The challenges in predicting the fatigue life of dissimilar brazed joints and initial finite element results for a tungsten to EUROFER97 steel brazed joint

This paper summarises the challenges in accurately predicting stress states in dissimilar brazed joints and presents initial results from a finite element analysis of a tungsten to EUROFER97 brazed joint. The residual stresses due to joint manufacture are presented and differences in stress distribution due to thermal and mechanical loading highlighted. The results from this analysis correlate well to experimental results from previous research however further validation is required. The challenges in developing fatigue assessment procedures for dissimilar brazed joints are also discussed. These fatigue assessment procedures are introduced and a validation strategy for such procedures is proposed.     

Corrosion of Ti-STS dissimilar joints brazed by a Ag interlayer and Ag-Cu-(Pd) alloy fillers

Corrosion behavior of dissimilar brazed joints between titanium Gr. 2 (Ti) and S31254 stainless steel (STS) was investigated. For the study, a Ag interlayer and two Ag-base eutectic alloys, 72Ag-28Cu and 66.2Ag-25.8Cu-8Pd (wt.%), were introduced as a diffusion control layer and fillers, respectively, between the base materials. The joints commonly had a layered structure of Ti(base)/TiAg/Ag solid solution/STS(base), but the one brazed by the Ag-Cu-Pd filler was slightly alloyed with the noble Pd elements over the Ag-rich solid solution region. A series of corrosion test experiments in a sea water revealed that a corrosion of TiAg layer and a stress-induced cracking at the TiAg/Ag solid solution interface were dominant due to a galvanic attack, but notably the Ti-STS dissimilar joint’s resistance to corrosion was significantly improved by alloying the Pd in the joint. The corrosion behavior of such dissimilar metal joints was discussed based on galvanic corrosion effect.     

Technological parameters of fine tube-sheet joints brazed by scanning electron beam heat

The present paper proposes the vacuum scanning EB brazing (SEBB) technology. Investigation on SEBB processing features of stainless steel tube-sheet structure was performed, the effect of SEBB technological factors (EB current, heating time, scanning area, EB current rising slope, filler metal mass preset, joint fit-up gap) on the brazed joints quality was analyzed, and the resulting optimal parameters array is presented in the paper.     

Corrosion and microstructural aspects of dissimilar joints of titanium and type 304L stainless steel

To link titanium and zirconium metal based (Ti, Zr-2, Ti-5%Ta, Ti-5%Ta-1.8Nb) dissolver vessels containing highly radioactive and concentrated corrosive nitric acid solution to other nuclear fuel reprocessing plant components made of AISI type 304L stainless steel (SS), high integrity and corrosion resistant dissimilar joints between them are necessary. Fusion welding processes produce secondary precipitates which dissolve in nitric acid, and hence solid-state processes are proposed. In this work, various dissimilar joining processes available for producing titanium-304L SS joints with adequate strength, ductility and corrosion resistance for this critical application are highlighted. Developmental efforts made at IGCAR, Kalpakkam are outlined. The possible methods and the microstructural-metallurgical properties of the joints along with corrosion results obtained with three phase (liquid, vapour, condensate) corrosion testing are discussed. Based on the results, dissimilar joint produced by the explosive joining process was adopted for plant application.     

Fatigue strength of dissimilar welded joints for the main vessel of an LMFBR

The employment of welded joints composed of dissimilar metals is one simple and inexpensive way to connect a main vessel made of austenitic stainless steel and a roof slab constructed of ferritic steel in the design of liquid metal fast reactors. Since dissimilar-metal welded joints have not been used for such large structures so far in Japan, the structural integrity of this type of joint should be carefully examined for such a design option to be selected. Here various kinds of tests were conducted for eleven types of welded joints of 50 mm thickness to obtain this fundamental strength characteristics. Type 304 stainless steel was used as one of the parent metals in all the joints. They differ from each other in regard to the type of ferritic steel, welding metal and welding procedure. Low-cycle fatigue tests were conducted for round-bar specimens made from these welded joints at room temperature. Fatigue crack-propagation tests were also conducted for some of the joints. Tests after manufacturing a large-scale shell model were also conducted. The results of these tests demonstrated that the present manufacturing technique can, produce welded joints of high quality and reliability. A trial calculation for actual design conditions showed the existence of large margins against fatigue failure or fatigue crack-propagation of a significant amount.     

