Life assessment for creep and fatigue of steam turbine components

Improvement of life assessment technologies for power plant materials and components is important in order to meet demands for economy and reliability. As for steam turbines, blade root and disc joints are one of the critical parts in turbines that experience the most severe creep and fatigue damage under high temperature or corrosive environment. In these parts, the structural stress concentration areas are close to the contact planes of blades and rotors, and this produces a complicated stress-strain field. Therefore, life assessment technologies based on precise stress analysis methods and damage mechanisms are necessary to ensure the reliability and economy of steam turbines. In this paper, creep and fatigue tests results by using component specimens simulating blade and rotor joint portions are described. Damage mechanisms of joint portions were investigated based on the observations of the micro-crack initiation and growth behaviors. Life assessment methodologies for joint structures are also discussed, based on the micro-damage, micro-crack or micro-cavity, and nonlinear finite element analyses of component specimens.     

Theoretical Considerations on Corrosion Fatigue Crack Initiation

Based on corrosion kinetics and fracture mechanics, it has been possible to determine quantitatively the corrosion fatigue life for three different types of corrosion behavior. Under general, active corrosion, corrosion fatigue life is controlled by the corrosion rate and the applied alternating stress range. If pitting corrosion occurs, corrosion fatigue life depends on the incubation time for nucleating a pit, the pit growth kinetics, and a critical pit depth, which is a function of the applied stress range. It has been assumed that under passive corrosion conditions, the passive layer has to be penetrated by slip steps, to form corrosion fatigue cracks. The corrosion fatigue crack initiation in this case is controlled by the repassivation kinetics of the material and also by a critical notch depth, depending on the applied stress range. It has been found that a critical current density exists, below which no corrosion fatigue cracks can initiate. Comparison of the theoretically calculated life times with experimental results showed a quite good correlation. M. MüLLER, formerly Research Engineer with Brown Boveri Research Center, Baden, Switzerland.     

激光熔覆IMC涂层的热疲劳性能

研究了激光熔覆和Ｎｉ－Ａｌ合金涂层及（Ｎｉ－Ａｌ）＋ＷＣ复合涂层的热疲劳性能。结果表明，涂层疲劳损伤形式为沿晶应力（氧化）腐蚀。腐蚀产物为Ａｌ２Ｏ３。每次热循环后，熔覆层中的最终残余应力是残余热应力和相变应力共同作用的结果。由于复合涂层中的残余应力为压应力，而合金涂层中的残余应力为拉应力，因此前热疲劳性能优于后者。     

Creep damage evaluation of a power plant header using combined FEM analysis and quantitative metallography

The actual operating conditions of a component may vary from the original design conditions either constantly or variably as a consequence of deviation from design parameters. Hence, the knowledge of operating temperatures and stresses, both historically and for anticipated future operation is required for an accurate life assessment. A number of high temperature and high stress components of Indian thermal power utilities are being monitored round-the-clock for damage assessment by acquiring real-time process data using an on-line damage monitoring system BOSSES, developed by BARC, Mumbai. A number of surface replicas have been obtained from the critical location of a shell-nozzle junction of a super-heater header and analyzed by quantitative metallographic methods. The aim of these activities is to assess service damage of critical components to prevent any unforeseen failure based on a closed loop on-line damage monitoring system-cum-surveillance-cum-life management programme.     

Estimating Fatigue Life of Bridge Components Using Measured Strains

The design fatigue life of a bridge component is based on the stress spectrum the component experiences and the fatigue durability. Changes in traffic patterns, volume, and any degradation of structural components can influence the fatigue life of the bridge. A fatigue life evaluation reflecting the actual conditions has value to bridge owners. Procedures are outlined in the AASHTO Guide Specifications for Fatigue Evaluation of Existing Steel Bridges to estimate the remaining fatigue life of bridges using the measured strain data under actual vehicular traffic. This paper presents the methodology with an actual case study of Patroon Island Bridge. The Patroon Island Bridge consists of ten spans. Spans 3 through 9 are considered the main spans and consist of steel trusses and concrete decks. Spans 1, 2, and 10 are considered approach spans and consist of plate girders. The overall bridge length is 1,795 feet. Strain data from critical structural members were used to estimate the remaining fatigue life of selected bridge components. The results indicate that most of the identified critical details have an infinite remaining safe fatigue life and others have a substantial fatigue life. Cracked floor beams were not addressed in this analysis, but have been recommended for retrofitting or replacement.     

