bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of the Master Curve Approach for Abnormal Material Conditions
Authors: Planman Tapio , Server William , Wallin Kim , Rosinski Stan
Published: August 2009

The range of applicability of Master Curve testing Standard ASTM E 1921 is limited to macroscopically homogeneous steels with “uniform tensile and toughness properties”. A majority of structural steels appear to satisfy this requirement by exhibiting fracture toughness data which comply with the assumed KJc vs. temperature dependence and scatter within the specified validity area. As indicated in ASTM E 1921 a criterion for material macroscopic inhomogeneity is often applied using the 2% lower bound (possibly also the 98% upper bound). Data falling below this 2% lower-limit curve may be an indication of material inhomogeneity or susceptibility to grain boundary fracture. When this situation occurs, it is recommended to analyze the material with the so-called SINTAP procedure, which is intended for randomly inhomogeneous materials to assure a conservative lower-bound estimate. When a data set distinctly consists of two or more different data populations instead of one (due to variation of irradiation dose or specimen extraction depth, for instance) adoption of a bimodal (or a multimodal) Master Curve model is generally appropriate. These modal models provide information if the deviation of distributions is statistically significant or if different distributions truly exist for values of reference transition temperature, T0, characteristic of separate data populations. In the case of data sets representing thick-walled structures (i.e., reactor pressure vessels), indications of abnormal fracture toughness data can be encountered such that material inhomogeneity or fracture modes other than pure cleavage should be suspected. A state-of-the-art review for extended, non-standard Master Curve data and techniques highlights limits of applicability in situations where the basic ASTM E 1921 procedure is not appropriate for material homogeneity or different fracture modes.
A Comparison of 2D and 3D Fracture Assessments in the Presence of Residual Stresses
Authors: Lewis Simon J. , Truman Christopher E. , Smith David J.
Published: August 2009

This paper presents an investigation into the effects of an initial residual stress field on fracture parameters, calculated via an energy-type integral method, in two and three-dimensional simulations. A residual stress field was introduced into a modified single edge notched bend, SEN(B), specimen using an in-plane compression procedure, such that a crack introduced into the specimen experienced opening displacement, even in the absence of external loading. J integral calculation was undertaken using standard two-dimensional area formulations and pointwise three-dimensional formulations, as well as using modified two- and three-dimensional routines developed to provide path independence in the presence of initial strain fields and non-monotonic plastic loading. The paper will describe the application of these modified J-integral techniques and use the results to re-interpret experimental fracture test data obtained from a set of A533B ferritic steel SEN(B) specimens. The implications for structural integrity assessments in the presence of residual stress fields, as well as the calculation route chosen for determination of fracture parameters, were explored in the context of the R6 assessment procedure. In particular, the different levels of conservatism in the assessments resulting from two- and three-dimensional simulations will be highlighted.
Early Developments for a Semi-Analytical Model of Weld Induced Residual Stress
Authors: Bromley Jess M. , Alizadeh Hassan , Smith David J. , Truman Christopher E.
Published: August 2009

The generation of plastic slip and residual stress by thermal processes is particularly difficult to understand and simulate. Modelling such problems is computationally expensive when approached numerically and extremely complex to approach analytically. ‘Semi-analytical’ models, in which analytical thermoelastic solutions are combined with approximate models of plasticity, offer a way to bridge this gap and have the potential to allow the rapid testing of parameter sensitivities before one launches a time-consuming full numerical model. However the construction of such models within such a thermal framework poses its own problems. An initial requirement for any such semi-analytical model is a complete solution to the elastic only response of the material to the given loading process. In this paper we focus on the formulation of such a solution for the simplest case relevant to welding or similar thermal processing. We verify the solution developed against finite element predictions and then further investigate it. In doing so we explain how the nature of this solution, especially its predicted yielding behaviour, has ramifications for the successful creation of a full semi-analytical solution.
Bending Modified J-Q Theory and Its Application to Fracture Constraint Analysis
Authors: Zhu Xian-Kui , Leis Brian N.
Published: August 2009

The J-Q theory [1,2] can characterize the crack-tip fields and quantify fracture constraints for various geometric and loading configurations in elastic-plastic materials, but it fails to do so for bending-dominant geometries at large-scale yielding (LSY). This issue significantly restricts its applications to fracture constraint analysis. A modification of the J-Q theory is thus proposed in this paper as a three-term solution with an additional term to address the global bending stress to offset this restriction. The nonlinear global bending stress is linearly approximated in the region of interest at LSY. To verify the bending-modified J-Q solution, detailed elastic-plastic finite element analysis (FEA) is carried out under plane strain conditions for three conventional bending specimens, i.e., single edge notched bend (SENB), single edge notched tension (SENT) and compact tension (CT) specimens for X80 pipeline steel. Deformation considered varies from small-scale yielding (SSY) to LSY. The results show that the bending modified J-Q solution can well match FEA results of crack-tip stress fields for the bending specimens at all deformation levels from SSY to LSY, and the modified parameter Q is a load- and distance-independent constraint parameter at LSY. Thus, the modified parameter Q can be effectively used to quantify the crack-tip constraint for bending geometries. Its application to fracture constraint analysis is demonstrated by ranking crack-tip constraint levels for fracture specimens and by determining constraint corrected J-R curves for the X80 pipeline steel.
Evaluation of Storage Tank Floating Roofs for Stress and Stability Due to Earthquake Induced Liquid Sloshing
Authors: Cacciatore Philip J. , Hantz Benjamin F. , Gustafsson L. Magnus
Published: August 2009

