where the behavior is quasi-elastic, but this impact appears to be overestimated in the upper shelf of the transition curve. Consequently, industry needs to develop criteria adapted to the problem of taking WRS into account in the analysis of fracture mechanics for fracture mechanics assessment, and to validate these criteria through experiments at different scales, then define the areas where these stresses must be taken into account or can be neglected. This paper present experimental results and first analysis of four points bending tests performed on large scale welded C-Mn steel pipe representative of the PWR secondary and auxiliary piping.

Framatome and EDF have initiated an R&D program to assess the impact of residual stresses on fracture initiation in representative welded pipes featuring a non-stress-relief weld and an included crack. The focus is on the upper shelf of the brittle to ductile transition, where material behavior behaves from quasibrittle to ductile. A review of the literature reveals that most available results pertain to relatively low temperatures, where the effect of welding residual stresses on brittle fracture is significant [1]. However, this temperature domain is less relevant for nuclear applications. Despite this, these findings provide critical insights and valuable guidance for the preparation of new experimental campaigns. This paper investigates the application of the Beremin model to C-Mn steel welds, with a focus on parameter identification using laboratory specimens and validation through structural-scale testing.

Tests on cracked CT and SENT specimens (152 tests) extracted from a nuclear pressure vessel steel (18MND5) were conducted between C and C together with tests on smooth and notched bars. Tests on tensile bars were used to calibrate a temperature-dependent hardening law. Tests on cracked specimens that exhibited brittle failure only were analyzed using the Beremin model. Special care was taken to evaluate local stresses using enhanced mixed finite elements. The Beremin model was able to represent the entire database provided the reference failure stress increases with temperature. Finally, the results were also interpreted using the Master Curve approach, which was adapted to account for different stress states using the -parameter.

Numerous fracture tests have been performed by EDF and Framatome on various specimen's geometry extracted from a 18MND5 ferritic steel plate, in the brittle-to-ductile transition regime. C(T), SE(T), SE(B), and non-standard specimens have been tested. This database has been recently updated with SE(T) tests performed in the lower shelf of the transition domain. This work highlights a robust methodology which allows to predict constraint effect with minor consequences of the choices of the Weibull stress model formulation (classic model, consideration of stress threshold.) and on m parameter on the final interpretation. A local approach based deterministic industrial method is proposed to consider this effect in Engineering Fracture Mechanics Assessment without considering any probability of failure or statistical approach. It allows to evaluate constraint effect at discrete locations along the crack tip. Its application on industrial cases shows that parameters set have limited impact on constraint effect prediction.

This paper presents some developments performed by EDF and FRAMATOME with the objective to define criteria for the consideration or not of Welding Residual Stresses within the Fracture Mechanics Assessment of nuclear components.The paper is organised in two parts. In the first part, a literature survey is proposed, showing that major part of available results is at low temperature in the Brittle to Ductile transition where the effect of Welding Residual Stresses is known to be significant. This temperature domain is not relevant for nuclear applications; however, all those results are providing very important recommendations regarding the preparation of new testing.In the second part, a numerical study is proposed for showing the relevance of the local approach for evaluating the impact of Residual Stresses on brittle fracture, and to initiate some criteria for defining the domain where they have a significant effect.

Numerous fracture tests have been performed by EDF and Framatome on various specimen geometries extracted from a 18MND5 ferritic steel plate, in the brittle-to-ductile transition regime. C(T), SEN(T), SEN(B), and non-standard specimens have been tested. Based on those data, the work presented here proposes a robust identification methodology which allows the prediction of constraint effect with minor consequences of the choices of the Beremin model formulation (classical model or more complex model with a stress threshold, or a plastic strain correction) and on m parameter on the final interpretation. Results demonstrate geometrical effects, resulting in a temperature shift of KIC distribution of most specimens to higher values than those obtained on C(T) specimens and the capability of the proposed strategy to correctly predict the experimental results. The final goal of this work is to propose a methodology applicable in Engineering Fracture Mechanics Assessment to gain margins without considering any probability of failure or statistical approach.

