316L不锈钢室温和高温单轴循环行为实验研究

对316L不锈钢分别进行了室温、高温单轴应变控制和应力控制下的系统循环试验,揭示和分析了循环应变幅值、为幅值历史以及温度历史对应变循环特性的影响以及应力幅值、平均应力和温度及其历史对循环棘轮行为的影响。研究表明,无论是单轴应变循环特性是还是非对称单轴应力下的棘轮效应不仅依赖于当前温度和加载状态,而且还于温度和加载的历史,并得到了关于316L不锈钢室温和高温单轴循环行为的若干有意义的结果。     

SS304不锈钢室温非比例多轴时相关棘轮行为实验研究

在室温下,对SS304不锈钢非比例多轴循环加载下的时相关棘轮行为进行了实验研究.揭示了材料在不同加载速率、不同保持时间以及不同加载路径下的棘轮变形特性.结果显示:即使在室温下,棘轮变形也有明显的时相关效应,其棘轮行为不仅强烈依赖于加载速率而且还明显依赖于保持时间;此外,材料的棘轮行为还对加载路径有强烈的依赖性.     

304不锈钢的单轴棘轮变形与失效行为研究

对304不锈钢在室温下进行了单轴应变控制下的应变棘轮变形与失效以及低周疲劳试验研究,系统地揭示了材料在循环过程中的材料变形与失效行为。研究表明：材料的应变棘轮变形与失效既不同于单轴拉伸,也不同于相同应变幅值下的对称应变循环加载时的变形与失效,而是强烈地依赖于应变幅值与每一循环周次在最大拉应变处的应变增加量。观察到了一些有意义的结果,     

304不锈钢在单轴应变棘轮变形下的随动硬化实验研究

对304不锈钢在室温下进行了单轴应变控制下的应变棘轮变形与失效以及低周疲劳试验研究,对材料在循环过程中材料的硬化行为进行了系统的揭示。在对称应变循环下,研究了不同应变幅值下弹性区尺寸和背应力演化规律;在给定工程应变幅值和循环棘轮应变增量组合的应变棘轮变形下进行了弹性区尺寸和背应力演化研究。观察到了各向同性硬化和随动硬化演化对加载历史的依赖性。     

Life prediction techniques for combined creep and fatigue

This paper contains a review of the techniques that are currently available for predicting the life of metallic materials when they fail at high temperatures under the combined action of creep and fatigue. The work concentrates on those theories that are relevant to engineering design without specifically employing fracture mechanics concepts. As a consequence this generally limits the prerequisite data to that of low cycle fatigue under high rate strain cycling conditions and monotonic creep at a given temperature. Some techniques require additional or alternative data on tensile ductility, high cycle fatigue, cyclic plasticity and creep.Each technique is appraised from the results of laboratory experiments. It is shown that life may be predicted quite reliably in one instance but not in another. Some attempt is made to provide explanations and recommendations where these anomalies occur. The range of application is also clarified. Many approaches are limited to strain controlled cycling under uniaxial stress conditions. Only a limited number may further be applied to predict life for cycles in which ratcheting occurs. At present it appears that very few techniques are available to predict the creep-fatigue life of materials under high temperature multiaxial stress states. under high temperature multiaxial stress states.     

Dynamic strain aging in stress controlled creep-fatigue tests of 316L stainless steel under different loading conditions

Stress controlled fatigue-creep tests were carried out for 316L stainless steel under different loading conditions, i.e. different loading levels at the fixed temperature (loading condition 1, LC1) and different temperatures at the fixed loading level (loading condition 2, LC2). Cyclic deformation behaviors were investigated with respect to the evolutions of strain amplitude and mean strain. Abrupt mean strain jumps were found during cyclic deformation, which was in response to the dynamic strain aging effect. Moreover, as to LC1, when the minimum stress is negative at 550 °C, abrupt mean strain jumps occur at the early stage of cyclic deformation and there are many jumps during the whole process. While the minimum stress is positive, mean strain only jumps once at the end of deformation. Similar results were also found in LC2, when the loading level is fixed at −100 to 385 MPa, at higher temperatures (560, 575 °C), abrupt mean strain jumps occur at the early stage of cyclic deformation and there are many jumps during the whole process. While at lower temperature (540 °C), mean strain only jumps once at the end of deformation.     

Thermal ratcheting under biaxial stress states

Thermal ratcheting, the phenomenon of net strain accumulation due to plastic straining of pressure vessels under cyclic thermal loading, is an important factor in the design of nuclear reactor pressure vessels. In this paper a cyclic thermal gradient loading idealized by means of a step distribution through the pressure vessel wall and biaxial thermoelastic-plastic stress-strain relations are used to calculate the strain accumulation under biaxial mechanical loads. It it shown that the uniaxial stress-strain models presently used in design calculations are adequate for spherical and cylindrical shells under internal pressure; however, for uniaxial loading and for torsional loading, the use of these models may underestimate the accumulated ratchet strain.     