Considerable interest has developed in the engineering community concerning the damage to the floating roof of oil storage tanks due to liquid sloshing from earthquake loading. Engineering groups in countries bordering the circum-Pacific seismic belt in particular are devoting extensive efforts to obtaining solutions capable of identifying vulnerable roof designs and developing modifications to improve strength. The recent efforts of the Japanese Fire Disaster and Management Authority (FDMA) as a result of 1995 Kobe and the 2003 Tokachi-Oki earthquakes are examples of recent work in this area. This paper focuses on efforts to analyze floating roof structures for stress and stability under typical earthquake velocity spectrums using advanced finite element methods. It employs ideas included in the Japanese FDMA studies, work done as part of the ASCE Committee on Gas and Liquid Fuel Lifelines, and some original methods developed at ExxonMobil. It has been applied to several tank designs and been submitted as a suitable advance analysis method to the Japanese FDMA. The paper provides both the theoretical foundation as well as an example covering typical tank geometry.
Residual Stress Relaxation in a Tube Attachment Weld Inside a Pressure Vessel Forging During Post Weld Heat Treatment
Authors: Hurrell P. R. , Davies J. , Leggatt N. A. , Dennis R. J. , Leggatt R. H.
Published: August 2009

This paper presents analyses done to determine residual stress relief achieved by post weld heat treatment (PWHT) of tube attachment welds inside a thick SA508 steel pressure vessel forging. Finite element (FE) analyses were performed modelling the manufacturing operations in detail including welding, machining and PWHT. The analyses demonstrate that PWHT at 600°C for 8 hours is effective in reducing as-welded residual stress levels from tensile yield magnitude (+500MPa approx) to <100MPa. The maximum residual stress was computed to be 90MPa sub-surface in a region of hydrostatic (tri-axial tensile) stress. Secondary creep was modelled using data from creep tests on SA508 steel uni-axial tensile specimens. Practically all of the stress relaxation is due to creep strain with minimal additional plastic strain. Most stress relief occurs during the first hour of soak, with diminishing benefit thereafter. Analysis results also indicate that PWHT effectiveness is more sensitive to soak temperature than hold time. These FE results are considered slightly pessimistic but are reasonably consistent with other analytical predictions. By comparison surface hole drilling stress measurements of <50MPa (10% yield strength) were recorded from a representative welded test block. Analysis pessimism was attributed to ignoring both primary creep and relaxation during the slow warm up phase of the heat treatment cycle.
Dynamic Analysis of the Bulk Tritium Shipping Package Subjected to Closure Torques and Sequential Impacts
Authors: Wu Tsu-Te , Blanton Paul S. , Eberl Kurt R.
Published: August 2009

This paper presents a finite-element technique to simulate the structural responses and to evaluate the cumulative damage of a radioactive material packaging requiring bolt closure-tightening torque and subjected to the scenarios of the Hypothetical Accident Conditions (HAC) defined in the Code of Federal Regulations Title 10 part 71 (10CFR71). Existing finite-element methods for modeling closure stresses from bolt pre-load are not readily adaptable to dynamic analyses. The HAC events are required to occur sequentially per 10CFR71 and thus the evaluation of the cumulative damage is desirable. Generally, each HAC event is analyzed separately and the cumulative damage is partially addressed by superposition. This results in relying on additional physical testing to comply with 10CFR71 requirements for assessment of cumulative damage. The proposed technique utilizes the combination of kinematic constraints, rigid-body motions and structural deformations to overcome some of the difficulties encountered in modeling the effect of cumulative damage. This methodology provides improved numerical solutions in compliance with the 10CFR71 requirements for sequential HAC tests. Analyses were performed for the Bulk Tritium Shipping Package (BTSP) designed by Savannah River National Laboratory to demonstrate the applications of the technique. The methodology proposed simulates the closure bolt torque preload followed by the sequential HAC events, the 30-foot drop and the 30-foot dynamic crush. The analytical results will be compared to the package test data.
A Theoretically-Based Statistical Model of Transition Toughness
Authors: EricksonKirk Marjorie A. , Wagenhofer Matthew
Published: August 2009

