Stéphane Marie

A16: guide for defect assessment at elevated temperature

Abstract A large program has been performed in France in order to develop, for the design and operation of High Temperature Reactor plants, defect assessment and Leak-Before-Break (LBB) procedures. As a result of the collaboration between CEA, EdF and Framatome, the A16 guide is now included in the RCC-MR 2002 edition. The main defect assessment procedures proposed in this A16 guide are: • fatigue or creep-fatigue crack initiation based on the σd method, • fatigue crack growth, • creep-fatigue crack growth, • ductile tear initiation and propagation, together with a LBB assessment. The calculations of the fracture mechanic parameters J and C ∗ (t) for plates, pipes, elbows, etc. and laboratory specimens under mechanical loading and thermal shock are based on new formulations of the reference stress and new KI handbooks for a large panel of geometries containing surface and through-wall defects. Some guidance on the experimental test methods for measuring the fracture mechanics properties of the materials are also included in the document. The procedures and analytical solutions of the A16 guide are consistent with those proposed at moderate temperature by the RSE-M for PWRs and cover also elevated temperature where creep is significant.

J Evaluation by Analytical Method for Cracked Elbows Under Mechanical Loading

The integrity of structures behaviour is an important subject for the nuclear reactor safety, specially for components containing a defect under increasing loading. In this case, most of assessment methods are based on the evaluation of the parameter J. This parameter may be difficult to obtain for complex structure like an elbow. Then to avoid complex elastic-plastic finite elements computations of J, a simplified (ie analytical) method has been jointly developped by CEA, EDF and Framatome-ANP. This method, called Js , is based on the reference stress approach and a new KI handbook using elastic stresses determined in the median section of the elbow. To validate this method, a complete set of 3D massive elastic-plastic finite element calculations of J have been performed on elbows (more than 140 calculations are available) for: • different types of longitudinal part-throughwall defect ending on either side of median section: semi-elliptical or long axial defect; • mechanical loading: pressure, bending moment, torsion moment; • different kind of materials: austenitic, ferritic. This paper presents a comparison between the simplified assessment of J and Finite Element results on these configurations for mechanical loading. Then, validity of the method is discussed and an applicability domain is proposed.Copyright

Brittle Fracture Analysis of Dissimilar Metal Weld Joints

Demonstration of large components integrity is based on the demonstration that they could never undergo brittle fracture. In parallel, considering a conventional defect somewhere in the component, failure must prevent ductile crack propagation.Connections between a ferritic component and an austenitic one have to respect these rules. The considered case is a Ni base alloy weld joint between a ferritic pipe and an austenitic one. For brittle fracture exclusion, the aim of the present study is to show that in the same loading conditions, the weld joint is less sensitive to the brittle fracture than the surrounding ferritic part of the component. That is to say that the demonstration should be focused on the ferritic base metal which is the weakest material.For that purpose, Chapuliot developed a stress-based criterion model, using a threshold stress (σth) below which the cleavage cannot occur. It can be used to define the brittle crack occurrence probability, which means that it is possible to determine the highest loading conditions without any brittle fracture risk.For the experimental part of this demonstration, two different mock-ups with DMW are used. The first one is composed of a 16MND5 (A508 cl. 3) steel pipe welded to a 316L stainless steel pipe with Ni base alloy 82. As for the second one, the materials are the 18MND5 (A533) steel, the 316L stainless steel and the Ni base alloy 52. Conventional defects have been considered in the ferritic part, close to the weld joint in the heat affected zone, and far away from the weld joint in the ferritic part. Two hundred specimens have been taken from the mock-ups: special tensile specimens, compact tensile specimens, single edge notch tension specimens, notched tensile specimens, smooth tensile specimens. All the materials have been characterized at −125°C/−130°C and −170°C/−175°C, even the heat affected zones. Finite element calculations have been done to complete the experimental investigations. The first results show that, due to the mismatch of the materials, the brittle fracture risk is much lower in the HAZ. Thus, DMW HAZ could not be a weak part concerning brittle fracture. This paper presents the criterion, the experimental work and the analyses made to evaluate the conservatism of the homogeneous ferritic case compared to the DMW.Copyright

Validation of ACE Analytical Criterion for Warm Pre-Stress Evaluation in RPV Integrity Assessment

