bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of Corrosion Fatigue for the Deaerator Heater Tanks in Nuclear Power Plants
Authors: Ghafouri-Azar Reza , Ho San S.
Published: July 2009

Throughout the world, failures of deaerators and associated storage vessels in thermal power plants have been reported. In 2004 a through wall crack was discovered at one of the Ontario Power Generation (OPG) thermal stations raising whether CANDU nuclear plant deaerators are at risk to similar failure mechanisms. The cause of cracking in the thermal plant was identified as corrosion fatigue cracking. The cracking area of the DA heater tank in the plant was located at the vicinity of the weld connecting the splash plate to the vessel wall. This plate divides the vessel into two compartments. The upper compartment of the tank is filled with condensate water coming from Low Pressure (LP) Heaters. The lower compartment contains steam fed by the LP turbine. In addition, condensate spray nozzles are mounted on this plate. To analyze this phenomenon and assess its potential risk for the nuclear plants, a Finite Element model of the DA heater tank cross section was created. Using the design conditions and operating transients measured during DA cycling, stress and fatigue analyses for the area susceptible for corrosion fatigue cracking were carried out. The criteria of ASME Sec VIII, Div. 2 were used for the analysis. It was concluded that CANDU nuclear plant deaerators are not susceptible to corrosion fatigue cracking during their life cycles because of their low operating cycles and less harsh working environment.
Finite Element Analysis of a Basic Wind Tank
Authors: Hari Yogeshwar , McCormick Joshua
Published: July 2009

The objective of this paper is to perform FEA analysis to establish a baseline analysis for the feasibility/optimization of the sidewalls for a simple wind box. A basic static finite element analysis (FEA) was run using Pro/Mechanica on the wind tank. The tank had corrugated walls, a flat bottom except for 3 sparse corrugations, and header that attaches to the top of all the tank walls around the perimeter of the tank. All components in the tank were constructed from 316L Stainless Steel material. Loads on the tank were then applied in the form of gravity (acting on all parts of the tank) and pressure (designated at 10 psi) acting on all the sidewalls and bottom of the tank. To make the analysis more efficient the model was idealized using shell elements, where applicable. The FEA analysis was completed to compare triangular, sinusoidal and trapezoidal sidewall designs for a simple wind box. The trapezoidal design was the best of the three designs, because it delivered the lowest stress and deformation levels. The sinusoidal and triangular corrugated walls provided a 6% reduction in needed material, but did not produce the same Factor of Safety as compared to the trapezoidal wall.
Advances in Research of Elevated and High-Temperature Structure Integrity of Process Pressure-Bearing Equipment in Chinese Petrochemical Industry
Authors: Chen Xuedong , Tu Shandong , Fan Zhichao , Yang Tiecheng , Guan Weihe , Jiang Jialing , Gao Zengliang
Published: July 2009

During 1995∼2000, investigations were carried out four times on the safety conditions of pressure-bearing equipments in petrochemical enterprises in China. As a result, it is found that many failure cases are related to elevated or high temperature environment. Because the service conditions of Chinese pressure vessels and process characteristics of petrochemical enterprises are somewhat different from those of the other countries, it is a challenge to set up Chinese standard for high-temperature structural integrity assessment, and these research work was started at 2000, namely the early stage of “10th five-year plan” (from 2000 to 2005), during which Hefei General Machinery Research Institute (HGMRI), East China University of Science and Technology, Zhejiang University, Zhejiang University of Technology, etc. have accumulated a lot of data on high-temperature performance of typical steels, set up various life prediction and damage assessment models, preliminarily formed the technical methods for defect safety assessment of high-temperature in-service pressure vessels, and thus lay a foundation for establishment of Chinese high-temperature defect assessment code. In this paper the present status, existing problems and future development direction of China are described in the aspect of non destructive test (NDT) of elevated and high-temperature defects, integrity assessment of defect-containing structures, life prediction and damage assessment etc.
Ratcheting of Inconel 718 at 649°C Under Axial/Torsional Loading
Authors: Kim Kwang Soo , Kim Hyo Shin
Published: July 2009

Ratcheting tests have been performed on Inconel 718 at 649°C for uniaxial loading, torsional loading and a few paths of axial-torsional loading. Ratcheting strain was measured to near failure cycles. The equivalent ratcheting strain is examined for different loading paths. Ratcheting strain is found to increase with cycles in the manner similar to the creep strain under static load, exhibiting steady-state behavior for much of the lifetime. The ratcheting rate depends greatly on the loading path, stress amplitude and mean stress as fatigue life does. Ratcheting rates are found higher for circular paths than for linear paths. The ratcheting strain at failure may be estimated from the steady-state ratcheting rate and cycles to failure for most test cases.
Micromechanical Modeling of Brittle Fracture of French RPV Steel: A Comprehensive Study of Stress Triaxiality Effect
Authors: Mathieu Jean-Philippe , Diard Olivier , Inal Karim , Berveiller Sophie
Published: July 2009

