bluuebunny/crossref_metadata_2025 huggingface dataset:
Study of Creep Cavitation in a Stainless Steel Weldment Using Small Angle Neutron Scattering and Scanning Electron Microscopy
Authors: Jazaeri H. , Bouchard P. J. , Hutchings M. T. , Lindner P.
Published: November 2014

Strain-relief cracking, also referred to as reheat cracking, is a generic creep failure mode that has been observed in many welded stainless steel structures operating at high temperatures in UK nuclear power plant. One of the challenges currently faced by nuclear utility EDF Energy is the need to predict, with high certainty, the life-time of ageing engineering plant operating in the creep regime.

A study of creep cavities near a reheat crack, ∼28 mm long, in an AISI Type 316H austenitic stainless steel header is reported. The cracked component was removed after 65,000 hours operation at a mean temperature of 525°C in a nuclear power plant. Creep cavities in the vicinity of the crack are observed using scanning electron microscopy (SEM) and found to be mainly surrounding inter-granular carbides. Optimized metallographic and image analysis procedures are used to quantify the size and area fraction of creep cavities as a function of distance from the crack along a line normal to the crack face. Small angle neutron scattering (SANS) is used to quantify creep cavitation, averaged over a large gauge volume, ∼20 mm3, at similar positions normal to the crack line. A systematic increase in the true fractional size distribution of cavities is observed approaching the crack. The cavity size distribution peaks between 100–300 nm diameter and this correlates closely with the quantitative metallography results of cavities in this sample. SANS has the advantage of being non-destructive and also the examined volume is several mm3, against an area of hundreds of μm2 in the case of SEM, therefore giving more statistically significant results. However the two techniques are complementary, as the detailed microscopic information from SEM is required to interpret the SANS data.
Study on Structural Integrity Assessment of Reactor Pressure Vessel Based on Three-Dimensional Thermal-Hydraulics and Structural Analyses
Authors: Katsuyama Jinya , Katsumata Genshichiro , Onizawa Kunio , Watanabe Tadashi , Nishiyama Yutaka
Published: November 2014

In the structural integrity assessment of a pressurized water reactor pressure vessel (RPV) during pressurized thermal shock (PTS) events, the thermal history of the coolant water and the heat transfer coefficient between the coolant water and RPV are dominant factors. These values can be determined on the basis of thermal-hydraulics (TH) analysis simulating PTS events and Jackson-Fewster correlation. Subsequently, using these values, structural integrity assessments of RPV are performed by structural analysis; e.g., loading that affects crack propagation is evaluated. Three-dimensional TH and structural analyses are recommended for precise assessments of the structural integrity of RPV. In this study, we performed TH and structural analyses simulating typical PTS events using three-dimensional models of cold-leg, downcomer and RPV to more accurately assess the structural integrity of RPV. From these analyses, we obtained loading histories from the reactor core region of RPV in which a crack is postulated in the structural integrity assessment. We discuss the conservativeness of current analysis methods on the structural integrity assessment of RPV through the comparison of loading conditions due to PTS events.
Seismic Performance Analysis of the Large Spherical Tank
Authors: Yang Zhirong , Zhang Dayong , Guo Longwei , Yang Baibing , Wang Guodong
Published: November 2014

The seismic safety problem of the spherical tank under seismic load has become one important subject in seismic research of special equipment. Based on the ANSYS finite element software, typical spherical tank mechanics model is established first of all, through precise time history response analysis under the seismic excitation to determine the significant location of the stress. Then, the seismic performance impact of the support structure design parameters is analyzed. Finally, the seismic performance of all kinds of spherical tank, such as the large, medium and small tank, is determined. This paper provides a reasonable basis for the anti-seismic safety security and design of the spherical tank.
Potential Drop Strain Sensor for Creep Monitoring
Authors: Corcoran Joseph , Davies Catrin , Nagy Peter , Cawley Peter
Published: November 2014

Creep is a high temperature damage mechanism of interest to the power industry and at present lacks a satisfactory monitoring technique. Existing material inspection techniques are extremely laborious while strain measurements rely on often infrequent off-load measurements. A quasi-DC directional potential drop technique is presented that is shown at both room temperature and at high temperature to act as a robust strain gauge. Resistivity changes that are expected to occur in high temperature environments are also discussed as a source of potential error in strain measurements.
Carburization of Ethylene Pyrolysis Furnace Tube in a Petrochemical Plant
Authors: Chen Tao , Chen Xuedong , Liu Chunjiao , Ye Juan , Nie Defu
Published: November 2014