Numerous experimental and numerical studies have been conducted in France to demonstrate the beneficial effect of warm pre-stress (WPS) on the brittle fracture resistance of reactor pressure vessel (RPV) steels and take into account this effect in French RPV integrity assessment. A large panel of experimental data is available, obtained on small, medium and large scale specimens. These data have been included in a specific EDF WPS experimental database.An analytical criterion — ACE criterion — is proposed by French organizations (AREVA, CEA and EDF) for analytical evaluation of warm pre-stress effect on the brittle resistance of RPV steels.After a description of ACE criterion and the EDF WPS database, the validation of the criterion is shown — based on this database — by comparison between experimental results and predictions of the model.Copyright

Cleavage Crack Propagation and Arrest Prediction in French PWR Vessel Steel

The integrity assessment of Reactor Pressure Vessels is mainly based on crack initiation. Nevertheless, in the frame of component life extension, crack arrest conception is investigated.This paper presents a local non-linear dynamic model to predict the propagation and arrest of cleavage crack in French PWR vessel steel (16MND5). The propagation criterion used in this model is a Ritchie Knott and Rice (RKR) fracture stress criterion: the crack propagates when the maximum stress ahead of crack tip reaches a critical level, which has been shown to depend on temperature and plastic strain rate.In the first step, the criterion has been identified from crack growth and arrest analysis on CT specimens at different temperatures. Then it was applied to predict the propagation and arrest of cleavage cracks on pre-cracked rings under mixed mode loading, at three different temperatures: −150°C, −125°C and −100°C.2D modeling was performed by using extended element method (XFEM) in CAST3M software. The propagation direction on pre-crack rings under mixed mode loading was determined from maximum hoop stress criterion. Numerical computation showed a good agreement with experiments, for both curved crack paths and crack arrest locations. Furthermore it showed that crack paths and crack arrest also depend on the level of the crack loading at initiation.Copyright

Cleavage Crack Path Prediction in a PWR Vessel Steel

Cleavage crack propagation has been tested for three different geometries of Compact Tensile (CT) specimens: CT25, CT50 and extended CT25 (CT25 with CT50 width) (Figure 3). The experimental results show that the crack paths are straight for CT25 and CT50, but they are unstable and curved for extended CT specimens (Figure 5 to 7).Numerical computation had been performed by extended finite element method (XFEM) in CAST3M software. 2D modeling was used in order to predict the crack path. The analysis was based on a local non-linear dynamic approach with a RKR fracture stress criterion depending on temperature and strain rate.In order to simulate the curvature of the cracks path, a statistical effect was introduced in the model to take into account the spatial distribution of cleavage initiators, which is the characteristic of cleavage fracture. At each step of propagation during the modeling, the direction was randomly chosen, according to a uniform defects distribution.The numerical results show a good agreement with experience. The different crack paths were curved in extended CT25, but remained almost straight in CT25 and CT50 specimens, despite of the instability introduced in the modeling in the propagation direction. These results show that the statistics of micro-defects can induce, jointly with the geometry of specimen, a large scatter of crack propagation paths.Copyright

Brittle Fracture Analysis of a Dissimilar Metal Weld

One important part of the integrity demonstration of large ferritic components is based on the demonstration that they could never undergo brittle fracture. Connections between a ferritic component and an austenitic piping (Dissimilar Metal Weld — DMW) have to respect these rules, in particular the Heat Affected Zone (HAZ) created by the welding process and which encounters a brittle-to-ductile transition.Within that frame, the case considered in this article is a Ni base alloy narrow gap weld joint between a ferritic pipe (A533 steel) and an austenitic pipe (316L stainless steel). The aim of the present study is to show that in the same loading conditions, the weld joint is less sensitive to the brittle fracture than the surrounding ferritic part of the component. That is to say that the demonstration should be focused on the ferritic base metal which is the weakest material.The bases of this study rely on a stress-based criterion developed by Chapuliot et al., using a threshold stress (σth) below which the cleavage cannot occur. This threshold stress can be used to define the brittle crack occurrence probability, which means it is possible to determine the highest loading conditions without any brittle fracture risk.Copyright

Cleavage Dynamic Propagation Analysis in a Nuclear Reactor Pressure Vessel Steel Using a High-Speed Camera