The present study describes a multiscale representation of mechanisms involved in brittle fracture of a french Reactor Pressure Vessel (RPV) steel (16MND5 equ. ASTM A508 Cl.3) at low temperatures. Attention will be focused on the representation of stress heterogeneities inside the ferritic matrix during plastic straining, which is considered as critical for further micromechanical approach of brittle fracture. This representation is tuned on experimental results [1]. Modeling involves micromechanical a description of plastic glide, a mean field (MF) model and a realistic three-dimensional aggregates Finite Element (FE) simulation, all put together inside a multiscale approach. Calibration is done on macroscopic stress-strain curves at different low temperatures, and modeling reproduces experimental stress heterogeneities. This modeling allows to apply a local micromechanical fracture criterion of crystallographic cleavage for triaxial loadings on the Representative Volume Element (RVE). Deterministic computations of time to fracture for different carbide sizes random selection provide a probability of fracture for an Elementary Volume (EV) consistant with the local approach. Results are in good agreement with hypothesis made by local approach to fracture. Hence, the main difference is that no phenomenological dependence on loading or microstructure is supposed for probability of fracture on the EV: this dependence is naturally introduced by the micromechanical description.
Measurement and Prediction of the Residual Stress Field in an Autogenously Welded Stainless Steel Plate
Authors: Alizadeh Hassan , Lewis Simon J. , Gill Christopher , Hossain S. , Smith David J. , Truman Christopher E.
Published: July 2009

There has been a concerted effort over recent years to develop and refine finite element models of welds in order to predict residual stresses. These residual stresses are required to ever improved accuracies in order to provide continued confidence in the safe operation of ageing plant. Not only have computing hardware and software developed at a rapid rate, but guidelines for weld modelling ‘best practice’ have started to be documented. In order to validate and verify weld modelling procedures, test specimens are required which may be subjected to a suite of residual stress measurement techniques in order to allow comparison and ‘benchmarking’ of the numerical predictions. An abundance of such test specimens have been developed over the last few years. These are typically studied via large multi-national ‘round robins’ and results used to fine tune methodologies. A specific example is the NeT ‘bead on plate’ specimen [1, 2] which considered a single weld bead on an austenitic stainless steel plate. Whilst the major thrust worldwide now is to fabricate and study test specimens more representative of real plant, by considering larger specimens, many weld passes, different materials (including ferritic steels and their associated phase change during welding), the research presented in this paper considers an even simpler test specimen. Thus, an autogenous (no filler material) weld on a stainless steel plate is considered. There were two principal motivations for this work. Firstly, numerical and experimental results were required to validate analytical models of welding induced residual stresses. These analytical models [3] are currently under development but, to date, have been formulated only for parent material. Secondly, the lessons learned on weld modelling from previous studies were desired to be tested on the simplest test specimen available.
Inelastic Seismic Test of the Small Bore Piping and Support System: Part 1 — Seismic Proving Test of the Small Bore Piping System
Authors: Shirai Eiji , Eto Kazutoshi , Umemoto Akira , Yoshii Toshiaki , Kondo Masami , Monde Masatsugu , Tai Koichi
Published: July 2009

Seismic safety is one of the major key issues of nuclear power plant safety in Japan. It is demonstrated that nuclear piping possesses large safety margins in the various piping ultimate test reports. But it is appeared that there still remain some technical uncertainties about the phenomenon when both piping and supports show inelastic behavior in the extremely high seismic excitation level. In order to obtain the influence of the inelastic behavior of the support to the whole piping system response, and the subsequent interaction when both piping and supports show inelastic behavior, the following two tests have been started. • Support element test: Load-displacement characteristics of the support system including U-bolt, support itself and concrete anchorage are obtained by the forced displacement test. • Seismic proving test of piping system: The small bore piping and support system consisted of three dimensional piping, supports, U-bolts, and concrete anchorages will be excited on the table by the extremely higher seismic level. This paper introduces the major results of seismic proving test of piping and support system. The support element test results is presented in the paper of part 2, and the simulation analyses of these tests are summarized in the paper of part 3 [1, 2].
Uncertainty in Residual Stress Measurements
Authors: Goudar D. M. , Hossain S. , Truman C. E. , Smith D. J.
Published: July 2009

