Machine-Learning Approach to Determine Surface Quality on a Reactor Pressure Vessel (RPV) Steel

Surface quality measures such as roughness, and especially its uncertain character, affect most magnetic non-destructive testing methods and limits their performance in terms of an achievable signal-to-noise ratio and reliability. This paper is primarily focused on an experimental study targeting nuclear reactor materials manufactured from the milling process with various machining parameters to produce varying surface quality conditions to mimic the varying material surface qualities of in-field conditions. From energising a local area electromagnetically, a receiver coil is used to obtain the emitted Barkhausen noise, from which the condition of the material surface can be inspected. Investigations were carried out with the support of machine-learning algorithms, such as Neural Networks (NN) and Classification and Regression Trees (CART), to identify the differences in surface quality. Another challenge often faced is undertaking an analysis with limited experimental data. Other non-destructive methods such as Magnetic Adaptive Testing (MAT) were used to provide data imputation for missing data using other intelligent algorithms. For data reinforcement, data augmentation was used. With more data the problem of ‘the curse of data dimensionality’ is addressed. It demonstrated how both data imputation and augmentation can improve measurement datasets

Neutron-Irradiation + Helium Hardening & Embrittlement Modeling of 9%Cr-Steels in an Engineering Perspective (HELENA)

This report provides a physically–based engineering model to estimate the radiationhardening of 9%Cr–steels under both displacement damage (dpa) and helium. The modelis essentially based on the dispersed barrier hardening theory and the dynamic re–solutionof helium under displacement cascades. However, a number of assumptions andsimplifications were considered to obtain a simple description of irradiation hardeningand embrittlement primarily relying on the available experimental data. As a result, twocomponents were basically identified, the dpa component that can be associated withblack dots and small loops and the He–component accounting for helium bubbles. Thedpa component is strongly dependent on the irradiation temperature and its dependencelaw was based on a first–order annealing kinetics. The damage accumulation law wasalso modified to take saturation into account. Finally, the global kinetics of the damageaccumulation kept defined, its amplitude is fitted to one experimental condition. Themodel was rationalized on an experimental database that mainly consists of ~9%Cr–steels irradiated in the technologically important temperature range of 50 to 600°C up to50 dpa and with a He-content up to ~5000 appm, including neutron and proton irradiationas well as implantation. The test temperature effect is taken into account through anormalization procedure based on the change of the Young's modulus and the anelasticdeformation that occurs at high temperature. Finally, the hardening–to–embrittlementcorrelation is obtained using the load diagram approach.Despite the large experimental scatter, inherent to the variety of the materials andirradiation as well as testing conditions, the obtained results are very promising.Improvement of the model performance is still possible by including He–hardeningsaturation and high temperature softening but unfortunately, at this stage, a number ofconflicting experimental data reported in literature should first be clarified

Prediction of radiation induced hardening of reactor pressure vessel steels using artificial neural networks

In this paper, we use an artificial neural network approach to obtain predictions of neutron irradiation induced hardening, more precisely of the change in the yield stress, for reactor pressure vessel steels of pressurized water nuclear reactors. Different training algorithms are proposed and compared, with the goal of identifying the best procedure to follow depending on the needs of the user. The numerical importance of some input variables is also studied. Very accurate numerical regressions are obtained, by taking only four input variables into account: neutron fluence, irradiation temperature, and chemical composition (Cu and Ni content). Accurate extrapolations in term of neutron fluence are obtained. © 2010 Elsevier B.V. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Size correction scheme to determine fracture toughness with mini-CT geometry in the transition regime

The mini-CT geometry is used to determine relevant fracture toughness data along with the master curve description within the transition regime. However, due to significant loss of constraint experienced at loadings close to J-levels corresponding to the reference temperature T0, the current size correction scheme from ASTM E1921 standard requires a better assessment in order to avoid censoring a large number of mini-CT specimens. Indeed, the current tight ASTM E1921 requirements on the KJc limit value set to ensure that the computed transition temperature T0 is reliable often implies that many samples have to be censored. A large number of censored specimens leads to a significant loss of time, money and material, and of unnecessary activated material in the case of irradiated specimens. Besides, because of the specimen size reduction, more mini-CT samples and lower test temperatures as compared to large sized samples are required to determine a reliable T0 value. In this study, an improved specimen size correction associated to the mini-CT geometry is proposed based on a local approach of brittle fracture combined with FEM simulations. The results show that the proposed size correction reduces the T0 bias compared to the 1T-CT geometry. It also contributes to relaxing the KJc limit for the mini- CT geometry as compared to the ASTM size corrected values

