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Thermal shock fracture experiments on large size plates of A533-B steel
Abstract
The paper gives a complete report on a five-year research program concerning the fracture behaviour of the pressure vessel steel A533-B under thermal shock conditions. Five tests on 140-mm thick plates carrying a surface through-thickness crack were performed with different material conditions and thermal histories. The results confirmed the applicability of the Linear Elastic Fracture Mechanics approach and of the arrest toughness criterion. Moreover, the confidence limits achievable by a material characterization procedure based on Charpy-V tests were assessed. The prevention of crack initiation under a decreasing stress intensity factor and crack-tip temperature (‘simple’ warm pre-stressing effect) was largely confirmed, while reinitiation prevention after thermal-mechanical histories including complete unloading could not be demonstrated.
... The second type of experiment is the thermal stress fatigue type (e.g. [6,10,17,18] ). In these experiments, the specimen (typically disk shaped) is subjected to a sudden change in temperature, usually by submersion in a heated/cooled medium. ...
... 3. These dimensions were chosen to give a central section suitable for 1D cooling similar to that achieved with the large plates used for thermal shock analysis by Vitale and Beghini [18]. To achieve the uniform 1D transient temperature ®eld during quenching, masses were attached to both sides of the specimen. ...
An experimental rig developed for investigating crack growth in pressure vessels and piping equipment is described. The rig allows full scale modeling of cyclic thermal shock conditions that occur in operating thermal power station pressure equipment. It has the ability to apply a primary steady state mechanical load and to control the quenching environment allowing the study of the effect of welds and stress concentrators under conditions that simulate operational loadings. These results can be used for lifetime assessment. Preliminary results from the test rig analysing the effect of primary load on crack initiation and growth are presented and discussed briefly. A comparison with prediction methods from the ASME Boiler and Pressure Vessel Code is made. It is concluded that the primary load has little or no effect on crack initiation times, however it significantly affects the crack growth rate. (C) 2000 Elsevier Science Ltd. All rights reserved.
... In the last decade, there has been a great deal of research into the simulation of the mechanical behavior of structural steels submitted to neutron-dose damage in radioactive environments (e.g., reactor pressure vessel (RPV) steels), in particular if a crack-like singularity exists (i.e., by fracture mechanics concepts). [1][2][3][4][5][6][7] Considering the high risk and cost inherent to the manipulation of materials in the irradiated state, simulation techniques have obvious safety and economic motivations, since they would allow the development of new experimental procedures in nonradioactive environments and would make feasible prototype and full-scale vessel hydrotests, which so far never have been attempted in as-irradiated materials. The simulation of irradiation damage effects on the mechanical behavior of metallic alloys consistently has been tried by heat [1][2][3][4][5][6] and/or thermomechanical [7] treatments. ...
... [1][2][3][4][5][6][7] Considering the high risk and cost inherent to the manipulation of materials in the irradiated state, simulation techniques have obvious safety and economic motivations, since they would allow the development of new experimental procedures in nonradioactive environments and would make feasible prototype and full-scale vessel hydrotests, which so far never have been attempted in as-irradiated materials. The simulation of irradiation damage effects on the mechanical behavior of metallic alloys consistently has been tried by heat [1][2][3][4][5][6] and/or thermomechanical [7] treatments. However, only a few of these studies have made reference to the effect of microstructural variables such as grain size and phase percentage, especially in quantitative terms, on the mechanical performance of materials. ...
The elastic-plastic fracture toughness and crack extension behavior under quasi-static loading regimes of the as-received
and several thermally embrittled states of a reactor pressure vessel (RPV) steel were assessed on the basis of microstructural
parameters. Through a simple rule of mixture that is typically applied for composite materials, it was found that the equivalent
grain size (EGS) of dual-phase annealed microstructures is the controlling parameter of the fracture properties. It was concluded
that a Hall-Petch type relationship correlates the J-fracture mechanics criteria to the EGS.
... There have been other experimental tests of the thermal cracking phenomena but for various reasons they have not mimicked the conditions in operating plant well. This has produced a number of limitations to the results from such testing; in particular they have not detected the importance of mechanical loading and environment on the growth rate of cracks [Vitale and Beghini, 1991; Marsh, 1981]. Figure 3. Experiment design. ...
... There have been other experimental tests of the thermal cracking phenomena but for various reasons they have not mimicked the conditions in operating plant well. This has produced a number of limitations to the results from such testing; in particular they have not detected the importance of mechanical loading and environment on the growth rate of cracks [Vitale and Beghini, 1991;Marsh, 1981]. The tests at Monash take several months to complete as the specimens are put through thousands of cycles which take 15 to 20 minutes each. ...
Repeated thermal shock loading is common in the operation of pressure equipment particularly in thermal power stations. Thermal shock can produce a very high stress level near the exposed surface that eventually may lead to crack nucleation and crack growth. This paper presents a unique experimental study and outlines the information being gained from this work. In the experiments, cracks are initiated and then grown in low carbon steel specimens exposed to repeated thermal shock. The test-rigs achieve large thermal shocks through the repeated water quenching of heated flat plate specimens. The effect of steady state loads on the growth and environmental effects due to the aqueous nature of the testing environment are found to be major contributors to the crack growth kinetics. The most important findings are that the conditions leading to the arrest of cracks can be identified and that the depth of a starter notch contributes little to the crack propagation.
