Development of inelastic constitutive model for austenitic stainless steel for design use

To prevent creep-fatigue failure or excessive deformation in high-temperature components of fast reactor plants, accurate estimation of inelastic deformation is essential. In performing inelastic analysis, employment of constitutive models, which can precisely reproduce inelastic deformation of the material is of critical importance. The authors have been engaged in the development of inelastic constitutive model for the use in structural design assessment of liquid metal-cooled fast reactor plants. Various improvements were made on the nonlinear hardening model proposed by Ohno and Wang, placing an emphasis on capability to simulate inelastic deformation behavior of austenitic stainless steels, under regular or irregular cyclic loading possibly with temperature variation and hold time. It was demonstrated that the model can simulate the inelastic deformation behavior under various loading conditions with a sufficient accuracy.     

Description of creep-plasticity interaction with non-unified constitutive equations: application to an austenitic stainless steel

We present constitutive equations able to account for time independent plasticity together with creep and creep-plasticity interaction. A classical decomposition of the inelastic strain into a time independent plastic strain and a time dependent viscoplastic part is assumed. The coupling between both deformation modes (i.e. creep and plasticity) is obtained through an interaction between the plastic and viscoplastic state variables. In a first part, the capabilities of the model are described, and qualitative identifications are given in order to characterize the behaviour of the model. The practical applicability of the model is then tested, mainly using test results from the literature, but also specific data including creep, relaxation and tensile tests with various loading rates, as reported in the paper. The model is found able to discriminate between the increase of hardening produced by plasticity or creep. The effect of the loading rate on the subsequent amount of relaxation is correctly described and a good general agreement is observed between experiment and model predictions, even for complex loading paths (monotonic with temporary unloading periods, multiaxial loading paths in the stress space).     

Determination of internal state variables and constitutive modeling for Type 316 stainless steel

The purpose of this study is to develop an approach for unified constitutive modeling based on experimentally determined back stress and overstress. Back stress and overstress were experimentally determined for Type 316 stainless steel at 600°C, by analyzing the unloading curve for the stress-strain response of cyclic strain tests. The result has indicated that the cyclic strain hardening behavior is mainly caused by hardening in the back stress. A phenomenological unified constitutive model in which the back stress and drag stress are taken as the internal state variables is proposed, and has been shown that this model is able to simulate the cyclic inelastic behavior.     

Development of constitutive equations for nuclear reactor core materials

A set of strain rate dependent constitutive equations has been described which is capable of predicting deformation behavior of anisotropic metals under complex loading conditions with or without the presence of a neutron flux. The important feature of the constitutive equations is that they describe history dependent plastic deformation behavior of anisotropic metals under three-dimensional stress states. Since the analytical model accounts for the effect of prior deformation history at all times, it is capable of handling consecutive or simultaneous loading histories, such as post-irradiation loading, in-pile loading, etc. It is demonstrated that the general form of the constitutive relations is consistent with experimental observations made for Zircaloys under both unirradiated and irradiated conditions.     

Systematic development of thermomechanical constitutive equations for inelastic analysis

In this paper some general concepts are outlined which are known from the literature and which may facilitate a systematic development of thermomechanical constitutive equations representing inelastic material properties. However, these concepts are as yet not sufficiently specialized in view of their application to inelastic analysis problems: In order to fill the gap between theoretical concepts and practical applications, further investigations are necessary.     

浅析不锈钢覆面的先贴法施工

为减少不锈钢覆面的现场施工工作量,有效地缩短整个工程项目的建造周期,不锈钢覆面施工采用分片分段在车间进行预制,现场进行拼装的施工方法,有效地缩短了不锈钢覆面在整个土建工程施工关键路径中的必需作业时间,加快了工程项目关键路径的建造进度,不锈钢覆面的先贴法施工可进一步推广。     

On establishing constitutive equations for use in design of high-temperature fast-reactor structures

The presentation describes the approach being used to establish constitutive equations for wide use in the design of fast breeder reactor (FBR) components in the US. The approach combines exploratory experiments, constitutive model studies, studies of computational techniques, and tests of simple structural configurations. Short-time (elastic-plastic) behavior, long-time (creep) behavior, and their interactions are considered, and some of the background to equations now identified for use in current FBR design applications involving three structural alloys is discussed. Comments are also given on current efforts aimed at identifying improved constitutive equations for these alloys and on properties data required for design applications. References are cited which have addressed the status of the process at various times.     