Initiation stage of cracks is considered as a key issue, but more and more component integrity analyses investigate the crack propagation and arrest possibility. This study deals with physical mechanisms of cleavage crack propagation and numerical computations related to brittle fracture. Dynamic effects, involved in unstable cleavage crack propagation, have to be taken into account to properly depict brittle crack propagation, arrest and possible propagation re-initiation events.Experiments were carried out on thin CT specimens made of 16MND5 PWR vessel steel at five temperatures (−150°C, −125°C, −100°C, −75°C, −50°C). In addition to standard crack gages, an innovative experimental technique has been used to determine crack propagation. By the means of developments on the experimental protocol (improvements of isolation and airtightness of the thermal chamber, optimization of the experimental protocol to eliminate ice in the thermal chamber and in order to have a good acquisition quality), use of a high-speed framing camera was made possible to measure crack propagation on a CT mirror polished surface. This optical device, combined with this optimized experimental process, has allowed the study of straight and branching crack paths with high accuracy. The framing camera (520 000 fps up to 1 100 000 fps) has allowed to have a very accurate estimation of crack speed even up to 1000 m.s−1 and also to detect some phases of crack branching during propagation and phases of arrest-re-initiation. Numerical computations, based on X-FEM and combining a local non linear dynamic approach with a RKR type fracture stress criterion, have been performed to depict experimental crack behavior.This paper describes this innovative experimentation and the interpretation by FE calculations and SEM observations associated with quantitative 3D optical microscopy.Copyright

Cleavage Crack Propagation and Arrest in a Nuclear Pressure Vessel Steel

The integrity assessment of Reactor Pressure Vessels, mainly based on crack initiation, can be completed by studying crack propagation and arrest. Whereas engineering approaches do not take into account dynamic effects, these effects are important in unstable cleavage crack propagation, arrest and possible propagation re-initiation events. This study deals with physical mechanisms of cleavage crack propagation and numerical computations related to brittle fracture in the framework of local approach to fracture.Experiments were carried out on thin CT 25 specimens made of 16MND5 PWR vessel steel at five temperatures (−150°C, −125°C, −100°C, −75°C, −50°C). Two kinds of crack path, straight or branching path, were observed. Branching cracks appear for the highest critical loadings at initiation, that increase the elastic stored energy and the effect of plasticity. The elastic-viscoplastic behavior of the ferritic steel was studied up to a strain rate of 104 s−1 and taken into account in the numerical simulations. The eXtended Finite Element Method (X-FEM) was used in CAST3M FE software to model crack propagation. Numerical computations combine a local non linear dynamic approach with a RKR type fracture stress criterion. The different physical micro-mechanisms, involved in cleavage fracture, were examined by the means of SEM fracture surface analyses at different temperatures and strain rates for the two kinds of crack path.The links of the critical fracture stress with both temperature and strain rate for straight crack path as well as analyses of branching crack phenomena were considered by the means of Scanning Electron Microscopy (SEM) fracture surface analyses, 3D quantitative optical microscopy and FE computations in order to aim at a robust physical justification of the propagation model which has already been developed at CEA in the frame of the B. Prabel PhD.Copyright

Mechanical Characterization for a Large Test Design of a Dissimilar Metals Welding With a Narrow Gap Nickel Alloy Weld: Experimental and Numerical Analysis on Specimens

Dissimilar metal welds are a common feature of light water reactors in connections between ferritic components and austenitic stainless steel piping systems. Inspection difficulties, variability of material properties and residual stresses all combine to create problems for structural integrity assessment.Within the framework of European project STYLE, a fracture test on a pipe containing a through wall crack in a narrow gap Nickel alloy Dissimilar Metals (DMWinc) is under preparation. The work is focusing on the nickel alloy - ferrite steel interface which is the weakest area of such welded pipes in front of ductile tearing. The study temperature is 300°C, which covers typical temperatures in components like hot pipes in the primary coolant system of pressurized water reactors.This paper gives an overview of the program and the first results of works which is been carried out by the French Atomic Energy Commission and Alternative Energies (CEA) in order to study the mechanical properties and integrity of component of the DMWinc pipes provided and designed by AREVA France.Copyright