... Thermal shocks or as in this case thermal fatigue combined with severe oxidation is a crucial mechanism leading to materials failure [11][12][13]. The stresses generated by cyclic shocks are usually high enough to exceed the fracture toughness of a material. ...
Slag pots are commonly used for the transportation of liquid slag from steel mills to slag dumps. A detailed failure analysis of slag pots is required to determine effects limiting their lifetime. Temperatures measurements were implemented throughout an ongoing operation to quantify thermal loading. Different microscopy methods, cross sectional analyses and hardness measurements were performed on a slag pot after prolonged usage. Material degradation mechanisms, wear and deformations found during and after slag pot operation were compared to performed finite element simulations of one transport cycle. Special focus was placed on weaknesses of the current design.
... Experimental and theoretical researches have been conducted in the recent years at the DCMN (see for instance Vitale (1989), Beghini and Vitale (1989), Beghini, Vitale and Milella (1991), Vitale and Beghini (1991), Beghini, Bertini and Vitale (1992)) on the structural behaviour of vessel during PTS accident In particular, numerical methods were developed and applied for Fracture Mechanics analysis which allow facing these typically multiparametric problems in an accurate and efficient way. Such an approach was applied to this PTS scenarious giving indication of the risk for a brittle rupture of the vessel during the transient. ...
A qualified nodalization for WWER-1000 is available at DCMN (Dipartimento di Costnizioni Meccaniche e Nucleari) of University of Pisa that is suitable for running with the thermohydraulic system code elap5/mod3.2. The nodalization consists of about 1400 hydraulic nodes and more than 5000 mesh points for conduction heat transfer. The four loops of the NPP are separately modelled.
Detailed information about the plant hardware has been got from contacts with Eastern Organizations in Bulgaria, Russia and Ukraine. The qualification of the nodalization has been achieved at a steady state level utilizing a procedure available at DCMN and at a transient level on the basis of operational (planned) transients performed in the Bulgarian Kozloduy-5 NPP and of the unplanned transient occurred at the Ukrainian Zaporosche NPP (April 1995). Data measured in steam generators have also been utilized.
The nodalization has been widely applied to the analysis of accident scenarios in WWER-1000, including Large Break LOCA, Small Break LOCA, ATWS, Loss of Feedwater and Station Blackout.
The present activity aims at evaluating the potential for PTS (Pressurized Thermal Shock) following a steam line break accident. The thermalhydraulic results were employed as input for a parametric Fracture Mechanics analysis based on conservative hypothesis of the shape and localization of a pre-existing defect. Stress analysis evidenced the effect of partial cooling of the vessel and gave some general indications of the risk for unstable crack propagation under the simulated PTS conditions.
The paper considers the elastic stresses and stress intensity factor at a geometrical discontinuity, a corner, when it is suddenly cooled or heated along its free surfaces. Two geometrical situations are examined. The first is when the vertex angle of the corner is less than π, an edge, when the stresses in the corner are finite. The second is when the vertex angle is greater than π, a notch, when the elastic stress field has a singular component. The paper deals mainly with the last case. Based on dimensional considerations and finite element (FE) analysis a solution for the stress singularity has been obtained. If, instead of a sharp corner, a notch with a tip radius is considered, an elastic solution for stresses at the notch is linked with the singular solution and the notch geometry.
Repeated thermal shock loading is common in the operation of pressure equipment particularly in thermal power stations. Thermal shock can produce a very high stress level near the exposed surface that eventually may lead to crack nucleation and crack growth. This paper presents a unique experimental study and outlines the information being gained from this work. In the experiments, cracks are initiated and then grown in low carbon steel specimens exposed to repeated thermal shock. The test-rigs achieve large thermal shocks through the repeated water quenching of heated flat plate specimens. The effect of steady state loads on the growth and environmental effects due to the aqueous nature of the testing environment are found to be major contributors to the crack growth kinetics. The most important findings are that the conditions leading to the arrest of cracks can be identified and that the depth of a starter notch contributes little to the crack propagation.
Thermal shock cracking is an important damage mechanism for many pressurised components which is not currently covered in design codes or fitness for purpose codes. This paper presents an analysis of the growth mechanisms involved and makes proposals as to the guidelines which might be used for design of vessels against the initiation of thermal shock cracking and the assessment of the importance of such cracking when it is found in service. The growth of thermal shock cracking is a combination of several factors including the size of the thermal shocks, the applied mechanical loads and the corrosion environment. In many cases in service the cracking will arrest and not be harmful. This work studies a wider range of conditions and provides a less conservative analysis than the current EPRI guidelines.
Repeated thermal shock cracking is common in the operation of pressure equipment where water and steam are present. Surprisingly it is not directly covered in the ASME Boiler and Pressure Vessel code nor in fitness-for-purpose recommended practice such as API 579. An example of thermal shock stresses occurs when hot surfaces exposed to splashing of cold water. This eventually may lead to crack nucleation and crack growth. However not all thermal shock cracks lead to failures (such as rupture, leak or in more brittle material fragmentation), indeed the most frequent situation is that the cracking arrests at a depth of a few millimeters. This paper presents a unique experimental study and analysis the information being gained from this study in terms of design guidelines and crack growth mechanisms. In the experiments, cracks are initiated and then grown in low carbon steel specimens exposed to repeated thermal shock. The test-rigs achieve large thermal shocks through the repeated water quenching of heated flat plate specimens. The effect of steady state loads on the growth and environmental effects due to the aqueous nature of the testing environment are found to be major contributors to the crack growth kinetics. The most important findings are that are that the conditions leading to both the initiation and the arrest of cracks can be identified and that the depth of a starter notch contributes little to the crack propagation.