Thermo-mechanical constitutive equations for the description of shape memory effects in alloys

A class of alloys show ‘shape memory effects’ which make them applicable for many tasks. For them it is possible to remove imposed deformations nearly entirely by heating. By cooling the material again under constant loads one nearly obtains the old deformations again. It will be shown that this effect as well as others can be described in three dimensions by means of an extended classical theory of plasticity. Two temperature-dependent yield criteria are used which respond under different conditions. For the one-dimensional case the constitutive equations can be simulated by a rheological model. An algorithm makes the material functions applicable for engineering purposes. Numerical results are given for spacial bending of bars.     

Development of , a seismic analysis program for FBR core components

A computer program, (seismic analysis program for fuel assemblies) has been developed to analyze core component vibration in fast breeder reactors (FBRs) during seismic excitation. Since an FBR core is composed of as many as 1000 core subassemblies (fuel assemblies, blanket assemblies, neutron shield assemblies, etc.), which are immersed in a coolant fluid, seismic analysis of FBR cores must consider the vibrations generated in a system with a large number of degrees of freedom with impacts under fluid-structure interactions. models subassemblies as finite beam elements. Fluid interaction forces are considered as added mass and time integration is done using mode superposition and the Nigam method. The load pad impact is modeled using a gap, a linear spring and a linear damper. The program also uses a new method to determine the nonlinear impact force, making it unnecessary to use convergence iteration. Comparison with experimental results confirms that the program can closely predict the seismic response of FBR cores.     

Thermophysical properties of mixed oxide fuel and stainless steel type 316 for use in transition phase analysis

For use in transition phase analysis, we have obtained thermodynamic and transport properties of mixed oxide fuel and stainless steel type 316, over a very wide range of temperatures. The majority of the thermodynamic properties for these materials in their solid and liquid states are based on either actual experimental data or extrapolation of low temperature data. However, a number of properties for these materials in the vapor state are based on very limited and scarce data. These properties include saturation vapor pressure, latent heat of vaporization and specific volume of saturated vapor. Due to complete lack of experimental data, a number of other properties such as specific heats, heat of vaporization (especially for stainless steel), transport properties, and the properties at the critical state had to be estimated by use of generally reliable empirical relations. It is recommended that efforts be initiated to obtain experimental data for these materials in the vapor state to verify both existing empirical relations and the relationships that are obtained here based on theoretical analyses.     

Irradiation creep of stainless steel in bending

The development is described of a test to measure irradiation enhanced creep in bending of 20% cold-worked Type-316 stainless steel. The test will be irradiated in the experimental fast reactor EBR-II. The rationale used in design selection is described. The selected beam designs, the supportive tests in other stress states and the measurement techniques are described in detail.     

Critical helium fluence for hydrogen trapping in pre-damaged stainless steel

Hydrogen depth profiles have been measured for stainless steel pre-implanted with 10 keV helium ions. An abrupt change in hydrogen retention has been observed in the helium fluence region between 2 × 10¹⁷ and 3 × 10¹⁷cm⁻². A marked difference in decay of hydrogen retention has also been found in the same region of helium fluence. The existence of helium critical fluence has been confirmed in the effect of helium pre-implantation to the hydrogen retention.     

