Stéphane Bugat

Transferability of Cleavage Fracture Parameters Between Notched and Cracked Geometries

In the recent past, the local approach to fracture has shown its capacity to be a predictive tool in many complex cases of structural integrity assessment [1]. Brittle fracture of steels is one of the fields where local approach has brought a lot of understanding (e.g. effect of prestraining, effect of dynamic loading [2]). In the nuclear industry, local approach to fracture is often used as a complementary tool to the code (e.g. ASME) in order to reduce the empirical margins and to contribute to a better understanding of the physical mechanisms which can lead to fracture.

PERFECT — Prediction of Irradiation Damage Effects in Reactor Components: Overview of RPV Mechanics Sub-Project

The EURATOM 6th Framework Integrated Project PERFECT (Prediction of Irradiation Damage Effects in Reactor Components) addresses irradiation damage in RPV materials and components by multi-scale modeling. This state-of-the-art approach offers many potential advantages over the conventional empirical methods used in current practice of nuclear plant lifetime management. Launched in January 2004, this 48-month project is focusing on two main components of nuclear power plants which are subject to irradiation damage: the ferritic steel reactor pressure vessel, and the austenitic steel internals. It is the purpose of the present paper to provide an overview of work being carried out in the RPV Mechanics Sub-project of PERFECT to predict the fracture behavior of PWR, BWR and WWER systems.Copyright

Cleavage Fracture Micromechanisms Related to WPS Effect in RPV Steel

Since the first investigations four decades ago, a large number of experiments on ferritic steels has confirmed the existence of a warm pre-stress (WPS) effect, which describes the effective enhancement of the cleavage fracture toughness at low temperature following the application, at a higher temperature, of a stress intensity factor (SIF) which exceeds the fracture toughness of the virgin material at low temperature. These experiments allowed for the establishment of the so-called ‘conservative principle’, which states that no fracture will occur if the applied SIF decreases (or is held constant) while the temperature at the crack-tip decreases, even if the fracture toughness of the virgin material is exceeded. In structural integrity assessments involving a prior overload or a thermal transient, such as that of a nuclear pressure vessel subjected to a pressurized thermal shock (PTS) consecutive to a loss of coolant accident (LOCA), such a principle is of great importance in the evaluation of the safety margins. Three main reasons have been advanced to explain the WPS effect: the blunting of the crack tip at high temperature, the formation of high compressive stresses on elastic unloading, and a change in the cleavage fracture micromechanisms induced by plastic deformation. While all these factors certainly contribute to the effective toughness enhancement following WPS, their relative incidence on the fracture risk is not easily established. In this paper, we choose to mainly focus on the cleavage fracture micromechanisms following WPS, as a first step towards better quantifying the individual contributions of crack tip blunting and residual stresses.

Modelling of irradiation embrittlement in the ductile to brittle transition range for an A508 pressure vessel steel

The aging behavior of structural steels used to manufacture nuclear pressure vessels is surveyed using Charpy V-notch specimens located in capsules inside the pressure vessel. The Charpy data are then used to assess the safety integrity of the structures based on semi-empirical relations relating Charpy impact transition curve shifts and the fracture toughness shifts due to irradiation. Using a computational strategy proposed in [1] which combines a deterministic model for ductile fracture and a statistical description of brittle fracture, this work aims at the prediction of the whole Charpy transition curve of irradiated steels. The actual strain hardening behavior of an A508 Cl.3 steel from the french surveillance program is considered in the simulations, contrarily to a previous work where a shift of the un-irradiated stress-strain curve to higher stress values was considered. Comparison with Charpy energy data for two levels of irradiation shows that irradiation possibly also affect brittle fracture. It is also shown that if a low increase of the yield stress is considered, the ductile fracture energy can decrease as a result of a compensation between the increase of dissipated energy due to a higher yield stress and a decrease of dissipated energy due to a faster ductile crack propagation.Copyright

KIa Crack Arrest Toughness Assessment Using Thermal Shock on Notched Disks

The aim of the study is to validate the KIa -T curve on a thermal shock experiment performed on a notched disk (DTSE) taken from a A533-B type steel. Several experiments have been performed. Non linear thermal analyses were carried out using the finite element method in order to obtain the full thermal field within the specimen during crack propagation. The results obtained are in excellent agreement with the experimental results. The DTSE is also interpretated in static terms to compare the obtained KIa (T) values with the limit curve. Finally, dynamic F.E. simulations allow to estimate the influence of dynamic effects in the DTSE and thus validate the methodology. According to the computations, the crack arrested when dK/da>0 and dKd /da = 0. The comparison between stress intensity factor computed from elastic-static analysis (or dynamic) and RCC-M code demonstrated the conservatism of the approach. Then static analysis is sufficient to analyse the result, since no wave interference with the crack propagation was identified.Copyright

PERFECT—Prediction of Irradiation Damage Effects in Reactor Components: Update of Progress in RPV Mechanics Sub-Project

The EURATOM 6th Framework Integrated Project PERFECT (Prediction of Irradiation Damage Effects in Reactor Components) addresses irradiation damage in RPV materials and components by multi-scale modeling. This approach offers many potential advantages over the conventional empirical methods used in current practice of nuclear plant lifetime management. Launched in January 2004, PERFECT is a 48-month project focusing on two main components of nuclear power plants which are subject to irradiation damage: the ferritic steel reactor pressure vessel (RPV), and the austenitic steel internals. It is the purpose of the present paper to provide an update of progress of work being carried out in the Mechanics Sub-project of PERFECT to predict the fracture behavior of RPVs in PWR and WWER systems.Copyright

Relation Between Crack Velocity and Crack Arrest

Initiation fracture toughness is a material parameter well established to predict crack initiation. An improved fracture design for security component could be obtained in predicting crack arrest. Arrest fracture toughness can be used for that purpose. However, this parameter although studied for a long time [1] is still questionable, even though a standard test procedure was developed in [2]. The influence of dynamic effects on crack arrest are often invoked and the assessment of arrest toughness under static analysis could lead to a dependence with specimen geometry [2].