Framatome and EDF have launched an important R&D program which aims at evaluating the effective impact of residual stresses on the fracture initiation in the brittle-to-ductile transition regime, on vessel low alloy steel plates including a surface defect. The test program is designed to allow a direct comparison of the tests results, with and without residual stresses, other factors being equal, to address the impact of the residual stresses in the brittle-to-ductile transition domain. High level of residual stresses of secondary nature have been generated on specimens, at the vicinity of defects, without modifying the local material toughness by avoiding the introduction of significant plastic deformation close to the defect. Specimens are pre-cracked and tested by 4 points bending. Results show an impact of residual stresses on the fatigue crack growth rate but none on the fracture risk in the transition and upper shelf of the brittle-to- ductile transition. Significant impact is only observed at the lower shelf of the material. It is shown that local approach using Beremin model clearly catches these results. It also demonstrates that residual stresses can be neglected for fracture assessment analysis in the upper part of the brittle-to-ductile transition for high toughness nuclear vessel low alloy steels

Framatome and EDF have launched an important R&D program which aims at evaluating the effective impact of residual stresses on the fracture initiation in the brittle-to-ductile transition regime, on vessel low alloy steel plates. The paper presented here deals with the process used for introduction the residual stresses in the selected mock up. The goal of the process is to generate residual stresses over 300 MPa in the thickness at the vicinity of the crack. The principle retained is to locally produce small inelastic compression strains, around the crack, using a thermal spot imposed in the centre of a plate on both sides. The vicinity of the defect is then quickly heated by induction so as its prevented thermal expansion induces compression stresses, which, in the case sufficiently high temperatures are reached, cause both inelastic creep and plastic strains, inducing tensile residual stressing at the end of the cooling phase. The process has been designed by finite elements modelling based on a fine material behaviour modelling in the plastic-viscous domain. The thermal cycle has been defined to reach a residual stresses level between 300 MPa and 400 MPa up to the mid-thickness of the plate. The experimental developments confirm the relevance of the thermal cycle defined through the numerical investigation, showing a good reproducibility, and expected residual stresses levels.

To reduce over conservative safety margins, toughness may be assessed by Single Edge Notched Tensile (SENT) testing in addition to Compact Tension (CT) testing. Higher crack growth resistance is found during SENT testing compared to CT testing for the studied 15NiCuMoNb5 (WB36) ferritic steel. To identify differences in damage mechanisms for the two samples that might explain these differences, synchrotron microtomography is carried out on stopped cracks cut from CT and SENT samples. They are complemented by post mortem fractography and elasto-plastic 3D finite element simulations. For the CT sample, substantial growth of voids is found, that primarily nucleated on MnS particles, leading to a rough crack with a diffuse crack tip. In contrast, the SENT sample shows limited void growth and a very defined and smooth crack with a damage free fan shaped zone ahead of the crack. Complementary high-resolution fractography also shows very small dimples for the SENT sample that is linked to nucleation on carbides. Damage quantification in regions of interest (ROIs) of 50 µm length showed void volume fractions up to 7

The main objective and mission of the ATLAS+ project is to develop advanced structural assessment tools to address the remaining technology gaps for the safe and long-term operation of nuclear reactor pressure coolant boundary systems. This is achieved by development and validation of: innovative quantitative methodologies to transfer laboratory material properties to assess the structural integrity of large components, enhanced treatment of weld residual stresses when subjected to long term operation, advanced simulation tools based on fracture mechanics methods using physically based mechanistic models, improved engineering methods to assess components under long term operation taking into account specific operational demands, integrated probabilistic assessment methods to reveal uncertainties and justify safety margins. Additionally, the objective is to disseminate the findings of the work through special training sessions and links to the NUGENIA association. The project scope of work focuses on piping systems of the reactor coolant pressure boundary components (RCPB) excluding the reactor pressure vessel (RPV). The project is aimed on an experimental proof of concept and validates the developed methodology both at the laboratory scale and the full-scale level. The ATLAS+ project contains 4 main technical WPs and one training and dissemination package. These are summarized next, in addition to a brief description of the progress.

Ductile tearing of a full size precracked pipe is experimentally investigated. In order to model and interpret the test, the pipe material is characterized using smooth and notched tensile bars and precracked C(T) specimens. This experimental database is used to fit the parameters of the non local Gurson-Tvergaard-Needleman (GTN) proposed in Zhang et al. (2018) and Chen et al. (2020). The model is used in finite element simulations using specific elements allowing for the control of strain/damage localization as well as volumetric locking. Mesh size independence is checked on notched tensile bars. The model is then able to represent the early stages of crack propagation in the pipe. In particular, experimentally observed crack branching is reproduced, whereas this appeared much more difficult to obtain using a local GTN model.