Repeated thermal shock cracking is common in the operation of pressure equipment where water and steam are present. Surprisingly, it is not fully covered in the ASME Boiler and Pressure Vessel code nor in fitness-for purpose recommended practice such as API 579. An example of thermal shock stresses occurs when hot surfaces are exposed to splashing of cold water This eventually may lead to crack nucleation and crack growth. However not all thermal shock cracks lead to failures (such as rupture, leak, or, in more brittle material, fragmentation), indeed the most frequent situation is that the cracking arrests at a depth of a few millimeters. This paper presents a unique experimental study and analysis of the information being gained from this study in terms of design guidelines and crack growth mechanisms. In the experiments, cracks are initiated and then grown in low carbon steel specimens exposed to repeated thermal shock. The test-rigs achieve large thermal shocks through the repeated water quenching of heated flat plate specimens. The effect of steady state loads on the growth and environmental effects due to the aqueous nature of the testing environment are found to be major contributors to the crack growth kinetics. The most important findings are that the conditions leading to both the initiation and the arrest of cracks can be identified and that the depth of a starter notch contributes little to the crack propagation.
A nuclear reactor pressure vessel steel was submitted to different annealing heat treatments aimed at simulating neutron irradiation damage. The obtained microstructures were mechanically tested with subsequent metallographic and fractographic characterization. The relevant micro-structural and fractographic aspects were employed in the interpretation of the mechanical behavior of the microstructures in both quasi-static (J-R curve) and dynamic (Charpy impact) loading re-gimes. A well defined relationship was determined between the elastic-plastic fracture toughness parameter J-integral and the Charpy impact energy for very most of the microstructures.
The Charpy impact energies of a reactor pressure vessel steel in the as received and several thermally embrittled conditions have been tentatively correlated to three J fracture toughness parameters derived under quasi-static loading regimen. Very good correlation has been achieved over the whole fracture resistance range of the structural steel, as obtained by the application of special heat treatments. It has been established that the parameters controlling the impact energy absorption capacity of the materials are the equivalent grain size of dual phase (ferrite/bainite) annealed microstructures and the bainite packet size of single phase quenched and tempered materials. The dependence of the Charpy impact energy of precracked, side grooved bend bars on the representative grain size of the microstructures tested has been disclosed as a Hall-Petch relationship.
The Charpy impact energies of a reactor pressure vessel (RPV) steel in the as-received and several thermally embrittled conditions have been assessed on the basis of microstructural parameters. It has been concluded that the parameters controlling the impact-absorbed energy of pre-cracked and side-grooved Charpy test specimens are the equivalent grain size of dual-phase annealed microstructures, and the bainite packet size of single-phase quenched and tempered materials. The Charpy energy has been correlated very well with the reduction in area and true fracture strain of tensile specimens, which could be inferred as grain size governed mechanical properties.
The evolution from original non-ductile failure considerations to the current pressurized thermal shock issue has accelerated the advancement of several emerging technologies to demonstrate that vessel failure is still a rare event for nuclear pressurized water reactors. The authors review how these technologies, which are associated with the design, analysis, and operation of PWR vessels for the prevention of nonductile failure, advanced through the evolution of the current PTS issue.
The appearance of cracks in structures, has an important influence on their life and safety, and imposes the development of a theory able to anticipate their behavior. In linear-elastic fracture mechanics the stress intensity factor is a fundamental parameter for the prediction of crack propagation. The weight function method evaluates that parameter through a function associated to the model, depending only on the geometry, and a stress field function depending on the load applied on the model. In this paper, determination of stress intensity factor is presented for two crack configurations: (1) internal radial crack in a circular ring subjected to a centrifugal load; (2) cracks at the edge of a hole in a finite plate subjected to uniaxial tension.
A survey of current code requirements (ASME and RCC-M) relating to the fracture toughness properties of the ferritic materials used in the fabrication of PWR reactor pressure vessels is presented, together with supporting test data published in the period 1970 to 1985. These fracture toughness properties are discussed in relation to the critical sizes of defect applicable to particular regions of a PWR primary vessel for given design basis transient conditions.
Within the pressurized thermal shock research program conducted at MPA Stuttgart, large-scale hollow cylinders have been tested under axial tensile loading and rapid inner surface cooling of the 320°C hot specimen wall.The experimental goal was the investigation of the fracture behaviour of circumferential fatigue sharpened flaw notches on the inner cylinder surface.By means of numerical simulation of the experiments, the fracture mechanics assessment of the crack extension could be validated with the experimental results.One of the five experiments is in detail described and experimental and computational results are compared.