Carburization phenomena of the ethylene pyrolysis furnace tube made of 25Cr35NiNb alloy was investigated after service for approximate 30,000h in a petrochemical plant. Three different zones, namely oxidation zone, carburization zone and aging product zone, were observed in the cross section of furnace tube by microscopic analysis. The oxidation zone near the inner surface has a thickness of about 700μm and possesses the characterization of many loose pores and voids. The inner surface has a continuous thin Cr2O3 layer and the grain boundaries near the inner wall are mainly composed of mixed oxides of Cr2O3 and SiO2. In addition, segregation of impurities S and P were detected at grain boundaries. The dark grey SiO2 distributes in front of the oxidation zone. Carbides distribute at grain boundaries in the carburization zone with lots of voids and micro cracks. The width of grain boundary is broadened. According to the experimental results, the initiation of cracks may be promoted by carbides at grain boundaries. The uneven distribution of the Cr element was found in the carburization zone, and direct experimental evidence of Cr element diffusion was detected. In the aging product zone, the carbides evolution was observed. The chromium carbides are composed of Cr7C3 and Cr23C6. The microstructure and composition evolutions of the furnace tube and their effects on service life of ethylene pyrolysis furnace tube were also discussed in this paper.
Material Selection and Safety Validation of Steel Tanks for Hydrogen Stations
Authors: Wada Yoru
Published: November 2014

This paper addresses the material selection and safety validation of hydrogen steel tanks for stationary applications. Tensile, fatigue and crack growth behavior of HSLA steels in high pressure gaseous hydrogen are briefly reviewed and material qualification procedure using fatigue analysis was proposed.
A Physically Based Damage Constitutive Model for Stress Relaxation
Authors: Zhou Yu , Chen Xuedong , Fan Zhichao , Han Yichun
Published: November 2014

A damage constitutive model for stress relaxation was proposed based on the continuum damage mechanics theory, in terms of the characteristics of damage induced by stress relaxation which is associated with reheat cracking. The effects of strain hardening and the precipitation and growth of carbides on the stress evolution behavior were considered in the present model. To validate the accuracy of the constitutive model, comparison of the predicted result provided by the model and the experimental data of stress relaxation for 2.25Cr-1Mo-0.25V ferritic steel was made. It is shown that the damage constitutive model can be used to predict the stress relaxation behavior and reheat cracking further for low alloy ferritic steels.
Buckling Behaviors of Metallic Columns Under Compressive Load at Extremely High Temperatures
Authors: Jo Byeongnam , Sagawa Wataru , Okamoto Koji
Published: November 2014

This study aims to investigate buckling behaviors of a slender stainless steel column under compressive loads in severe accident conditions, which addresses the accidents in Fukushima Daiichi nuclear power plants. Firstly, buckling load, defined a load which generates a failure of the column (plastic collapse) was experimentally measured in a wide range of temperatures from 25 °C up to 1200 °C. The buckling load values measured were compared to numerical estimations for both an ideal column and for a column initially bent. Secondly, creep buckling tests were also performed for extremely high temperatures (800 °C, 900 °C, and 1000 °C). Creep buckling was found to occur very quickly compared to general creep times under tensile stresses. Time to creep buckling was exponentially increased with decrease of loads applied. Lateral deflection of a test column was estimated using captured images by a high speed camera. It was suggested to represent creep buckling behaviors as a time-lateral deflection curve. Moreover, an empirical correlation was developed to predict creep buckling time, based on the Larson-Miller model with experimental results obtained in present study.
An Improved 1D Model for Liquid Slugs Travelling in Pipelines
Authors: Tijsseling Arris S. , Hou Qingzhi , Bozkuş Zafer
Published: November 2014

An improved one-dimensional (1D) model — compared to previous work by the authors — is proposed which is able to predict the acceleration and shortening of a single liquid slug propagating in a straight pipe with a downstream bend. The model includes holdup at the slug’s tail and flow separation at the bend. The obtained analytical and numerical results are validated against experimental data. The effects of the improvement and of holdup are examined in a parameter variation study.
Prediction of Stress Relaxation Behavior of 2.25Cr-1Mo-0.25V Steel Based on Continuum Damage Mechanics
Authors: Zhou Yu , Chen Xuedong , Fan Zhichao , Han Yichun
Published: November 2014

The functional features of the CDM-based damage constitutive model for stress relaxation which has been recently proposed were analyzed. Due to the highly nonlinear hyperbolic sine function adopted in the function and the large difference in the orders of magnitude among the material constants, an efficient genetic algorithm based optimization scheme was applied to obtain the global minima for the least square function. In addition, a procedure for the preliminary evaluation of material constants in the model was developed to better converging to the minima. The user-defined subroutine implementing the damage constitutive model was developed. It is validated that the predicted result provided by the developed ANSYS program agrees well with the experimental data of the stress relaxation for ferritic steels, providing preliminary results for the prediction of reheat cracking.
Creep Crack Growth Predictions in 316H Steel Over a Wide Range of Stresses and Temperatures
Authors: Mehmanparast A. , Davies C. M. , Nikbin K. M. , Webster G. A.
Published: November 2014

