汽轮机结构元件的损伤和寿命评估

提出了汽轮机转子在高应力疲劳与高温蠕变变互作用下的损伤本构模型与寿命评估方法,以及用于汽轮机转子结构分析的基本方程与变分原理,利用“间置加载”型荷载谱的特点,提出两个基本假设,并根据初始循环应力应变场对损过程中的循环应力应变场进行了有效描述,可对汽轮机转子结构寿命进行评估。     

A creep life assessment method for boiler pipes using small punch creep test

The small punch creep (SPC) test is considered as a highly useful method for creep life assessment for high temperature plant components. SPC uses miniature-sized specimens and does not cause any serious sampling damages, and its assessment accuracy is at a high level. However, in applying the SPC test to the residual creep life assessment of the boiler in service, there are some issues to be studied. In order to apply SPC test to the residual creep life assessment of the 2.25Cr–1Mo steel boiler pipe, the relationship between uniaxial creep stress and the SPC test load has been studied.     

Remaining creep life estimation by strain assessment on plant

By utilising a generalised damage parameter first introduced in the Rabotnov-Kachanov equations for tertiary creep a model is proposed for remanent creep life prediction based on in situ strain assessment. It is shown that for relatively ductile materials the rupture life can be accurately related to a single strain or strain rate measurement without a knowledge of the rupture strain. Materials data are required only in the form of the minimum creep rate-rupture life product. Considerable evidence suggests that the latter is approximately constant for relatively low stresses in a range of materials so that the specific creep response of the component material is not required. It is also demonstrated that an assessment of strain at more than one stage of the life negates the need for materials data. Consideration is given to the effect of multiaxial stressing and the model is applied to the life prediction of low alloy ferritic steel tubes and pipes.     

Creep crack growth testing of P91 and P22 pipe bends

Laboratory component tests play an important role in the development of life assessment procedures for high temperature crack initiation and growth. Thus, the working programme of the project BE 1702 HIDA, which addressed the validation, expansion and harmonisation of existing procedures for high temperature defect assessment, included a comprehensive experimental programme with feature tests of components as its core. Because of their relevance for the high temperature industry, P91 and P22 were included in HIDA among five materials. This paper presents laboratory creep crack growth tests of P91 and P22 pipe bends, discusses the test experience and draws some conclusions for laboratory component tests in general. The components were prepared with spark-eroded notches at the outer surface. The test temperature was 625°C for P91 and 565°C for P22.     

Experimental creep life assessment of tubular structures with geometrical imperfections in welds with reference to fast reactor plant life

Creep damage is a major life limiting factor for components operating at high temperature. For the fast breeder reactor (FBR), the hot sodium pool components and the steam generator are critical in determining the operating temperatures from structural mechanics considerations. Therefore, creep fatigue damage at various critical locations is estimated based on viscoplastic analyses, in compliance with the high temperature design rules of RCC-MR. In addition, welded joints of steam generator tubes with spigots made of modified 9Cr–1Mo and the welds of fuel pins made of 20% cold worked stainless steel to annealed end plugs, which have certain geometrical imperfections in the welds are also critical and the applicability of existing design rules to these components needs to be investigated carefully. Towards assessing the life of such tubular components realistically, accelerated tests were conducted simulating the creep damage to rupture. The generated test data were investigated by applying the existing creep damage assessment procedure recommended in the design code RCC-MR and the results demonstrated that with the recommended multiaxial creep damage assessment rule, the creep crack initiation lives have been predicted accurately. Further, the extrapolated life predicted by applying an appropriate Larson–Miller parameter indicates that the circumferential welds in the steam generator tubes and fuel pins are not life limiting factors in the design.     