The 4-years European project ATLAS+ (Advanced Structural Integrity Assessment Tools for Safe long Term Operation) has been launched in June 2017. One of its objectives is to study the fransferability of material ductile properties from small scale specimens to large scale components and validate some advanced tools for structural integrity assessment. The study of properties fransferability is based on a wide experimental program which includes a full set of fracture experiments conducted on conventional fracture specimens and large scale components (mainly pipes). Three materials are considered in the program: a ferritic steel WB36 typical from secondary feed water line in German PWR reactors, an aged stainless steel austenitic weld representative of EPR design and a typical VVER austenitic dissimilar weld (DMW). This paper describes preparation, realization, fractographic expertise and analysis of large scale tests performed on the ferritic steel WB 36 (15NiCuilloNb5). These large scale tests consisted of four point bending tests on piping's conducted at room temperature. Two configurations of cracks (shape, size and location) were tested. With the first configuration (FP1), with a through wall crack, a large ductile tearing was obtained. For the second configuration (FP2), with an external semi-elliptical crack, cleavage was obtained after a limited ductile tearing.

The 4-year European project ATLAS+ (Advanced Structural Integrity Assessment Tools for Safe long Term Operation) was launched in June 2017. One of its objectives is to study the transferability of ductile material properties from small scale specimens to large scale components and validate some advanced tools for structural integrity assessment. The study of properties transferability is based on a wide experimental program - within the framework of work-package 1 (WP 1) - which includes a full set of fracture experiments conducted on conventional fracture specimens and on large scale components (mainly pipes). Three materials are considered in the program: a low-alloy ferritic steel 15NiCuMoNb5 (WB36) typical from feedwater line in German PWR, an aged austenitic stainless steel weld typical (narrow gap) from EPR and a typical VVER austenitic stainless steel dissimilar weld (DMW). Several European organizations are involved in the experimental work: EDF, CEA, Framatome, ARMINES, KIWA, Framatome GmbH, VTT, BZN, MTA-EK, and CIEMAT.

Abstract The 4-year European project ATLAS+ (Advanced Structural Integrity Assessment Tools for Safe long Term Operation) was launched in June 2017. One of its objectives is to study the transferability of ductile material properties from small scale specimens to large scale components and validate some advanced tools for structural integrity assessment. The study of properties transferability is based on a wide experimental program — within the framework of work-package 1 (WP 1) — which includes a full set of fracture experiments conducted on conventional fracture specimens and on large scale components (mainly pipes). Three materials are considered in the program: a low-alloy ferritic steel 15NiCuMoNb5 (WB36) typical from feedwater line in German PWR, an aged austenitic stainless steel weld typical (narrow gap) from EPR and a typical VVER austenitic stainless steel dissimilar weld (DMW). Several European organizations are involved in the experimental work: EDF, CEA, Framatome, ARMINES, KIWA, Framatome GmbH, VTT, BZN, MTA-EK, and CIEMAT.

The 4-years European project ATLAS+ (Advanced Structural Integrity Assessment Tools for Safe long Term Operation) has been launched in June 2017. One of its objectives is to study the transferability of material ductile properties from small scale specimens to large scale components and validate some advanced tools for structural integrity assessment. The study of properties transferability is based on a wide experimental programme which includes a full set of fracture experiments conducted on conventional fracture specimens and large scale components (mainly pipes). Three materials are considered in the programme : a ferritic steel WB36 typical from secondary feed water line in German PWR reactors, an aged stainless steel austenitic weld representative of EPR design and a typical VVER austenitic dissimilar weld (DMW). This paper describes the experimental work conducted on the ferritic steel WB 36 (15NiCuMoNb5) and summarizes the experimental results available after 2 years of work. Numerous mechanical tests have been conducted on a wide panel of fracture mechanics specimens for a full characterization of the ferritic steel: Tensile properties, Hardness, Charpy Energy, pre-cracked Charpy PCC, Master curve on CT and SENT specimens, ductile tearing properties on CT and SENT specimens. In parallel, it is planned to test three 4PB large scale tests on pipings (FP1, FP2 and FP3) at room temperature on the EDF test facility with 3 configurations (shape, size and location) of cracks: through wall crack (TWC), internal and external ½ elliptical cracks. Progress of these large scale experiments is described including first results.

