脉冲强磁体中应力的有限元分析

提高脉冲磁场强度的主要障碍是磁体中巨大的应力,而脉冲磁体中应力的产生和作用过程比较复杂,要准确地计算出来很困难。采用有限元法对脉冲磁体中的应力进行分析,计算中全面考虑了预应力、电磁力、热应力等情况。通过计算,得出了,一些设计脉冲磁体的基本原则。     

Finite element formulation for thermal stress analysis of thin reactor structures

This paper describes the formulation of a finite-element procedure for the thermal stress analysis of thin wall reactor components. A general temperature-dependent constituent relationship is derived from a Gibbs potential function and a temperature-dependent yield surface. This form of constitutive relationship is applicable to problems of small strain. A similar form of a hypoelastic-plastic type is also developed for large strains. The variation of the yield surface with temperature is based upon a temperature-dependent, work-hardening model. The model uses a temperature-equivalent stress-plastic strain diagram which is generated from isothermal unaxial stress-strain data.The above constitutive relationships are incorporated into two computer codes and a previously developed numerical algorithm is used with minor modifications. A set of problems is presented validating the thermal analysis capability of the computer codes to a variety of problem types.     

Integrity assessment of the high temperature engineering test reactor (HTTR) control rod at very high temperatures

The high temperature engineering test reactor (HTTR) is the first high temperature gas-cooled reactor (HTGR) in Japan with a reactor outlet coolant temperature of 950°C at high temperature test operation. The HTTR contains 16 pairs of control rods for which Alloy 800H is chosen of the metallic parts. Because the maximum temperature of the control rods reaches about 900°C at reactor scrams, structural design guideline and design material data on Alloy 800H are needed for the high temperature design. The design guideline for the HTTR control rod is based on ASME Code Case N-47-21. Design material data is also determined and shown in this paper. Under the guideline, temperature and stress analysis was conducted, and it is confirmed that the target life of the control rods of 5 years can be achieved.     

The analysis of dynamically loaded non-linear structures

Small models of a proposed prestressed concrete containment structure for a sodium cooled fast breeder reactor have been made and tested. They were partly filled with water and loaded internally by detonating explosive charges. Prior to the tests the model was analysed by an axisymmetric dynamic relaxation computer program. No difficulties occurred with the analyses which were arranged to print out, inter alia, stresses, deflections and cracks at suitable time intervals during and after the pressure excursions which lasted for up to 3000 μsec. A previous paper (ref. [7]), describing the program, has shown how cracks can be calculated simply and accurately. The correlation of computer and test results is discussed.     

Particle generation at very high energies

Values are obtained for the intensities of beams of various particles, including -quanta, neutrinos, and -mesons, formed in inelastic particle interactions in the energy range 1–10³ Bev. These values may be useful in designing accelerators for very high energies.We are grateful to I. Silin, an assistant at the Calculational Center of the Joint Institute of Nuclear Studies for the large amount of assistance rendered in making the calculations, as well as to V. I. Veksler for his critical comments and valuable advice, much of which was made use of in our calculations. We are especially happy to thank D. I. Blokhintsev and M. A. Markov, on whose initiative our calculations were made, for numerous discussions on very high energy particle interactions.     

Finite element elastic-plastic analysis of LMFBR components

The present effort involves the development of computationally efficient finite element methods for accurately predicting the isothermal elastic-plastic three-dimensional responses of thick and thin shell structures subjected to mechanical and thermal loads. This work will be used as the basis for further development of analytical tools to be used to verify the structural integrity of liquid metal fast breeder reactor (LMFBR) components. The methods presented here have been implemented into the three-dimensional solid element module (HEX) of the Grumman PLANS finite element program. These methods include the use of optimal stress points as well as a variable number of stress points within an element. This allows monitoring the stress history at many points within an element and hence provides an accurate representation of the elastic-plastic boundary using a minimum number of degree of freedom. Also included is an improved thermal stress analysis capability in which the temperature variation and corresponding thermal strain variation are represented by the same functional form as the displacement variation. Various problems are used to demonstrate these improved capabilities.     