Strength design and life assessment of welded structures subjected to high temperature creep

Premature failures of weldments have taken a large percentage among the failure cases of high temperature components. The accurate prediction of the high temperature creep behavior of welded components is hence becoming increasingly important in order to realize an optimized design and management of a plant life. In the present paper, design and life assessment procedure for welded structures at elevated temperature is discussed. A concept of structural creep-rupture strength is proposed to define the weldment creep strength reduction factor. Multiaxial effect is taken into account. A damage mechanics approach is introduced to study the life reduction and ductility reduction due to the presence of a weld in structures at high temperature.     

Creep and damage analysis of a serviced tee intersection in a boiler header: Comparison between numerical and experimental results

In the last few years modelling of damage phenomena under creep condition has been developed in order to take into account the microstructural material evolution in life predictions of high temperature components. The new analytical methods based on “Continuum Damage mechanics” and experimental creep and creep-rupture data aim at describing both stress-strain and damage field in structures in order to predict crack initiation. These models are implemented in computer finite element programs and should be subjected to rigorous experimental verification for a practical use in power plant assessment activities.

In the present paper the numerical results obtained from some creep and damagement analyses of a header component (10 CrMo 910 steel) are shown and compared with the experimental ones. The creep analyses have been performed by the computer code ABAQUS and the damage evaluation has been carried out by means of proper in-house developed user's subroutine and post processor.     

Creep life assessment by low strain rate tensile testing

A testing method, originally proposed by Rajakovics,⁵ has been investigated for its ability to extrapolate creep rates from the results of short-term tests. Low constant strain rate tensile tests have been performed on a number of high temperature materials in the virgin, service-aged and welded conditions in the temperature range from 525°C to 1050°C. A technique has been developed that permits one to determine the steady-state creep rate under a known stress or the creep stress for a given creep rate. A step towards a quantitative residual life assessment of service-aged materials has been made. The results of the proposed extrapolation technique are compared with results from creep tests.     

Hardness–fracture toughness–creep rupture strength relationship for welds in P91 main steam pipe

Abstract

An experimental study on the impact toughness, hardness, fracture toughness and creep rupture strength of P91 steel weld metals under two conditions (R and D) has been carried out. The results show that weld metal having higher hardness, lower impact toughness and lower fracture toughness (sample R) has higher creep rupture strength. The creep rupture strength is closely related to creep life of component. Thus, assessment of the weld hardness of P91 steel is of value in assessing the creep life of pipes in service. The lower toughness of sample R is attributed mainly to the higher welding current and input line energy used. The lower hardness and strength of sample D, which received a second post-weld heat treatment, partially result from too high a secondary tempering temperature being employed.     

3D creep cavitation characteristics and residual life assessment in high temperature steels: a critical review

Abstract

The need for a new paradigm to estimate remaining creep life of service exposed steels is critically assessed. New approaches to residual life assessment are proposed, in the light of a decade’s experience of the use of micro-tomography to characterise the three-dimensional (3D) nature of cavitation damage in structural materials. Imaging of conventional structural materials such as steels with high absorption to X-rays has been realised by synchrotron micro-tomography (SR-μCT), providing new insights into phenomena such as creep failure. The unique feature of SR-μCT studies is the direct imaging in 3D of cavities (hundreds of micrometres in size) present in the bulk, revealing the spatial characteristics and morphology of the creep voids. Quantitative analyses of the cavitation characteristics revealed by 3D datasets, when scaled with respect to time, stress and temperature, provide functional information suitable for developing constitutive equations for creep. The application of SR-μCT, a non-destructive technique providing high fidelity data, significantly reduces the ambiguity in developing functional relationships to predict creep failure. The explicit use of such constitutive equations to estimate the residual life of components in creep, and the consequent assessment of structural integrity, would prove invaluable. Micro-tomography studies related to creep in materials are reviewed, with special emphasis on a 10·86%Cr heat resistant steel, to demonstrate the type of data available for life assessment and design against creep failure. A brief discussion of current methods to estimate residual life in the light of recent 3D micro-tomography data follows. Finally, the possibility of new approaches, using micro-tomography data in conjunction with destructive 3D approaches such as serial sectioning, to formulate advanced residual life estimates, is briefly considered.     