Accurate characterization of residual stress in engineering components is important in structural integrity assessment. Two commonly used methods of measuring residual stress include the neutron diffraction technique and the deep-hole drilling (DHD) technique. The former is a well-known nondestructive measurement method and the latter is a semi-invasive technique which is readily available and portable. Both these measurement techniques depend on a number of physical quantities and are therefore sensitive to errors associated with the measured data. The resulting stress uncertainties can easily become significant and compromise the usefulness of the results or lead to misinterpretation of the behaviour of stress distribution. This paper describes briefly the error analysis for both techniques. Results from earlier neutron diffraction and deep hole drilling measurements are used to illustrate the errors. It is found that the average error for both techniques is about ±20MPa. In the case of the neutron diffraction method this error is acceptable for path lengths less than a few centimetres. At greater path lengths the errors become unacceptably large. In contrast the error in the DHD is independent of depth.
Development of High Chromium Steel for SFR in Japan and Creep-Fatigue Assessment of the Welded Joint
Authors: Wakai Takashi , Isobe Nobuhiro , Date Shingo , Asayama Tai , Kubo Shigenobu
Published: July 2009

This paper describes the provisional material specifications of the high chromium (Cr) ferritic steel for the Sodium cooled Fast Reactor (SFR) and development of creep-fatigue assessment procedure for the welded joint made of the steel. Based on the test results, it was revealed that tungsten (W) should be diminished to achieve better creep-fatigue strength and toughness after long term aging at elevated temperature. Metallurgical examinations using a scanning electron microscope showed that W precipitated on the grain boundaries as “Laves phase” during aging process. The toughness of the steel which contained much W might be degraded by such coarse precipitations on the grain boundaries. As a result, provisional specifications of the high Cr ferritic steel for SFR pipes and tubes were proposed. Creep-fatigue strength assessment procedure for the welded joints made of the steels was also investigated. An assessment procedure using 2-element model was proposed and verified by comparing with some creep-fatigue test results. The creep-fatigue lives observed in the experiments were well predicted by the proposed assessment procedure, but the failure of the welded joints really occurred in the heat affected zone (HAZ) in some creep-fatigue tests. Since the HAZ was not taken into account in the procedure, there were obviously some rooms for improvement. Creep-fatigue failure mechanisms of the welded joint must be investigated and the characteristics of the HAZ must be formulated for more precise creep-fatigue strength assessment.
Uncertainty of Preloads in Closure Bolts for Transportation Casks for Hazardous and Radioactive Materials
Authors: Shah Vikram N.
Published: July 2009

Maintaining closure bolting integrity under normal and accident conditions is essential for the safe transportation of shipping casks bearing hazardous and radioactive materials. Bolt preload introduces a clamping force in the joint that plays a critical role in ensuring a less-than-specified rates of leakage from such casks. Precise control over bolt preload is essential to introduce adequate clamping force in the joint. Uncertainty is introduced in the closure bolt preloads when an individual bolt is tightened and then when the other bolts are tightened in a specified sequence. Tightening an individual bolt can introduce uncertainty as high as ±35% mainly because of the variation in the coefficient of friction. Tightening the subsequent bolts in a specified sequence introduces uncertainty because the elastic interactions between bolts reduce the preloads already installed. This paper reviews the literature related to the methods employed for installing the preloads, factors introducing uncertainty in the preloads, methods for reducing the uncertainty, and the techniques available for measuring the preloads. It is concluded that the use of an elastic interaction matrix has a potential for providing the specified preload in the bolted joint and that ultrasonic measurements can be used to monitor the preload with ±10% or less uncertainty.
FITNET FFS Methodologies for the Assessment of Low Constraint Conditions: Overview, Contents and New Contributions
Authors: Cicero Sergio , Gutie´rrez-Solana Federico , Kocak Mustafa
Published: July 2009

In the last years constraint has represented one of the major issues on fracture mechanics and structural integrity research. The need of more adjusted less conservative (and still safe) assessments requires, in many cases, the consideration of the constraint conditions in the crack tip in order to make better predictions of the load bearing capacity or the critical crack dimensions. The newly developed FITNET FFS Procedure gathers a comprehensive set of methodologies for the assessment of low constraint conditions, from those ones that have their origin on the crack shallowness and/or the type of loading (something that is also treated in other procedures), to those related to the notch effect (included for first time in an assessment procedure). Moreover, one of the biggest difficulties that has been traditionally found when performing constraint assessments, has been the obtainment of the different material and constraint parameters that have to be implemented in the calculations. FITNET FFS has dedicated special efforts in providing values of these parameters for a wide variety of materials and geometries, so that the application of its constraint assessment methodologies is noticeably simplified if compared to other procedures. This paper presents these FITNET FFS constraint methodologies and the advances achieved by the procedure in the constraint field.
Local Approach Studies of the Effect of Load History on Ductile Fracture
Authors: Howells R. O. , Jivkov A. P. , Beardsmore D. W. , Sharples J. K. , Watson C. T.
Published: July 2009