Finite element analysis of inelastic structural behaviour

The paper examines computational aspects of the numerical solution of dissipation and diffusion problems which describe inelastic mechanical processes and transport mechanisms of transient environments. Both phenomena are rate processes which lead in general to a system of coupled first order differential equations after finite element idealization of the spatial dependence.     

Finite element analysis of moderately thick shells

Thick shells of revolution find application in nuclear pressure vessel technology. In this paper, a finite ring shell element, based on the stress-strain relations of moderately thick shells, is developed. The element has 20 degrees of freedom. Cubic polynomials are assumed for all the generalized displacements, i.e. the in-plane forces, the normal displacements and the total rotations of the normal about the co-ordinate lines. This ensures faster convergence and better accuracy in the results. The effectiveness of the element is demonstrated by numerical examples.     

Finite element analysis of ion-implanted diamond surface swelling

We present experimental results and numerical finite element analysis to describe surface swelling due to the creation of buried graphite-like inclusions in diamond substrates subjected to MeV ion implantation. Numerical predictions are compared to experimental data for MeV proton and helium implantations, performed with scanning ion microbeams. Swelling values are measured with white-light interferometric profilometry in both cases. Simulations are based on a model which accounts for the through-the-thickness variation of mechanical parameters in the material, as a function of ion type, fluence and energy. Surface deformation profiles and internal stress distributions are analyzed and numerical results are seen to adequately fit experimental data. Results allow us to draw conclusions on structural damage mechanisms in diamond for different MeV ion implantations.     

Finite element analysis of a finite-strain plasticity problem

A finite-strain plasticity analysis was performed of an engraving process in a plastic rotating band during the firing of a gun projectile. The aim was to verify a nonlinear feature of the NIFDI/RB code: plastic large deformation analysis of nearly incompressible materials using a deformation theory of plasticity approach and a total Lagrangian scheme.     

Finite element analysis of mixed-mode thermoelastic fracture problems

This paper presents a rigorous hybrid finite element procedure for the analysis of the thermoelastic problems with mixed-mode cracks. The singular character of the temperature gradient which significantly affects the distribution of thermal stresses near the crack-tip is precisely described. The calculations of temperature and thermal stress fields are carried out by the finite element assemblage in which hybrid singular elements are used around the crack-tip and high-order isoparametric regular elements are taken elsewhere.To determine the mixed-mode stress intensity factors, both the direct extrapolation method based on the finite element solutions and the indirect method using the modified integrals accounting for thermal effects have been proposed. To avoid the underestimation of stress intensity factors obtained by quarter-point singular elements, the important role of the hybrid singular elements developed is also demonstrated.For verification purposes, several pure mode and mixed-mode problems are solved. Excellent correlations between the computer results and available referenced solutions are drawn.     

Finite element fracture mechanics analysis of creep rupture of fuel element cladding

A finite element fracture mechanics technique is applied for simulating the elevated temperature creep rupture behavior of initially defected austenitic stainless steel fuel element cladding. The basic analytical approach consists of determining total instantaneous strain energy release rates G_T, and the corresponding values of the stress intensity factor K_T from sequential linear elastic finite element solutions and relating these to either an effective creep fracture toughness parameter G_ec (or K_ec) or to creep crack growth rates , obtained from test results.An initial application of this approach has been made to simulate the creep rupture behavior of initially defected type 316 austenitic stainless steel fuel element cladding in the 20% cold worked condition, tested at 650°C. This application has provided a relationship in the simple familiar form: , where σ is the nominal loop stress, a is the initial depth of a longitudinal crack, h is the cladding thickness, t_r is the time to rupture, and q is a structure sensitive parameter which accounts for the influence of the environment. is a function, obtained from finite element solutions, which accounts for the geometric differences between the present structure and the classical Griffith plate. The function ) is obtained from creep rupture tests of cladding with varying initial flaw depths and times to rupture under corrosive as well as inert environments.Performing time-dependent analyses, a preliminary relationship is obtained between the instantaneous values G_T and K_T, and crack growth rates under corrosive and non-corrosive environments. The analytical predictions of critical combinations of cladding flaw configurations, stresses, times to rupture and crack growth rates are in good agreement with the limited test data available for comparison. Current applications are aimed at the long-term cyclic creep fracture behavior of fast reactor fuel elements, using a nonlinear finite element code. In addition, multiple intergranular fracture configurations are being investigated.     