A program was undertaken to develop a fully predictive model of the scatter in toughness across a wide range of transition temperatures based on a physical understanding of deformation and fracture behavior. The temperature dependence of the proposed model is taken from previous work in which the local mechanisms of cleavage fracture were used to define the plastic work to fracture. The local to global stress transference is achieved by a dislocation-mechanics based examination of the interaction between the globally applied stresses, a macroscopic crack and a nearby accumulation of dislocations blocked by a second phase particle, i.e. slip band, whose position relative to the macroscopic crack tip is variable. The scatter of toughness values at each temperature is captured through variation of this macro-crack / micro-crack geometry, and of the particle size. Once the local stress field is determined using the dislocation-based transference equations, an energy balance criterion for fracture is applied that incorporates the temperature-dependent fracture work term and the local stresses determined from the transference equations. This paper summarizes this multiscale fracture model, which serves as a foundation for more detailed descriptions of the mathematics of the quantitative model, its temperature dependence and scatter characteristics and coding efforts. These latter topics will be addressed in greater detail in subsequent papers.
Residual Stress Improvement in Repair Welded Plates Using Water-Shower Cooling During Welding Process
Authors: Yanagida Nobuyoshi , Saitou Kouichi , Koide Hiroo , Kawakami Mitsuo
Published: August 2009

To reduce tensile residual stress at a repair-welded area, a method that applies water-shower cooling behind a torch was developed. The width of the repair-welded area in this study was 27 mm, while the maximum width of welded areas in previous studies was 18 mm. To examine how much the welding method reduced residual stress at large-width welds, we first applied the method to multipass bead on plate specimens. The first layer consisted of a five-pass bead. A single large-width weld pass was applied with water-shower cooling. To cover the five-pass bead on plate area with a large-width weld, the torch was moved in a weaving motion. Residual stresses were measured. The measurements showed that tensile stresses remained in the five-pass bead on plate specimen. The tensile residual stresses were improved to compressive when our welding method was applied at the large-width weld pass. Then, we applied our method to a repair-welded specimen. The specimen was a butt-welded joint with an X-shaped groove. Repair welding was performed around the boundary of the welded area. A single large-width weld pass with water-shower cooling was applied at the surface. Residual stresses were measured. The measurements showed that tensile stresses remained on the surface of the X-shaped groove welding. Tensile residual stresses increased around the repair-welded area. When our welding method was applied, residual stresses were improved to compressive. Therefore, our welding method can reduce tensile stress in a repair-welded plate.
Resolution of Creep Strain Measurements Using the ARCMAC Strain Monitoring System
Authors: Morris Andy , Kourmpetis Miltiadis , Dear John , Puri Amit , Maharaj Chris
Published: August 2009

Essential is that creep strain monitoring sensors need to be of a rugged design well able to endure the demanding conditions related to the environment of high-pressure steam pipes and other power plant components. A useful monitoring method employs the E.ON UK ARCMAC gauge system with a capability of both uniaxial and biaxial strain measurements. The choice of monitoring location, for example, can be to welded pipe sections and other known early failure locations on steam pipes and other plant components. The ARCMAC system has been refined over the years to improve the accuracy of measurement. In this paper, recent developments have allowed the strain measurement accuracy to be improved to 64 micro-strain with an error of <10%. Also, recently, high temperature trials of the combined ARCMAC/DIC have commenced and early findings are reported.
Nuclear PWR 3-Loop Plants: Reactor Coolant Circuit Strategic Evaluation for Life Management of Primary Circuit Components
Authors: Bezdikian Georges
Published: August 2009

The French utility has organized the life management program of Nuclear Plants in function of several actions of expertises of knowledge on the long term experience feedback and the maintenance program for life. This program is a strategic stake based on technical point of view considering the aging assessment of the key components on reactor coolant circuit components — elbows, laterals — of the plant, combining the economic aspects, the life management of each components. The actual life evaluation is the results of prediction of life assessment from important program of expertises for the 3-loop PWR and 4-loop PWR plants in operation. For all of inlet and outlet Steam generators elbows and other elbows on coolant circuit, it was assessed the toughness characteristics and prediction to maintain components in operation for 40 years and 60 years. To optimize the strategic in order to achieve the best possible performance and to prepare the technical and economical choice and decision, the paper presents the association of life management strategy and the program of replacement of several elbows that 60 years life management will be difficult and the association with steam generators replacements to chose the right period to replace some of them. This assessment is performed considering: • the life evaluation of Steam generators on the plants and alternative maintenance actions, • the large database from cast reactor coolant component expertised after removed from nuclear power plants, • the identification of degradation for different components and prediction criteria proposed, periodic maintenance and volume of expertises.
Welding Repair Procedure of Defects in WWER Reactor Pressure Vessels
Authors: Brumovsky Milan , Brynda Jiri , Ellinger Jiri
Published: August 2009