Creep-fatigue damage assessment of special type of sodium to air heat exchanger having toroidal shape headers

In Prototype Fast Breeder Reactor, which is under construction, a dedicated Safety Grade Decay Heat Removal (SGDHR) is employed for removing decay heat during shut down (in case the normal path is not available). Each SGDHR loop consists of a sodium to sodium heat exchanger (DHX) and sodium to air heat exchanger (AHX). Two types of AHX are employed for diversity and this paper discusses creep-fatigue damage evaluation of a special type of sodium to air exchanger. It is a vertical cross flow type heat exchanger with finned tubes connecting two toroidal sodium headers. Five different types of tube configurations, each 50 in numbers (total 250 Nos.) connect both the headers and the entire component is supported through the top header with the bottom outlet header floating. During the normal reactor operating conditions the sodium temperature at the hot end is nearly 823 K. The normal operation, SGDHR operation and shut down condition lead to cyclic thermal stresses. 3-D analyses of the exchanger have been completed for self weight, pressure and thermal loadings. The FEM code CAST-3M, issued by CEA France has been used. The toroidal header has been modeled using shell elements and all the tubes are modeled using pipe elements. The peak stress at the critical junction is 96.4 MPa. The resulting fatigue damage is found to be negligible and the total creep damage is estimated as 0.88 which is conservative, as the stress relaxations are not accounted in the analysis. More details are discussed in the paper.     

应力与环境耦合条件下高强钢S135的疲劳行为北大核心CSCD

为评价应力与环境耦合(交变应力、腐蚀介质和温度耦合)对高强度钢S135疲劳性能的影响,探索应力与环境耦合对高强度钢S135疲劳的作用机制,测试了S135材料在空气中和不同温度下(25、40、60和80℃)有机盐钻井液中的疲劳寿命,分析了S135在有机盐钻井液中的腐蚀疲劳敏感性,并使用扫描电镜观察了不同温度下的断口形貌,研究了有机盐钻井液中S135腐蚀疲劳的断裂机理。结果表明,同一应力(低应力)水平下,随温度升高(25、40、60和80℃),腐蚀疲劳寿命逐渐降低,腐蚀疲劳敏感指数和温度敏感指数逐渐增加。研究认为,交变应力、腐蚀介质和温度耦合对S135材料疲劳有极大的影响,交变应力是疲劳失效的主要因素,腐蚀介质会使S135表面产生腐蚀坑,导致疲劳裂纹的萌生,温度通过加速腐蚀对疲劳破坏产生影响。     

Failure Diagram and Chemical Driving Forces for Subcritical Crack Growth

Kitagawa-Takahashi diagram that is modified for fatigue is now extended to the subcritical crack growth behavior under stress-corrosion crack growth. The analogy with the fatigue helps us to identify several regimes of interest from both the point of understanding of the material behavior as well as quantification of the failure process for structural design of components that are subjected to stress-corrosion and corrosion fatigue crack growths and failure. In particular, the diagram provides a means of defining the mechanical equivalent of chemical stress concentration factor and the chemical crack-tip driving forces to crack growth or its arrest. In addition, threshold stresses, crack arrest, and nonpropagating crack growth conditions can be defined, which help in developing sound design methodology against stress corrosion and corrosion fatigue. Chemical crack driving forces under corrosion fatigue can be similarly defined using the inert behavior as a reference.     

High temperature low cycle fatigue behaviour of P92 tungsten added 9Cr steel

Modified 9Cr-1Mo steel is being used extensively as structural material for steam generator components of liquid metal cooled fast breeder reactor and fossil fired power plants. The selection of this material is primarily based on a good combination of mechanical properties, high thermal conductivity, low thermal expansion coefficient and good resistance to stress corrosion cracking in water-steam and sodium environment systems compared to austenitic stainless steels. A further improvement of the thermal efficiency of modern steam turbines can be achieved by increasing the steam operating temperature. It has been found that the substitution of W for Mo enhances the high temperature tensile strength, fracture toughness and creep strength. This had led to the development of P92 Steel. The components of the steam generators are often subjected to repeated thermal stresses as a result of temperature gradients that occur on heating and cooling during start-ups and shut-downs or during temperature transients. This leads to low cycle fatigue (LCF) damage in the component. However, the data on LCF properties of the W added 9Cr steel (designated as P92 steel) is scarce and this paper aims in understanding the high temperature low cycle fatigue behaviour of P92 Steel. Total axial strain controlled low cycle fatigue tests have been conducted in air in the temperature range 823 and 873 K to ascertain the influence of temperature on the LCF behaviour of the steel. A constant ramp rate of 3 × 10^?3 s^?1 was employed for all the tests. The alloy exhibited a continuous softening before the final load drop that occurred due to the propagation of macro fatigue cracks. The softening rate is more for the tests conducted at higher strain amplitudes compared to that of lower strain amplitudes. Fatigue life decreased with increasing temperature and strain amplitude. In this paper the observed LCF results have been explained based on the detailed metallography and fractography conducted on the failed samples.     