Computer modelling of the sealing behaviour of gaskets in flanged joints

A computer program is described which is designed to predict the gasket stress distribution in flanged joints. The program incorporates finite element modules for calculation of flange deflections in response to loads applied by bolts, gasket and fluid. Account is taken of non-linear gasket load-compression curves and hysteresis, thermal deflections, etc.     

A contribution to the 1D modelling of fuel element mechanics

A fuel element mechanics model must involve such effects as fuel cracking, the friction contact of the fuel and the cladding and so on. These effects are however essentially 3D ones and far from being trivial. This paper makes an attempt to model the fuel element mechanical behavior in the simplest way as a system of 1D problems depending only on one space variable. The presented general theories deal with a boundary (contact) value problem for one fuel rod section, a homogenized fuzzy-crack problem and a 1D axial friction contact problem. An interconnection of particular submodels is discussed. Numerical examples illustrating the particular submodels and the model as a whole are presented.     

Probability-based assessment and maintenance of welded joints damaged by fatigue

This paper presents a probabilistic reliability assessment procedure for steel components damaged by fatigue. The study combines the structural reliability theory with a maintenance strategy. The fatigue assessment model is based on a modelisation of the fatigue phenomenon issued from the principles of fracture mechanics theory. The safety margin includes the crack growth propagation and allows to treat fatigue damage in a general manner. Damaging cycles and non damaging cycles are distinguished. The sensitivity study of the different parameters shows that some variables can be taken as deterministic. Applications are made on a welded joint ‘stiffener/bottom-plate' of a typical steel bridge. The model is then used for taking into account inspection results. Non destructive inspection (NDI) techniques are also used for updating failure probabilities. The results show their ability to be inserted in a maintenance strategy for optimizing the next inspection time. This has led to define cost functions related to the total maintenance cost; this cost is then minimized for determining the optimal next inspection time. An example of welded joint cracked by fatigue highlights the different concepts. The approach presented in the paper is not only restrained to fatigue problems, but can be applied to a wide variety of degrading phenomena.     

Fracture mechanics evaluation of the structural material for fast breeder reactors

Results of fracture mechanics investigations on austenitic steels used for LMFBRs (Liquid Metal Fast Breeder Reactors) are presented. A summary of reported tests on straight piping and elbows with through wall flaws is given which agree well with predictions made by using a plastic instability model. Crack growth experiments and calculations indicate that initial flaws will not extend significantly during service. Even if considerable crack growth is postulated cracks will penetrate the piping wall with a high safety margin to unstable crack configurations. Theoretical investigations of flawed structures under high strains show that the effect of crack size can be discussed similarly to the elastic range. The information demonstrate that with respect to the design requirements and operating conditions of LMFBRs a sudden rupture of the piping can be excluded. The integrity of the coolant boundary is given also in case of initial flaws.     

Determination of fracture mechanics properties of welded joints and comparison with the failure behaviour of wide plates

Fracture behaviour of different welded joints of 15 MnNi 6 3 and 20 MnMoNi 5 5 steel is studied by testing Charpy-V impact, fracture mechanics specimens as well as wide plate specimens of different sizes. The influence of welding conditions and stress relief heat treatment on the failure behaviour of the heat affected zones is examined. Fracture mechanics test data are used in order to predict failure loads of the wide plates by the R6-procedure and by FE-calculations.     

Fundamental study on applications of damage mechanics to evaluation of material degradation

Damage mechanics theory based on continuum mechanics has recently been attracting attention. This branch of physics describes the mechanical behavior of materials damaged through the production of microscopic voids. Nuclear structural components during operation seldom generate such voids. Damage mechanical theory may be of use for evaluating microscopic material behavior, and confirmation of this point was the purpose of this study. A comparison was made of calculation results, using constitutive equations of Chow and Wang, and of Lemaitre to describe ductile behavior of notched tension bars. The results were also compared with experimental results.     

Fracture mechanics concepts for evaluation of irradiated reactor pressure vessel material behavior

Within the German research program Forschungsvorhaben Komponentensicherheit (FKS), irradiation experiments were performed with ferritic reactor pressure vessel (RPV) steels and welds. The materials cover a wide range of chemical composition and initial toughness to achieve different susceptibility to neutron irradiation. Different neutron flux was applied and the neutron exposure extended up to 8×10¹⁹ cm⁻². The change in material properties was determined by means of tensile, Charpy impact, drop-weight and fracture mechanics tests, including crack arrest. The results have provided more insight into the acting embrittlement mechanisms and shown that the fracture mechanics concept of the Code provides in general an upper bound for the material which can be applied in the safety analysis of the RPV.     