The prediction of the creep crack growth (CCG) behaviour in engineering materials is of great importance in the life assessment of power plant components. The conventional technique to predict CCG is to employ uniaxial creep properties and appropriate damage models in finite element (FE) simulations or analytical CCG prediction models. Uniaxial creep trends for Type 316H SS have been recently estimated for a wide range of stresses and temperatures in [1] and FE CCG predictions have been made at 550 °C and validated through comparison with the experimental data. In this paper, FE CCG predictions using the developed uniaxial creep trends for a wide range of stresses and temperatures are presented and the results are compared with the predicted CCG trend at 550 °C and also with the analytical constant creep ductility NSW CCG prediction models. The results from FE predictions are discussed in terms of the temperature effects on the creep deformation and crack growth behaviour of components operating at elevated temperatures.
Further Development of the Nonlocal Damage Model of Rousselier for the Transition Regime of Fracture Toughness and Different Stress States
Authors: Gehrlicher Sarah , Seidenfuss Michael , Schuler Xaver
Published: November 2014

In nuclear power engineering failure has to be excluded for components with high safety relevance. Currently, safety assessments mainly use fracture mechanics concepts. Especially in the transition region of fracture toughness where limited stable crack extension may appear before cleavage fracture the currently applied methods are limited.

This Paper deals with the development and verification of a closed concept for safety assessment of components over the whole range from the lower shelf to the upper shelf of fracture toughness. The results of classical used local damage mechanics models depend on the element size of the numerical model. This disadvantage can be avoided using an element size depending on microstructure. With high stress gradients and small crack growth rates usually smaller elements are required. This is in conflict with an element size depending on microstructure. By including the damage gradient as an additional degree of freedom in the damage mechanics model the results depend no longer at the element size. In the paper damage mechanics computations with a nonlocal formulation of the Rousselier model are carried out for the evaluation of the upper transition area. For the prediction of fracture toughness from the ductile to brittle transition area the nonlocal Rousselier model is coupled with the Beremin model. Thus ductile crack growth and failure by brittle fracture can be described in parallel. The numerical prediction of the behaviour of fracture toughness specimens (C(T)-specimens and SE(B)-specimens with and without side grooves) and the experimental results are highly concordant. The load displacement behavior of the specimens and the developed crack front from the ductile to brittle transition area can be well calculated with the nonlocal damage model. The instability in relation to temperature calculated with the coupled damage mechanics model predicts the variations of the experimental results very well.

For further application of the nonlocal Rousselier model experiments and numerical calculations of specimens with different stress states and multi-axiality are carried out. Modified fracture toughness specimens like CTS-specimens (compact tension shear specimens) are taken to investigate the applicability of the nonlocal damage model of Rousselier to mixed mode fracture.
Numerical Analysis of Fracture Mechanics of SENB Specimens Prepared From HDPE Pipes
Authors: Gonzalez Marco , Teixeira Paulo , Gonzalez Jeanette , Machado Raul
Published: November 2014

High density polyethylene (HDPE) is commonly used in pipe fabrication for water and natural gas systems, due to its versatility, low cost and lightweight. A piping system is subject to service conditions such as impact and cyclic loads as a consequence of internal pressure or external pressure fluctuations, and the existence of discontinuities in the material. These conditions cause material damage, cracking and weakening, and have to be considered in the piping design. The Boundary Element Method (BEM) is a numerical method based on integral equations that consider only the contour of the solid (meaning an easier meshing). Crack modeling is one of the most important applications for the BEM, since it allows an accurate stress analysis around the crack tip. In this work, a computational study based on the BEM in two dimensions whose aim is to determine the stress intensity factors (SIFs) in order to evaluate the mechanical resistance to fracture of HDPE PE100 pipes and its comparison with the results obtained by previous experimental tests, is developed. Numerical simulations of specimens subject to three point bending loads (SENB specimens) using the characteristics of the linear elastic fracture mechanic (LEFM), are developed. As a first attempt, the numerical models of different SENB geometries are validated comparing the numerical solution versus the results given by a reference solution from literature. The results show that the BEM under the LEFM approach is valid for loads within the linear range of HDPE since LEFM gives an upper bound of the fracture load of HDPE specimens; however, an Elastic-Plastic fracture analysis could be required for loads in the plastic range of the material.
Fasteners: Strength and Quality Requirements
Authors: Kirkemo Finn
Published: November 2014

Heavy hex nuts may be selected with similar hardness to the studs to avoid stripping of nut threads for pressure containing (closure bolting) and primary load bearing bolting. However, industry specification for subsea production systems, e.g. such as API Specifications 6A, 16A, 16C, and 17D, allows the use of low strength heavy hex nuts, e.g. ASTM A194 Grade 2HM or 7M, to be applied together with high strength studs, e.g. ASTM A320 Grades L43 or L7. In the refining and chemical industries the common practice is to use ASTM A193 B7 studs with 2H nuts. Calculations and testing of nuts have been performed and capacity formulas have been established for nut structural capacities. Guidance is given on selecting nut strength/hardness to avoid stripping of nut threads. ASTM specifications give minimum quality requirements during manufacture. A brief review of standard quality requirements is given and guidance for additional requirements for high integrity fasteners in order to have equivalent quality as pressure containing forgings is given. The results from this paper may be used as background for requirements in code updates and purchaser specifications.
Hydrogen-Assisted Subcritical Crack Growth Rate in Cr-Mo Steels
Authors: Konosu Shinji , Uemori Ryuji , Yuga Masao , Yamamoto Hiroshi
Published: November 2014