The exhaustion of creep ductility in 1CrMoV steel

The calculation of creep damage due to secondary (self equilibrating) stresses for the lifetime assessment of critical locations in components subject to creep-fatigue loading at high temperatures may be determined in terms of ductility exhaustion. A new Spindler type stress-modified creep ductility exhaustion model has been developed for 1CrMoV steel in which the rupture ductility is dependent not only on creep strain rate, stress and temperature, but also on a material pedigree function.     

Modelling crack growth for creep and fatigue loading

Estimates of creep crack growth in engineering components under steady load conditions are usually based on the application of fracture mechanics concepts. In particular the creep parameter C* has become widely used together with creep crack growth data obtained from laboratory tests. There are now a number of practical methods to utilise experimental data. For high temperature components, which are subjected to cyclic (fatigue) as well as creep loading, the estimation of the fracture mechanics parameters becomes much more difficult, and consequently the extent to which the growth of pre-existing cracks grow by creep and fatigue is difficult to quantify. In this paper the response of Type 316L stainless steel is examined. This material progressively strain hardens under reversed cyclic loading, and the creep behaviour also changes. Using uniaxial fatigue and creep results, fracture parameter maps are developed to establish the appropriate regimes for creep-fatigue crack growth. Using the maps a model is developed which can predict the combined effect of fatigue and creep on crack growth. The implications of the model are discussed in relation to the limitations of obtaining results from laboratory tests at short times, and the assessment of practical engineering components.     

Recent advance in life prediction for HTGR applications

Key issues in design methods at high temperatures for an HTGR regime are creep constitutive equations. The life in service of structural components is controlled by creep damage. A creep constitutive equation is then needed to calculate inelastic stress-strain components. The method for life prediction, applicable to this temperature regime, has been investigated. The ductility exhaustion rule in conjunction with the creep constitutive equation is confirmed to be useful from the point of view of methodology. Creep-fatigue damage for Hastelloy XRs was assessed by this method in conjunction with the Miner's rule. It is found that the ductility exhaustion for creep damage has a tendency to estimate creep damage larger than the time faction that is often used conventionally. Creep damage under compressive stress should be evaluated at high temperatures.     

Some issues in life assessment of longitudinal seam welds based on creep tests with cross-weld specimens

In order to reduce production costs, it is of great interest to use longitudinal seam welds when manufacturing large diameter pipes. The cost reduction can be as high as 30%. However, severe inservice accidents for this type of pipes working in the creep regime have occurred mainly due to mismatch in weldment creep properties.

In many cases, creep tests of cross-weld specimens, taken from the seam weld, are used to predict the behaviour of the seam weld, assuming that the creep behaviour of specimen and weldment is equivalent. Experiences of this procedure indicate that further knowledge is required before translation between specimen and component can be made.

In the present paper, both full scale seam welded pipes and cross-weld specimens are studied with the damage mechanics concept using finite element, FE, technique. The same mechanical model of multiple material zones is used for the two components. Both the influence of differences in creep properties between the weldment constituents and the size effect of the cross-weld specimen, are studied.

It is found that the cross-weld test results can not directly be translated to the full scale component. Factors such as the creep properties and the relative geometry of the weldment constituents and the size of the cross-weld specimen have to be considered when performing creep life assessment.     

Creep strain growth behaviour of Hastelloy X above 800°C under thermal-stress cyclings

Actual structural components at high temperature are usually not only subjected to steady mechanical and cyclic loads but also accompanied by temperature variation. Creep ratcheting due to cyclic thermal stress is one of the typical examples. The creep strain growth behaviours of Hastelloy X above 800°C under thermal stress cyclings have been discussed. (1) Cycle dependent creep ratchet strain growth can be estimated to be ΔT, where is the thermal expansion coefficient and ΔT is temperature difference across a body. (2) Time dependent strain growth with temperature variation can be derived by time fraction, virtual activation energy and creep strain of the base temperature. (3) Decreasing creep rate will appear in the neighbourhood of 15–20% of rupture time and increasing creep rate will occur after this, which is almost linear on a logarithmic time scale and Hastelloy X shows softening behaviour on creep after exposure at the higher temperature.     

