Controlling the microstructure distribution in the final product is challenging and demanding work for the aluminium extrusion industry. This can only be achieved when the microstructure evolution ...during deformation is fundamentally understood. In this paper, some physical models which are based on dislocation density, subgrain size, and misorientation have been integrated into a commercial finite element modelling program simulating extrusion. The volume fraction recrystallised after press quench and the recrystallised grain size after solution treatment are studied. The influences of die configuration, container temperature and die temperature on the recrystallisation behaviour are detailed. Following this successful prediction, a uniform distribution of microstructure along the length of the extrudate has been obtained by adjusting the ram speed according to the calculated subgrain size at die exit. The calculated histories of temperature and subgrain size during deformation are presented. The problems of this new process are illustrated. Further work on simulation and control of microstructure is finally discussed.
The major objective of the present paper is to explore the complicated interactions between die design, forming parameters (i.e. ram speed, container temperature, billet temperature and extrusion ...ratio) and the product qualities (extrudate shape, surface condition and microstructure) by the use of finite element modelling (FEM). The various models (such as recrystallisation, damage criteria, etc.) have been integrated into the commercial codes, FORGE2
® and FORGE3
®, through user routines. The physical recrystallisation model proposed by Sellars and Zhu Mater. Sci. Eng. A280 (2000) 1 and Vatne et al. Acta Mater. 44 (1996) 4463 have been compared. The predicted distributions of the volume fraction recrystallised were also compared with the experimental results from the literature. The influences of forming parameters on the occurrence of surface cracks were studied by the combination of the Taguchi method with the FEM. It was found that the choked die could significantly reduce the possibility of producing surface cracks. Through simulating a shape extrusion process using two different die structures, it was found that the use of an expansion chamber can significantly reduce the degree of non-uniformity in terms of the extruded product shape and properties. The character of the complex material flow is also identifiable, which is very useful to help improve die design.
•Weight function-based COD solutions were proposed for circumferential-cracked pipe.•Parametric 3D FEA were performed to develop the weight functions.•Geometry Hi functions were calculated and ...compared with those in the literature.•The developed solution was used to calculate COD caused by weld residual stress.
This paper presents a direct weight function-based method to evaluate the crack opening displacement (COD) which is a key parameter in leak-before-break (LBB) assessment and in probabilistic fracture mechanics (PFM) assessment of nuclear piping systems. The focus of this study is a circumferential-cracked pipe with non-linearly through-wall distributed stress loaded on the crack face. Parametric finite element analyses (FEA) were performed to calculate COD associated with different crack face pressure (CFP) loadings. The analyses matrix covers a wide range of pipe size and crack lengths. The crack mouth COD values at three locations: at outside surface, inner surface and mid-thickness were considered. The numerical results were then used to determine the coefficients that construct the direct weight functions. The proposed solution can be used to predict the COD resulted from given weld residual stress (WRS) field with the general prediction error being less than 8%. The research outcome can provide more computational efficiency in both deterministic LBB assessment as well as PFM analyses of pressurized pipes.
► A probabilistic methodology is proposed to evaluate the rupture frequency of feeder pipes due to flow accelerated corrosion. ► A strategy for developing risk-informed feeder life management plans ...is presented. ► Inspection priority of the feeder is ranked by the calculated rupture frequency. ► Target rupture frequency can be satisfied by properly planning the inspection scheme.
Feeder pipe wall thinning due to flow accelerated corrosion (FAC) has been identified as a degradation mechanism that may affect the operating life of outlet feeder pipes. A large number of feeders are expected to require costly repair or replacement over the remaining life of the station if a conservative FAC rate is used in the deterministic structural integrity assessment. The paper presents a preliminary probabilistic framework for determining the rupture frequency of feeders subject to FAC based on commercial probabilistic fracture mechanics code WinPRAISE 2007. The obtained information can be used as inputs for developing risk-informed feeder life management plans. Darlington Unit 2 is selected to demonstrate the proposed method.
Four major local microstructural parameters (local strength difference and the volume fraction of soft and hard phases) involved in the heterogeneous models are studied by the Taguchi method12 to ...determine their individual contribution to the calculated forming limit strain. A number of heterogeneous finite element models have been developed to treat the effect of local microstructure (i.e., texture, grain size, particle distributions, etc.) on the forming limit strains.
•Two loop analysis to deal with uncertainties in the structural integrity assessment of steam generator tubing.•The comparison between deterministic assessment and probabilistic assessment with ...varying confidence levels.•Two acceptance standards for the probabilistic structural integrity assessment.
A probabilistic structural integrity assessment framework utilizing a two-loop Monte Carlo simulation process is proposed. The process and interpretation of the results are demonstrated using an example of Steam Generator (SG) tubing with fretting flaws. Acceptance standards established using a 95/90/90 criterion (which represents 95% probability at 90th percentile with 90% one-sided confidence) is proposed and compared with the generally accepted deterministic and traditional probabilistic acceptance standards.
Probabilistic fracture mechanics (PFM) simulates the behavior of cracked structures and propagates uncertainties from input parameters to a failure probability or its uncertain estimate. In nuclear ...technology, this approach supports the assessment of the rupture probability of highly reliable pipes, which is an important parameter for the safety analysis of a nuclear power plant. For the appropriate probabilistic modelling of a structure with consideration of uncertainties, but also for the analysis of PFM application cases, the question arises, which input parameter of a probabilistic model has a higher impact on the estimate of computed failure probability, and which has a minor impact. This question is associated with the sensitivity measures or importance factors of the input parameters and their ranking concerning their influence.
