Low cycle fatigue tests are performed on specimens of niobium stabilized austenitic steel AISI 347 (1.4550) at ambient temperature. During the test, the fatigue specimens are equipped with eddy ...current probes, and it can be seen here that the impedance phase shift changes significantly at very early stages of fatigue (i.e. before cracking). Electron backscattering diffraction investigations were carried out to better connect microstructure evolution with impedance phase shifts. Machine learning techniques are employed to relate the impedance shift to the fatigue degradation. This approach allows also the derivation of fatigue life curves with few specimens.
•Eddy current sensors are sensitive to fatigue-induced microstructural changes•The fatigue degradation can be identified by artificial neural networks•Fatigue life curves can be defined based on machine learning techniques
The assessment of metallic materials used in power plants’ piping represents a big challenge due to the thermal transients and the environmental conditions to which they are exposed. At present, a ...lack of information related to degradation mechanisms in structures and materials is covered by safety factors in its design, and in some cases, the replacement of components is prescribed after a determined period of time without knowledge of the true degree of degradation. In the collaborative project “Microstructure-based assessment of maximum service life of nuclear materials and components exposed to corrosion and fatigue (MibaLeb)”, a methodology for the assessment of materials’ degradation is being developed, which combines the use of NDT techniques for materials characterization, an optimized fatigue lifetime analysis using short time evaluation procedures (STEPs) and numerical simulations. In this investigation, the AISI 347 (X6CrNiNb18-10) is being analyzed at different conditions in order to validate the methodology. Besides microstructural analysis, tensile and fatigue tests, all to characterize the material, a pressurized hot water pipe exposed to a series of flow conditions will be evaluated in terms of full-scale testing as well as prognostic evaluation, where the latter will be based on the materials’ data generated, which should prognose changes in the material’s condition, specifically in a pre-cracked stage. This paper provides an overview of the program, while the more material’s related aspects are presented in the subsequent paper.
AISI 347 austenitic steel is, as an example, used in nuclear energy piping systems. Piping filled with superheated steam or cooled water is particularly exposed to high stresses, whereupon local ...material properties in the pipes can change significantly, especially in the case of additional corrosive influences, leading to aging of the material. In the absence of appropriate information, such local material property variations are currently covered rather blanketly by safety factors set during the design of those components. An increase in qualified information could improve the assessment of the condition of such aged components. As part of the collaborative project “Microstructure-based assessment of the maximum service life of core materials and components subjected to corrosion and fatigue (MiBaLeB)”, the short-time procedure, StrainLife, was developed and validated by several fatigue tests. With this procedure, a complete S–N curve of a material can be determined on the basis of three fatigue tests only, which reduces the effort compared to a conventional approach significantly and is thus ideal for assessing the condition of aged material, where the material is often rare, and a cost-effective answer is often very needed. The procedure described is not just limited to traditional parameters, such as stress and strain, considered in destructive testing but rather extends into parameters derived from non-destructive testing, which may allow further insight into what may be happening within a material’s microstructure. To evaluate the non-destructive quantities measured within the StrainLife procedure and to correlate them with the aging process in a material, several fatigue tests were performed on unnotched and notched specimens under cyclic loading at room and elevated temperatures, as well as under various media conditions, such as distilled water and reactor pressure vessel boiling water (BWR) conditions.
•Leak-before-break (LBB) analysis of a list of BWR and PWR components.•Comparison of American and (renewed in 2014) German LBB assessment.•Verification of the generic pipe size influence to LBB ...behavior.
Components of primary circuit piping of light water reactor (LWR) plants fulfil high safety requirements. Beside prevention against any damage, cracking, and leaking, pipes are especially designed to exclude double-ended ruptures, which may cause a high loss of coolant. One option in this strategy, supported by regulations and standards in several countries, is the leak-before-break (LBB) behavior: a crack-like leakage should be detected before it becomes a break.
Evidently, many factors influence the LBB behavior of a specific piping section. Material properties, plant operation parameters, assumed accident scenarios and the local geometry determine the result of a LBB analysis as well as the associated safety margins. Therefore, an analysis is traditionally done case by case, and site by site, according to the applicable standard.
In this article, typical components of pressure water reactor (PWR) and boiling water reactor (BWR) primary circuits are examined, and their LBB behavior is investigated. The German standard (KTA) and the US approach (U.S.NRC) are compared. The quantitative results are obtained with the GRS codes PROST and WinLeck, which include different methods for fracture assessment and leak rate calculation. A clear trend concerning the pipe size is found, resulting in large safety margins for large-diameter piping.