This paper summarizes the design calculations performed by Framatome, EDF, KIWA INSPECTA and VTT for three large scale tests on ferritic pipes made of material WB 36 (15 NiCuMoNb 5). The large scale tests will be performed on a 4-point bending test facility provided by EDF under displacement control at room temperature. The overall goal of the planned large scale tests is to demonstrate the effect of the crack tip constraint on the fracture toughness at the component level. Results of those tests will be utilized to develop and validate advanced tools for structural integrity assessment within WP 3 particularly with respect to the transferability of material properties from small scale specimens to large scale components as well as for the development and validation of a procedure for the determination of component fracture resistance curves. Three configurations of the initial defect with different constraint conditions (one through-wall and two surface cracks) are considered. The design calculations are divided into two parts. In the first part an optimization of three different crack shapes is performed on basis of the standard fracture mechanics approach (based on J-Integral) without consideration of the constraint effect. In the second part a quantification of the crack tip constraint for the selected crack configurations from part I is performed. The effect of the constraint on the crack initiation and propagation for the selected crack configurations shall be assessed and compared between each other. Based on these calculations the final flaw configuration for each large scale experiment is selected.

This paper will assess the capability of the shear modified Gurson model developed by Nahshon and Hutchinson which is used by Kiwa Inspecta within the ATLAS+ project. This is done by comparison to experimental results from SENTfracture tests performed by EDF and ARMINES. The procedurefor parameter identification for the standard and shear modified Gurson model is also summarized. The work presented in this paper is part of Work Package 3 within the ATLAS+ project. WP3 focus mainly on ductile tearing predictions for large defects in components. Models exists to accurately predict ductile tearing and to consider phenomena such as stress triaxiality effects. These advanced models include local approach coupled models or advanced energetic approaches. However, there is a need to validate these models for use in industrial applications. This will be done within the ATLAS+ project by predicting the results of the large scale component tests where input to the models are given from small size laboratory specimens. Within the paper a description ofthe shear modified Gurson model is given, as developed by Nashson and Hutchinson [1]. Furthermore, the procedure in determining the material model parameters is discussed. To determine the material parameters for the shear modified Gurson model a uniaxial tensile test, a fracture test and shear tests are used. The material that is used is the ferritic steel WB 36 (15 NiCuMoNb 5) which will be used for the large scale component tests within the ATLAS+ project. The procedure is also evaluated by comparing predictions done with the shear modified Gurson model to experimental results from SENT specimens performed by EDF and ARMINES. A comparison of the capability in predicting the ductile tearing in the SENT experiments between the standard Gurson model and the shear modified Gurson model is also presented within the paper.

The main objective and mission of the ATLAS+ project is to develop advanced structural assessment tools to address the remaining technology gaps for the safe and long term operation of nuclear reactor pressure coolant boundary systems. This is achieved by development and validation of: • innovative quantitative methodologies to transfer laboratory material properties to assess the structural integrity of large components, • enhanced treatment of weld residual stresses when subjected to long term operation, • advanced simulation tools based on fracture mechanics methods using physically based mechanistic models, • improved engineering methods to assess components under long term operation taking into account specific operational demands, • integrated probabilistic assessment methods to reveal uncertainties and justify safety margins. Additionally, the objective is to disseminate the findings of the work through special training sessions and links to the NUGENIA association. The project scope of work focuses on piping systems of the reactor coolant pressure boundary components (RCPB) excluding the reactor pressure vessel (RPV). The project is aimed on an experimental proof of concept and validates the developed methodology both at the laboratory scale and the full scale level. The ATLAS+ project contains 4 main technical work packages and one training and dissemination package. These are summarised here.

The 4-years European project ATLAS+ project was launched in June 2017. Its main objective is to develop advanced structural assessment tools to address the remaining technology gaps for the safe and long term operation of nuclear reactor pressure coolant boundary systems. The transferability of ductile material properties from small scale fracture mechanics specimens to large scale components is one of the topics of the project. A large programme of experimental work is to be conducted in support of the development and validation of advanced tools for structural integrity assessment within the framework of the work-package 1 (WP 1): Design and execution of simulation oriented experiments to validate models at different scales. The experimental work is based on a full set of fracture mechanics experiments conducted on standard specimens and large scale components (several pipes and one mock-up), including a full materials characterization. Three materials are considered: • a ferritic steel 15NiCuMoNb5 (WB 36) • an aged austenitic stainless steel weld • a VVER (eastern PWR) dissimilar metal weld (DMW) The paper presents the WP 1, the experimental programme and summarizes the first results.