Hydrogen substantially reduces fracture properties such as threshold stress intensity factor KIH and tearing resistance dJ/da in conventional Cr-Mo steels. In order to enable the life assessment of a reactor with a hydrogen-induced crack using a failure assessment diagram (FAD), an experimental database of hydrogen-assisted subcritical crack growth rates da/dt is requisite. However, there are very few studies concerning the effects of hydrogen- and temper-embrittlement on da/dt at ambient temperatures in 2.25Cr-1Mo steels with high and low impurity levels. In this paper, vacuum melted lab heats of 2.25Cr-1Mo steel were supplied with compositional controls. Some specimens were embrittled by step cooling heat treatment (SCHT). Subcritical crack growth rate at a constant load was obtained by means of the potential drop method for 2.25Cr-1Mo steel with initial internal hydrogen (3.2 mass ppm).
Volumetric Examination After PWHT on a Carbon Steel Pressure Vessel
Authors: Becker James W. , Schulte Matthew L.
Published: November 2014

Carbon steel pressure vessels may require post weld heat treatment (PWHT) for stress relief in accordance with global pressure vessel safety codes. When stress relieved, most of these vessels receive a full volumetric exam, such as radiography or ultrasound. Many supplementary design specifications require the volumetric exam take place after PWHT; however, when a volumetric exam after PWHT is required, vessel fabricators commonly perform an additional examination before PWHT to mitigate the risk of a second PWHT cycle. The increased cost associated with performing two full volumetric exams is passed on to the end-user of the pressure vessel.

This investigation examines the need for a volumetric exam after PWHT on a carbon steel pressure vessel. Following a literature review and after analysis of radiography results from vessel fabricators in the United States, there is little evidence supporting a volumetric examination requirement after PWHT. PWHT does not cause sub-surface flaw development or sub-surface flaw propagation. A surface examination after PWHT is still recommended to confirm flaws have not developed near the surface during PWHT; however, a full volumetric examination on a carbon steel vessel after PWHT may be unnecessary.
Nonlinear Normal Modes and the Dahl Friction Model Parameter Identification
Authors: Hadji Abdallah , Mureithi Njuki
Published: November 2014

Fretting wear of steam generator tubes due to vibration induced by fluid flow remains a serious problem in the nuclear industry. Azizian and Mureithi [1] have recently developed a hybrid friction model to simulate the friction behavior of tube-support interaction. However, identification of the model parameters remains unresolved.

To identify the parameters of the friction model, the following quantities are required: contact forces (tangential force (friction) and normal force (impact)), the slip velocity and displacement in the contact region. Direct measurement of these quantities by using a steam generator tube interacting with its supports is difficult. To simplify the problem, a beam, clamped at one end and simply supported with consideration of friction effect at the other is used. The beam acts as a mechanical amplifier of the friction effects at the microscopic level. Using this simple setup, the contact forces, the sliding velocity and the displacement can be indirectly obtained from the beam’s vibration response measurements.

A new method based on nonlinear modal analysis was developed to calculate the contact forces. This method is based on the modal superposition principle and Fourier series expansion. The nonlinear normal modes (NNMs) and the generalized coordinates (GCs) have been identified experimentally as functions of the excitation level, the frequency, the preload in the contact area, with and without lubrication.

Three hypotheses and related analyses to identify the NNMs and GCs were tested; the analysis based on the harmonic balance method gives the best results for reconstructing the accelerometer signals with an error less than 2% for all excitation levels compared to more than 2% for other methods.

The successful signal reconstruction makes it possible to accurately identify the parameters of the Dahl friction model. This is also the first step to identify the parameters of the hybrid friction model.
Dropped Object Impact Analysis of Subsea Tree Frame
Authors: Karpanan Kumarswamy , Hamilton-Smith Craig
Published: November 2014

Subsea oil and gas production involves assemblies such as trees, manifolds, and pipelines that are installed on sea floor. Each of these components is exposed to severe working conditions throughout its operational life and is difficult and expensive to repair or retrieve installed. During installation and operation, a rig/platform and several supply vessels are stationed on the waterline directly above the well and installed equipment below. If any object is to be dropped overboard, it presents a hazard to the installed equipment. A subsea tree comprises of a number of critical components such as valves and hydraulic actuators, in addition to several electrical components such as the subsea control module and pressure/temperature gauges. Their ability to operate correctly is vital to the safe production of oil and gas. If an object were to impact and damage these components, resulting in their inability to operate as intended, the consequences could be severe. In this paper, a typical subsea tree frame is analyzed to ensure its ability to withstand the impact from an object accidentally dropped overboard. This was accomplished using nonlinear dynamic Finite Element Analysis (FEA). In this study, the framework was struck by a rigid body at terminal velocity, resulting in a given impact energy. Displacements and resultant strain values at critical locations were then compared to allowable limits to ensure compliance to the design requirements.
Ultrasonic Sound Field Mapping Through Coarse Grained Cast Austenitic Stainless Steel Components
Authors: Crawford Susan L. , Prowant Matthew S. , Cinson Anthony D. , Larche Michael R. , Diaz Aaron A. , Anderson Michael T.
Published: November 2014

