A knowledge based system for creep–fatigue assessment

A knowledge based system was developed in the BRITE-EURAM C-FAT project to store the material property information necessary to perform complex creep–fatigue assessments and to thereby improve the effectiveness of data retrieval for such purposes. The C-FAT KBS incorporates a multi-level database which is structured to contain not only ‘reduced’ deformation and fracture test data, but also to enable ready access to the derived parameter constants for the constitutive and model equations used in a range of assessment procedures. The data management scheme is reviewed. The C-FAT KBS also has a dynamic worked example module which allows the sensitivity of predicted lifetimes to material property input data to be evaluated by a number of procedures. Complex cycle creep–fatigue endurance predictions are particularly sensitive to the creep property data used in assessment, and this is demonstrated with reference to the results of a number of large single edge notched bend specimen feature tests performed on a 1CrMoV turbine casting steel at 550°C.     

CEA and JNC approaches for creep–fatigue evaluation of weldments and inter-comparison through benchmark analyses

Creep–fatigue is an important failure mode in elevated temperature components especially for weldments. To establish rational design-by-analysis methods for weldments, the CEA and JNC have developed creep–fatigue evaluation methods. Both organizations have compared data and ideas through two kinds of benchmark analyses. The CEA benchmark is fatigue and creep–fatigue evaluation of welded plates under reverse bending at 550 °C. The JNC benchmark is creep–fatigue evaluation of a welded vessel under cyclic thermal transients. The main differences in the methods are the strain concentration factors for base metal fatigue strength reduction factors of weldments. To evaluate inferior performance of weldments to base metal, the CEA pays attention to material strength reduction in the weld metal, while the JNC examines strain redistribution between the base and weld metals. Both the CEA and JNC calculations gave good results for welded plates. Both methods obtained conservative results for a welded vessel because evaluation methods for base metal are conservative.     

Ratcheting deformation of advanced 316 steel under creep–plasticity condition

A series of mechanical ratcheting tests under tension–torsion biaxial conditions has been conducted with an advanced 316 stainless steel at 923 K. Accumulation of torsional ratcheting strain was measured with a cyclic axial strain ranges of 0.005–0.02, cyclic axial strain rate of 10⁻⁵ to 10⁻³ s⁻¹ and steady torsional stresses of 24.5/ to 73.5/ MPa. The accumulation of ratcheting shear strain is mainly affected by the cyclic axial strain range and the steady shear stress, and increases with an increase of both these parameters. A simple evaluation of the accumulation of ratchet strain is proposed. Although this procedure is based on the separation of creep and plasticity and uses experimental data to skip the plasticity analysis, the obtained results show good agreement with the experimental results.     

Weld metal creep–fatigue life prediction by modeling the microstructure degradation due to the exposure to high temperature and load

A new analytical method to evaluate creep–fatigue strength of stainless weld metals that were suffering from microstructure degradation was proposed. Based on the observation that creep–fatigue crack initiated adjacent to the interfaces of σ and δ-ferrite, an FE-model that consisted of matrix, σ and δ-ferrite was developed. The volume fraction of the σ in the model corresponded to the maximum amount of precipitation expected, which means that the model represents the degraded microstructure after long-term exposure to high temperature and load. Using the model, microscopic concentration of stress and strain adjacent to the interfaces were calculated. Fatigue and creep damage were consequently evaluated which allowed creep–fatigue life evaluation. The predicted results reproduced experimental results with sufficient accuracy in a relatively higher strain region. Validation in lower strain region is expected.     