XAFS study of europium chloride at high temperatures

Local structural changes in europium dichloride around europium ion at high temperature are evaluated by using X-ray absorption fine structure technique. A distinct phase shift in EXAFS oscillation observed between 700 and 950K arises from Eu²⁺ to Eu³⁺ oxidation. Referring to the difference in EXAFS oscillation between oxide and oxychloride of lanthanum, the chemical state of europium on oxidation is identified mainly as Eu₂O₃.     

Finite element analysis of linear and nonlinear heat transfer

The finite element method is rapidly becoming a popular procedure for the evaluation of thermal stresses in complex structures. In linear analysis the method has been used extensively and has been coupled with stress analysis computer programs in order to automate thermal stress analysis. However, for the method to be effective, efficient numerical techniques need to be used. The purpose of this paper is to survey the recent developments in linear heat transfer analysis and, specifically, to present the techniques that permit the practical analysis of large and complex three-dimensional heat conduction problems. Typical practical problems are described and solution times are presented. In the analysis of systems with nonlinear thermal properties the method has had limited application. In this paper the general formulation of the incremental equations used in nonlinear heat transfer analysis are presented. An efficient numerical solution of the equations is given. Several types of nonlinearities are discussed and the solutions of some typical problems are presented.     

Finite element methods in the analysis of reactor vessels

The application of the finite element method to linear and non-linear problems in pressure vessel technology is presented. New developments for dealing with components such as liners, prestressing cables and reinforcement are outlined and some improvements possible in thin shell situations are discussed. A general solution technique for non-linear analysis is presented and applied firstly to the problem of the plastic behaviour of steel pressure vessels. The failure of PCRVs by concrete cracking is then considered. Finally, the time-dependent phenomenon of creep is discussed. In all cases the theory is illustrated by practical examples.     

Finite element analysis of thick annular and sector plates

A finite element displacement formulation based on Reissner's thick plate theory is presented for the bending analysis of annular and sector plates. An annular element and an annular sector element, associated with six and twenty degrees of freedom, respectively, are developed. One of the significant features of these elements, which include both bending and shear deformation effects and which conform with respect to all degrees of freedom considered, is the ability to satisfy exactly the condition of vanishing shear stress resultant along a free edge. Numerical results are presented for substantiating the proposed procedure.     

Finite element analysis of steady-state nonlinear heat transfer problems

A method of analysis and the associated computer program are presented for the purpose of solving steady-state nonlinear heat transfer problems in two-dimensional structures. The nonlinearity arises from the dependence of the thermal conductivities on temperature as well as from the presence of rediative heat transfer between parts of the structure. The problem is formulated in terms of an integral of conductivity and solved in an iterative way via the finite element concept. Several examples are given to illustrate the validity and practicality of the suggested solution technique.     

Hydrogen absorption by zirconium alloys at high temperatures

At high temperatures, e.g. during a hypothetical severe accident, zirconium and its alloys are not stable to other materials and to oxidising atmospheres. Exothermic reactions with steam cause the production of hydrogen which will be released to the atmosphere and, thus, endanger the containment or may be absorbed by the remaining metal. The hydrogen solubility in Zircaloy-4 and Zr-1Nb was measured in the temperature range of 1230-1730 K and at hydrogen partial pressures between 10 and 100 kPa. The parameters of the Sieverts constant were determined. No significant differences between the two alloys were observed. The hydrogen solubility of oxygen containing Zircaloy-4 decreases with increasing oxygen content.