Reactor pressure vessels are components that are practically not replaceable and thus they determine the lifetime of the whole NPP. The lifetime of reactor pressure vessels is determined by the level of he material degradation as well as by the existence of defects found during in-service inspection and their size and location. According to some codes, it is necessary to have a procedure for the repair of potential defects found during in-service inspection that can be non-acceptable from the point of view of assurance of required RPV lifetime. A wide experimental program was realized to propose, check and approve a procedure for repair of potential defects in the underclad region of WWER-440 (WWER = Water-Water-Energy-Reactor = PWR type) reactor pressure vessels during operation. The experimental program was realized on as-received materials as well as on materials subjected to simulated ageing to a level representing the expected material embrittlement after design reactor lifetime. The main requirements for this procedure were: the RPV will not be pre-heated during repair welding, and no post-weld heat treatment will be applied. The maximum repair depth will be 40 mm including austenitic cladding thickness (8 to 10 mm). The procedure was developed and checked by series of mechanical testing (hardness, fracture toughness, impact notch toughness as well as by detailed metallography studies).
Residual Stresses in Austenitic Cladding
Authors: Brumovsky Milan , Pistora Vladislav , Kupka Ivan
Published: August 2009

Reactor pressure vessels (RPV) are usually manufactured with austenitic cladding on their inner surface as a protection against corrosion from the primary circuit water environment. Thus, they are not included into the strength calculations of pressure vessels due to their lower strength properties and much smaller thickness in comparison with those of vessels as they are taken only as a corrosion layer. In the same time, due to different thermal coefficients and Young moduli, welding of austenitic cladding results in a high residual stresses in the cladding and also in the adjacent area in the base ferritic metal. These residual stresses as well as stresses resulted from the temperature field in the vessels represent necessary inputs into pressurized thermal shock calculations. WWER (Water-Water Energy Reactor = PWR type) reactor pressure vessels have relatively thick cladding — nominally 8 mm — made from two layers: first layer of 25/10 type welded by one pass while the second layer of 18/10/Ti typed is usually welded by three passes. The main part of the vessels was performed by strip welding with strips of 60 mm wide. Results of residual stresses measurements are given in the paper. Method with incremental milling of beams was used for the measurements and determination of residual stresses. Tests were performed on specimens in as-welded state and also after final heat treatment of the vessels, i.e. after several stress relieves including first hydrotest in shop. As residual stresses depends strongly also on direction of welding, beams were oriented in both directions — parallel and perpendicular to the welding direction. Results of these measurements are shown and discussed in the paper.
The Effect of Modelling Simplifications on the Prediction of Residual Stresses in a Multi-Pass Plate Groove Weld
Authors: Warren A. P. , Bate S. K. , Hurrell P.
Published: August 2009

The inherent complexity of modelling welding processes and the lack of computational power available to analysts have resulted in simplified methods being commonly utilised when predicting residual stresses in welded components. Despite considerable advances in computational power, it is still often not possible to run detailed 3D analyses of complex welded geometries within practical timescales. Against this background, a programme of work has been undertaken in order to investigate the effect these simplifications will have on the prediction of residual stresses on a number of test-pieces. The geometry investigated in this paper is a plate containing a “V” groove, which runs the length of the plate and is filled with eight mechanised TIG weld passes. This paper presents the results of a number of finite element analyses conducted of this geometry. The analyses presented have been conducted using the commercial finite element packages SYSWELD and ABAQUS, using a number of modelling simplifications. These simplifications include modelling the plate in 2D and the use of the bead lumping technique to idealise a number of beads as a single bead. Also considered are various methods of heat source representation, namely; a moving ellipsoidal heat source, prescribed temperature and block dumping. These analyses are compared and qualitative conclusions are drawn.
Autofrettage of a Spherical Pressure Vessel
Authors: Parker Anthony P. , Huang Xiaoping
Published: August 2009

There is a numerical procedure for modeling autofrettage of thick-walled cylinders that incorporates Bauschinger effect as a function of prior plastic strain and Von Mises’ yield criterion. In this paper the numerical procedure is extended to solve the analogous problem of a spherical, thick walled steel vessel. An equivalent new analytical solution for the case of a spherical vessel is also formulated. The analytical and numerical solutions are shown to be in close agreement. It is demonstrated numerically that a re-autofrettage procedure, previously proposed for cylindrical vessels, may be extremely beneficial for spherical vessels. Additional commentary is provided on the limitations of certain analytic solutions.
IAEA Coordinated Research Projects on Master Curve Technology: An Overview
Authors: Kang Ki-Sig
Published: August 2009