Multiaxial fatigue analysis for IMIC of ITER upper ELM coil

Inconel Jacketed Mineral Insulated Conductor (IMIC) is a very important component of International Thermonuclear Experimental Reactor (ITER) Edge Localized Modes (ELM) coils, which are located between the vacuum vessel (VV) and blanket shield modules and subject to high radiation levels, high temperature and high magnetic field. These coils will experience thermal pulsed, cyclic electromagnetic (EM) load during operation. They are designed to sustain at 1.5e8 total stress cycles and shall have sufficient strength and excellent fatigue to transport and bear the high cyclic load. For IMIC, multiaxial fatigue analysis is used to evaluate failure. Two methods based on the alternating stress and mean stress in American Society of Mechanical Engineers (ASME) code provide the design codes for multiaxial fatigue evaluation: constant principal stress direction and variation of principal stress direction. Results show that using the two methods obtains basically the same equivalent alternating stress. Both of them can be recommended for the ELM coils and IMIC can meet the fatigue criteria.     

Multiaxial creep-fatigue damage

This paper presents an alternative method for extending the various methods of creep-fatigue damage assessment from the uniaxial to general multiaxial regimes. Specifically, the method gives a procedure for determining a reasonable strain range in multiaxial situations where loading may not be proportional. In determining this strain range, factors which influence crack initiation and growth, such as the plane of maximum sheer strain range and the strain normal to that plane, are considered.     

Control of welding residual stress for ensuring integrity against fatigue and stress–corrosion cracking

The availability of several techniques for residual stress control is discussed in this paper. The effectiveness of these techniques in protecting from fatigue and stress–corrosion cracking is verified by numerical analysis and actual experiment. In-process control during welding for residual stress reduction is easier to apply than using post-weld treatment. As an example, control of the welding pass sequence for multi-pass welding is applied to cruciform joints and butt-joints with an X-shaped groove. However, residual stress improvement is confirmed for post-weld processes. Water jet peening is useful for obtaining a compressive residual stress on the surface, and the tolerance against both fatigue and stress–corrosion cracking is verified. Because cladding with a corrosion-resistant material is also effective for preventing stress–corrosion cracking from a metallurgical perspective, the residual stress at the interface of the base metal is carefully considered. The residual stress of the base metal near the clad edge is confirmed to be within the tolerance of crack generation. Controlling methods both during and after welding processes are found to be effective for ensuring the integrity of welded components.     

Effects of thermal aging on the low cycle fatigue behavior of austenitic–ferritic duplex cast stainless steel

Thermal aging is observed in a primary reactor cooling system (RCS) made of cast stainless steel. The observation occurs when the RCS is exposed for a long period of time to a reactor operating temperature between 290 and 330°C. An investigation of the effect of thermal aging on the low cycle fatigue characteristics of CF8M is required. The purpose of the present investigation is to find the effect of thermal aging of CF8M on a low cycle fatigue life. The specimen of CF8M is prepared by an artificially accelerated aging technique maintained for 300 and 1800 h at 430°C, respectively. The low cycle fatigue tests for the virgin and two aged specimens are performed at room temperature for the strain amplitudes (_ta) of 0.3, 0.5, 0.8, 1.0, 1.2 and 1.5%. In the experiment, it is found that the fatigue life is rapidly reduces with an increase of aging time. The experimental fatigue life estimation formula, between the virgin and two aged specimens are obtained.     

Multiaxial loading tests of high temperature reactor components

This paper provides background information on the theory and tests to develop stress and strain analysis procedures employed in the high temperature component design of the helium cooled reactor prototype plant THTR.     

Phase transition temperature in the Zr-rich corner of Zr–Nb–Sn–Fe alloys

The influence of small composition changes on the phase transformation temperature of Zr–1Nb–1Sn–0.2(0.7)Fe alloys was studied in the present work, by electrical resistivity measurements and metallographic techniques. For the alloy with 0.2 at.% Fe we have determined T_↔+β=741°C and T_+β↔β=973°C, and for the 0.7 at.% Fe the transformation temperatures were T_↔+β=712°C and T_+β↔β=961°C. We have verified that the addition of Sn stabilized the β phase.     

