Thermo-Mechanical Loading of Full-Scale Welded Piping Components in High Temperature Water Environment

2017 ◽  
Author(s):  
Matthias C. Kammerer ◽  
Xaver Schuler ◽  
Stefan Weihe ◽  
Michael Seidenfuß ◽  
Mi Zhou ◽  
...  
Keyword(s):  
High Temperature ◽  
Fatigue Life ◽  
Fatigue Testing ◽  
Water Environment ◽  
Hot Water ◽  
Component Level ◽  
High Temperature Water ◽  
Weld Material ◽  
Nickel Base ◽  
Temperature Water

The effect of high-temperature water environment on the fatigue life of steels used for pressure retaining components has been discussed controversially for the last 20 to 30 years. Fatigue testing of laboratory specimens for typical steels showed significant drops in fatigue life when tested in high temperature water environment compared to air environment. Based on these findings the applicability of fatigue design curves such as those enclosed to ASME code Section III NB are questionable concerning their degree of conservatism. Nevertheless, experience from components experiencing power plant operation does not match up with laboratory fatigue testing of small uniaxial specimens. Fatigue life estimations based on models representing laboratory tests do highly overestimate the fatigue life reduction resulting from high temperature water environments compared to the analyses of components having reached their postulated fatigue life. To overcome this disagreement component testing under defined laboratory conditions is highly desired to achieve “gap closure”. At MPA University of Stuttgart a test facility was set up where environmental fatigue testing on component level can be realized within a hot water test loop. Within the framework of a research project sponsored by the German Federal Ministry of Education and Research (BMBF) piping modules containing a dissimilar metal weld are exposed to water environments with alternating temperature conditions. At specific locations water at about room temperature is injected to a hot pipe segment which results in thermal induced loading situations. Consequently thermal stratification and shocks cause localized stresses and strains in the tested modules. Within this paper an overview of the testing procedure, the tested materials and results from both experimental measurements and fractographic analyses are presented and discussed. In addition to experimental investigations the results drawn from a coupled computational fluid dynamics (CFD) and structural mechanics finite-element-analysis (FEA) including a fatigue life assessment are shown. Finally, this work states on the applicability of common fatigue assessment procedures including the fatigue life reducing factors based on the results from realistic fatigue testing on component level. Within low cycle fatigue tests a nickel-base weld material was characterized regarding its fatigue life in air and high temperature water environment in comparison. It was found that the effect of environmentally assisted fatigue is in good agreement with what is known from literature for smooth specimens made from austenitic steels. Results from tests using notched specimens showed a significant change in the environmental effect compared to tests using smooth specimens. During component testing within a hot water loop modules which contain a dissimilar metal weld were exposed to alternating water temperature conditions between 20 °C and 65 °C. At the end of the component test cracks were found in the regions where the highest temperature changes were measured and calculated. The numerical analysis of the fluid-structure-interaction pointed out that the transition region between the austenitic steel and the nickel base weld material is the highest loaded section within the module. Finally the fatigue assessment of the pipe sections containing cracks showed that based on common fatigue hypothesis the loading state is regarded to be subcritical.

Study of Fatigue Initiation of Austenitic Stainless Steel in a High Temperature Water Environment and in Air Using Blunt Notch Compact Tension Specimens

2015 ◽  
Author(s):  
Norman Platts ◽  
David R. Tice ◽  
Jennifer Nicholls
Keyword(s):  
High Temperature ◽  
Fatigue Life ◽  
Crack Growth ◽  
Water Environment ◽  
Compact Tension ◽  
Life Assessment ◽  
Blunt Notch ◽  
High Temperature Water ◽  
Fatigue Initiation ◽  
Temperature Water