The linear matching method applied to the high temperature life integrity of structures. Part 1. Assessments involving constant residual stress fields

Design and life assessment procedures for high temperatures are based on ‘expert knowledge’ in structural mechanics and materials science, combined with simplified methods of structural analysis. Of these, R5 is one of the most widely used life assessment methods internationally with procedures based on reference stress techniques and shakedown calculations using linear elastic solutions. These have been augmented by full finite element analysis and, recently, the development of a new programming method, the linear matching method (LMM), that allows a range of direct solutions that include shakedown methods and simplified analysis in excess of shakedown. In this paper, LMM procedures are compared with calculations typical of those employed in R5 for cyclic loading problems when the assumption of a constant residual stress field is appropriate including shakedown and limit analyses, creep rupture analysis and the evaluation of accumulated creep deformation. A typical example of a 3D holed plate subjected to a cyclic thermal load and a constant mechanical load is assessed in detail. These comparisons demonstrate the significant advantages of linear matching methods for a typical case. For a range of cyclic problems when the residual stress field varies during the cycle, which include the evaluation of plastic strain amplitude, ratchet limit and accumulated creep strains during a high temperature dwell periods, the corresponding LMM and R5 procedures are discussed in an accompanying paper.     

The use of damage concepts in component life assessment

The damage concepts introduced by Kachanov several decades ago have rarely been used in practical application, in spite of their simplicity. Because there are no alternatives of note to Kachanov's ideas, this paper re-examines the basic premises involved in damage concepts, with a view to deriving results which will be credible to those responsible for component life management. This has led to possible schemes for economical data acquisition as well as to simplified life assessment procedures which are described in the paper.

For metallurgically stable materials, long term creep strain and rupture data can be described simply in terms of two material constants and the results from short term tests for initial creep rates and strength. These data are all bounded by a single term β which depends on the stress level in relation to the short term strength. Examples are given for stiffness and strength reductions caused by damage. A simple relationship between current strain rate and remaining life should be of use in routine plant measurements.     

Introduction of SCRI model for creep rupture life assessment

A “service condition-creep rupture property” interference model (SCRI model for short) based on Z parameter method is proposed to predict the reliability of creep rupture life for high temperature components. The dispersibility of creep rupture property and the fluctuation of service conditions such as servicing temperature and stress are considered in this model. The scattering of creep rupture data is generally supported by normal distribution. Monte-Carlo method is applied to evaluate the reliability at a certain operating condition by simulating the fluctuation of service temperature and stress. HK40 steel has been used as an example to demonstrate the influence of creep rupture property dispersibility and operating condition fluctuation on the result of reliability evaluation.     

Long-term evolution of mechanical performance of solid oxide fuel cell stack and the underlying mechanism

Mechanical performance analysis is important for ensuring the long-term reliability of solid oxide fuel cells (SOFCs). Thermal-mechanical models are constructed to conduct time-dependent mechanical performance analysis of SOFC stack with temperature field obtained by multiphysics modeling. The volume-averaged temperature field is used as comparison. The creep strains are examined with a time step of 10 h for 10,000 h. It reveals: (1) Uniform temperature significantly decreases the stresses, strains, failure probabilities of all stack components. (2) The failure probability of sealant reduced rapidly and the sealant becomes mechanically safer for long-term operation. (3) Creep strain is dominant for anode/sealant/interconnect, but negligible for electrolyte/cathode. All components are predictably safe against strain failure for 100,000 h (4) Creep strains of stack components interact with each other. Coupled analysis of creep strains of anode/sealant/interconnect is mandatory, but the creep strains of electrolyte/cathode may be neglected for studying mechanical evolutions.