In this article, a partial selection of experiments on enhancing the impact resistance of structural components with non-metallic, textile-reinforced concrete is discussed. The focus is on the ...experimental investigations in which the impact resistance of thin, textile-reinforced concrete plates is characterized. The article discusses the materials, fabrics and test setup used. For the experimental work, a drop tower from the Otto Mohr Laboratory, which belongs to the Technische Universtät Dresden, was used. Furthermore, the experimental results are presented and evaluated using different methods. Based on the collected data, a suitable approach to determining the perforation velocity of an impactor through the investigated thin, textile-reinforced concrete plates is shown.
•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.
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
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.
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.
•Using FEM it was possible to simulate measured ring compression test data.•The FEM provides burst stresses from Zry-4, M5 and ZIRLO cladding.•The ratio of burst stresses to yield stresses was ...correlated.•The ratio depends linearly on the state of oxidation and hydriding.•The ratio of stresses at unity can be applied as embrittlement criterion.
It is of regulatory interest to prevent the breaking of fuel rods in LOCA transients. In current regulations this is accomplished by limiting the oxidation during LOCA to such an extent that still some residual ductility is preserved in the fuel rod cladding. The current oxidation limit in German as well as in US regulations is set to 17% ECR (Equivalent Cladding Reacted) which aims at maintaining a residual ductility for oxidized claddings.
Recent ANL tests have shown that the combination of both oxidation and additionally hydrogen up-take affects the transition to zero-ductility. Furthermore, the oxidation during LOCA transient is accompanied by a significant up-take of hydrogen (secondary hydriding) if the fuel rod bursts during this transient. This secondary hydriding affects the cladding in the vicinity of the burst opening. These findings necessitate a new criterion for preserving cladding's strength.
This paper describes a method how to derive a criterion which assures the required residual mechanical strength of the cladding for LOCA transients. This method utilizes the experimental results of 102 ring compression tests (RCT) conducted at ANL and KIT. RCTs of various cladding materials, oxidation levels and hydrogen content were considered.
The basic approach was to compare the RCT test data with finite element analyses using the code ADINA. Starting with the cladding oxidation model of Leistikov, both the layer structure of the cladding and the distribution of the oxygen among these layers were determined. The mechanical properties of these layers were taken from MATPRO/FRAPCON models and adapted if necessary.
For each RCT the calculated load/displacement curve followed the measured curve up to that displacement at which the first leaping drop appears which indicates the first crack in the test. For this displacement the maximum equivalent stress in the computational mesh is considered as burst stress. The burst stresses were determined for cladding materials Zircaloy-4, Zircaloy-4 HBR, ZIRLO and M5 at different levels of oxidation, hydrogen content and test temperatures.
The ratio burst stress to yield stress (R) shows a linear dependency on the levels of oxidation and hydrogen up-take. The stress ratio R=1 describes the transition from ductile to brittle behavior dependent on the levels of oxidation and the hydrogen up-take. This ratio can be applied for a conservative safety assessment of fuel rod claddings undergoing LOCA transients with fuel rod burst including secondary hydriding.
•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.
•RCS failure modes and material behavior under high-pressure core melt conditions.•Analytical/FE gross boundary failure methods successfully compared on experiment.•Application on accident scenario, ...parametric studies to evaluate factors of influence.•Methods for gross boundary failure mature, different advantages and disadvantages.•Methods for local failure could improve knowledge of containment-bypass probability.
During postulated high-pressure core melt accidents, high temperatures can be reached by the reactor coolant system (RCS) pressure boundary due to hot gases from the core melt process, which may lead to failure in terms of leak or break and subsequent depressurization. Location, time and size of the failure can have large implications on the further accident progression. Nevertheless, not for all possibly affected components and failure modes suitable integrity assessment methods, in-depth structure mechanical analyses or in some cases even simple models for the integration into severe accident codes are available. Investigations on the events in Fukushima Daiichi emphasize the importance of methods for RCS failure assessment within severe accident analysis.
This work concentrates on the assessment of gross boundary failure, e.g. of the RCS piping, from a mechanical point of view. First, failure modes of RCS components as well as the deformation and fracture behavior of typical RCS materials in the relevant loading regime are discussed. Established assessment methods based on linear damage accumulation and the use of Finite Element (FE) codes as well as a newly developed method based on a zero-dimensional approach are presented. The methods are compared among each other and to a large-scale experiment with a DN700 pipe and international benchmark analyses within the OECD/NEA Components and Structures under Severe Accident Loading (COSSAL) activity. Results of the application to an exemplary severe accident scenario in a PWR and corresponding parametric studies are summarized. An outlook is given on the analysis of local boundary failure of components, potentially responsible for small early leaks.
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