Development of guidelines for inelastic analysis in design of fast reactor components

The interim guidelines for the application of inelastic analysis to design of fast reactor components were developed. These guidelines are referred from “Elevated Temperature Structural Design Guide for Commercialized Fast Reactor (FDS)”. The basic policies of the guidelines are more rational predictions compared with elastic analysis approach and a guarantee of conservative results for design conditions. The guidelines recommend two kinds of constitutive equations to estimate strains conservatively. They also provide the methods for modeling load histories and estimating fatigue and creep damage based on the results of inelastic analysis. The guidelines were applied to typical design examples and their results were summarized as exemplars to support users.     

纯化氢用多孔不锈钢表面的修饰技术

采用浆料涂敷法,将纳米不锈钢粉末沉积在多孔不锈钢载体上,形成孔径依次减小的微孔不锈钢载体,其孔径约为200nm.在压差为0.1MPa时,其干空气渗透率约为200cm3·cm-2·min-1.再经溶胶-凝胶涂敷法,在此载体上沉积TiO2薄膜,其孔径约为100nm,此时的干空气渗透率,在压差为0.1MPa时,约为100cm3·cm-2·min-1.最后经化学镀技术,得到表面较光洁、合金化较完全的钯银合金膜,该膜在压差为0.1MPa、温度为300℃时的渗氢速率在标准状态下达35cm3·cm-2·min-1,氢氦分离因子达500.     

Material equations for steel fibre reinforced concrete members

The application of steel fibre reinforced concrete is a supplement to existing methods used in concrete constructions. In combination with conventional or prestressed reinforcement, structural members made of steel fibre reinforced concrete display high load bearing capacity, reliability and durability, making them well suited to withstand high dynamic forces (impact, vibration). Steel fibre reinforced concrete is an expensive material. Its use can only prove economical if its favourable properties are taken into account in design. In this contribution suitable material equations and results of selected tests and presented.     

Oxidation of 304 stainless steel in high-temperature steam

An experiment on oxidation of 304 stainless steel was performed in steam between 900°C and 1350°C, using the spare cladding of the reactor of the nuclear-powered ship Mutsu. The temperature range was appropriate for a postulated loss of coolant accident (LOCA) analysis of a LWR. The oxidation kinetics were found to obey the parabolic law during the first period of 8 min. After the first period, the parabolic reaction rate constant decreased in the case of heating temperatures between 1100°C and 1250°C. At 1250°C, especially, a marked decrease was observed in the oxide scale-forming kinetics when the surface treated initially by mechanical polishing and given a residual stress. This enhanced oxidation resistance was attributed to the presence of a chromium-enriched layer which was detected by use of an X-ray microanalyzer. The oxidation kinetics equation obtained for the first 8 min is applicable to the model calculation of a hypothetical LOCA in a LWR, employing 304 stainless steel cladding.     

Reflection of hydrogen from stainless steel and Nb

Polycrystalline targets of stainless steel and Nb have been bombarded at normal incidence by protons and deuterons with energies between 1.0 and 15 keV. The energy distributions of neutral, positive, and negative particles backscattered at 135° have been measured. For bombardment energies above 2.5 keV the spectra of all three components of the backscattered particles and hence their sum show a maximum at low energies. By integrating the spectra absolute particle and energy reflection coefficients were determined. For stainless steel they agree with theoretical expectations while for Nb they are considerably lower.     

Development of CrN precipitates during the initial stages of PIII nitriding of stainless steel thin films

Stainless steel thin films produced by ion beam sputtering (IBS) were used as a model system to investigate the nitrogen diffusion and CrN formation after 10 min of nitrogen plasma immersion ion implantation (PIII) at 350 °C and 450 °C. At 350 °C, a thin nitrided layer of 70 nm is formed, with additional diffusion of nitrogen along grain boundaries and the growth of CrN precipitates along these grain boundaries. For 450 °C, a complete nitriding of the whole 400 nm thick layer was observed, with the lower 75 nm consisting of an expanded phase and the upper 330 nm of a decomposed phase with CrN precipitates formed inside the original grains. Such a layered structure capturing the transformation process has not been observed before. A determination of time–temperature dependencies of this process and the transfer of these results for bulk material should be possible.