Thermal-shock experiments TSE-5, TSE-5A, and TSE-6 were conducted for the purpose of investigating the behavior of surface flaws under pressurized-water-reactor (PWR) overcooling-accident conditions. These experiments were the fifth, sixth, and seventh in a series of thermal-shock experiments conducted with large steel cylinders (A 508 class-2 chemistry; 991-mm OD by 76- and 152-mm wall by 1.2-m length) as a part of the Heavy-Section Steel Technology (HSST) Program for this purpose. For each of these experiments, the initial flaw was on the inner surface and extended the full length of the cylinder. The thermal shock was applied to the inner surface only, and this was accomplished by effectively dunking the test cylinder, initially at approx.93°C, into a large volume of liquid nitrogen. Results of the experiments have confirmed that (1) linear-elastic fracture-mechanics is valid for thermal-shock loading; (2) crack arrest will take place in accordance with recently developed crack-arrest concepts; (3) crack-arrest toughness values for rising and falling K/sub I/ fields are the same; (4) warm prestressing is effective in preventing crack initiation; (5) thermal shock alone cannot drive a flaw all the way through the wall; (6) dynamic effects for PWR-vessel thermal-shock loading conditions are negligible; (7) in the absence of cladding and under severe thermal-shock loading conditions, finite-length flaws will extend on the surface to become very long; and (8) there can be very large scatter in small-specimen fracture-toughness data. 24 refs., 114 figs., 29 tabs.
The VISA (Vessel Integrity Simulation Analysis) code was developed as part of the NRC staff evaluation of pressurized thermal shock. VISA uses Monte Carlo simulation to evaluate the failure probability of a pressurized water reactor (PWR) pressure vessel subjected to a pressure and thermal transient specified by the user. Linear elastic fracture mechanics are used to model crack initiation and propagation. parameters for initial crack size, copper content, initial RT/sub NDT/, fluence, crack-initiation fracture toughness, and arrest fracture toughness are treated as random variables. This report documents the version of VISA used in the NRC staff report (Policy Issue from J.W. Dircks to NRC Commissioners, Enclosure A: NRC Staff Evaluation of Pressurized Thermal Shock, November 1982, SECY-82-465) and includes a user's guide for the code.
The LOCA-ECC Thermal Shock Program was established to investigate the potential for flaw propagation in pressurized-water reactor (PWR) vessels during injection of emergency core coolant following a loss-of-coolant accident. Studies thus far have included fracture mechanics analyses of typical PWRs, the design and construction of a thermal shock test facility, determination of material properties for test specimens, and four thermal shock experiments with 0.53-m-OD (21-in.) by 0.15-m-wall (6-in.) cylindrical test specimens. In the first experiment, initiation was not expected and did not occur, although there was a small amount of subcritical crack growth. In the second experiment, initiation of a semicircular flaw took place as expected; the final length along the surface was about four times the initial length, but there was no radial growth. The third and fourth experiments were similar, and the long axial flaw initiated in good agreement with predictions.
The Second Marshall Report (1982) presented a detailed analysis of the integrity of PWR pressure vessels. As part of that study theoretical calculations of failure probabilities were made. Since the publication of that Report modifications have been made to the theoretical model to extend the failure criterion into stable crack extension, to update the knowledge of the distribution of various parameters, to more accurately represent the stress intensities and crack shapes, and to consider a different representation of pre-service detection of defects.In this paper these modifications are summarised and the application of the model to the calculation of failure frequencies for the most severe accident conditions, the large loss-of-coolant accident and the steam break, is presented. The results indicate that the probability of vessel failure is one to two orders of magnitude lower than previously predicted.
The phenomenon of the pressurized thermal shock on the reactor pressure vessel is expected to occur in the case of such an accident as the small loss of the coolant accident in the PWR nuclear plant. In order to study the structural integrity of the reactor pressure vessel under the pressurized thermal shock, the cleavage thermal shock fracture experiment was conducted here using an initially corner-cracked nozzle type specimen made of the pressure vessel steel A508 class 3. The fracture mechanics analysis was performed to asses bthe crack behaviors in the experiment using the time dependent stress intensity factor deduced from the three-dimensional J integral with the thermal effect.
Three extensive experimental and analytical programmes are underway at the HDR, the MPA and ORNL, whose aim is to investigate fracture mechanics phenomena parameters. The quantification of these parameters is important for a fuller assessment of the safety margins in the pressure boundary of a nuclear reactor system in the event of occurrence of a pressurized thermal shock (PTS) accident. The three programmes are reviewed and it is shown that between the three, a full range of material properties is adequately covered.
The recent operating experience of the Pressurized Water Reactor (PWR) Industry has focused increasing attention on the issue of reactor vessel pressurized thermal shock (PTS). Previous reactor vessel integrity concerns have led to changes in vessel and plant system design and to operating procedures, and increased attention to the PTS issue is causing consideration of further modifications. Events such as excess feedwater, loss of normal feedwater, and steam generator tube rupture have led to significant primary system cooldowns. Each of these cooldown transients occurred concurrently with a relatively high primary system pressure. Consideration of these and other postulated cooldown events has drawn attention to the impact of operator action and control system effects on reactor vessel PTS.