In this paper, six different approaches for the quantification of the sensitivity of parameters PFM evaluations are investigated: the amplification ratio, the direction cosine, the degree of separation, the analysis of the most probable failure point, the separation of uncertainty method, and the simple sample-based sensitivity study. Each method is described, visualized, applied to a common test case, and compared. The application case and the comparison are part of the Coordinated Research Project (CRP), “Methodology for Assessing Pipe Failure Rates in Advanced Water-Cooled Reactors (AWCRs)” by the International Atomic Energy Agency (IAEA), which is dedicated to the development of failure rates of piping in AWCRs. The participants used different PFM computer codes to analyze the test case and individual sensitivity methods to rank the input parameters, which motivated the comprehensive survey.
The predicted parameter ranking of the approaches is consistent between the methods and between different PFM codes, but the approaches differ in the scope and the required effort. A conclusion is drawn and recommendations for the six different approaches are given.
•Sensitivity measures in probabilistic fracture mechanics are compared.•Six parameter ranking methods with different starting points are evaluated.•The proposed sensitivity measures agree in their ranking.•The sensitivity ranking approaches differ in their scope and the required effort.
•Computation of failure frequency in the absence of operational experience.•Mechanistic models that account for the factor of improvement (FOI) concept.•Three different piping reliability modeling ...approaches for the computation of failure frequency.
Probabilistic failure metrics such as leak frequency and rupture frequency are commonly used to characterize piping reliability. The methodologies for calculating the failure metrics rely on a complex set of input parameters. Operating experience data and experimental data play an important role in informing the different input parameters. The paper describes results and conclusions of a coordinated research project to benchmark three different reliability models using a four-step procedure: reference case definition of relevance to advanced reactor designs, input parameter calibration, validation of results, and application of different methodologies upon completion of the calibration and validation steps. The reference case is a weld consisting of nickel-base alloy 152/52 and located within a primary pressure boundary of an advanced reactor. This alloy is a class of structural materials known to be highly resistant to stress corrosion cracking. Synergies between the different methods are noted and the importance of a multi-disciplinary approach to input parameter development is underscored. A key conclusion is that the three methods are equally suitable for estimating failure frequencies. In any specific application, a selection of the most practical or effective computational tool can be considered. The comparison of alternative models confirms and helps to gain confidence in the computed failure frequency estimates. The study was part of a coordinated research project organized by the International Atomic Energy Agency.
This paper presents the first output of the International Atomic Energy Agency (IAEA) Coordinated Research Project (CRP), titled Methodology for Assessing Pipe Failure Rates in Advanced Water-Cooled ...Reactors (AWCRs), launched in 2018. This IAEA CRP is aimed at developing a good-practices document for estimating the probabilistic failure metrics of piping in AWCRs. The reliability of piping that comprise the Reactor Coolant Pressure Boundary (RCPB) is important for maintaining safe and stable operations of Nuclear Power Plants (NPPs) because failure of those piping components could lead to undesirable consequences, such as plant shutdown, costly repair, the occurrence of Loss-of-Coolant Accidents (LOCAs) and, possibly, subsequent core damage or large release events. Probabilistic failure metrics (e.g., failure rate, failure frequency, or failure probability) of RCPB components are the key inputs to the Probabilistic Safety/Risk Assessment (PSA/PRA) and risk management of NPPs. The estimation of probabilistic failure metrics, however, is challenging, especially for AWCRs, due to the lack of operating experience. Therefore, as the first step of the IAEA CRP activities, this paper is developed to provide a literature review of the existing studies (from 2000 to April 2019) on the estimation of probabilistic failure metrics for RCPB piping and Steam Generator (SG) tubes of NPPs and to categorize them based on four criteria: (1) explicitness of incorporation of physical failure mechanisms; (2) types of failure characterization; (3) types of physical models for degradation; and (4) explicitness of consideration of maintenance. The existing studies are also analyzed from the viewpoint of the following key aspects: (i) uncertainty analysis, (ii) sensitivity analysis, (iii) validation strategies, and (iv) the areas of applications.
•Existing studies on probabilistic failure metrics of piping and tubes are reviewed.•The existing studies are categorized based on a structured scheme.•Uncertainty/sensitivity analyses and validation strategies are also reviewed.•Practical application areas in the existing studies are summarized.
This article presents the first output of the International Atomic Energy Agency (IAEA) Coordinated Research Project (CRP), titled Methodology for Assessing Pipe Failure Rates in Advanced ...Water-Cooled Reactors (AWCRs), launched in 2018. This IAEA CRP is aimed at developing a good-practices document for estimating the probabilistic failure metrics of piping in AWCRs. The reliability of piping that comprise the Reactor Coolant Pressure Boundary (RCPB) is important for maintaining safe and stable operations of Nuclear Power Plants (NPPs) because failure of those piping components could lead to undesirable consequences, such as plant shutdown, costly repair, the occurrence of Loss-of-Coolant Accidents (LOCAs) and, possibly, subsequent core damage or large release events. Probabilistic failure metrics (e.g., failure rate, failure frequency, or failure probability) of RCPB components are the key inputs to the Probabilistic Safety/Risk Assessment (PSA/PRA) and risk management of NPPs. The estimation of probabilistic failure metrics, however, is challenging, especially for AWCRs, due to the lack of operating experience. Therefore, as the first step of the IAEA CRP activities, this paper is developed to provide a literature review of the existing studies (from 2000 to April 2019) on the estimation of probabilistic failure metrics for RCPB piping and Steam Generator (SG) tubes of NPPs and to categorize them based on four criteria: (1) explicitness of incorporation of physical failure mechanisms; (2) types of failure characterization; (3) types of physical models for degradation; and (4) explicitness of consideration of maintenance. The existing studies are also analyzed from the viewpoint of the following key aspects: (i) uncertainty analysis, (ii) sensitivity analysis, (iii) validation strategies, and (iv) the areas of applications.
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