Boiling heat transfer behavior of lead–bismuth–steam–water direct contact two-phase flow

The boiling heat transfer behavior of lead–bismuth (Pb–Bi)–steam–water direct contact two-phase flow was experimentally investigated. Experimental study was performed using Pb–Bi–steam–water direct contact boiling two-phase flow loop. The heat transfer rate was estimated from data of one-dimensional flow direct contact boiling of water in Pb–Bi. It is assumed in the analysis that film boiling occurs at the surfaces of a small water droplet after water is injected into hot Pb–Bi flow, because of the large temperature difference between water and Pb–Bi (i.e. 493 K and 733 K for injection water and Pb–Bi temperature, respectively). The heat transfer occurs between Pb–Bi and steam without phase change after all water completely evaporates. The overall heat transfer coefficient decreased with the superheat at low injection flow rate and was nearly constant for high injection flow rate. The local heat transfer coefficient was higher than average one in the whole tube, which means that the direct contact boiling heat transfer coefficient was high and it decreased in the downstream direction. Almost all the water vaporized in the test tube at high pressure according to the local and total heat transfer rate.     

Thermodynamic evaluation of the Nb–O–Zr system

Thermodynamic properties of the ternary system Nb–O–Zr have been evaluated by means of the CALPHAD (CALculation of PHAse Diagrams) method. The pertinent experimental data are surveyed and the thermodynamic models based on the previous assessments of the binary systems Nb–O, Nb–Zr and O–Zr are delineated. The results of our computations indicate that the models describe the zirconium rich portion of the ternary phase diagram satisfactorily, however, in the niobium rich part, the calculations differ from the experimental data and should be verified by new experiments.     

Distillation of cadmium from uranium–plutonium–cadmium alloy

Uranium–plutonium alloy was prepared by distillation of cadmium from U–Pu–Cd ternary alloy. The initial ternary alloy contained 2.9 wt% U and 8.7 wt% Pu other than Cd, which were recovered by molten salt electrolysis with liquid Cd cathode. The distillation experiments were conducted in 10 g scale of the initial alloy using a small-scale distillation furnace equipped with an evaporator and a condenser in a vacuum vessel. After distillation at 1073 K, the weight of the residue was in good agreement with that of the loaded actinides, where the content of Cd decreased to less than 0.05 wt%. The uranium–plutonium alloy product was recovered without adhering to the yttria crucible. The cross section of the product was observed using electron probe micro-analyzer and it was found to consist of a dense material. Almost all of the evaporated Cd was recovered in the condenser and so enclosed well in the apparatus.     

Thermal–hydraulic feasibility analysis on uprating the HTR–PM

In order to meet energy demand in China, the high temperature gas-cooled reactor–pebble-bed module (HTR–PM) is being developed. It adopts a two-zone core, in which graphite balls are loaded in the central zone and the outer part is fuel ball zone, and couple with a steam cycle. Outer diameter of the reactor core is 4.0 m and height of the core is 9.43 m. The helium inlet and outlet temperature are 250 and 750 °C, respectively. The reactor thermal power is 380 MW. Preliminary studies show that the HTR–PM is feasible technologically and economically. In order to increase the reactor thermal power of the HTR–PM, some efforts have been made. These include increasing the height of reactor core, optimizing the thickness of fuel zone and better selection of the scheme of central graphite zone, etc. Basic design concepts and thermal–hydraulic parameters of the HTR–PM are given. Measures to increase the thermal power are introduced. Thermal–hydraulic analysis results are presented. The results show that, from the viewpoint of thermal–hydraulics, it is possible to increase the reactor power.     