The Second Report of the Marshall Study Group [1] reviewed the design of a pressure vessel to identify those regions which were most sensitive to the expected loading conditions. The design and loading conditions, including the pressure test, were also reviewed. A stress analysis data base of sufficient detail was established to permit a full fracture analysis to be carried out. After reviewing currently available methods for fracture assessment the CEGB R6 method was adopted. Selected normal and upset, emergency and fault transients were considered. Initiation defect sizes for normal and upset transients were generally large. In the beltline and nozzle shell course, for example, the minimum initiating height for an extended line defect was approximately 70 mm. For emergency and fault conditions, providing upper shelf material properties apply, the height to initiate semi-elliptical defects exceeds 25 mm and cracks substantially larger than this will extend by only small amounts in a ductile manner and remain stable. If during the large LOCA upper shelf material properties cannot be maintained semi-elliptic defects 6 to 10 mm high may propagate transversely parallel to the inside surface of the vessel. This may be limited by warm prestressing.
This article summarizes the results of a study whose purpose was to identify and analyze events at U.S. pressurized water reactors (PWRs) which either resulted in or could be considered as precursors to pressure vessel thermal shock (PVTS). The data base includes ∼ 16,000 Licensee Event Reports on 49 PWRs, covering ∼ 329 reactor-years from 1963 through 1981. The 99 identified events were assigned to one of five categories of severity. On this basis, 34 of the events were considered significant to PVTS. The events are categorized by cause and by vendor of the plant at which the event occurred. Some probabilistic risk assessment data are also developed.
The reactor pressure vessel in a pressurized water reactor is normally subjected to temperatures and pressures that preclude propagation of sharp, crack-like defects that might exist in the wall of the vessel. However, if certain postulated accidents, referred to as overcooling accidents, were to occur, the pressure vessel could be subjected to severe thermal shock while the pressure is substantial. As a result, vessels containing high concentrations of copper and nickel, which enhance radiation embrittlement, may possess a potential for extensive propagation of preexistent inner-surface flaws prior to the vessel's normal end of life. A fracture-mechanics analysis for a typical postulated accident and also related thermal-shock experiments indicate that very shallow surface flaws that extend through the cladding into the base material could propagate. This is of particular concern because shallow flaws appear to be the most probable and presumably are the most difficult to detect.
Spherical coordinates are r, θ, φ. The half-space extends in θ < π/2. The crack occurs along φ = 0. The region to be investigated is the solid space-triangle (or cone) between the three planes θ = π/2, φ = +0 and φ = 2π − 0, which planes are to be taken stress-free.In this space-angle a state of stress is considered in terms of the cartesian stress components Possible values λ are determined from a characteristic (or eigenvalue) equation, expressing the condition that a determinant of infinite order is equal to zero. The root of λ which gives the most serious state of stress in the vertex region (the region r → 0) is the root closest to the limiting value . Knowledge of this state of stress, or at least of this value of λ is essential in the determination of the three-dimensional state of stress around a crack in a plate for distances of order of the plate thickenss.Along the front of the quarter-infinite crack (z-axis) the so called stress-intensity factor behaves like (z → 0) and thus tends to zero, respectively to infinity, accordingly to Re λ being or . But in the region z → 0 the notion stress-intensity factor loses its meaning. The required state of stress passes into the well-known state of plane strain around a crack tip if Poisson's ratio (v) tends to zero. The computed state of stress for the incompressible medium () is confirmed by experiment.
The purpose of the document is to present schemes for flaw preparations in heavy section steel. The ability of investigators to grow representative sharp cracks of known size, location, and orientation is basic to representative field testing to determine data for potential flaw propagation, fracture behavior, and margin against fracture for high-pressure-, high-temperature-service steel vessels subjected to increasing pressurization and/or thermal shock. Gaging for analytical stress and strain procedures and ultrasonic and acoustic emission instrumentation can then be applied to monitor the vessel during testing and to study crack growth. This report presents flaw preparations for HSST fracture mechanics testing. Cracks were grown by two techniques: (1) a mechanical method wherein a premachined notch was sharpened by pressurization and (2) a method combining electron-beam welds and hydrogen charging to crack the chill zone of a rapidly placed autogenous weld. The mechanical method produces a naturally occurring growth shape controlled primarily by the shape of the machined notch; the welding-electrochemical method produces flaws of uniform depth from the surface of a wall or machined notch. Theories, details, discussions, and procedures are covered for both of the flaw-growing schemes.
The second pressurized-thermal-shock experiment (PTSE-2) of the Heavy-Section Steel Technology Program was conceived to investigate fracture behavior of steel with low ductile-tearing resistance. PTSE-2 was designed primarily to reveal the interaction of ductile and brittle modes of fracture and secondarily to investigate the effects of warm prestressing. A test vessel was prepared by inserting a crack-like flaw of well-defined geometry on the outside surface of the vessel. The flaw was 1 m long by approx.15 mm deep. The instrumented vessel was placed in a test facility in which it was initially heated to a uniform temperature and was then concurrently cooled on the outside and pressurized on the inside. These actions produced an evolution of temperature, toughness, and stress gradients relative to the prepared flaw that was appropriate to the planned objectives. The experiment was conducted in two separate transients, each one starting with the vessel nearly isothermal. The first transient induced a warm prestressed state, during which K/sub I/ first exceeded K/sub Ic/. This was followed by repressurization until a cleavage fracture propagated and arrested. The final transient was designed to produce and investigate a cleavage crack propagation followed by unstable tearing. During this transient the fracture events occurred as had been planned. 7 refs., 13 figs., 2 tabs.