The Pacific Northwest National Laboratory (PNNL) has been involved with nondestructive examination of coarse-grained cast austenitic stainless steel (CASS) components for over 30 years. More recent work has focused on mapping the ultrasonic sound fields generated by low-frequency phased-array probes that are typically used for the evaluation of CASS materials for flaw detection and characterization. The casting process results in the formation of large-grained material microstructures that are nonhomogeneous and anisotropic. The propagation of ultrasonic energy for examination of these materials results in scattering, partitioning, and redirection of these sound fields. The work reported here provides an assessment of sound field formation in these materials and provides recommendations on ultrasonic inspection parameters for flaw detection in CASS components.

Confirmatory research conducted at PNNL consisted of acquiring sound field data from four CASS components containing columnar, equiaxed, and banded grain structures, and a fine-grained wrought stainless steel specimen used for benchmarking. Phased-array probes with center frequencies of 0.5, 0.8, and 1.0 MHz were used for sound field formation, with a pinducer being raster scanned over the end of the specimen face to capture the sound field energy. Data were collected at multiple refracted and skew angles, and imaging performed for analyses. A 6.4-mm (0.25-in.) thick slice of material was removed from the end of the CASS components and the beam mapping repeated. This slicing and mapping sequence was performed three times to produce multiple beam images through the specimens. Grain sizes were also measured at each mapped specimen face and compared to sound field characteristics. The acquired sound field images were characterized in terms of beam redirection from the theoretical position, beam scatter or coherence, and partitioning. A comparison of the fine-grained beam data to the CASS data is made and conclusions are presented.
Comparative Study of Acoustic Emission Characteristics for Intergranular Corrosion of 316L Stainless Steel Parent Material and Weldment
Authors: Bai Wenjie , Chai Mengyu , Li Lichan , Duan Quan
Published: November 2014

The 316L stainless steel parent material and weldment specimens were made to carry out intergranular corrosion(IGC) test using the method of boiling nitric acid. During the corrosion experiment, the acoustic emission(AE) signals were collected. Through the comparative analysis of corrosion rate and metallographic structure, the results showed that the IGC of parent material and weldment can be divided into the preliminary corrosion stage and the rapid corrosion stage. The AE parameters and spectrum characteristics of the two corrosion stages of the parent material and weldment were analyzed. The results showed that: in preliminary and rapid corrosion stages, the AE signal amplitude and energy of weldment were higher than that of parent material; the spectrum characteristics of weldment was more abundant than that of parent material. Based on the results of the comparative analysis, the AE sources of parent material and weldment IGC and the possibilities of monitoring IGC using AE technique were analyzed.
Probabilistic Pressurized Thermal Shocks Analysis for a Reactor Pressure Vessel
Authors: Qian Guian , Niffenegger Markus
Published: November 2014

Both deterministic and probabilistic methods are used to analyze the integrity of a reactor pressure vessel (RPV) subjected to pressurized thermal shocks (PTSs). The FAVOR code is applied to calculate the probabilities for crack initiation and failure of the RPV subjected to two transients, by considering crack distributions based on cracks observed in the Shoreham and pressure vessel research user facility (PVRUF) RPVs. The crack parameters, i.e. crack density, depth, aspect ratio, orientation and location are assumed as random variables following different distributions. KI of the cracks with the same depth increases with its aspect ratio. Both KI and KIC at the crack tip increase with crack depth, which is the reason why a deeper crack does not necessarily lead to a higher failure probability. The underclad crack is the most critical crack and the deeper crack is the least critical one in this study. Considering uncertainties of the transients results in higher failure probabilities.
Evaluation of Residual Stresses in Small Diameter Stainless Steel Pipe Welds by Two-Dimensional and Three-Dimensional Finite Element Analyses
Authors: Ku Francis H. , Hicks Trevor G. , Mabe William R. , Miller Jason R.
Published: November 2014

Two-dimensional (2D) and three-dimensional (3D) weld-induced residual stress finite element analyses have been performed for 2-inch Schedule 80 Type-304 stainless steel pipe sections joined by a multi-layer segmented-bead pipe weld. The analyses investigate the similarities and differences between the two modeling approaches in terms of residual stresses and axial shrinkage induced by the pipe weld. The 2D analyses are of axisymmetric behavior and evaluate two different pipe end constraints, namely fixed-fixed and fixed-free, while the 3D analysis approximates the non-axisymmetric segmented welding expected in production, with fixed-free pipe end constraints.