The viscoplasticity theory applied to the inelastic analysis at elevated temperatures

Monotonic and cyclic loading tests on strain rate changes are conducted on — 1Mo steel at 600°C. The examination of the tensile stress-strain response suggests that the viscosity function which characterizes the rate-dependency in the viscosity theory used previously at room temperature should not only depend on the overstress but also on the strain. A new extended viscosity control function is introduced to represent such strain dependency.The material constants of this modified viscoplasticity model are determined at temperatures of 25°C to 600°C and the model is applied to deformation tests on — 1Mo steel carried out under time-varying temperature conditions and other conditions.The extended viscoplasticity theory is shown to reproduce such various experimentally observed stress-strain behavior at elevated temperatures.     

不锈钢316L单轴棘轮行为及其影响因素的研究

对不锈钢３１６Ｌ材料进行了多种平均应力以及多种应力幅值下的单轴循环加载棘轮效应试验，分析研究了平均应力大小及其历史对其棘轮效应的影响，以及应力值大小及其历史对其棘轮效应的影响，揭示了不锈钢３１６Ｌ的单轴棘轮效应演化的特征及导致这些特征的原因，得到一些有意义的结果，对建立能较为精确地估计结构的循环累积变形的本构模型具有一定的参考作用。     

Safety concept for an HTR liner design for combined stress and strain-controlled load

The liner of a high-temperature reactor is strained by the concrete deformation and the temperature difference between the liner and the concrete. This is a strain-controlled compression load. At some points of the liner loads from closures or components must be transfered through the liner into the concrete. These loads originate from internal pressure, gravity, earthquake etc. and are stress-controlled loads. At these points the liner is loaded by a combination of strain and stress-controlled loads.For the liner dimensioning there are regulations for strain or stress controlled loads, which could not be used in an acceptable way for a combination of strain and stress-controlled load.In this contribution a concept for the dimensioning of liner parts exposed to combined strain and stress-controlled loads is presented. This concept is based on the safety theory. It is practicable with only little additional computation effort.     

室温下304不锈钢的单轴循环棘轮行为实验研究

在室温下对固溶处理的304不锈钢进行了系统单轴应变与应力循环实验研究了具有平均应力的工程应力幅值历史以及相同工程应力幅值下平均应力历史、每一循环平均应力有一固定应力增量的应力幅值历史对单轴棘轮变形行为的影响：得到了一些有意义的结果．．     

Characteristics of microscopic strain localization in irradiated 316 stainless steels and pure vanadium

Characteristics of localized dislocation glide were investigated for 316 and 316LN stainless steels and pure vanadium after ion or neutron irradiation near room temperature and deformation by a uniaxial tensile load or by a multiaxial bending load. In the irradiated 316 stainless steels, both the uniaxial tensile loading and the multiaxial bend loading produced straight localized bands in the form of channels and twins. In vanadium specimens, on the other hand, curved channels were observed after tensile deformation, and these became a common feature after multiaxial bend deformation. No twin was observed in vanadium. A river pattern of channels was observed in the bent samples after irradiation to a high dose of 0.69 dpa. A highly curved channel can be formed by successive cross slip of screw dislocations. Also, the channel width was not constant along the channels; channel widening occurred when weak defect clusters were removed by the gliding screw dislocations changing their paths by cross slip. It is believed that the dissociation of dislocations into partials and high angles between easy glide planes suppresses the formation of curved channels, while a multiaxial stress state, or a higher stress constraint, increases the tendency for channel bending and widening.     

SS304不锈钢单轴时相关棘轮行为的本构描述

在SS304不锈钢室温和高温单轴时相关棘轮行为实验研究的基础上,基于Abdel-Karim-Ohno非线性随动硬化律,发展了一个新的统一粘-塑性本构模型.由于该模型考虑了具有应力保持时间和高温下不可忽略的静力恢复或热恢复效应,通过与实验结果的比较发现,新发展的统一粘-塑性本构模型对室温和高温下的时相关棘轮行为均能给予合理的预测结果.     

基于一元应力参量的钛合金T225NG单轴棘轮演化模型研究

基于T225NG钛合金的单轴常温棘轮试验，本文研究了峰值棘轮应力对T225NG钛合金棘轮应变的影响。试验结果表明，常温单轴应力循环条件下材料棘轮应变随峰值棘轮应力的变化关系为一簇类线性的变化曲线，存在常值棘轮应力门槛值，峰值棘轮应力与该门槛值的关系可决定材料是否产生棘轮变形。由此，建立了一套一元应力能量控制的T225NG钛合金棘轮应变演化模型。该模型建模容易，适合棘轮应变预测的工程应用。     

Inelastic behaviour of steel under plasticity-creep interaction condition: An interim report of the Bench Mark project by the subcommittee on inelastic analysis and life prediction of high temperature materials, JSMS

Some of the interim results of the Bench Mark Project by the Subcommittee on the Inelastic Analysis and Life Prediction of High Temperature Materials, JSMS, is presented. The purpose of the present bench mark study is to review and evaluate the inelastic constitutive models relevant to material response under the plasticity-creep interaction.By specifying normalized and tempered steel at 600°C, sixteen bench mark problems of four categories are first established: (I) tensile stress-strain relations and creep curves, (II) material response under mixed modes of plastic and creep loading, (III) ratcheting and deformation under program loads, and (IV) cyclic deformation behaviour under the combination of different strain rates. Then, the outline of seventeen inelastic constitutive models of nine types discussed in this project is presented. Finally, the interim results of these bench mark tests are compared with the corresponding predictions of the constitutive models to evaluate their accuracy in simulating the actual behaviour of the material.     