Thermal-shock experiment TSE-7 was conducted for the purpose of investigating the behavior of surface flaws under pressurized-water reactor (PWR) overcooling-accident conditions. This experiment was the eighth in a series of thermal-shock experiments conducted for this purpose with large steel cylinders (A 508, class-2 chemistry; 991-mm OD x 152-mm wall x 1.2-m length) as a part of the Heavy-Section Steel Technology (HSST) Program. The initial flaw for TSE-7 was a shallow, semielliptical, inner-surface, axially oriented, sharp crack located at midlength of the test cylinder. The thermal shock was applied to the inner surface only, and this was accomplished by effectively dunking the test cylinder, initially at approx.93/sup 0/C, into a large volume of liquid nitrogen. The specific purpose of TSE-7 was to determine whether, in agreement with analysis, a short and shallow surface flaw, in the absence of cladding, would extend on the surface to effectively become a very long flaw as a result of severe thermal-shock loading. During the experiment, there were three major initiation-arrest events. The first event consisted of some radial propagation and very extensive surface extension, with many bifurcations taking place. The second and third events consisted primarily of radial propagation. A fourth initiation event was prevented by warm prestressing. These results were in good agreement with predictions. 50 refs., 77 figs., 13 tabs.
Thermal-shock experiments with steel cylinders have been conducted at ORNL to investigate the behavior of long axial inner-surface flaws during loading conditions similar to those imposed by a PWR LOCA. The scope of the three most recent experiments was dictated by results of the PWR-LOCA fracture-mechanics analysis and included: initiation and arrest of shallow and deep flaws, a series of initiation-arrest events with deep penetration of the wall, long crack jumps, arrest in a rising K/sub 1/ field, and warm prestressing. For the purpose of designing the experiments and evaluating their results, the static initiation and arrest toughness curves were determined for the test cylinders, and this was done using compact specimens. The experiments demonstrated WPS and arrest in a rising K/sub 1/ field and indicated that LEFM is valid for thermal-shock loading.
The first pressurized-thermal-shock test of a 148-mm-thick steel pressure vessel with a 1-m-long flaw was performed to investigate fracture behavior of a vessel under conditions relevant to a flawed nuclear reactor pressure vessel during an overcooling accident. The objectives were to observe crack arrest and stability on the ductile upper shelf and the effects of warm prestressing on crack initiation. Three coordinated pressure and thermal transients were imposed on the vessel, which was preheated to approx.290/sup 0/C. Two episodes of crack propagation and arrest occurred. The thermal transients were induced by coolant at -29 to 15/sup 0/C. Pressure transients were as high as 94.4 MPa. The experimental objectives were attained. The inhibiting effects of warm prestressing were definitely demonstrated. Crack propagation was nearly pure cleavage. Fracture-mechanics analysis of brittle fracture based on small-specimen toughness measurements was reasonably accurate.
The VISA-II (Vessel Integrity Simulation Analysis code was originally developed as part of the NRC staff evaluation of pressurized thermal shock. VISA-II uses Monte Carlo simulation to evaluate the failure probability of a pressurized water reactor (PWR) pressure vessel subjected to a pressure and thermal transient specified by the user. Linear elastic fracture mechanics methods are used to model crack initiation and propagation. Parameters for initial crack size and location, copper content, initial reference temperature of the nil-ductility transition, fluence, crack-initiation fracture toughness, and arrest fracture toughness are treated as random variables. This report documents an upgraded version of the original VISA code as described in NUREG/CR-3384. Improvements include a treatment of cladding effects, a more general simulation of flaw size, shape and location, a simulation of inservice inspection, an updated simulation of the reference temperature of the nil-ductility transition, and treatment of vessels with multiple welds and initial flaws. The code has been extensively tested and verified and is written in FORTRAN for ease of installation on different computers. 38 refs., 25 figs.
Thickness size effects and large amounts of data scatter often occur in the cleavage fracture toughness testing of steel. It is shown that the Irwin βIc equation provides an effective adjustment for thickness size effects as well as reduces data scatter. Examples of applying the Irwin βIc adjustment to both static and dynamic toughness data are given. The significance of cleavage microcracking in the initiation of fast fracture is discussed and it is reasoned that this phenomenon is closely related to the sensitivity of the cleavage fracture toughness to triaxiality and strain rate.
A superposition method, in which analytical and finite element solutions are combined through the variational principle, is presented for determining the three-dimensional stress intensity factors of the through-wall crack. Cumbersome volume integral due to the singular terms at the crack tip is carried on by applying the least squares methods to the results obtained by the Legendre-Gauss quadrature. Several numerical calculations are made based on the present method, showing that the three-dimensional effects cannot be neglected especially for the cases of larger Poisson's raito. The comparison of the present method with the other finite element methods proposed for determining the three-dimensional stress intensity factors indicates that the former has the obvious superiority to the latters in respect to the numbers of elements and nodal points necessary to obtain reasonable results.