Based on the results presented, the following conclusions can be drawn. The welding temperature contour results between the 2D and 3D analyses are very similar. Only the 3D analysis is capable of simulating the non-axisymmetric behavior of the segmented welding technique. The 2D analyses yield similar hoop residual stresses to the 3D analysis, and closely capture the maximum and minimum ID surface hoop residual stresses from the 3D analysis. The primary difference in ID surface residual stresses between the 2D fixed-fixed and 2D fixed-free constraints cases is the higher tensile axial stresses in the pipe outside of the weld region. The 2D analyses under-predict the maximum axial residual stress compared to the 3D analysis. The 2D ID surface residual stress results tend to bound the averaged 3D results. 2D axisymmetric modeling tends to significantly under-predict weld shrinkage. Axial weld shrinkage from 3D modeling is of the same magnitude as values measured in the laboratory on a prototypic mockup.
Fatigue Behavior of Butt Weld Seams: Experimental Investigation and Numerical Simulation
Authors: Langschwager Kay , Bosch Alexander , Lang Eliane , Rudolph Jürgen , Vormwald Michael , Scholz Alfred , Oechsner Matthias
Published: November 2014

Austenitic stainless steel of type X6CrNiNb18-10 (1.4550) is a widely used material in piping and components of nuclear power plants. The fatigue behavior of these components is often operationally determined by thermomechanical strains and corresponding stresses. Welded structures lead to complex stresses in the component and potential fatigue lifetime reductions. Various geometrical and microstructural inhomogeneities in welded structures represent the main factors of influence. Nevertheless, clear identification and quantification of various factors of influence are issues still to be resolved.

Within the framework of an ongoing research project, the experimental investigation comprises uniaxial and biaxial fatigue experiments on welded joints which cover temperatures from 25°C to 350°C. Furthermore, a key issue deals with the thermomechanical fatigue behavior of machined and unmachined butt weld seams. A special focus is set on typical low cycle fatigue (LCF) tests in order to explain the behavior of the base material and the weld material to identify the influence of microstructural inhomogeneities. In addition, specimens manufactured directly from the pipe components are tested to examine the influence of the butt weld seam geometry. For a better understanding of the local strain effects, optical strain field measurements (OSFM) are conducted and used to validate numerical simulation. The finite element method (FEM) is utilized to expand the parameter space and identify the main parameters.

Experimental and numerical results show that fatigue failure occurs either in the base metal in the vicinity of the welded zone or in the top layer of the weld, depending on the loading conditions. This knowledge is used to develop an approach to fatigue lifetime estimation.
Simplified Approach for Fracture Integrity Assessment Procecedure of Flawed Weld Joint Under Non-Conventional Condition
Authors: Carlucci Antonio , Mcirdi Kamel
Published: November 2014

Welded pipes are widely employed in many oil and gas applications. Engineering Critical Assessment (ECA) shall be performed in order to establish acceptance levels for revealed or postulated flaws in new or existing constructions. Although methods for assessing the acceptability of flaws in all types of structures are presented in codes and standards, such as BS7910 [1], API579-1 [2], R6 [3], DNV OS-F101 [4], the approaches are typically derived by simplified geometries as plate solutions, simplified material assumption, and simplified load condition as uniaxial load condition. Dedicate numerical solution are more accurate and would improve the assessed results. But the use of appropriate conditions in the full ECA requires several specific Finite Element Analysis (FEA) which are able to identify the Crack Driving Force (CDF) for each postulated defect geometry, material assumption and load conditions. The purpose of this paper is to propose a simplified method, into a standard procedure (similar to BS7910 [1]), minimizing numerical analyses, to guaranty the safety against fracture of many kind of weld joint under non-conventional condition (such as generic weld joint geometry and/or weld joint subjected to combined axial force, bending moment and internal over pressure which are not contemplate in current code and for which dedicated FEA are recommended).
Limit Plastic Collapse on Remaining Ligament of Flawed Welds for ECA Application
Authors: Carlucci Antonio , Mcirdi Kamel
Published: November 2014

Engineering Critical Assessments (ECAs) are routinely used to provide defect acceptance criteria for pipelines girth welds. The Failure Assessment Diagram (FAD) concept is the most widely used methodology for elastic-plastic fracture mechanics analysis of structural components and adopted by standards/documents including BS7910 [1], API579-1/ASME FFS-1 [2], R6 [3]. It is defined by two criterion Kr and Lr which describe the interaction between brittle fracture and fully ductile rupture: Kr measures the proximity to brittle fracture whereas Lr reflects the closeness to plastic collapse.

The BS7910 FAD level 2B is the most employed for assessment of flaws under mechanical strain lower than 0.4%, the FAD associated is material-specific and it based on single toughness value obtained from CTOD test, the latter-on gives no information about the tearing initiation.