Fatigue life assessment procedures for components exposed to high temperature pressurised water environments are typically based on NUREG/CR-6909 or broadly similar codes (e.g. proposed ASME code case N-792). The effects of a high temperature water environment on the fatigue life are accounted for by simply adjusting the fatigue life in ambient temperature air by an environmental factor (FEN). This adjustment assumes that the environment affects both initiation (nucleation) and propagation equally, which is potentially over-conservative. Blunt notch compact tension (CT) specimens (along with direct current potential drop (DCPD) crack detection) have been proposed as a means of determining the “true” fatigue initiation life, enabling the relative impact of the environment on initiation and growth to be characterised and the level of conservatism in the FEN approach assessed. The current work uses a combination of finite element analysis and fatigue testing in both air and water to assess the feasibility of blunt notch CT testing to detect initiation and to quantify the environmental impact. This work indicates significant difficulties with the blunt notch CT test methodology both in terms of quantifying the applicable strain and in terms of detection of the very early stages of initiation which preclude the quantitative application of the technique to study true initiation. Qualitatively, the results suggest that there is still a significant impact of the high temperature water environment on the earliest detectable stages of crack growth in austenitic stainless steels; however the earliest defects detectible by DCPD techniques still involve a significant contribution from short crack growth. Nevertheless, the technique provides a valuable insight into initiation and residual life of components subjected to through wall strain gradients.

Effect of Surface Condition on the Fatigue Life of Austenitic Stainless Steels in High Temperature Water Environments

2018 ◽  
Author(s):  
Andrew Morley ◽  
Marius Twite ◽  
Norman Platts ◽  
Alec McLennan ◽  
Chris Currie
Keyword(s):  
High Temperature ◽  
Fatigue Life ◽  
Rough Surface ◽  
Surface Finish ◽  
Water Environment ◽  
Austenitic Stainless Steels ◽  
Design Curve ◽  
Fatigue Design ◽  
High Temperature Water ◽  
Temperature Water

High temperature water environments typical of LWR operation are known to significantly reduce the fatigue life of reactor plant materials relative to air environments in laboratory studies. This environmental impact on fatigue life has led to the issue of US-NRC Regulatory Guide 1.207 [1] and supporting document NUREG/CR-6909 [2] which predicts significant environmental reduction in fatigue life (characterised by an environmental correction factor, Fen) for a range of actual and design basis transients. In the same report, a revision of the fatigue design curve for austenitic stainless steels and Ni-Cr-Fe alloys was proposed [2]. This was based on a revised mean curve fit to laboratory air data and revised design factors to account for effects not present in the test database, including the effect of rough surface finish. This revised fatigue design curve was endorsed by the NRC for new plant through Regulatory Guide 1.207 [1] and subsequently adopted by the ASME Boiler and Pressure Vessel (BPV) Code [3]. Additional rules for accounting for the effect of environment, such as the Fen approach, have been included in the ASME BPV Code as code cases such as Code Case N-792-1 [4]. However, there is a growing body of evidence [5] [6] [7] and [8] that a rough surface condition does not have the same impact in a high temperature water environment as in air. Therefore, application of Fen factors with this design curve may be unduly conservative as it implies a simple combination of the effects of rough surface and environment rather than an interaction. Explicit quantification of the interaction between surface finish and environment is the aim of a number of recent proposals for improvement to fatigue assessment methods, including a Rule in Probationary Phase in the RCC-M Code and a draft Code Case submitted to the ASME BPV Code as described in References [9] and [10]. These approaches aim to quantify the excessive conservatism in current methods due to this unrecognised interaction, describing this as an allowance for Fen effectively built into the design curve. A number of approaches in various stages of development and application are discussed further in a separate paper at this conference [11]. This paper reports the results of an extensive programme of strain-controlled fatigue testing, conducted on two heats of well-characterised 304-type material in a high-temperature simulated PWR environment by Wood plc. The baseline behaviour in environment of standard polished specimens is compared to that of specimens with a rough surface finish bounding normal plant component applications. The results reported here substantially add to the pool of data supporting the conclusion that surface finish effects in a high-temperature water environment are significantly lower than the factor of 2.0 to 3.5 assumed in construction of the current ASME III fatigue design curve. This supports the claim made in the methods discussed in [9] [10] and [11] that the fatigue design curve already incorporates additional conservatism for a high-temperature water environment that can be used to offset the Fen derived by the NUREG/CR-6909 methodology. At present, this observation is limited to austenitic stainless steels.