Local Approach methods provide alternative routes for carrying out fracture mechanics assessments. These methods use a finite element analysis of the cracked component that incorporates a micro-mechanical model of material behaviour in the region of the crack tip. The development of damage, and eventual failure of the material for particular loading conditions, is calculated directly by the micro-mechanical model. A potential advantage of such models is that they can automatically incorporate the effects of loss of constraint and load history. This paper describes studies that compare the predictions of the Gurson-Tvergaard-Needleman (GTN) local approach model of ductile fracture with traditional fracture mechanics parameters. The model is used to consider the effect of load history on ductile fracture initiation in a typical fracture mechanics Compact Tension (CT) specimen. The aim is to investigate the benefits of applying such models to situations where events in the plant loading history can have a significant effect on crack driving force for typical defects that subsequently develop during service. It is anticipated that the results will be used to make improvements to the accuracy of the traditional assessment procedures. The results show that in certain circumstances, for instance where residual stresses are present in the vicinity of a defect, load excursions on the specimen can have a beneficial effect on global load carrying capacity and that load history effects can be captured by adopting the local approach for the assessment of ductile fracture. It is also noted that the trends observed in other local parameters, e.g. J and CTOD, must be treated with caution, probably due to near crack-tip softening associated with the implementation of the GTN model in a region of high stress concentration.
Load History Effects on Crack Driving Force for Cracks in Residual Stress Fields
Authors: Charles Richard , Beardsmore David W. , Teng Huaguo , Watson Chris T.
Published: July 2009

Fracture mechanics assessments of engineering components and structures containing defects are made by comparing an estimate of the crack driving force KJ with an effective fracture toughness KJc. The assessments must account for the combined effect of primary loads, such as internal pressure in pressurised components, and secondary stresses arising from welding and/or thermal loading. Elastic-plastic finite element analysis, or simplified methods set out in standard assessment procedures, can be used to estimate the crack driving force KJ as a function of the applied primary load on the component. The effective fracture toughness KJc should take account of the material fracture toughness and the crack tip constraint. For the assessment of defects in weld residual stress fields, it is usually assumed that the defect is inserted into the as-welded stress distribution in such a way that traction free crack surfaces are created simultaneously at all positions on the crack faces. However, it may be beneficial to take account of any relaxation in the residual stress field that might arise during proof-testing or in-service cyclic loading, and to consider a more gradual, progressive introduction of the defects. These benefits could, in principle, result in a reduction in the crack driving force. This paper describes work that has been undertaken to provide estimates of the crack driving force KJ for a fully-circumferential defect in a circumferential repair weld in a cylindrical pipe. Calculations have been carried out to establish KJ for a number of cases where different pressure overloads are applied to the uncracked pipe and different methods of crack insertion are applied. Estimates of the margin of safety on fracture toughness and pressure loading were calculated. At the outset, it was assumed that the fracture toughness of relevance for the defects is the material fracture toughness KJc* derived from strain free, high constraint fracture toughness specimens. No allowance was made for constraint effects associated with the finite geometry or initial strains in the pipe. The values of KJ were derived from values of J calculated using the JEDI post-processing code; this allows for initial inelastic strains present in the model prior to the start of the crack insertion process.
Statistical Analysis of Residual Stress Profiles Using a Heuristic Method
Authors: Teng H. , Bate S. K. , Beardsmore D. W.
Published: July 2009

In this paper we present a recently developed heuristic method for statistical analysis of residual stress that is based on a combination of the weighted least-squares method and the application of expert judgement. The least-squares method allows a model of the best residual stress profile to be determined as a linear combination of basis functions; the expert knowledge gives the flexibility of applying expert judgement to determine the weights from the observed scatter in the residual stress data. The heuristic method has been applied to a set of measurement data of a Welded Bead-on-Plate specimen. The results show that with the heuristic method, it is possible to obtain less conservative residual stress profile to a known confidence level.
Applicability of the Wavy-Wall Model of Fluidelastic Instability to a Normal Triangular Tube Array
Authors: Harel Julie , Meskell Craig
Published: July 2009

The Yetisir and Weaver formulation of the Lever and Weaver “wavy-wall” model for fluidelastic instability in tube arrays has been implemented for both normal triangular and in-line square arrays. The sensitivity of this model to the input parameters (i.e. attachment and separation points, decay function and phase lag function) has been examined. It was found that variations in the decay function were most significant and that the model behaved similarly for both array types. The predicted surface pressure perturbation due to tube displacement has then been compared with experimental data. For the in-line array the model behaviour compared well, while for the normal triangular array, the predictions were not representative of the experimental data. It is concluded that while the Yetisir and Weaver model can be applied successfully to in-line square arrays, it is not appropriate for densely packed normal triangular arrays.
Axially Compressed Cylindrical Shell Containing Axisymmetric Random Imperfections: Fourier Series Techniques and ASME Section VIII Division 2 Rules
Authors: Brar Gurinder Singh , Hari Yogeshwar , Williams Dennis K.
Published: July 2009