Exact results for the stress intensity factor are presented for an external circular crack with oppositely directed concentrated loads applied to the crack surfaces. This result is specialized to the case of a semi-infinite crack in an infinite body with concentrated loads on the crack. A procedure is then suggested by which one can obtain from the corresponding plane result the approximate three-dimensional Green's function (concentrated load result) for any straight crack in an infinite elastic body. This procedure is used to determine the Green's functions for a finite-length crack in an infinite body, and is then used in conjunction with a suggested "slicing" procedure to obtain approximate three-dimensional Green's function for plates of finite thickness and infinite extent, containing finite length cracks. Previously existing solutions for crack problems are compared with results obtained by application to plate tension and bending problems of the three-dimensional Green's functions. The results indicate that the procedure yields satisfactory results when stress gradients through the plate thickness are not excessive. However, an accurate assessment of the validity of the slicing procedure awaits further progress in three-dimensional crack analysis.
The method of superposition of analytical and finite-element solutions is proposed for determining three-dimensional distributions of the stress intensity factor; the singular part of the solution is expressed by a linear combination of analytical solutions, and the rest by a finite-element solution. The method is applied to a round bar with a circumferential crack and plates with penetrating cracks. Detailed distributions of the stress intensity factor near the plate surfaces are investigated with the aid of Benthem's theory, which shows that less than 0.5% of the plate thickness is severely influenced by the plate surfaces in the case of a compact tension specimen. Computations for the present method can be performed with a general purpose program for finite element analysis without using special elements.La mthode de superposition des solutions analytiques et par lments finis est propose pour dterminer les distributions tri-dimensionnelles du facteur d'intensit des contraintes; la partie singulire de la solution est exprime par une combinaison linaire des solutions analytiques et le reste de la solution par une solution lments finis. La mthode est applique aux cas d'une barre ronde comportant une fissure circonfrentielle, et de tles comportant des fissures pntrantes. Les distributions dtailles du facteur d'intensit de contrainte au voisinage des surfaces de la tle sont analyses l'aide de la thorie de Benthem, qui montre que moins de 0,5% de l'paisseur de la tle est svrement influence par l'effet de surfaces dans le cas d'prouvette de tension compacte. Les calculs de la mthode prsente peuvent tre excuts l'aide d'un programme objectifs gnraux destin l'analyse par lment fini sans recourir des lments spciaux.
The analytical work performed in the framework of the Pressurized Thermal Shock (PTS) experimental research at the JRC Ispra, Italy, is described in the paper. In particular, the development of the FRAP preprocessor and development and implementation of a methodology for analysis of local non-stationary heat transfer coefficients during a PTS, have been tackled. FRAP is used as a front-end for the finite element code ABAQUS, for the heat transfer, stress and fracture mechanics analyses. The ABAQUS results are used further on, for the probabilistic fatigue crack analysis performed by the JRC Ispra code COVASTOL. Only the preliminary results of application of FRAP, ABAQUS and COVASTOL codes in the experiment are given in this paper, in order to illustrate the applied analytical procedure.
This paper presents some general and practical aspects of the analytical and experimental work performed within the JRC Ispra PTS (Pressurised Thermal Shock) research. Main elements of the structural reliability assessment performed for the scaled pressure vessel model exposed to repetitive PTS loads have been highlighted, and first results regarding the pre-experiment analysis and the start-up phase of the experiment given (stress analysis, damage mechanics, fracture mechanics, etc.). The paper also presents the basic method and first applications regarding treatment of uncertainties, and use of artificial intelligence tools in planning and management of the experiment.
The aim of this paper is to review recent trends, improvements and validations of methodologies for the assessment of reactor pressure vessel (RPV) integrity against the risk of leak or catastrophic failure, mainly deriving from the possible presence of crack-like defects at critical locations in the vessel wall.The first part of the work gives an overview of the input parameters, namely loading conditions, material properties and possible crack shape and dimensions, which are needed for a comprehensive fracture analysis of RPVs, discussing recent findings and still open questions about them.The next two sections are concerned with reviews of the presently available fracture approaches, related to both brittle and ductile fracture behaviour, and of probabilistic fracture mechanics methodologies.As conclusion, present limitations of methodologies for evaluation of RPV structural integrity and areas which need further improvements are outlined.
The integrity of PWR pressure vessels during overcooling accidents. Paper presented at International Meeting on Thermal Nuclear Reactor Safety
- R D Cheverton
Cheverton, R. D., et al., The integrity of PWR pressure vessels during overcooling accidents. Paper presented at International Meeting on Thermal Nuclear Reactor Safety, Chicago, Aug. 29-Sept. 2, 1982.
Finite element modelling of ductile crack extension under pressurized thermal shock loading
- K Kussmaul
- A Sauter
- T Nguyen-Huy
- D Feyhl
Kussmaul, K., Sauter, A., Nguyen-Huy,T. & Feyhl, D., Finite element modelling of ductile crack extension under pressurized thermal shock loading. Proc. 8th SMiRT Conference, Brussels, Belgium, August 19-23, 1985, Vol. G, ed. J. Stavpaert, North-Holland Publ., Rotterdam, 1987, pp. 169-175.
Pressurized thermal shock loading of HDR RPV-nozzle and cylindrical wall
- E Roos
- S Stegmeyer
- G Katzenmeier
- M Klein
Roos, E., Stegmeyer, S., Katzenmeier, G. & Klein, M., Pressurized thermal shock loading of HDR RPV-nozzle and cylindrical wall. Proc. 10th SMiRT Conference, Anaheim, California, August 14-18, 1989, Vol.