The IAEA has sponsored a series of Coordinated Research Projects (CRPs) that have led to a focus on reactor pressure vessel (RPV) structural integrity. In the previous CRP on the application of the Master Curve (MC), three key areas were identified to need further works. These three tasks are: (1) test specimen bias, constraint, and geometry effects on the measurement and application of To values; (2) effects of loading rate up to impact loading conditions on To; and (3) potential changes in the shape of the MC for highly embrittled RPV materials. These three areas are focused on the new CRP (here below CRP-8). A new CRP is follow-on to previous successful CRPs on resolving technical issues associated with application of the MC approach. Overall objectives of CRP-8 include: 1) Better quantification of fracture toughness issues related to testing surveillance specimens for application to RPV integrity assessment, and 2) Development of approaches for addressing MC technical issues in integrity evaluation of operating RPVs. 15 organizations from 11 countries participate in the CRP and the final report will be issued in the end of 2008.
Crack Growth Behavior of Nickel Alloy Welds in a PWR Environment
Authors: Alexandreanu B. , Chopra O. K. , Shack W. J.
Published: August 2009

A program is under way at Argonne National Laboratory to evaluate the resistance of Ni alloys and their welds to environmentally assisted cracking in simulated Light Water Reactor (LWR) coolant environments. This paper focuses on the cracking behavior of Ni-alloy welds in simulated pressurized water reactor (PWR) environment at 290–350°C. Crack growth tests have been conducted on both field- and laboratory-produced welds. The results are compared with the existing crack-growth-rate (CGR) data for Ni-alloy welds to determine the relative susceptibility of specific Ni-alloy welds to environmentally enhanced cracking. To analyze the CGRs, a superposition model was used to establish the individual contributions of mechanical fatigue, corrosion fatigue, and stress corrosion cracking.
Evaluation of “Linearized” Stresses Without Linearization
Authors: Strzelczyk Andrzej T. , Ho San S.
Published: August 2009

ASME Code stress assessment of pressure vessels in the power generation industry is usually done by finite element analysis using one of the two approaches. In the first, “shell-element” approach, vessels are modeled out of shell elements; primary plus bending and primary plus secondary stresses are taken directly from the finite element analysis results and the alternating stresses are based on primary plus secondary stresses prorated by respective stress concentration factors. The strength of the “shell-element” approach is its simplicity; its weakness is problematic modeling of the stress concentration and some modeling difficulties (varying wall thickness, nozzle/vessel connectivity, pressure applied to the mid-surface instead of to the inner surface.) In the second, “solid-element” approach, vessels are modeled out of solid elements; “linearized” stresses can not be taken directly from the finite element analysis results, first they must be linearized, and only then, can be compared against their allowable counterparts; the alternating stresses can be based directly on the outer/inner-surface-node-stresses, provided that the mesh of the model is fine enough to account for the stress concentration effect. The strength of the “solid-element” approach is its high accuracy; its weakness is the time consuming, sometimes ambiguous, stress linearization process. This paper proposes a modification of the “solid-element” approach, in which the time consuming linearization process is replaced by a modification of the original model. To do so, a vessel must be modeled out of quadratic 20 node solid elements; the mesh density of the model (on its surface and through thickness) must be adequate for stress concentration representation and the mesh lines in the thickness direction must be more or less normal to the surfaces. The results from this original model can be taken directly for fatigue evaluation. To obtain the “linearized” stresses the original model must be slightly modified, specifically the number of elements through thickness must be reduced to one, and the reduced integration technique is recommended. For such a modified model, the nodal stresses are equivalent to the “linearized stresses” of the original model. The equivalence is discussed on a model of a circular nozzle attached to a cylindrical vessel. The vessel loads are pressure and thermal expansion.
Failure Analysis of a Ruptured Final Reheat Tube
Authors: French S. M.
Published: August 2009

Two damaged final reheat tubes from a 30 year old supercritical unit were submitted to the laboratory for evaluation following the discovering of a failure of one of the tubes after deslagging operations; a third, dented tube was left in service. The 304H stainless steel tubes were installed in 1990 when the reheater was replaced. The bulk microstructure of both tubes shows evidence of sensitization, which is not unusual given this application (reheater). The failed tube appears to be an intergranular separation that started either subsurface or at the ID, propagating to the OD surface. The sensitization of the steel apparently made the material susceptible to corrosion as well as significantly reduced the impact strength of the material to 10–15% of its estimated original level (verified by Charpy impact test). Examination of the dented tube (#101A) showed a subsurface plane of damage some 30 mils from the ID surface, running parallel to the surface. The damage consisted of intergranular separation, caused by the impact loading event, and referred to in the literature as an “attached spalling failure”. Spalling failures occur when the shock wave is reflected from the back surface (the ID surface of the tube), interacting with the incident shock wave as a stress wave. When the magnitude of this tensile stress exceeds the inherent strength of the material, failure occurs. The overall area of the attached spalling failure is being investigated; the concern there is if it is exceptionally large, it may provide a thermal barrier to heat transfer from the OD to the ID and result in a local overheating failure. Within the metallographic sample, however, the damage area was quite small and therefore did not appear to be an immediate issue. The long-term suitability of tube 105A, which remains in service with a dent induced by the same deslagging process that damaged tubes 101A and 103A, is doubtful and should be addressed during the Fall 2006 boiler overhaul. For the shortterm, the assumption was made that cracking due to the deslagging impact would be oriented similar to non-failed tube and extension of these fissures to failure between Spring 2006 and the Fall outage is not expected.
Simulation of Shear Spinning Process for Pressure Vessel Components
Authors: Zhao Jian-Hua , Champliaud Henri , Dao Thien-My
Published: August 2009