High temperature deformation and damage behavior of RAFM steels under low cycle fatigue loading: Experiments and modeling

Structural materials of fusion reactors are subjected to complex creep-fatigue loading and high irradiation doses. Correct modeling of their deterioration is a precondition of a sufficiently reliable lifetime prediction procedure. In the continuum mechanics approach selected for lifetime prediction of RAFM steels under creep-fatigue conditions, the inelastic strain rate modified (ISRM) damage model is coupled with a modified viscoplastic deformation model taking into account the complex non-saturating cyclic softening of RAFM steels. The resulting coupled model is a powerful prediction tool, which can be applied to arbitrary creep-fatigue loading provided that the material, temperature and possibly irradiation dose-dependent parameters of the model have been determined. Therefore, a fitting procedure has been developed for the parameters identification on the base of deformation and lifetime data from strain-controlled low cycle fatigue (LCF) tests without and with hold time as well as creep tests. The coupled deformation-damage model has been applied to F82H mod and EUROFER 97 in the reference (unirradiated) state under isothermal cyclic loading at 450, 550 and 650 °C. The comparisons between model and experiment show that the observed lifetimes in the LCF tests could be fairly well calculated even for the tests with hold time, which were not considered for the identification of the damage model parameters.     

Constitutive laws of materials in dynamics—outline of a programme of testing on small and large specimens for containment of extreme dynamic loading conditions

A sound foundation of stress and/or strain criteria for the mechanical design of fast breeder reactor structures capable of bearing extreme dynamic loading conditions, passes through the experimental determination of dynamic mechanical properties of materials in end-of-life conditions with respect to the damaging processes to which the structures are submitted.Calculation codes must be implemented by constitutive equations describing the dynamic mechanical response of the materials, without the knowledge of which any calculation code is liable to important inaccuracies which provoke the use of high safety coefficients and, often, the uncertainty as to the effective capability of the structures to withstand the accidental loads.The results of a screening programme of dynamic tensile tests performed on AISI 304 and AISI 316 austenitic stainless steels using small specimens (7 mm²) showed that the dynamic response of such steels at temperatures of 400 and 550°C is not univocal, passing from a substantial dynamic hardening behaviour to a dynamic softening behaviour, probably due to residual microstructural differences caused by the transformation processes.From the discussion of the results obtained, follows the development of a testing programme on small (up to 20 mm² cross section) and large specimens (up to 5000 mm² cross section) focused on: (i) developing and calibrating dynamic constitutive equations founded on basic physical aspects, for uniaxial and multiaxial loading conditions under particular deformation and loading histories; (ii) verifying in dynamics the existing yielding criteria, hardening rules and failure theories; (iii) investigating by means of a high-load apparatus (5 MN) the influence of near real-size thickness, welding, defects and notches, on dynamic strength and deformation capability of large reactor structures.     

应力循环下T225NG合金塑性累积行为研究

对应力循环下T225NG合金的塑性累积行为进行了试验研究,提出了预测棘轮饱和应变的本构关系及描述棘轮应变演化规律的指数型演化方程。讨论了蠕变效应对T225NG合金棘轮行为的影响．结果表明．应力幅越低,循环蠕变分量在塑性累积中的贡献越大。     

Elastic–plastic analysis of the steel-to-tungsten transition joint for a high performance divertor

The use of tungsten as a plasma-facing material necessitates a transition joint to the oxide dispersion strengthened (ODS) steel or ferritic steel (FS) structural material of the primary coolant loop at the end of the divertor target plate where the surface heat flux is very low. A critical issue in the transition joints is the coefficient of thermal expansion (CTE) mismatch between the tungsten (or tungsten-alloy) and ODS steel, which can lead to unacceptably high thermal stresses during steady state and ratcheting during cyclic loads. Detailed 2D and 3D thermo-mechanical analyses were conducted to study the behavior of a transition from tungsten to FS with an intermediate layer of tantalum, located outside of the high heat flux region. The results include plastic strains under various loading conditions including fabrication processes, warm and cold shutdown, and allow for plastic behaviors leading to stress relaxation. The accumulation of plastic deformation may cause ratcheting. Modifications were proposed to the transition joint design in order to eliminate stress concentration and ratcheting under cyclic loading. The results of the modified design exhibited less plastic deformation in the joints as well as no ratcheting caused by warm and cold shutdown.