Numerical simulation of stress and deformation in a railway crossing

Stress/strain analysis is the key to understanding and predicting wear and fatigue behavior of a crossing. By taking into account non-linear material properties, a three dimensional elastic–plastic finite element model, which is composed of wheel, crossing and ties, is presented for the simulation of stress/deformation in a railway crossing. The influences of dynamic wheel load and wheel–crossing contact are examined. Stress, plastic strain and vertical displacement of the simulated crossing under dynamic wheel load at different wheel–crossing contact positions are investigated. The maximum vertical displacement occurs at the wheel–crossing contact region, and decreases gradually from the wheel–crossing contact region to the toe-end and heel-end of the crossing. The maximum von Mises stress and maximum equivalent plastic strain in the crossing increase remarkably with the increase of the train speed. The maximum vertical displacement in the crossing increases obviously and varies linear approximately with the train speed. The maximum von Mises stress, maximum vertical displacement and maximum equivalent plastic strain in the crossing are very sensitive to the axle load and are linear approximately with the axle load.     

径向流分子筛吸附器流场数值模拟

介绍了径向流分子筛吸附器的主要结构和工作原理。采用CFD软件对径向流分子筛吸附器流场进行了模拟计算,分析了径向流分子筛吸附器流场压力分布和气体流动速度分布,比较分子筛层和氧化铝层中不同位置处的速度分布曲线,预测径向流分子筛吸附器吸附工作时吸附饱和临界突破位置。     

A local-domain based phase field method for crack propagation

Abstract

The phase field method transfers the crack surface area into a domain integral through a smeared damage parameter. As a result, the fracture energy release rate can be directly incorporated into the energy form without tracking of crack surface, and a minimization approach to optimize the crack shape becomes possible. This nonlocal damage approach requires a fine mesh discretization to capture the damage gradient in the smeared crack zone, which produces a significant degree of freedom problem to optimize over the entire domain. In this research, a local-domain based phase field method is proposed. This method considers only elements in critical regions for energy minimization, whose internal energy are dominant, thus significantly reducing the degree of freedom to be solved. A numerical verification is carried out to demonstrate the feasibility of this approach, and simulations are presented to verify the accuracy of the new method.     

端部霍尔离子源的磁场设计与数值模拟

首先简单介绍了用于离子束辅助沉积的端部霍尔离子源的工作原理，然后分析了其性能对磁场的要求，据此介绍了磁路组件的设计。最后,采用ANSYS大型有限元分析软件对一个条形的端部霍尔离子源的磁场进行了模拟计算，并与实验结果进行了比较，获得了满意的结果。通过对模拟结果的分析，获得了对该离子源磁场分布的直观深入的认识，为具体的端部霍尔离子源的改进设计提供了理论指导。     

Numerical Simulations of Equiaxed Dendrite Growth Using Phase Field Method

Phase field method offers the prospect of being able to perform realistic numerical experiments on dendrite growth in a metallic system.In this paper, the equiaxed dendrite evolution during the solidification of a pure material was numerically simulated using the phase field model.The equiaxed dendrite growth in a two-dimensional square domain of undercooled melt(nickel) with four-fold anisotropy was simulated.The phase field model equations was solved using the explicit finite difference method on a uniform mesh.The formation of various equiaxed dendrite patterns was shown by a series of simulations,and the effect of anisotropy on equiaxed dendrite morphology was investigated.     

Numerical simulation of the temperature field in the electromagnetic semi-continuous casting of slab

Electromagnetic casting of aluminum is a semi-continuous casting process and the corresponding mathematical model is a non-steady non-homogeneous heat-transfer equation. This paper indicates and testifies that the dynamic temperature field can be calculated with a three-dimensional non-steady homogeneous heat-transfer equation. The software program has universal use. Further, a casting process of 1860 × 510 mm aluminum slab is simulated, the results agreeing well with those reported in the literature.     