The objective of this paper is to propose an approach for determination of the critical fracture toughness (associated to zero-tearing: JΔa=0). This approach is based on the comparison between the load-CMOD curve provided from a fracture toughness test to the one obtained by Finite Element Analysis (FEA). The goals is to propose a conservative guidance on how to identify a remote strain level below which it may be considered guaranteed the integrity of the remaining ligament.
Measurement of Fracture Properties for Ferritic Steel in High-Pressure Hydrogen Gas
Authors: Iijima Takashi , Itoga Hisatake , An Bai , San Marchi Chris , Somerday Brian P.
Published: November 2014

Recently, the measurement of threshold stress intensity factors for various low alloy ferritic steels in high-pressure hydrogen gas of 103 MPa was performed, and it was revealed that the subcritical cracking threshold under rising displacement was lower than the subcritical cracking threshold for crack arrest under constant displacement. These experimental results demonstrate the importance of the testing method for evaluating the fracture properties in high-pressure hydrogen gas. We measured the subcritical cracking threshold under rising displacement for ASME SA-372 Grade J ferritic steels in high-pressure hydrogen gas at pressure up to 115MPa. In contrast to other reported procedures where the applied displacement was increased continuously, in this study crack length was determined using an unloading elastic compliance method. The values of the subcritical cracking threshold measured by the unloading elastic compliance method are consistent with previous measurements in which the applied displacement continuously increased. These results suggest the possibility that subcritical cracking thresholds do not depend on the applied displacement path, i.e., periodic unloading vs. continuously rising displacement.
Simplified Toughness Resistance Curve
Authors: Carlucci Antonio , Mcirdi Kamel , Auvret Pierre-Louis , Li Jun
Published: November 2014

This paper presents a method in order to plot a simplified toughness resistance curve (R-Curve) from three single conventional toughness tests.

The simplified toughness resistance curve will be used to carry out a tearing assessment as part of Engineering Criticality Assessment (ECA) (ref. to Level 3B, [2]). This level of assessment takes into account of stable tearing in order to reduce over-conservatism, but it usually requires additional toughness tests to identify the toughness resistance curve. This present method allows to reach this assessment level without impacting the welding procedure qualification (WPQ).
Simplified Dissimilar Metal Weld Through-Wall Weld Residual Stress Models for Single V Groove Welds in Cylindrical Components
Authors: Sommerville Daniel , Qin Minghao , Walter Matthew
Published: November 2014

Two simplified models are developed and benchmarked for predicting through-wall axial and hoop weld residual stress (WRS) distributions in single V groove dissimilar metal welds (DMWs) joining cylindrical components such as piping or nozzles. The models can be used to predict WRS distributions for different pipe mean radius to wall thickness ratios (Rm/t) without an inside surface repair and WRS distributions at a single Rm/t for various inside surface weld repair depth to pipe thickness ratios (x/t). The models are developed by approximating the through-wall WRS distribution using a finite Fourier series where the coefficient of each term in the Fourier series is determined using a linear equation in which the Rm/t or x/t is the independent parameter. The model for the unrepaired condition has been benchmarked against two plant specific finite element WRS analyses of BWR nozzle to safe end welds as well as experimental and FEA WRS data from the PWR pressurizer safety/relief nozzle to safe end weld documented in MRP-317. The weld repair model has been benchmarked against the pressurizer surge nozzle experimental data presented in MRP-316. The models have been used to perform numerous plant specific DMW residual life calculations and can save significant time and money when performing weld specific fracture mechanics analyses.
Structural Performance of Steel Pipe Tee-Junctions
Authors: Papatheocharis Theocharis , Diamanti Kalliopi , Varelis George E. , Perdikaris Philip C. , Karamanos Spyros A.
Published: November 2014

The behavior of steel pipe junctions (Tees) subjected to strong loading in the presence of internal pressure is examined in the present study. The analysis is based on a set of monotonic and cyclic out-of plane bending tests under constant and increasing amplitude displacement-controlled loading schemes leading to low-cycle fatigue failure.

Rigorous finite element models are developed to support the experiments, accounting for detailed dimensional measurements and material testing results obtained prior to testing. A parametric analysis is also conducted focusing on the effect of the geometrical characteristics on the overall junction behavior. The performance of the Tee-junctions with varying geometries under out-of plane bending, in-plane bending and axial loading is also examined numerically accounting for the presence of internal pressure.
Measurement of Plastic Strain Distribution in Dissimilar Metal Weld by Micro-Hardness Mapping
Authors: Yu Xinghua , Qiao Dongxiao , Crooker Paul , David Stan , Feng Zhili
Published: November 2014

Measuring plastic strains is very useful method for validating finite element model of weld residual stress, which is very important for understanding welding process and facilitating other engineering applications. In this work, the distribution of plastic strains in a multi-pass dissimilar metal weld comprised of Nickel Alloy 82 and austenitic stainless steel 304L is evaluated quantitatively through micro-hardness mapping. An experiment procedure was developed to separate the contribution to hardness from the plastic strain (work hardening) that forms the chemistry variation in the dissimilar metal weld. It is found that high equivalent plastic strains are predominately accumulated in the buttering layer, the root pass, and the heat affected zone, which experience multiple welding thermal cycles. The final cap passes, experiencing only one or two welding thermal cycles, exhibit less plastic strain accumulation. Moreover, the experimental residual plastic strains are compared with those predicted using an existing weld thermo-mechanical model with two different strain hardening rules.
Effect of Weld Geometry on Mis-Match Limit Load Analyses for Circumferential Surface Cracked Pipes in the Centre of Welds
Authors: Kim Sang-Hyun , Han Jae-Jun , Lee Han-Sang , Kim Yun-Jae
Published: November 2014