Spinning process for the medium- and small-lot production of axisymmetric components has many advantages, such as smaller deformation force, simpler tool-and-die design, and lower investment in equipment, over conventional forming processes. It has been widely used in aerospace, energy and defense industry. This paper presents a 3-D elastoplastic finite element (FE) model for the simulation of the shear spinning process, in order to study deformation behaviour and design proper process parameters for the spinning process. The proposed model has the following characteristics: i) a shell element is used to mesh the contact pairs between the workpiece (a metal blank) and the tool sets (a pressing roller and a mandrel); ii) the offset thickness of the shell element is considered during contact treatment; iii) the movement of the roller and the fixing of the central blank and mandrel are treated as the boundary condition; iv) relative movement between the roller and the blank is treated as a spiral feeding process. FEM simulations for shear spinning with aluminum have been implemented, using a dynamic explicit scheme, on ANSYS/LS-DYNA software. Spinning force, thickness distribution and stress distribution have been studied under various roller feeds and inclined angles of mandrel. The simulation results have provided good confirmation with the experiments.
Use of Cohesive Elements in Fatigue Analysis
Authors: Lucas Leonard J. , Black T. M. , Jones D. P.
Published: August 2009

Cohesive laws describe the resistance to incipient separation of material surfaces. A cohesive finite element is formulated on the basis of a particular cohesive law. Cohesive elements are placed at the boundary between adjacent standard volume finite elements to model fatigue damage that leads to fracture at the separation of the element boundaries per the cohesive law. In this work, a cohesive model for fatigue crack initiation is taken to be the irreversible loading-unloading hysteresis that represents fatigue damage occuring due to cyclic loads leading to the initiation of small cracks. Various cohesive laws are reviewed and one is selected that incorporates a hysteretic cyclic loading that accounts for energetic dissipative mechanisms. A mathematical representation is developed based on an exponential effective load-separation cohesive relationship. A three-dimensional cohesive element is defined using this compliance relationship integrated at four points on the mid-surface of the area element. Implementation into finite element software is discussed and particular attention is applied to numerical convergence issues as the inflection point between loading and unloading in the cohesive law is encountered. A simple example of a displacement-controlled fatigue test is presented in a finite element simulation. Comments are made on applications of the method to prediction of fatigue life for engineering structures such as pressure vessels and piping.
Application of Failure Event Data to Benchmark Probabilistic Fracture Mechanics Computer Codes
Authors: Simonen F. A. , Gosselin S. R. , Lydell B. O. Y. , Rudland D. L. , Wilkowski G. M.
Published: August 2009

This paper describes an application of data on cracking, leak and rupture events from nuclear power plant operating experience to estimate failure frequencies for piping components that had been previously evaluated using the PROLOCA and PRAISE probabilistic fracture mechanics (PFM) computer codes. The calculations had addressed the failure mechanisms of stress corrosion cracking, intergranular stress corrosion cracking and fatigue for materials and operating conditions that were known to have failed components. The first objective was to benchmark the calculations against field experience. A second objective was a review of uncertainties in the treatments of the data from observed failures and in the structural mechanics models. The database PIPExp-2006 was applied to estimate failure frequencies. Because the number of reported failure events was small, there were also statistical uncertainties in the estimates of frequencies. Comparisons of predicted and observed failure frequencies showed that PFM codes correctly predicted relatively high failure probabilities for components that had experienced field failures. However, the predicted frequencies tended to be significantly greater than those estimated from plant operating experience. A review of the PFM models and inputs to the models showed that uncertainties in the calculations were sufficiently large to explain the differences between the predicted and observed failure frequencies.
Analysis of Heat Flow Around Bolted Joints and Variations of Axial Bolt Force
Authors: Fukuoka Toshimichi , Nomura Masataka , Shino Keiichi
Published: August 2009

A bolted joint is frequently used under thermal load in practical applications, such as pressure vessels, internal combustion engines, etc. In order to accurately evaluate the thermal stresses thus produced, the effects of thermal contact resistance at the interface and the heat flow through small gaps, which exist around the objective bolted joint, must be taken into account. In this paper, a numerical approach with high computation efficiency is proposed, where empirical equations for thermal contact coefficient and apparent thermal contact coefficient are incorporated into commercial engineering software. By conducting systematic three-dimensional FE analyses, it is quantitatively elucidated how the supplied heat flows through each part of a bolted joint and how the bolt stress varies with time. Bolted joints made of the materials with low thermal conductivity exhibit specific behaviors on the heat flow pattern around the bolted joint and the variations of axial bolt stress and bolt bending stress.
Failure Analysis of Inlet Branch Pipes of a Reformer
Authors: Lu Zhi-Ming , Gao Zeng-Liang , Zhang Wei
Published: August 2009