转速对快速法生长KDP晶体影响的实验与数值模拟研究

采用快速生长法在不同转速下生长了KDP晶体,对晶体生长过程中的现象进行了研究;运用fluent软件,采用多重参考模型和标准k-ε湍流模型对籽晶架和晶体在溶液中旋转产生的速度场进行了数值模拟,分析了不同转速对速度场和晶体长大对溶液稳定性的影响;结合实验现象与模拟结果分析可知,初始阶段适于KDP晶体快速生长的最优转速是100 r/min,随着晶体的长大,转速应适当降低。     

An object-oriented finite element framework for multiphysics phase field simulations

The phase field approach is a powerful and popular method for modeling microstructure evolution. In this work, advanced numerical tools are used to create a framework that facilitates rapid model development. This framework, called MARMOT, is based on Idaho National Laboratory’s finite element Multiphysics Object-Oriented Simulation Environment. In MARMOT, the system of phase field partial differential equations (PDEs) are solved simultaneously together with PDEs describing additional physics, such as solid mechanics and heat conduction, using the Jacobian-Free Newton Krylov Method. An object-oriented architecture is created by taking advantage of commonalities in the phase field PDEs to facilitate development of new models with very little effort. In addition, MARMOT provides access to mesh and time step adaptivity, reducing the cost for performing simulations with large disparities in both spatial and temporal scales. In this work, phase separation simulations are used to show the numerical performance of MARMOT. Deformation-induced grain growth and void growth simulations are also included to demonstrate the muliphysics capability.     

Temperature dependence of impact deformation behavior of single crystal Ni based superalloy

The impact test was carried out to investigate the intermediate temperature brittleness of single crystal Ni based superalloy. The samples were impacted at the velocity of ～5?m?s^??1. The results showed that the impact toughness also exhibited intermediate temperature brittleness, which is similar to the situation in tensile. The samples showed the highest impact toughness at 600°C but exhibited the lowest impact toughness at 760°C. Results showed that the variety of impact toughness was due to the deformation mechanism. At 600°C, a/2? dislocation slips on the {111} slip system were attributed to the high impact toughness; however, a/2? dislocation slips from octahedral {111} planes to cubic {100} planes resulted in significant work hardening, leading to the decrease of impact toughness.     

Simulation of epitaxial growth on convex substrate using phase field crystal method

Phase field crystal （PFC） model is employed to simulate the process of growth of epitaxial layer on plane-convex substrate with a lattice mismatch and a small inclination angle. The variation of the systematic free energy, the total atomic number of the epitaxial layer, and the effect of the curvature and the angle of the substrate are analyzed. The results show that when the surface of the substrate is plane, the free energy increases with the increase of the substrate inclination angle, and also the total atomic number of the epitaxial layer increases; while the surface of the substrate is convex, the free energy decreases with the increase of substrate angle and so also the total atomic number of the epitaxial layer decrease. This is the reason that the frontier of surface of epitaxial layer changes from the step bunching to the hill-and-valley facet structure with the increasing of the inclination angle of convex substrate. These results are in good agreement with the other method results.     

室内指向性声源的声场数值模拟

为了利用计算机对室内指向性声源的声场进行模拟,文中将声线跟踪法,虚源法和Monte Carlo法相结合,编制了一种场场数值模拟仿真程序,该程序能模拟有（或无）指向性声源在室内给定点的声压级等参数值,还能计算出该点的脉冲响应函数,作为例子,分别利用该法和声压叠加法研究了点源和偶极子声源的指向性。     

Phase field crystal simulation of grain boundary movement and dislocation reaction

