David Lidbury

Assessment of Local Approach Methods for Predicting End-of-Life Toughness of RPV Steels

Local approach methods are becoming increasingly popular as practical tools for cleavage fracture toughness prediction. Their application involves two distinct elements: calculation of ‘individual’ probabilities of failure, dictated by the local mechanical fields; and summation of these failure probabilities to predict the probability of component failure. In this work, we demonstrate that development of the local approach methods to date has been essentially focused on improving the criterion for predicting local failure as a function of the local mechanical fields. Yet, the existing methods fail to predict with sufficient accuracy the effects of irradiation and defect geometry on fracture toughness when the calculations are based on a common set of model parameters. A possible reason for this, common to all methods, is found in the calculation of the cumulative failure probability, which is based on the weakest-link argument. We discuss the implications of the weakest-link assumption, identify those situations where it needs to be reconsidered, and propose future work that will increase our understanding for improving the calculation of global failure probability.Copyright

Introducing Heterogeneity into Brittle Fracture Modeling of a 22NiMoCr37 Ferritic Steel Ring Forging

Microstructural observations of the 22NiMoCr37 “EURO” reactor pressure vessel (RPV) steel ring forging reveal that there is a banded structure along the radial direction, composed of alternate layers rich in bainite and ferrite of wavelength ∼1.5 ±0.75 mm. Heterogeneity at this meso (millimetre)-scale as well as at the micro (micrometre)-scale is currently not considered by conventional fracture mechanics. This paper describes the development of two numerical approaches aimed at incorporating such heterogeneity into the Beremin local approach model of cleavage failure, a model that has been used extensively for predicting the brittle fracture of ferritic RPV steels. The approaches developed combine the crystal plasticity finite element method (CPFEM) with continuum finite element analysis (FEA). CPFEM is applied to predict stress distributions at the microscale and to obtain phase-specific yield and flow properties for continuum FEA that derives stresses at the mesoscale. The results confirm that deformation heterogeneity on the micro- and mesoscales influences the local development of stress. At the microscale, the stress distribution within a representative volume of material located within the crack-tip plastic zone is shown to follow a normal distribution with a ratio of mean stress to standard deviation tending towards 0.1. These results indicate local stress levels that are within approximately ±20 % of those derived using continuum FEA. At the mesoscale, a periodic variation of stress is predicted within the larger representative volume. This variation is less dramatic than that observed at the microscale, though it still gives a spatial variation in maximum principal stress of approximately ±7 % between bainite- and ferrite-rich microstructural regions. These results suggest a significant influence of deformation heterogeneity on local stress levels, particularly at the microscale. However, the conventional Beremin cleavage fracture model, modified to account for microscale stress distribution, predicts only a modest influence of deformation heterogeneity on cleavage fracture probability, increasing Pf by just 5 %. This highlights the need to account for both the spatial variation in cleavage initiation sites as well as the distribution in stress throughout the microstructure. The paper describes one approach for this development.

SMILE Project — The Effect of Warm Pre-Stress in RPV Assessment: Synthesis of Experimental Results and Analyses

Within framework of 5th EC European SMILE project, some R&D actions have been conducted to demonstrate, to model and to validate the beneficial warm pre-stress (WPS) effect regarding the risk of brittle failure in a RPV assessment. An important experimental work has been conducted including classical WPS type experiments on CT specimens on usual RPV steels, and one PTS type transient on a large cracked cylinder. All experimental results confirm the beneficial effect of warm pre-stress, with a significant increase of the materials resistance regarding the risk of brittle failure. The experiments have been analyzed using fracture mechanics, including both engineering methods (Chell, Haigh, Wallin) and more refined analyses based on local approach to fracture (‘modified Beremin’ model). Following a short description of WPS concept, the paper summarizes the main experimental results and presents a synthesis of mechanical analyses involving engineering approaches and numerical analyses.Copyright

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

Effects of Neutron Irradiation on the Fracture Toughness of RPV Materials: Prediction of Material Property Changes for Irradiated Euro Reference Material ‘A’ and Other RPV Materials

A micromechanistic model is used to estimate the irradiation-induced change in the Master Curve reference temperature for cleavage fracture as a function of the associated change in material yield stress relative to the yield stress in the unirradiated condition. The model is shown to predict well the behaviour of Euro Reference Material A (quenched and tempered 22NiMoCr37 ring forging) irradiated at temperatures of T = 285°C and T = 150°C with neutron fluences of 4.3E+19 n/cm2 (En > 1MeV) and 3.1E+19 n/cm2 (En > 1MeV) respectively. Further validation of the model is provided with reference to published data for a range of irradiated RPV plate, forging and weld materials, where measured and predicted values of the change in the Master Curve reference temperature are successfully compared. For the LWR materials and irradiation conditions considered, the fitted parameters of the model are consistent with the view that the primary effect of neutron irradiation is to increase the friction stress for plastic flow of crack-tip material, whereby the irradiation-induced change in yield stress may be associated with a change in a non-hardening, athermal term e.g. as described in the Zerelli-Armstrong constitutive equation. The model predictions compare well with trend curves due to Sokolov and Nanstad, and Wallin and Laukkanen. A particular advantage of the model, compared with these more general formulations, is that it is potentially better suited to a more detailed analysis and interpolation of RPV material datasets covering a range of irradiation conditions where flow properties are reasonably well characterised.Copyright