This paper presents the comparison of reliability technique that employ Fourier series representations of random axisymmetric imperfections in axially compressed cylindrical shells with evaluations prescribed by ASME Section VIII, Division 2. The ultimate goal of the reliability type technique is to predict the buckling load associated with the axially compressed cylindrical shell. The representation of initial geometrical imperfections in the cylindrical shell requires the determination of appropriate Fourier coefficients. The buckling of cylindrical shells in any type of loading is sensitive to the form and amplitude of geometric imperfections present in the structure. Initial geometric imperfections have significant effect on the load carrying capacity of axisymmetrical cylindrical shells. Many deterministic and probabilistic techniques are there to predict shell behavior during buckling. Fourier decomposition is used to interpret imperfections as structural features can be easily related to the different components of imperfections. The mean vector and the variance-covariance matrix of Fourier coefficients are calculated from the simulated shell profiles. Recommendations for further use of Fourier coefficients through simulation by Monte Carlo Method are laid down. Large number of shells thus created can be used to calculate buckling stress for each shell. The probability of the ultimate buckling stress exceeding a predefined threshold stress can be calculated.
Practical Application of Fastener Preload Guidance Research
Authors: Ludman John , Waterland A. Fitzgerald
Published: July 2009

Bolted flanged connections (BFC) in North America and throughout the world are designed, fabricated and constructed according to several design codes and component standards including the ASME Boiler and Pressure Vessel Code and ASME B31 Piping Codes. Despite the considerable engineering controls in place to ensure proper equipment design, bolted flange connections are all too often the source of leaks, fugitive emissions and process shutdowns in the process industries. A significant contributing cause of flange leaks is the limited understanding, of and improper selection and application of optimum assembly preloads on flange fasteners. Currently there are several “rule of thumb” approaches for identifying assembly bolt preload; each of these has limitations and short-comings that practitioners of these approaches need to be aware of. A new approach providing guidance to the selection of assembly preload bolt stresses is detailed.
The Effect of Clamping Force and Material Properties on the Tightness Performance a Flexible Box-Shaped Flange Bolted Joint
Authors: Kurosawa Ryou , Sawa Toshiyuki , Nagata Satoshi , Tatsumi Yasuaki
Published: July 2009

Bolted joints with gaskets such as non-circular flange connections have been widely used in mechanical structures, nuclear and chemical industry, and so on. They are usually used under internal pressure as well as other loadings such as thermal, impact loadings and so on. In designing the non-circular flange connections with gaskets, it is important to evaluate the sealing performance of the non-circular flange connections with gaskets subjected to internal pressure. An important issue is how to evaluate the sealing performance in the box-shaped bolted flange connections by using the contact gasket stress distributions at the interfaces, how to reduce the weight and how to determine the initial clamping bolt force (preload) by using the new gasket constants. In this paper, the stresses of box-shaped flange connection with gaskets subjected to an internal pressure are analyzed using the finite element method (FEM), taking account a hysteresis in the stress-strain curves of the gasket. The reduced contact gasket stress distributions are analyzed when the internal pressure is applied to the connection. The leakage tests were conducted using an actual box-shaped flange connection with a gasket. Using the reduced contact gasket stress distributions at the interfaces under an internal pressure (Helium gas was used) and the amount of the leakage measured in the experiment, the sealing performances are evaluated experimentally and theoretically by changing the initial clamping force and cover material in the connections. In addition, the sealing performance of the connection with raised face is also examined. Discussion is made on the effects of the initial clamping force, cover material and the types of facing on the sealing performance in the above connections.
Effects of Temperature Change on Bolt Load and Gasket Load of Bolted Flange Connection With Ring Type Joint Gasket
Authors: Nagata Satoshi , Sawa Toshiyuki
Published: July 2009

Finite element analysis is carried out to clarify the effects of temperature change on the bolt load and gasket load of the bolted flange connections with ring type joint gasket so called RTJ. The RTJ type bolted flange connections are usually utilized for such severe operating conditions as high pressure and/or elevated temperature. It is important for the RTJ type flange connections to be clarified its behavior under internal pressure and thermal load. In this paper, finite element analysis is performed for the piping flange connection of 12 inch class 900 RTJ subjected to the combinations of internal pressure 10 MPa and/or uniform temperature 450°C. Three material combinations are also studied such as both low alloy flange and bolt, stainless steel flange and low alloy bolt and both stainless steel flange and bolt. Elastic analysis and elastic-plastic analysis are both computed and compared. From the analysis results, it is observed that the elastic analysis and elastic-plastic one give completely different behaviors especially in the bolt load change under the loading and the unloading conditions. The difference in coefficients of thermal expansion is essential on the bolt load change when the different materials are used for the flanges and the bolts. As a conclusion, in order to establish a simplified calculation of the RTJ type flange connection for its optimal design, it is necessary to consider the elastic-plastic material behavior at the contact between the gasket and the groove. It is also required to incorporate the difference of thermal expansion and stiffness between flange and bolt with temperature dependency.
A Probabilistic Evaluation Model for Welding Residual Stress Distribution at Piping Joint in Probabilistic Fracture Mechanics Analysis
Authors: Itoh Hiroto , Katsuyama Jinya , Onizawa Kunio
Published: July 2009