Studies on material properties and structural mechanics of engineered barriers for HLW geological isolation

A massive engineered barrier system (EBS) composed of vitrified waste, carbon steel overpack and buffer material (compacted sodium-bentonite) can be expected to isolate hazardous radionuclides from the human environment.Corrosion, leaching and migration studies of EBS materials have supported the performance assessment of the system. Natural analogue is expected to be a method for the validation of the long-term durability of EBS materials. Geochemistry study of groundwater evolution in EBS supports the site generic performance assessment. Coupled thermo-hydro-mechanical process, mechanical stability and hydrogen gas behavior in EBS are also research items for more realistic evaluation of the EBS.     

The physical metallurgy of high strength zirconium alloys

The requirements of materials for use in the core section of unenriched uranium type nuclear reactors are, to a large extent, fulfilled by zirconium alloys. However, the operating temperature of such reactors is circumscribed by the inability of the presently available alloys to maintain their properties at temperatures exceeding 350–400° C. This paper is concerned with the selection of alloying elements for the improvement of the high temperature strength of zirconium alloys and reports the results obtained from a study of the phase transformations in dilute Zr-Cr alloys.     

建筑材料矿床放射性评价的应用研究

采用石材产品放射防护分类控制标准，应用地面γ能谱测量，γ总量测量、采集样品后在室内进行定量核素分析等方法对全国２１个石材矿床进行了放射性评价，并介绍了评价指数、进行此种评价的意义及工作方法，详述了在广西岑溪县红色花岗石矿床上进行的工作。     

Failure probability assessment of wall-thinned nuclear pipes using probabilistic fracture mechanics

The integrity of nuclear piping system has to be maintained during operation. In order to maintain the integrity, reliable assessment procedures including fracture mechanics analysis, etc., are required. Up to now, this has been performed using conventional deterministic approaches even though there are many uncertainties to hinder a rational evaluation. In this respect, probabilistic approaches are considered as an appropriate method for piping system evaluation. The objectives of this paper are to estimate the failure probabilities of wall-thinned pipes in nuclear secondary systems and to propose limited operating conditions under different types of loadings. To do this, a probabilistic assessment program using reliability index and simulation techniques was developed and applied to evaluate failure probabilities of wall-thinned pipes subjected to internal pressure, bending moment and combined loading of them. The sensitivity analysis results as well as prototypal integrity assessment results showed a promising applicability of the probabilistic assessment program, necessity of practical evaluation reflecting combined loading condition and operation considering limited condition.     

Numerical assessment of functionally graded tungsten/steel joints for divertor applications

In the current helium cooled divertor concept tungsten and its alloys are considered as refractory as well as structural materials. Due to the brittleness of tungsten at low temperatures its use as a structural material is limited to the high temperature region (>650 °C) of the component and a joint to another structural material, the ferritic martensitic steel EUROFER97-ODS, is necessary. However, the remarkable difference in thermal expansion between tungsten and steel causes a thermal mismatch between them resulting in stresses, which would yield failure of the joint. An idea to reduce these stresses is to introduce a functionally graded layer between the materials to be joined.In this work a functionally graded tungsten/steel joint is analyzed using the finite element method and considering a simplified model of the proper divertor component. Therefore elasto-plastic and elasto-viscoplastic simulations are performed varying the thickness of and the transition function within the graded layer and considering application relevant loadings. The resulting stresses and deformations are then evaluated to determine the optimal joining parameters. On the base of these parameters the realization of the joint is investigated by screening the methods applicable for the fabrication of functionally graded materials and selecting the most suitable ones.     

个性化人眼模型及屈光手术视觉质量评估

人眼是一台精巧而复杂的光学成像系统,其复合结构存在像差,它的空间极限分辨能力要小于无像差的同口径的小孔所对应的衍射极限。眼球也并非完美对称的,各介质曲面由于不规则性或位置异常等因素对视觉质量产生影响,为精确修正眼球的屈光不正带来困难。而常规的角膜手术虽然可以较好地修正以离焦为主的低阶像差,但却不能改善高阶像差,在手术实施的过程中反而会因为改变角膜的面型因子,引入较大的球差,严重影响暗视场的视力。本文依据人眼的基本结构及统计的典型人眼参数,实现全眼球数字化模拟并考虑了个性化因子,分析影响视力的主要因素,针对眼球屈光不正设计出个性化的角膜手术方案,使术后眼球的像差得到全面修正,从而实现最佳的人眼成像能力。