Validation of Constraint Based Methodology in Structural Integrity: Project Overview and Update

VOCALIST (Validation of Constraint Based Methodology in Structural Integrity) is a shared cost action project co-financed by the European Commission under the Fifth Framework of the European Atomic Energy Community (EURATOM). The motivation for VOCALIST is based on the observation that the pattern of crack-tip stresses and strains causing plastic flow and fracture in components is different to that in test specimens. This gives rise to the so-called constraint effect. Crack-tip constraint in components is generally lower than in test specimens. Effective toughness is correspondingly higher. The fracture toughness measured on test specimens is thus likely to underestimate that exhibited by cracks in components. The purpose of VOCALIST is to develop validated models of the constraint effect, and associated best practice advice, with the objective of achieving (i) an improved defect assessment methodology for predicting safety margins; (ii) improved lifetime management arguments. The work is intended to be of benefit to both plant operators and their regulators. The project started in October 2000 and all experimental and analytical work was completed by February 2004. The final report and an updated Best Practice Handbook are currently being prepared. This paper provides an overview of VOCALIST, describes its various Work Packages, and provides an up-to-date summary of results.Copyright

Engineering Assessment of Constraint Effects on Cleavage Fracture

It is well known that material fracture toughness is influenced by factors including loading mode and crack size that influence the level of stress triaxiality ahead of the crack tip. This so-called “constraint effect” has been demonstrated both experimentally and analytically, with low constraint (low stress triaxiality) conditions leading to enhanced fracture toughness. Two-parameter fracture mechanics has been developed to provide a framework within which to assess the influence of constraint on safety margins for shallow structural defects. However, this requires the availability of a significant amount of plant-specific material with which to measure the materials’ constraint sensitivity experimentally. This paper presents a case study where constraint effects on cleavage fracture toughness of a shallow-cracked biaxially loaded bend specimen are assessed through a combination of modelling and miniaturised testing. The assessment is performed using the Failure Assessment Diagram approach of R6. It is concluded that the approach provides a practical engineering method for assessing the likely magnitude of constraint effects for low constraint configurations.Copyright

Validation of Constraint-Based Methodology in Structural Integrity: Current Issues and Best Practice Handbook

The paper presents an assessment of existing constraint-based procedures for as-received and aged nuclear power plant components, which has been performed as part of the European project VOCALIST. Current analytical procedures and gaps in understanding and limitations in constraint-based procedures are summarized and a First Best-Practice Handbook for constraint-based procedures is outlined. This handbook is based on case studies provided by the VOCALIST partners. The cases are selected to provide an overview of the state-of-the art methods include two-parameter and local approach analyses for cleavage and ductile tearing of different structural components.Copyright

Grain-scale heterogeneity effect on mechanistic modelling of cleavage fracture of a ferritic rpv steel forging material

Improving brittle fracture prediction is crucial for structural integrity assessment. In current safety assessments, fracture mechanics treats polycrystalline steels as homogeneous continua. In reality, deformation of structural steels is heterogeneous. Part of this heterogeneity is due to the elastic and plastic anisotropy of their constituent (often randomly orientated) grains. This paper will compare the predicted failure stresses from tensile tests performed on a ferritic pressure vessel steel using the crystal plasticity finite element approach alongside measured carbide distribution and classical Beremin cleavage model. Available tensile data of 22NiMoCr37 steel at low temperature (−91°C and −154°C) were analysed using Bridgman solutions to account for the necking effect on the stress state at the centre of necking where brittle cracking initiates. This stress state imposed on representative volume element (RVE) made up of 10×10×10 randomly orientated grains, whose deformation is simulated using crystal plasticity finite element modelling (CPFEM). Randomly distributed carbides were produced based on the measured carbide size distribution and density for this steel. By assuming carbides as Griffith microcracks, the cleavage fracture stress in each grain can be assessed based the maximum principal stress on the cleavage crystal plane and an assumed surface energy. By repeating the random carbide distribution 1,000 times, brittle fracture probability can be calculated. Detailed examination shows that the above approach is actually a verification of the BEREMIN local approach model for cleavage fracture. The modelling results were compared with the available ductility data at −91°C and the interpolated ductility data at −154°C at the centre of necking. It is foreseen that this approach will lead to improvements in brittle fracture modelling in heterogeneous ferritic steels by introducing realistic surface energies and real defect distributions in specific materials, when used alongside the CPFEM submodelling approach.Copyright

NULIFE Network of Excellence: Progress on Structural Integrity Issues Relating to Safety of Ageing Nuclear Power Plants

The NULIFE (Nuclear Plant Life Prediction) network is a European network of excellence funded by the European Commission’s 6th Framework Programme [1] together with in-kind contributions of the participants. The network is made up of 11 work package leader organisations (contractors) and about 30 associate contributors and collaborators. NULIFE kicked-off in October 2006 and is working over a 5-year period to create a single organisation structure, capable of providing harmonised R&D at European level to the nuclear power industry and the related safety authorities in the area of lifetime evaluation methods for critical components.Copyright