Stress corrosion cracking (SCC) has been observed at some piping joints made by Austenitic stainless steel in BWR plants. In JAEA, we have been developing probabilistic fracture mechanics (PFM) analysis methods for aged piping based on latest aging knowledge and an analytical code, PASCAL-SP. PASCAL-SP evaluates the failure probability of piping at aged welded joints under SCC by a Monte Carlo method. We proposes a simplified probabilistic model which can be applied to the failure probability analysis based on PFM for welded joint of piping considering the uncertainty of welding residual stress. And the probabilistic evaluation model is introduced to PASCAL-SP. A parametric PFM analysis concerning uncertainties of residual stress distribution using PASCAL-SP was performed. The PFM analysis showed that the uncertainties of residual stress distribution largely influenced break probability. The break probability increased with increasing the uncertainties of residual stress.
Oxidative Stability of Carbon by Thermal Gravimetric Analysis
Authors: Melane Chris , Badenhorst Hein , Holo Luxolo , Vuroinen Eino , Rand Brian , Focke Walter W.
Published: July 2009

The Pebble Bed Modular Reactor (PBMR) is a High Temperature Gas Cooled nuclear Reactor (HTGR) with numerous inherent passive safety features. Graphite is the most important material of construction for the reactor core and the fuel pebbles. PBMR accident scenarios include uncontrolled air ingress into the reactor. Understanding the high temperature behavior of the graphite materials under such conditions is vital for design and accident modeling purposes [1]. Graphitic materials have a very high thermal stability compared to ordinary organics. The high degradation temperatures are beyond the capability of Differential Scanning Calorimetry (DSC) and thus necessitate the use of Differential Thermal Analysis (DTA) or Thermogravimetric Analysis (TG). The oxidative stability of two graphite samples was investigated using temperature scanning TG. It was found that air oxidation of a natural graphite sample commenced at temperatures that were significantly lower than those observed for a synthetic graphite sample. The natural graphite also showed peculiar bimodal mass loss rates.
Determination of Creep Properties From Small Punch Test
Authors: Li Yingzhi , Sˇturm Roman
Published: July 2009

The small punch test is one of the techniques, which can directly assess the current material properties of components on situ. The main advantages of this method are that only small amount of material is needed and no repair is required afterwards. In addition, the test itself is rather simple to perform. Since early 1980’s, the technique has been developed and applied to nuclear reactors, electric power plants for safety assessment. European researchers have carried out the pioneer work on small punch testing at the creep region. Recently the European Code of Practice documents are available for both high and low temperature properties, which summarizes the international experiences in last 20 years and provides a guide line to perform small punch test for metallic materials. This paper gives an overview of the research work on determination of creep properties from small punch test. Verification is performed by comparison of creep properties derived by small punch test and uni-axial tests. An agreement is found and shows that small punch test is able to predict reasonable creep properties of materials. Considerations how to improve the approach are also addressed. Further work is needed as available tests are still limited. This paper makes a starting point that others can improve further.
Research and Development of Intelligent Seismic Isolation System Using Air Bearing
Authors: Fujita Satoshi , Minagawa Keisuke , Miyazaki Mitsuru , Tanaka Go , Takahashi Osamu
Published: July 2009

This study aims at research and development of the intelligent seismic isolation system using air bearings as isolation device and Earthquake Early Warning (EEW) as trigger of isolation system. In October 2007, EEW was started providing to resident of Japan. The EEW system expects earthquake intensity and arrival time at particular place by analysis of seismic wave that was observed near the earthquake center. Therefore social and technical application of the system is strongly expected for suppression of disaster scale. On the other hand, long period seismic waves having predominant period of from a few seconds to a few ten seconds have recently been observed in various earthquakes. Also resonances of high-rise buildings and sloshing of petroleum tanks in consequence of long period seismic waves have been reported. In metropolises of Japan such as Tokyo, Osaka and Nagoya, it is expected that long period seismic waves are excited in large earthquakes because these are located on sedimentary layers. Therefore the isolation system having very long natural period or no natural period is required. In this study, we propose an isolation system having no natural period by using air bearing as isolation device. Air bearing is a bearing that can reduce contact friction between floor and the bearing by thin air film produced by compressed air. In general, the air bearing is used as heavy machinery moving equipment. The approximate friction coefficient is 0.0005 to 0.001, so that the system using air bearing almost isolates seismic wave. In addition, the EEW is applied as trigger of isolation. The EEW is applied for turning gas and electrical heater off, too. P-wave sensor is also equipped and it can operate as trigger in case of near-field earthquake or when EEW system could not work properly. Furthermore, steel plate equipped at the bottom of the air bearing can operate as friction bearing when air bearing does not work. In this paper, we describe results of earthquake response analysis of the intelligent isolation system using air bearing. From results of the analysis, it was confirmed that response acceleration depends on friction coefficient only, and the system has good isolation performance not only against typical seismic wave, such as El Centro wave, but also against long period seismic wave. However residual displacement remains after seismic input stopped. Additionally, experimental test was executed so as to investigate basic performance of isolation. As a result, it was confirmed that the isolation system has good isolation performance.
Delamination Identification in Quasi-Isotropic CFRP Laminate Using Electric Potential Technique
Authors: Ueda Masahito , Todoroki Akira , Kato Yasuyuki
Published: July 2009