Ductile–brittle transition of polycrystalline iron and iron–chromium alloys

Fracture toughness of polycrystalline Fe, Fe–3%Cr and Fe–9%Cr was measured by four-point bending of pre-cracked specimens at temperatures between 77 K and 150 K and strain rates between 4.46 × 10⁻⁴ and 2.23 × 10⁻² s⁻¹. For all materials, fracture behaviour changed with increasing temperature from brittle to ductile at a distinct brittle–ductile transition temperature (T_c), which increased with increasing strain rate. At low strain rates, an Arrhenius relation was found between T_c and strain rate in each material. At high strain rates, T_c was at slightly higher values than those expected from extrapolation of the Arrhenius relation from lower strain rates. This shift of T_c was associated with twinning near the crack tip. For each material, use of an Arrhenius relation for tests at strain rates at which specimens showed twinning gave the same activation energy as for the low strain rate tests. The values of activation energy for the brittle–ductile transition of polycrystalline Fe, Fe–3%Cr and Fe–9%Cr were found to be 0.21, 0.15 and 0.10 eV, respectively, indicating that the activation energy for dislocation glide decreases with increasing chromium concentration in iron.     

304不锈钢非比例多轴时间相关棘轮行为的本构模型研究

基于304不锈钢在不同加载速率和不同保持时间下得到的非比例多轴时相关棘轮行为的实验研究结果,在Abdel-Karim-Ohno模型的基础上,建立了一个非比例多轴时间相关循环本构模型.该模型通过在背应力演化律中引入与非比例度相关的静力恢复项,在各向同性硬化律中引入Tanaka非比例度来考虑非比例路径对时间相关棘轮行为的影响.模拟结果与实验结果的比较表明:该本构模型对室温和高温下多轴时间相关棘轮行为的演化规律均能给予较合理的描述.     

Effect of γ-ray irradiation on Nd–Fe–B alloys

We report on how γ-ray irradiation affects the magnetic properties of a powder sample of Nd–Fe–B, which was irradiated at room temperature with doses up to 700 kGy. Both the magnetic properties and surface morphology were changed by the effects of the γ-ray irradiation. The unirradiated and irradiated samples were then characterized using the VSM, XRD and SEM techniques.     

Core thermal–hydraulic design

The core thermal–hydraulic design for the HTTR is carried out to evaluate the maximum fuel temperature at normal operation and anticipated operation occurrences. To evaluate coolant flow distribution and maximum fuel temperature, we use the experimental results such as heat transfer coefficient, pressure loss coefficient obtained by mock-up test facilities. Furthermore, we evaluated hot spot factors of fuel temperatures conservatively.As the results of the core thermal–hydraulic design, an effective coolant flow through the core of 88% of the total flow, is achieved at minimum. The maximum fuel temperature appears during the high-temperature test operation, and reaches 1492 °C for the maximum through the burn-up cycle, which satisfies the design limit of 1495 °C at normal operation. It is also confirmed that the maximum fuel temperature at any anticipated operation occurrences does not exceed the fuel design limit of 1600 °C in the safety analysis.On the other hand, result of re-evaluation of analysis condition and hot spot factors based on operation data of the HTTR, the maximum fuel temperature for 160 effective full power operation days is estimated to be 1463 °C. It is confirmed that the core thermal–hydraulic design gives conservative results.     

On the formation of U–Al alloys in the molten LiCl–KCl eutectic

U–Al alloy formation has been studied in the temperature range of 400–550 °C by electrochemical techniques in the molten LiCl–KCl eutectic. Cyclic voltammetry showed that underpotential reduction of U(III) onto solid Al occurs at a potential about 0.35 V more anodic than pure U deposition. Open circuit potential measurements, recorded after small depositions of U metal onto the Al electrode, did not allow the distinction between potentials associated with UAl_x alloys and the Al rest potential, as they were found to be practically identical. As a consequence, a spontaneous chemical reaction between dissolved UCl₃ and Al is thermodynamically possible and was experimentally observed. Galvanostatic electrolyses were carried out both on Al rods and Al plates. Stable and dense U–Al deposits were obtained with high faradic yields, and the possibility to load the whole bulk of a thin Al plate was demonstrated. The analyses (by SEM-EDX and XRD) of the deposits indicated the formation of different intermetallic phases (UAl₂, UAl₃ and UAl₄) depending on the experimental conditions.