The phase field crystal （PFC） model is used to simulate the premelting dislocation movement of the symmetric tilt grain boundary （STGB） under strain action when the system temperature is at far from the melting point and close to the melting point, respectively. The results show a local premelting occurs surrounding the dislocations as the premelting temperature is approached to from below temperature. The premelting dislocations of the STGB can glide under strain action, and the premelting region is a companion for dislocation gliding. The process of STGB decay is very similar at the two high temperature conditions. As premelting presents, it diminishes the gliding resistance for the dislocations and leads to a faster movement of dislocations, and also brings about more energy reduction of the system during the decay process of STGB. In spite of applying strain to these premelting samples in whole decay processes of STGB, the premelting dislocation region does not obviously develop and extend. This indicates that the external strain action does not promote the premelting at the high temperature, and cannot induce more premelting dislocation, which can be owed to the premelting phase around the dislocation exhibit fluid-like properties and to the premelting dislocation easily gliding and relaxing the strain energy; this is in agreement with the results of experiments and molecular dynamics.     

Visualization of deformation in atomistic simulation

A general and simple method is developed of characterizing deformation in an atomistic model by simulations. In the method, the deformation index is defined as the maximum relative displacement of an atom to its nearest neighbors during deformation. The deformation index can be applied to quantitatively visualize lattice defects such as dislocations and stacking faults during twin or slip deformation in a single image. As an example, the cracking processes of nickel and iron are presented and analyzed by the method.     

Hybrid numerical scheme for time-evolving wave fields

Many problems in geophysics, acoustics, elasticity theory, cancer treatment, food process control and electrodynamics involve study of wave field synthesis (WFS) in some form or another. In the present work, modelling of wave propagation phenomena is studied as a static problem, using finite element method and treating time as an additional spatial dimension. In particular, WFS problems are analysed using discrete methods. It is shown that a fully finite element-based scheme is very natural and effective method for the solution of such problems. Distributed WFS in the context of two-dimensional problems is outlined and incorporation of any geometric or material non-linearities is shown to be straightforward. This has significant implications for problems in geophysics or biological media, where material inhomogeneities are quite prevalent. Numerical results are presented for several problems referring to media with material inhomogeneities and predefined absorption profiles. The method can be extended to three-dimensional problems involving anisotropic media properties in a relatively straightforward manner. Copyright © 2006 John Wiley & Sons, Ltd.     

Coupling phase field with creep damage to study evolution and creep deformation of single crystal superalloys

A phase-field model coupling with elastoplastic deformation and creep damage has been built to study the microstructural evolution and deformation behavior for Ni-Al single crystal alloy during the whole creep processing.The relevant experiments were conducted to verify the model validity.The simulation results show that under the tensile creep at 1223 K/100 MPa,cubic γ'phases coarsen along the direction parallel to the axis of tensile stress during the first two creep stages;and spindle-shaped and wavy γ'phases are formed during tertiary creep,similar to the experimental results.The evolution mechanism ofγ'phases is analyzed from the perspective of changes of stress and strain fields.The island-like γ phase is observed and its formation mechanism is discussed.With the increase of creep stress,the directional coarsening of γ'phase is accelerated,the steady-state creep rate is increased and the creep life is decreased.The comparison between simulated and experimental creep curves shows that this phase-field model can effectively simulate the performance changes during the first two creep stages and predict the influence of creep stresses on creep properties.Our work provides a potential approach to synchronously simulate the creep microstructure and property of superalloys strengthened by γ'precipitates.     

A robust and efficient substepping scheme for the explicit numerical integration of a rate‐dependent crystal plasticity model

This paper describes the development of efficient and robust numerical integration schemes for rate‐dependent crystal plasticity models. A forward Euler integration algorithm is first formulated. An integration algorithm based on the modified Euler method with an adaptive substepping scheme is then proposed, where the substepping is mainly controlled by the local error of the stress predictions within the time step. Both integration algorithms are implemented in a stand‐alone code with the Taylor aggregate assumption and in an explicit finite element code. The robustness, accuracy and efficiency of the substepping scheme are extensively evaluated for large time steps, extremely low strain‐rate sensitivity, high deformation rates and strain‐path changes using the stand‐alone code. The results show that the substepping scheme is robust and in some cases one order of magnitude faster than the forward Euler algorithm. The use of mass scaling to reduce computation time in crystal plasticity finite element simulations for quasi‐static problems is also discussed. Finally, simulation of Taylor bar impact test is carried out to show the applicability and robustness of the proposed integration algorithm for the modelling of dynamic problems with contact. Copyright © 2014 John Wiley & Sons, Ltd.     