Carbon fiber reinforced plastic (CFRP) laminate is very sensitive to an impact. Even a low impact creates a delamination, which deteriorates the compression strength of the laminate. Monitoring for delamination is indispensable to maintain the reliability of CFRP structures. Electric potential change method (EPCM) has been applied for CFRP as a nondestructive testing method although it has not yet been reported for the successful application to a quasi-isotropic CFRP laminate which is commonly used for various applications. Strong electrical anisotropy and inhomogenity of the laminate makes simple application of the method difficult. In this paper, a new concept was introduced to resolve the problem of the conventional method. The new method utilizes the piezoresistivity of CFRP woven fabric. Variation of strain on laminate surface due to delamination was measured as electric potential change of CFRP woven fabric. As CFRP woven fabric is generally stacked on the laminate on purpose to protect laminate surface, no additional sensor is required in the method. Delaminations were estimated from the electric potential changes as an inverse problem. Finite element studies were performed to investigate the applicability of the method. The simulation results indicated the validity of the method for delamination identification in quasi-isotropic CFRP laminate.
Bayesian Inference Method for Failure Probability of Wall Thinning Pipe in Corrosion Rate Fluctuation Model
Authors: Okajima Satoshi , Izumi Satoshi , Sakai Shinsuke
Published: July 2009

To rationalize the inspection interval for the wall-thinning piping element, the linear-Bayes method was proposed in the previous paper. To derive the simple formula, the linear-Bayes method ignores the corrosion rate change against time. However, this change may be caused by the one of the operational environment. Therefore, without the sufficient monitoring of the environment, the linear-Bayes method may underestimate the failure probability. In this paper, the linear-Bayes method is extended for the wall-thinning model with the corrosion rate fluctuation, which imitates the unexpected change of the corrosion rate. The extension is carried out through following two approaches: the “correction-term” and the “error-term” approaches. The correction-term approach can evaluate the change of corrosion rate, however, it requires sufficient number of inspections. The error-term approach evaluates the failure probability conservatively.
Effective Stress Ratio of Triangular Pattern Perforated Plates
Authors: Kasahara Naoto , Takasho Hideki , Kawasaki Nobuchika , Ando Masanori
Published: July 2009

Tubesheet structures utilized in heat exchangers have complex perforated portions. For realistic design analysis, axisymmetric models with equivalent solid materials of perforated plate are conventionally adopted to simplify perforated area (figure1). Sec.III Appendix A-8000 (ASME 2004) provides elastic equivalent solid materials for flat tubesheets. Plastic properties were studied by Porowski et al. (1974), Gorden et al. (2002) and so on. Elevated temperature design of tubesheets requires plastic and creep properties in addition. The purpose of this study is to develop a general determination method of non-linear equivalent material properties for perforated plates and to confirm their applicability to both flat and spherical tubesheets. Main loadings of tubesheets in fast reactor heat exchanges are inner pressure and thermal stress at transient operations. Under above conditions, average stress of perforated area becomes approximately equi-biaxial. Therefore, average inelastic behaviors of various perforated plates subjected to equi-biaxial field were investigated by inelastic finite element method. Though above investigations, Authors clarified that perforated plates have their own effective stress ratio (ESR). ESR is a function of geometry and is independent from their materials. ESR can determine non-linear equivalent material properties of perforated plates for any kind of constitutive equations of base metals. For simplified inelastic analysis of perforated plates, the brief equations were proposed to determine equivalent plastic and creep material properties for perforated plates. It is considered that physical meaning of ESR is an effective stress ratio between perforated plates and equivalent solid plates. ESR is a function of geometry and is independent from constitutive equations. ESR can determine non-linear equivalent material properties for perforated plates from any kind of constitutive equations of base materials. Assumptions in ESR are von Mises’s equivalent stress-strain relationship and equi-biaxial loadings. Applicability of ESR was investigated through finite element analyses of various flat and spherical tubesheets.
Reliability-Based Material Selection for Glycol Evaporator of PTA Installation
Authors: Wang Heng , Zhao Jianping
Published: July 2009