The behaviour of a reactor pressure vessel nozzle under pressurized thermal shock loading
- M Klein
- G E Neubrech
- E Roos
Klein, M., Neubrech, G. E. & Roos, E., The behaviour of a reactor pressure vessel nozzle under pressurized thermal shock loading. Proc. 10th SMiRT Conference, Anaheim, California, August 14-18, 1989, Vol. G, pp. 25-30.
A spinning cylinder tensile test facility for pressure vessel steels
- A M Clayton
- R E Leckenby
- K Reading
- E J Robbins
- W P White
Clayton, A. M., Leckenby, R. E., Reading, K., Robbins, E. J. & White, W. P., A spinning cylinder tensile test facility for pressure vessel steels. Proc. 8th SMiRT Conference, Brussels, Belgium, August 19-23, 1985, Vol. G, ed. J. Stavpaert, North-Holland Publ., Rotterdam, 1987, pp. 21-26.
Determination of upper-shelf fracture resistance in the spinning cylinder test facility
- Lacey
Lacey, D. J. & Leckenby, R. E., Determination of upper-shelf fracture resistance in the spinning cylinder test facility. Proc. 10th SMiRT Conference, Anaheim, California, August 14-18, 1989, Vol. G, pp. 1-6.
PTS integrity study in Japan
- H Okamura
- G Yagawa
- T Kawakami
- T G Funada
- Ed J Stavpaert
Okamura, H., Yagawa, G., Kawakami, T. & Funada, T., PTS integrity study in Japan. Proc. 8th SMiRT Conference, Lausanne, Switzerland, August 17-21, 1987, Vol. G, ed. J. Stavpaert, North-Holland Publ., Rotterdam, 1987, pp. 395-400.
First results related to the JRC Ispra PTS Experiment: Nozzle corner crack behaviour
- A Jovanovic
- A C Lucia
- G Volta
Jovanovic, A., Lucia, A. C. & Volta, G., First results related to the JRC Ispra PTS Experiment: Nozzle corner crack behaviour. Proc. 8th SMiRT Conference, Lausanne, Switzerland, August 17-21, 1987, Vol. G, pp. 375-80.
Thermal shock experiments on large size plates with surface flaws: Tests TSP01 and TSP02. In Fracture Mechanics Verification by large-scale testing
- E Beghini
- P P Milella
- E Vitale
Beghini, E., Milella, P. P. & Vitale, E., Thermal shock experiments on large size plates with surface flaws: Tests TSP01 and TSP02. In Fracture Mechanics Verification by large-scale testing, ed. K. Kussnaul, EGF/ESIS Publication & MEP, London, 1991, pp. 339-56.
Crack initiation and arrest during thermal shock tests on large size plates with surface cracks
- Beghini
Beghini, E. & Vitale, E., Crack initiation and arrest during thermal shock tests on large size plates with surface cracks. Proc. 10th SMiRT Con-ference, Anaheim, California, August 14-18, 1989, Vol. F, ed. A. H.
Scaling of thermally driven fluid mixing and heat transfer associated with pressurized thermal shock of reactor vessels. Paper presented at Joint NRC/ANS Meeting on Basic Thermal Hydraulics Mechanisms in LWR Analysis
- P H Rothe
- G B Wallis
Rothe, P. H. & Wallis, G. B., Scaling of thermally driven fluid mixing and heat transfer associated with pressurized thermal shock of reactor vessels. Paper presented at Joint NRC/ANS Meeting on Basic Thermal Hydraulics Mechanisms in LWR Analysis, Sept. 14-15, 1982 Bethesda, USA.
PWR downcomer fluid temperature transients due to high pressure injection at stagnated loop flow. Paper presented at Joint NRC/ANS Meeting on Basic Thermal Hydraulics Mechanisms in LWR Analysis
- T G Theofanous
- H P Nourbakhsh
Theofanous, T. G. & Nourbakhsh, H. P., PWR downcomer fluid temperature transients due to high pressure injection at stagnated loop flow. Paper presented at Joint NRC/ANS Meeting on Basic Thermal Hydraulics Mechanisms in LWR Analysis, Sept. 14-15, 1982 Bethesda USA.
Thermal shock tests on A533-B plates: Final report
- E Beghini
- C Carmignani
- E Vitale
Beghini, E., Carmignani, C. & Vitale, E. Thermal shock tests on A533-B plates: Final report. DCMN-RL358 (88), University of Pisa, 1988 (in Italian).
Initiation and arrest of cracks due to thermal loading
- Beghini
Beghini, M., Initiation and arrest of cracks due to thermal loading. PhD Thesis, Pisa, 1989 (in Italian).
Analysis of flawed plates loaded by severe thermal transients
- Beghini
Beghini, M. & Vitale, E., Analysis of flawed plates loaded by severe thermal transients. Proc. IX National Congress AIMETA, Bari, Italy, 1988, pp. 181-4 (in Italian).
First results related to the JRC Ispra PTS Experiment: Nozzle corner crack behaviour
- Jovanovic
Flaw preparation for HSST program vessel fracture mechanics testing: Mechanical-cycling pumping and electron-beam weld-hydrogen-charge cracking schemes
- Holtz
Finite element modelling of ductile crack extension under pressurized thermal shock loading
- Kussmaul
Thermal shock experiments on large size plates with surface flaws: Tests TSP01 and TSP02
- Beghini