Thermal Fatigue Behaviour of Co-Based Alloy Coating Obtained by Laser Surface Melt-Casting on High Temperature Alloy

Inthefieldofaviation,aswormgearvanesofanaircraftenginerevolveathighspeed,theyarebeatenbydustsandparticlesintheair.Afterservingforalimitedtime,theywouldgraduallyproducecorrosionpits,anditispossiblethatthesetinycorrosionpitsbecomecrackinitiation.Therefore,itisconsideredtocoatalayerofalloybylasermelt-castingonthedamagedplacesofvanes,inordertomakethepropertiesofthecastinglayerapproachorsurpassthoseofsubstrate,andgetthevanestoservenormally.Thuscostscanbereducedandapossiblecomprehensiveeconomicbenef…     

Plasma-facing materials for thermo-nuclear fusion devices

The plasma facing materials and components in existing and future fusion devices are strongly affected by plasma wall interaction processes. These components, in particular the first wall (FW), the limiters and the divertor are subject to intense quasi-stationary thermal loads during plasma operation. While the resulting thermal loads to the first wall will remain below 1 MW·m⁻², special attention has to be paid to high heat flux components like limiters and the divertor. Here the expected power densities will be at least one order of magnitude above the ones at the FW, with expected peak heat fluxes of up to 20 MW·m⁻² for future magnetic confinement devices. Beside quasi-stationary heat loads, short transient thermal pulses with deposited energy densities up to several tens of MJ·m⁻² are another serious concern for next step tokamak devices, in particular for ITER. The most serious events are plasma disruptions, vertical displacement events, and Edge Localized Modes (so-called ELMs). These requirements make high demands on the selection of qualified materials and reliable fabrication processes for actively cooled plasma facing components. High heat flux test facilities based on intense electron and ion beams have been utilized successfully to assess the efficiency and the fatigue life time of different material solutions and design concepts. Modeling and experiments with both normal operation scenarios and transient events, are being performed to evaluate and to quantify the resulting material erosion or damage and thus to assess the life time of the components. Additional research activities are focused on the degradation of materials and joints due to energetic neutrons. In order to investigate irradiation induced property changes, materials samples and actively cooled plasma facing components have been irradiated in fission reactors and tested in thermal load tests. The technical solutions which are considered today are mainly based on beryllium, carbon materials or tungsten as armor materials and copper alloys or stainless steel for the heat sink. Furthermore, the needs for extensive quality control methods and non-destructive analyses during the procurement phase will be highlighted.     

Q345R钢湿硫化氢预腐蚀低周疲劳行为

对Q345R钢在湿硫化氢环境中预腐蚀低周疲劳性能进行了研究,以硫化氢溶液的浓度和预腐蚀时间为环境因素.低周疲劳试验在MTS-809疲劳试验机上进行,对各试验组的结果数据进行回归分析,得到各试验组环境下Q345R钢的循环应力-应变曲线、应变-寿命曲线等低周疲劳特性.不同试验环境下的预腐蚀低周疲劳结果表明,材料循环应力应变响应特性不受环境因素影响,为循环硬化特性;预腐蚀时间因素对材料低周疲劳寿命的影响比硫化氢溶液的浓度因素显著.试样断口为典型的低周疲劳断裂形貌,裂纹扩展阶段为脆性准解理断裂特征.     

Dynamic Response and Fatigue of Steel Tied-Arch Bridge

The Toutle River Bridge is a steel tied-arch bridge, one that vibrates extensively and has sustained significant fatigue cracking. An experimental study into the cause of this behavior is described. Computer analyses of the bridge behavior are used to estimate the expected response and to establish appropriate locations for instrumentation. The instruments were installed and field tests were performed. Controlled tests were performed with trucks of known axle weight and spacing. Some controlled tests were performed with trucks traveling at known speed and in a specific driving lane with no other traffic on the bridge. Controlled tests were used to calibrate the instrumentation and establish the basic bridge behavior. The results showed that composite action had been lost in the heavily loaded stringers, and little amplification of dynamic response was noted. The measured periods of vibration generally compared well with computer predictions. Uncontrolled truck traffic was then measured for approximately one month. This data was used to establish load spectra and to estimate the fatigue life of critical components. Fatigue, which is caused by calculated stress ranges, should not be important on this bridge for another 20 to 30 years. Existing fatigue damage is driven by distortional fatigue caused by the large bridge deformations. Several options for dealing with the problem are presented.     