The weld metal compared to the base metal, is known to be more vulnerable to cracking. Thus, the strength mismatch limit load analysis is necessary for defect assessment. In our previous paper, closed-form solutions of mis-match limit loads were proposed for idealized butt weld configuration as a function of the strength mis-match ratio with geometry-related slenderness parameter. The actual weld geometry is not idealized band configuration but shape of V-grooved welds. The surface crack is more common and significant than through wall crack. The present work provides mis-match limit loads for circumferential surface cracked pipes in the centre of V-groove welds using finite element (FE) analysis. Various crack depth (a/t=0.35∼0.8) and groove angles (20°∼90°) are considered for systematic investigation. With regards to loading conditions, axial tension is applied for all cases.
Turbulent Buffeting of Helical Coil Steam Generator Tubes
Authors: Xu Heqin , Mallet Matthew , Liszkai Tamas
Published: November 2014

Turbulent buffeting is the most common flow-induced-vibrating mechanism in the nuclear power industry since turbulent flow is present in virtually all power plant components and will always apply a random pressure to the surface of the structures wherever there is a flow over, along, inside or cross a structure. For each specific component subjected to turbulent buffeting, the applicable potential degradation mechanisms, such as wear, fretting, or fatigue must be identified so that appropriate evaluations can be performed. For SG tubing, the two degradation mechanisms associated with turbulent buffeting response are tube-to-support wear and tube fatigue, both of which can be evaluated using the root mean square (RMS) response of turbulence induced vibration. In this paper, a methodology is proposed and implemented using ANSYS APDL command objects to estimate the RMS response for multiply supported SG tubes, such as the helical coil SG tubes in the NuScale design. As expected, the random vibrations due to the lower velocities associated with the natural circulation design result in lower RMS responses, which help prevent wear and fatigue failures in reactor module components, such as SG tubes. The number of modes required to adequately capture the RMS response is examined. The cross-modal contributions to RMS response are also examined to provide a justification to exclude the cross-modal terms in future evaluations. In order to capture the dynamic behavior accurately, the hydrodynamic mass and mass inside the helical tubes are fully accounted for in the total effective mass along the SG tubes.
Comparison Between ASME Code-Case N-761, NUREG/CR-6909 and Stainless Steel Component Fatigue Test Results
Authors: Harrison H. T. , Gurdal Robert
Published: November 2014

For Class 1 components, the consideration of the environmental effects on fatigue has been suggested to be evaluated through two different methodologies: either NUREG/CR-6909 from March 2007 or ASME-Code Case N-761 from August 2010. The purpose of this technical paper is to compare these two methods. In addition, the equations from Revision 1 of the NUREG/CR-6909 will be evaluated.

For these comparisons, two stainless steel component fatigue test series with documented results are considered. These two fatigue test series are completely different from each other (applied cyclic displacements vs. insurge/outsurge types of transients). Therefore, they are producing an appropriate foundation for these comparisons.

In general, the severities of the two methods are compared, where the severity is defined as the actual number of cycles from the fatigue tests, including an evaluation of the scatter, divided by the number of design cycles from the two methods. Also, how stable the methods are is being evaluated through the calculation of the coefficient of variation for each method.
Numerical Investigation of Stress Concentration Factor at Irregular Corrosion Pit Under Tension-Torsion Loading
Authors: Huang Yuhui , Wang Chengcheng , Tu Shan-Tung , Xuan Fu-Zhen , Itoh Takamoto
Published: November 2014

Finite element analysis is adopted to study the stress concentration of pit area under tension-torsion loading. The stress concentration factors under regular evolution and irregular evolution of pits are investigated by conducting a series of three-dimensional semi-elliptical pitted models. Based on the finite element analysis, it can be concluded that pit aspect ratio (a/2c) is a significant parameter affecting stress concentration factor (SCF) for regular evolution pits. Pits, having higher aspect ratio, are very dangerous form and can cause significant reduction in the load carrying capacity. When local dissolution occurs in the pitting area, SCF will have a sharp increase, it is more probable for a crack to initiate from these areas compared with pits for regular evolution. Furthermore, local dissolution coefficient is proposed to study effect of local dissolution within the pit on SCF.
Fatigue Life of BWR Feedwater Nozzles Considering Actual Plant Transients
Authors: Evon Keith , Gilman Timothy , Walter Matthew
Published: November 2014

Feedwater (FW) nozzles are often considered to be locations with the highest cumulative fatigue usage in boiling water reactors (BWRs). Despite the lack of fatigue failures of these components, application of fatigue life environmental correction factors to the usage factors, originally computed by the designers, typically result in environmentally assisted fatigue (CUFen) values greater than 1.0. The reason for this disconnect may be attributable to the differences in analytical methods, as well as the differences in severity between the idealized design transient definitions and the actual transient behavior. This paper investigates these differences by comparing the traditional design fatigue analyses with those computed using modern fatigue monitoring methods. The results support the position that the level of conservatism in those original analyses account for some, if not all, of the environmentally-assisted fatigue effects.