The inlet branch pipes connected the aggregate pipes and furnace tubes of a reformer, the outlines of them extended as “S” shapes, and they were made of austenitic steel 1Cr18Ni9Ti. After about 60,000 hours service, a lot of cracks were found near the elbows of inlet branch pipe and joints with the furnace tubes, many of them propagated through the pipe walls, and lead to leakage of raw material. In order to explore the failure causes, the microstructure of cracks and corrosion ingredients remained on fracture surface were analyzed by scanning electron microscopy (SEM), chemical composition, metallographic and mechanical properties of the branch-pipe steels were examined. The test result showed that the cracks propagated from the outside to inside of the pipe wall, the fracture surfaces were almost flat, there was no obvious deformation existed at the fracture section. Certain percent of chlorine and sulphur elements could be found and the secondary cracks could be observed on the microstructure fracture surfaces. The inlet branch pipes were elongated when they were connected with the aggregate pipes and furnace tubes, the stress were analyzed by finite element method (FEM) under the tensile deformation, the results showed that the stresses near the elbow were high. It could be concluded that the crack of the inlet branch pipe were stress corrosion cracks accelerated by the aggressive medium rich of CI and S ion and high stresses. Based on the causes analysis results, the corresponding countermeasures and improvements were suggested.
Study on Explosive Forming Using Pressure Vessel and Metal Die
Authors: Iyama Hirofumi , Itoh Shigeru
Published: August 2009

Explosive forming is one particular forming technique, in which, most commonly, water is used as the pressure transmission medium. An explosive is set at the top of the pressure vessel filled with water and is exploded by an electric detonator. An underwater shock wave propagates through the water medium and impinges on the metal plate causing it deformation. If a metal die is used, the metal plate deforms to a specified form. Generally, explosive forming has little spring back of the metal plate, because sufficient plastic deformation is obtained. There are two pressure actions to the metal plate on explosive forming. The first pressure pulse is from the shock loading and the second pressure pulse is caused by the expansion gas generated by the detonation of the explosive. The secondary pressure pulse duration is longer, but the peak pressure is lower than the primary shock pressure. The intensity of the pressure pulse is based on the conditions of the pressure vessel. We have also been using the metal die for explosive forming. The deformation shape of the metal plate is effected by a distribution shape of shock loading. In this technique, the pressure vessel has a desired internal shape. The pressure vessel has an internal shape of a parabola. In order to understand the deformation mechanism of the metal plate, some experiments and numerical analysis were carried out.
Some Applications of Piping Failure Database to Nuclear Safety Issues in Korea
Authors: Choi Sun Yeong , Choi Young Hwan
Published: August 2009

Korean applications of a piping failure database to some nuclear safety issues related to nuclear piping such as a domestic initiating event frequency for a probabilistic safety assessment (PSA), an enhanced in-service inspection (ISI) program, a new strategy for the ISI program, leak before break (LBB) screening criteria, a risk-informed ISI (RI-ISI) program, and aging management for a periodic safety review (PSR) and/or a life extension are introduced in this study. By using a piping rupture frequency and a piping damage frequency obtained from a piping failure database, a domestic initiating frequency for a PSA, an enhanced ISI program, and a new strategy for an ISI are established. Some safety issues such as the LBB screening criteria, ISI program, RI-ISI program, and the aging management of nuclear piping are reviewed based on the information and insights obtained from a case-by-case analysis of the piping failure events in the piping failure database.
Stress Analysis and Sealing Performance of Pipe Flange Connections With NiTi Shape Memory Alloy Gasket
Authors: Takagi Yoshio , Tatsuoka Teruhisa , Kawasaki Naoki , Sawa Toshiyuki
Published: August 2009