Effect of the pre-crack non-uniformity on the initiation of brittle fracture in mini-CT specimen

Non-uniform pre-crack fronts often develop during fatigue pre-cracking of mini-CT specimen, resulting in invalid measurement when following the tight requirements of the ASTM standards regarding fatigue pre-crack curvature. Invalid measurements lead to a waste of test resources which is a major issue for irradiated materials. Previous studies have demonstrated a weak effect of excessive pre-crack curvature on the experimentally measured fracture toughness in the transition regime. In this paper, experimental investigations as well as finite element simulations were performed to provide additional confirmation of this conclusion. In order to demonstrate that meaningful results can still be generated in the brittle regime from non-uniformly precracked mini-CT specimens, a local approach of brittle fracture was adopted to quantify the effect of pre-crack front non-uniformity on the initiation of brittle fracture. The final goal is to contribute to consolidate relevant requirements on the pre-crack front curvature

Effect of pre-crack non-uniformity for mini-CT geometry in ductile tearing regime

peer reviewedThe mini-CT specimen, as one of the geometries that offers significant advantages, attracts the attention from all over the world for application to fracture toughness measurement. However, one of the shortcomings of this geometry is related to the required tight accuracy of the specimen dimensions, inparticular the fatigue pre-crack curvature which often violates the requirements of the ASTM standards. Given the limited thickness of mini-CT geometry, a non-uniform pre-crack tends to develop during fatigue pre-cracking, resulting in a large proportion of mini-CT specimens being considered invalid. Previous investigations have demonstrated that mini-CT specimens with excessive crack front curvature can still provide meaningful fracture toughness results. In this paper, the effect of pre-crack front non-uniformity on ductile fracture is studied: first, the difference of macro parameters such as the applied load, J-integral and crack tip stress-strain field are investigated to illustrate the varying fracture behavior related to non-uniform pre-crack. Next, two micro-mechanical-based approaches, the Rice-Tracey void growth model and Thomason void coalescence model, are integrated to compare the ductile fracture initiation conditions associated with uniform and non-uniform fatigue pre-crack. Finally, the experimental verification of the ductile fracture simulations is performed for mini-CT specimens with uniform and 30° tilted initial cracks. The results indicate that the pre-crack non-uniformity plays a major role in the redistribution of local J-integral and stress-strain state, further affects the position of crack initiation and the way of crack propagation. Nevertheless, the pre-crack non-uniformity has limited effect on the global properties that are usually expected from fracture toughness tests, such as applied load, J-R curve and critical fracture toughness. The requirements in the ASTM E1820 regarding pre-crack front curvature is believed to need to be relaxed

The Effect of Loss of Constraint on the Initiation of Ductile Fracture in a Mini-CT

As one of the most appealing miniaturized geometries for fracture toughness characterization, the miniature compact tension (mini-CT) geometry was demonstrated to provide reliable fracture toughness measurement in the ductile-to-brittle transition regime and Master Curve assessment. However, when using a mini-CT in the upper shelf (fully ductile) regime, the ductile crack initiation and the crack growth resistance tend to be underestimated for most of the investigated RPV steels. Previous experimental investigations have attributed the underestimation of the crack resistance properties to two concurrent processes related to loss of constraint and cold working occurring in the ligament ahead of crack tip during cracking, and gave an engineering size correction that mainly accounts for the thickness of the specimens. In this study, finite element simulations and two micromechanics-based approaches: the Rice-Tracey void growth model and Thomason void coalescence model are combined, aimed at investigating the role that the loss of constraint plays in the initiation of ductile fracture in a mini-CT and providing numerical evidence to support the proposal of the size correction on the critical fracture toughness. The final objective of this paper is to estimate the ductile crack initiation behavior that would be obtained from a standard 1T-CT specimen by applying appropriate size correction on a mini-CT. The results show that the method incorporating the finite element simulations and two micromechanics-based approaches can well describe the effects of loss of constraint for different materials, thus contributing to the size correction of critical fracture toughness of mini-CT in various conditions