The glycol evaporator is the key equipment of the Purified Terephthalic Acid (PTA) unit to control the glycol component concentration by evaporation of water. The operation medium includes glycol, water, and a small quality of corrosive impurity, which causes the glycol evaporator to fail; the solution is to select the optimal vessel material such as the 20R, the 304, or the 316L steel. Based on the basic failure mechanism of the glycol evaporator, it was concluded that the main failure mechanism included corrosion thinning and stress corrosion cracking due to the corrosive solution. Then the calculation to solution of the failure probability is discussed based on the first order second moment theory and the method recommended by API581 standard, and the failure probability of the three kinds of material evaporator with the same operating condition and the different inspection effectiveness is carried out. From these figures, it is found that the stainless steel 316L is the best suitable material, and it is shown that the material selection based on the reliability index is a feasible verification method.
Evaluation of Creep Properties of Non-Asbestos Joint Sheet Gaskets at Elevated Temperature by Three-Dimensional Viscoelasticity Model
Authors: Yamaguchi Atsushi , Tsuji Hirokazu
Published: July 2009

When the gasket in a flange joint is used over the long term at an elevated temperature, the gaskets experience creep/relaxation. The creep of the gaskets may cause leakage of the internal fluid. Many gaskets are used at elevated temperatures, so the clarification of their creep properties at elevated temperatures is urgently needed. The creep of non-asbestos gaskets at an elevated temperature was tested using four-inch flanges and compressed non-asbestos joint sheet gaskets. The test conditions are 180°C and 500 hours. A three-dimensional viscoelasticity model that yields more accurate results compared to the viscoelasticity model, which uses the conventional single axis, was applied to the elevated temperature creep properties. Using the three-dimensional viscoelasticity model, the gasket creep is divided into the viscoelasticity component that converges on a certain strain and the volume change component that increases with time. The gasket strain is evaluated by the three-dimensional viscoelasticity model that considers the stress reduction. It is shown that the gasket strain is divided into the pure creep component of the gasket and the volume change due to the weight loss and chemical factor.
Semi-Large Scale Experiments Performed on Specimens With Underclad Cracks
Authors: Pistora Vladislav , Brumovsky Milan
Published: July 2009

During years 2005–2006, a series of semi-large scale experiments on specimens containing underclad (embedded) crack were performed in NRI Rez. The experiments were performed within EU PHARE project EUROPAID/116529/D/SV/CZ, in cooperation with VTT, FNS, TVONS (Finland) and Tecnatom (Spain). The aim of the project was to investigate fracture mechanics properties of cladding, in particular, to establish the role of cladding in the fracture/failure process of the specimen, and based on obtained results to improve the current procedure for evaluation of integrity of WWER reactor pressure vessels. The specimens were manufactured from artificially aged material of decommissioned (not operated) reactor pressure vessel of WWER 440 Type NPP. Each of the specimens contained through-thickness crack embedded in the base material (with approx. 3 mm ligament separating the upper crack front from cladding), and was loaded by four-point-bending loading at room temperature. During loading, majority of specimens exhibited pop-ins followed by ductile tearing of cladding and final failure; only 3 specimens fractured through suddenly, without preceding pop-ins. Evaluation of the experiments concentrated on both process of cleavage fracture in base material and process of ductile tearing in cladding. In the presented paper, the results of experiments are in detail described, including photos from the experiments and photos taken with using high-speed video camera during some of experiments.
Evaluation of Experiments on Semi-Large Scale Specimens With Underclad Cracks With Focus on Constraint Evaluation
Authors: Lauerova Dana , Brumovsky Milan
Published: July 2009

During years 2005–2006, a series of semi-large scale experiments on specimens containing underclad (embedded) crack were performed in NRI Rez. The experiments were performed in the frame of EU PHARE project EUROPAID/116529/D/SV/CZ, in cooperation with VTT, FNS, TVONS (Finland) and Tecnatom (Spain). The aim of the project was to investigate fracture mechanics properties of cladding, in particular, to establish the role of cladding in the fracture/failure process of the specimens, and to exploit the obtained knowledge in the procedure for evaluation of integrity of WWER reactor pressure vessels. Experimental results of the project are described in details in parallel paper [1]. In the present paper, brief summary of experimental results is provided, and FE evaluation of the experiments is described, with focus on constraint evaluation. FE evaluation covers, among others, evaluation of J-integral in the moment of cleavage fracture initiation (1st pop-in or sudden fracture through) for both upper and lower crack fronts, and evaluation of some type of constraint parameter. Three types of constraint parameter were calculated: T-stress based on elastic calculation, T-stress based on elastic-plastic calculation and Q-parameter. In the paper, detailed development of elastic-plastic T-stress as well as Q-parameter with loading is presented for one specimen that sustained the highest load. Besides of that, the approach of effective stress intensity factor KIeff is examined in the paper, and results of sensitivity analysis with respect to uncertainty concerning the way in which the effect of constraint is included into the KIeff approach are presented.