Due to the long-term durability and the excellent resistance to aging degradation, metal gaskets are expected to give a longer life compared to conventional polymer gaskets. However, the nature of high elastic modulus of the metal reduces the sealing performance at interface. Nickel-Titanium shape memory alloy (NiTi alloy) shows an excellent elasticity and is expected to be applicable as a gasket. The authors have already evaluated the sealing performance of the pipe flange connections with NiTi gaskets. The results revealed that NiTi alloy showed a superior sealing performance to the conventional aluminum gasket. However, the sealing performance of the connection depended on the ambient temperature due to the change in physical properties like the elastic modulus and the thermal expansion coefficient of NiTi alloy. In this paper, the stress distribution was analyzed with 3D FEM and the effect of temperature in the sealing performance was evaluated. In the analysis, the particular element code was adopted for analyzing the deformation behavior of NiTi alloy gasket. The elements worked well in the analysis and helped to evaluate the sealing behavior of the flange connections. According to the analysis, the gasket contact stress decreased as increasing the temperature because of the lower thermal expansion of NiTi alloy than that of austenitic stainless steel of which the flange and bolt material were made. On the other hand, the FEM analysis suggested that the increment of elastic modulus of NiTi alloy compensate the reduction of compression strain of the gasket due to the difference of thermal expansion between gasket and flange. As a result of the ex-experimental study and FE analysis, the recommendable assembling process for NiTi alloy gasket was proposed.
Acoustic Power Calculation in Deep Cavity Flows: A Semi-Empirical Approach
Authors: Oshkai P. , Yan T. , Velikorodny A. , VanCaeseele S.
Published: August 2009

Acoustic power generated by turbulent flow over a coaxial side branch (deep cavity) resonator mounted in a rectangular duct is calculated using a semi-empirical approach. Instantaneous flow velocity is decomposed into an irrotational acoustic component and vorticity-bearing hydrodynamic field. The total velocity at several phases of the acoustic oscillation cycle is measured using digital particle image velocimetry. The acoustic velocity field is calculated numerically. The emphasis is on the effect of the accurate geometry representation for the acoustic field modeling on the calculated acoustic power. Despite the generally low levels of acoustic radiation from the coaxial side branches, when the main duct is incorporated into the model for calculation of the acoustic velocity, the acoustic velocity exhibits substantial horizontal (streamwise) components in the vicinity of the cavity corners. This streamwise acoustic velocity correlates with hydrodynamic horizontal velocity fluctuations, thus contributing to the calculated acoustic power. In addition, spatial structure and strength of the acoustic source changes as the distance between the side branches varies. The transformation of the acoustic source structure is characterized in terms of patterns of instantaneous and phase-averaged flow velocity, vorticity, and streamline topology.
Self-Loosening Mechanism of Nuts Due to Lateral Motion of Fastened Plate
Authors: Shoji Yasumasa , Sawa Toshiyuki , Yamanaka Hiroshi
Published: August 2009

As self-loosening of nuts is really a problem for bolted joints in practical use, countermeasures for the loosening is highly required. In this situation non-loosening fasteners are one of the resolutions for any fastened machinery as an essential mechanical element. Self-loosening of threaded bolt/nut systems has been researched in number of works and most researches were based on experiment and a few were based on the finite element analysis in these years. Using this new approach, various types of nuts can also be examined. Among these nuts eccentric nuts and slit nuts are especially expected to be the solution, as these nuts are reported to endure NAS vibration tests and were not loosened. In the authors’ previous paper, an eccentric nut and a normal nut were analyzed and compared in the aspect of loosening property. In this paper degree of loosening of various nuts was investigated by experiment and the FEA.
Sensitivity of Stress-Strain Parameters in Leak-Before-Break Evaluations
Authors: Cofie Nathaniel G. , McGill Robert O. , Miessi G. Angah , Wu Jim
Published: August 2009

Leak-before-break (LBB) evaluations involve the use of deterministic fracture mechanics to establish the margin between critical and leakage flaw sizes in order to assure that leaks can be detected by the plant leak detection system before a through-wall flaw reaches critical flaw size. When the material is semi-ductile, the fracture mechanics evaluations involve the use of elastic-plastic fracture mechanics (EPFM) consisting of the J-integral and tearing modulus (J/T) analyses. An important input into the J/T analyses is the Ramberg-Osgood (R-O) material stress strain parameters which describe the stress-strain curve of the material of interest. These are also key inputs in the determination of leakage associated with through-wall flaws. If the stress-strain curve of the material of interest is available, the R-O parameters can be determined from power law curve fit. However, in most cases, archival material of existing plant piping is not readily available to determine the actual stress-strain curve. In the absence of the actual stress-strain curve, several approximate methods for determining the R-O parameters from basic mechanical properties have been proposed in the literature. These approximate methods however produce different sets of R-O parameters. In this paper, the effect of using different sets of R-O parameters from three R-O formulations on LBB analyses results is investigated. EPFM analyses are performed to determine the critical through-wall flaw lengths with the various sets of the R-O parameters for various materials and various pipe sizes. The same sets of parameters are then used to determine the leakage associated with through-wall flaws. The results of the evaluation indicate that different sets of R-O parameters can yield different critical flaw sizes as well as leakage flaw sizes, thus resulting in different margins in LBB evaluations. Considering the margins involved in LBB evaluations (factor of two on critical flaw size and factor of 10 on leakage), it is believed that these differences are small enough that any of the three correlations presented in this paper for determination of the R-O parameters can be adequately used in LBB evaluations employing EPFM analyses.