Liquid phase electro epitaxy growth kinetics of GaAs—A three-dimensional numerical simulation study

A three-dimensional numerical simulation study for the liquid phase electro epitaxial growth kinetic of GaAs is presented. The kinetic model is constructed considering (i) the diffusive and convective mass transport, (ii) the heat transfer due to thermoelectric effects such as Peltier effect, Joule effect and Thomson effect, (iii) the electric current distribution with electromigration and (iv) the fluid flow coupled with concentration and temperature fields. The simulations are performed for two configurations namely (i) epitaxial growth from the arsenic saturated gallium rich growth solution, i.e., limited solution model and (ii) epitaxial growth from the arsenic saturated gallium rich growth solution with polycrystalline GaAs feed. The governing equations of liquid phase electro epitaxy are solved numerically with appropriate initial and boundary conditions using the central difference method. Simulations are performed to determine the following, a concentration profiles of solute atoms (As) in the Ga-rich growth solution, shape of the substrate evolution, the growth rate of the GaAs epitaxial film, the contributions of Peltier effect and electromigration of solute atoms to the growth with various experimental growth conditions. The growth rate is found to increase with increasing growth temperature and applied current density. The results are discussed in detail.     

相场方法模拟氙枝晶生长研究

基于相场方法对纯物质的凝固过程进行了二维数值模拟.以氙为例,展示了界面形貌的演化过程,研究了相场参数对枝晶界面形貌的影响.结果表明,随着过冷度值的增加,晶核由一圆核逐渐发展为发达的枝晶,相应的枝晶稳态尖端速度明显加快;随着耦合系数的增大,晶核由一圆核发展为细长的枝晶,枝晶的稳态尖端速度呈单调递增趋势,界面的稳定性逐渐变差.     

热顶电磁成型系统磁场的实验与数值模拟

根据电磁场理论,建立了热顶-电磁连铸成型系统的物理和数学模型,用小线圈法实测了不同电源功率时系统内的磁场强度,用有限元软件数值模拟了感应线圈高度、结晶器高度、热顶结构等对成型系统内磁感应强度和分布的影响,结果表明:1)降低结晶器高度使系统内磁感应强度增强,但不十分明显;2)感应线圈的高度对系统内磁场影响显著,采用20mm线圈较40mm线圈的磁感应强度提高约85%;3)有载时的模拟结果显示,系统内的磁场呈现更明显的趋肤效应;热顶结构对磁场的强度和分布规律无明显的影响.研究结果表明热顶电磁连铸法将有利于提高电源效率和稳定液柱高度,改善铸坯的内外部质量.     

超重力场下球形炸药水下爆炸实验及数值模拟

常重力条件下,Mach数和Froude数无法同时满足相似。采用离心模型实验,研究水下爆炸冲击波特性及气泡脉动规律,并采用数值模拟方法重现了超重力场下的水下爆炸过程。实验及数值模拟的结果表明,重力的改变基本不影响冲击波的峰值,COLE理论在超重力环境下仍然适用,而超重力场下,小当量炸药可以模拟气泡的脉动迁移过程,其结果可以用于预测大当量炸药深水爆炸的特性。此外,为减少计算时间并提高计算精度,采用将二维冲击波作为初始条件映射到三维模型的建模方法。对于冲击波计算,网格尺寸宜取药包半径的1/20¹/10;对于气泡脉动,宜取药包半径的1/21/10;对于气泡脉动,宜取药包半径的1/2¹。