Evaluation of Environmental Effects on the Fatigue of Notched Specimen of Austenitic Stainless Steel Using Modified Rate Approach Method

2004 ◽  
Author(s):  
Katsumi Sakaguchi ◽  
Yasuhide Asada ◽  
Masao Itatani ◽  
Toshiyuki Saito
Keyword(s):  
Stainless Steel ◽  
High Temperature ◽  
Fatigue Life ◽  
Stress Concentration ◽  
Austenitic Stainless Steel ◽  
Notch Root ◽  
Notched Specimen ◽  
High Temperature Water ◽  
Approach Method ◽  
Temperature Water

Fatigue testing was conducted on notched specimens of austenitic stainless steel 316NG in high temperature water. Specimens were notched round bar with elastic stress concentration factors Kt of 1.4 and 3. For the specimen of Kt = 3, fatigue test was also performed in high temperature air. Environmental correction factor Fen recently proposed by Environmental Fatigue Tests (EFT) project in Japan Nuclear Safety Organization (JNES) was applied to the result of fatigue test to evaluate the environmental effects on fatigue life of notched specimen. Since the notch root strain varies non-proportionally to nominal strain in the elastic-plastic region, the modified rate approach method was applied to predict the fatigue life of notched specimen in the water, which was proposed to account for the environmental effect on fatigue life of nuclear component materials under varying conditions. Notch root strain and strain rate were calculated by FEM analysis. The difference between predicted and experimental fatigue lives in high temperature water was within factor of 2 for Kt = 3. The relationships between fictitious stress amplitude at notch root (= notch root strain amplitude multiplied by elastic modulus) and corrected fatigue life shows good coincidence with best fit curve for austenitic stainless steels. It is concluded that the modified rate approach method and current environmental correction factor Fen proposed by EFT project is applicable to predict fatigue life of the stress concentration when the notch root strain is adequately estimated.

Experimental Study on the Wear Behavior of Nuclear Fuel Rod in High Temperature Water Environment

2005 ◽  
Vol 297-300 ◽  
pp. 1395-1400
Author(s):  
Young Ho Lee ◽  
Hyung Kyu Kim ◽  
Youn Ho Jung
Keyword(s):  
High Temperature ◽  
Nuclear Fuel ◽  
Wear Behavior ◽  
Water Environment ◽  
Wear Particle ◽  
Fuel Rod ◽  
High Temperature Water ◽  
Particle Layer ◽  
Increasing Temperature ◽  
Temperature Water

In this study, the variation of spring characteristics with increasing temperature was examined and the effect of their variations on the wear behavior of a nuclear fuel rod in both room and high temperature (300°C) water conditions was evaluated. From the results of the load-displacement tests, the spring stiffness was remarkably varied with increasing temperature. The results of the wear tests indicated that the wear damages are decreased at high temperature water when compared with the room temperature result. These results indicated that the removal mechanisms of wear debris at high temperature water are dependent on not only the formation of the wear particle layer but also on the changed contact conditions such as the contact length or area due to the stiffness drops.

Effect of Oxide Film Thickness on Stress-Strain Field at the Tip of Stress Corrosion Cracking

2011 ◽  
Vol 474-476 ◽  
pp. 1128-1131
Author(s):  
Wei Tang ◽  
He Xue ◽  
Dan Zhao
Keyword(s):  
High Temperature ◽  
Oxide Film ◽  
Film Thickness ◽  
Strain Field ◽  
Water Environment ◽  
Structural Materials ◽  
Stress Strain ◽  
High Temperature Water ◽  
Oxide Film Thickness ◽  
Temperature Water

Environmentally assisted cracking (EAC) or stress corrosion cracking (SCC) of key structural materials in the environments of the light water reactor is one of the main problems for the management of the structural safety and service life of nuclear power plants. To understand the effect of oxide film thickness on SCC growth in structural materials in a high temperature water environment, the stress-strain field at the tip of a SCC in nickel base alloy constituted by base metal and oxide film was analyzed in this paper by finite element method using commercial software. The effects of oxide film thickness on the stress-strain field at the tip of the SCC was obtained, which provides a new insight into the research of the mechanism of SCC growth in structural materials in a high temperature water environment.

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