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•Phenomena of subcooled flow through leaks in thin walls are studied.•Laboratory tests on flow through thin-walled apertures with mass flow and optical measurements are performed.•A ...computational model assuming a jet of metastable liquid water is proposed and applied.•Comparative analysis of leak rates with different models on selected tests.
The modeling and prediction of flow rates through leaks in steam generator tubes requires an understanding of the rapid decompression of a subcooled fluid flowing through a small and short flow path. The thin walls of steam generator tubes may lead to the phenomenon that the coolant water cannot evaporate within the leak flow path, which is a key difference compared to leak flow in thick-walled components. Experimental investigations in the literature were investigated and additional laboratory tests on dedicated specimens were performed, including mass flow measurements and optical measurements, which study the flow pattern at the outlet of the leak. These experiments cover a ratio of flow length (wall thickness) to hydraulic diameter L/dh range from 0.4 to 12. Based on the findings, a metastable jet model is proposed, which is based on the formation of a jet of metastable liquid water. The model parameters are fitted with the experiments. The model is compared with alternative approaches, which demonstrates that it is suitable to approximate the mass flow of leaks in steam generator tubes with a sufficiently large leak width or cross section. Due to the small absolute wall thickness of steam generator tubes only very narrow flaws are found to be successfully modeled with ordinary two-phase flow models.
Appropriate target reinnervation and functional recovery after spinal cord injury depend on longitudinally directed regrowth of transected axons. To assess the capacity to promote directed axon ...regeneration, alginate-based highly anisotropic capillary hydrogels (ACH) were introduced into an axon outgrowth assay in vitro and adult rat spinal cord lesions in vivo. In an entorhino-hippocampal slice culture model, alginate-based scaffolds elicit highly oriented linear axon regrowth and appropriate target neuron reinnervation. Coating of alginate-based ACH with the extracellular matrix components collagen, fibronectin, poly
l-ornithine and laminin did not alter the axon regrowth response as compared to uncoated alginate-based ACH. After implantation into acute cervical spinal cord lesions in adult rats, alginate-based ACH integrate into the spinal cord parenchyma without major inflammatory responses, maintain their anisotropic structure and in parallel to findings in vitro induce directed axon regeneration across the artificial scaffold. Furthermore, adult neural progenitor cells (NPC), which have been shown to promote cell-contact-mediated axon regeneration, can be seeded into alginate-based ACH as a prerequisite to further improve the regenerative capacity of these artificial growth supportive matrices. Thus, alginate-based ACH represent a promising strategy to induce directed nerve regrowth following spinal cord injury.
We propose that the amino acid residues 57/58 and 60/61 of eukaryotic release factors (eRF1s) (counted from the N-terminal Met of human eRF1) are responsible for stop codon recognition in protein ...synthesis. The proposal is based on amino acid exchanges in these positions in the eRF1s of two ciliates that reassigned one or two stop codons to sense codons in evolution and on the crystal structure of human eRF1. The proposed mechanism of stop codon recognition assumes that the amino acid residues 57/58 interact with the second and the residues 60/61 with the third position of a stop codon. The fact that conventional eRF1s recognize all three stop codons but not the codon for tryptophan is attributed to the flexibility of the helix containing these residues. We suggest that the helix is able to assume a partly relaxed or tight conformation depending on the stop codon recognized. The restricted codon recognition observed in organisms with unconventional eRF1s is attributed mainly to the loss of flexibility of the helix due to exchanged amino acids.
•The influence of crack location on crack size in fatigue tests is investigated.•A given load drop in a test corresponds to a range of possible crack depths.•Initiated fatigue cracks may have a crack ...depth larger or smaller than 3 mm.•The derived crack size assumptions are relevant in fracture mechanical assessments.
The simulation and assessment of fatigue damage of metallic materials are strongly dependent on the definition of critical damage initiation, i.e. the formation of a crack after a certain number of cycles under fatigue loading. To get this described in an appropriate way classical fatigue analysis needs to be combined with fracture mechanics to obtain a realistic size of an initiating macro crack in a component in the end. Ultimately, this size depends on the procedure of how fatigue life curves are generated from the fatigue tests performed. In this paper strain-controlled fatigue tests and fractographic results have been combined with a computational assessment to better understand the relationship between a traditional load drop criterion characterizing the size of a crack being initiated. It is stated that the location of an initiating crack versus the location of an extensometer applied to the specimen can have an influence on the crack size being initiated and thus the fatigue life for a load drop criterion being fixed. A relationship is proposed to link a crack size dependent on the fatigue test evaluation scheme with the fatigue life, which can be used in engineering assessments. The relevance of practical application cases is discussed.
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.
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