Remaining life assessment (RLA) of gas based units — case studies

Gas based power plants, particularly the hot gas path components, are subjected to very harsh and rigorous environments. Many time dependent degradation mechanisms such as creep, fatigue, erosion, corrosion, electro mechanical fretting and embrittlement act in combination. The plants are, therefore, designed for a finite life. In order to avoid premature failures, margins of safety are provided and a number of critical components are recommended for replacement at fixed intervals. However, in view of the in-built safety factors, variation in operating conditions, use of lower bound material properties etc. can result in the components recommended for replacement to have a large percentage of their life still remaining. On the other hand, premature failures can occur due to operational and other factors not envisaged in the original design. Methodologies and techniques have been developed to assess the remaining life of gas based power units. Case studies related to the work carried out on land based and marine gas turbines are covered in this paper.     

2100BL电铲上机架体的修复设计

2100BL型电铲是美国P&H采矿设备公司研制的大型矿用机械式正铲挖掘机,因工况条件恶劣,负载变化很大,受冲击载荷频率高,其零部件极易受损。从结构上来说,上机架体是主要受力机构,也是最易受损的结构件之一,在工作中受力最大,必须同时具有抗压、抗弯、抗扭等特性。本文主要介绍了如何应用焊接和精加工技术来修复电铲上机架体的工艺。     

Q700D热影响粗晶区疲劳寿命与小裂纹扩展分析

摘要：为提高焊接构件的动载疲劳寿命,以热模拟为试验手段,对Q700D高强钢进行了焊接热模拟,研究了粗晶热影响区的疲劳寿命、小裂纹扩展行为以及组织软化特征。利用Paris方程和轴向拉伸疲劳试验数据,建立了ΔKth值与模拟粗晶区疲劳寿命的对应关系,利用ΔKth值实现了快速预估粗晶区疲劳寿命。研究表明：相同应力幅值下的lgN值与ΔKth值存在一定的线性拟合关系,即ΔKth值越大,则疲劳寿命N越长。小裂纹扩展微观机理在于所形成的大角度晶界（不小于15°）对小裂纹尖端的止裂性较强,可迫使小裂纹尖端转向耗能。CGHAZ的软化与第二相粒子回熔与粗化有关,粗化的第二相粒子易萌生小裂纹,可通过提高大角度晶界抑制裂纹扩展。     

Roll cooling and its relationship to roll life

Combined experimental and numerical research has been conducted to investigate the roll cooling system used in steel rolling mills and its relationship to roll life. Roll cooling has been identified as a critical factor in the problems of excessive roll wear or spalling, which cause shortened roll life. A special laboratory apparatus resembling the cooling systems used in the steel mills has been developed to evaluate the corresponding heat transfer coefficients. These coefficients have then been utilized for numerical simulation of the rolling processes. In simulation, the thermal behavior of both the roll and the strip have been considered with emphasis on roll temperature and the induced cyclic thermal stresses. An understanding of the cyclic stress can be utilized to evaluate roll wear, and leads to reduction of the roll spalling, or to extension of the roll life by minimizing the cyclic stress or the resultant fatigue stress. As indicated by the present study, in order to minimize the cyclic or fatigue stresses, the roll should be subjected to uniform cooling, as the sharper the heat transfer coefficient distribution, the higher the thermal stress induced.     

连铸机预应力结构拉矫辊的优化设计

针对连铸机拉矫辊的破坏原因，提出一种预应力结构拉矫辊的设计方法，其原理是由机械应力抵消一部分工作时的热应力，从而降低了辊子工作中的应力幅值及平均应力，提高了辊子的热疲劳强度及使用寿命。     

Laboratory Testing Material Property and FE Modeling Structural Response of PAM-Modified Concrete Overlay on Bridges

Portland cement concrete overlay on bridge deck is subjected to distresses of cracking and interface debonding under the effects of repeated vehicle loading and temperature cycling. In order to improve the overlay performance, this research used the polyacrylamide (PAM) polymer to modify the mechanical properties of concrete. The direct shear and impact resistance tests were designed to measure the interface bonding strength and dynamic performance, respectively. The comprehensive and flexural strength and three-point bending fatigue tests were conducted following the standards. Meanwhile, the three-dimensional finite-element (FE) models of the T-girder and box-girder bridges under the moving traffic loadings were built to analyze the stress response and improve the structural design. An analytical model of flexural stress was developed and validated the FE modeling results. A rubber cushion was designed in the FE model to “absorb” the flexural stress. Laboratory testing results indicate that PAM can significantly improve the flexural strength, bonding strength, impact resistance, and fatigue life of concrete. The modified concrete with 8% PAM by mass of cement poses higher flexural strength and impact resistance than concretes with other PAM percentages. FE simulation results indicate that there exists a critical overlay thickness inducing the maximum interface shear stress, which should be avoided in the structural design. The rubber cushion can effectively relieve the flexural stress.