爆炸柱壳动态断裂准则的研究

本文以一种典型的FAE装置为实际研究对象，通过对其薄柱壳在内部爆炸载荷下的动态膨胀断裂过程进行数值模拟，推导出一种实用的动态断裂准则；且在推导中引入了一个损伤演化方程．准则表明最终的结构破裂取决于与临界损伤变量密切相关的断裂应变．经算例验证，该准则的理论解和数值结果吻合较好．     

腰椎椎间植骨融合有限元分析

为了评估植骨融合术对腰椎运动和应力分布的影响, 利用影像诊断技术(CT扫描) 和CAD三维重建技术获得人体腰椎三维模型, 椎间盘纤维环采用各向异 性纤维加强超弹性本构模型, 建立了包括椎体、椎间盘和韧带的正常有限元模型以 及L3-L4融合的腰椎L2-L4节段有限元模型, 并通过对比模拟和实验结果验证其有效 性. 利用商业有限元软件ABAQUS/Standard 进行前屈、后伸和轴向旋转载荷情况下 的模拟分析, 对比两种模型在不同状态下的运动范围及应力分布情况. 模拟结果显 示: 在相同的载荷情况下, 融合模型的运动范围和正常模型相比明显偏小; 相邻椎 体的应力分布与正常模型明显不同, 但植骨融合对相邻椎间盘的影响相对较小. 此 外, 小关节对维持脊椎正常生理功能起着重要作用, 小关节功能丧失会使相应节 段的椎间盘髓核压力增大.     

Modelling and numerical approach to a class of metal‐forming problems—Quasi‐steady case

A class of quasi‐steady metal‐forming problems, with rigid‐plastic, incompressible, strain and strain‐rate dependent material model and with unilateral frictionless and nonlinear, nonlocal Coulomb's frictional contact conditions is considered. A coupled variational formulation, constituted of a variational inequality, with nonlinear and nondifferentiable terms, and a strain evolution equation, is derived and under a restriction on the material characteristics and using a variable stiffness parameters method with time retardation, existence, uniqueness and convergence results are obtained and presented. An algorithm, combining this method and the finite element method, is proposed and applied for solving an example strip drawing problem. Copyright © 2011 John Wiley & Sons, Ltd.     

Fabrication of <1 1 0> oriented tungsten nano-tips by field-assisted water etching

We report the field-assisted H₂O etching that enabled us to fabricate nano-tips from polycrystalline <1 1 0> oriented tungsten wires at room temperature. We optimized the sharpening procedure in order to obtain field emissions (FEs) with high collimation. The typical tip apex was composed of a large base and a nano-protrusion with a radius of curvature less than 3.5 Å. The narrowest opening angle (full width at half maximum) of the FE was 4.3° at 150 pA. We prepared two types of tips using two different applied bias voltages during the H₂O etching. The electron microscope images and the analysis of Fowler-Nordheim (FN) plots revealed that the sizes of the individual bases depended on the fixed bias voltages during the H₂O etching and affected their FE properties. In addition, we could confirm that the FE current from the nano-tip was more stable than that of the normal tip.     

New approach based on continuum damage mechanics with simple parameter identification to fretting fatigue life prediction

A new continuum damage mechanics model for fretting fatigue life prediction is established. In this model, the damage evolution rate is described by two kinds of quantities. One is associated with the cyclic stress characteristics obtained by the finite element(FE) analysis, and the other is associated with the material fatigue property identified from the fatigue test data of standard specimens. The wear is modeled by the energy wear law to simulate the contact geometry evolution. A two-dimensional(2D) plane strain FE implementation of the damage mechanics model and the energy wear model is presented in the platform of ABAQUS to simulate the evolutions of the fatigue damage and the wear scar. The effect of the specimen thickness is also investigated. The predicted results of the crack initiation site and the fretting fatigue life agree well with available experimental data. Comparisons are made with the critical plane SmithWatson-Topper(SWT) method.     

KTX真空室支撑焊接设计及有限元分析

提出了一种KTX 真空室支撑焊接方案,并利用有限元方法模拟计算了焊接温度场分布及焊接残余应力场分布。通过理论分析和有限元分析相结合的方法,对焊缝结构进行了强度分析。分析结果验证了焊接结构的可靠性,并为真空室支撑结构的焊接工艺提供了理论依据。     

A constitutive model of aluminum foam for crash simulations

A new constitutive model for metallic foams is developed to overcome the deficiency of existing models in commercial finite element codes. The proposed constitutive model accounts for volume changes under hydrostatic compression and combines the hydrostatic pressure and von Mises stress into one yield function. The change of the compressibility of the metallic foam is handled in the constitutive model by allowing for shape changes of the yield surface in the hydrostatic pressure-von Mises stress space. The backward Euler method is adopted to integrate the constitutive equations to achieve numerical accuracy and stability. The model is implemented into LS-DYNA as a user-defined subroutine, verified with existing solutions, and validated with foam testing data. The verified and validated model is then utilized in the crushing simulations of foam-filled columns with square and hexagonal cross-sections. Two constitutive models are studied: the first using an exponential function to describe the relationship of plastic Poisson's ratio with respect to true strain and the second using linear interpolation function as an alternative approximation. The new foam model provides satisfactory prediction of crushing forces and deformed shapes compared to experimental results. Additionally, the new foam model was shown to have better numerical stability and accuracy than existing LS-DYNA built-in material models.     

Superconvergence analysis of Crank‐Nicolson Galerkin FEMs for a generalized nonlinear Schrödinger equation

The purpose of this article is to apply nonconforming finite element(FE) to solve a generalized nonlinear Schrödinger equation. First, a new important property of nonconforming FE (see ( 2.3 ) of Lemma 2 below) is proved by use of BHX lemma and the integral identities techniques. Second, a linearized Crank‐Nicolson fully discrete scheme is constructed and the superclose error estimate of order for original variable u in broken H¹‐norm is also derived by using the properties of element and the splitting argument for nonlinear terms, while previous works always only obtain convergent error estimates with this element. Furthermore, the global superconvergence is arrived at by the interpolated postprocessing technique. Finally, two numerical experiments are provided to confirm the theoretical analysis. Here, h is the subdivision parameter and τ is the time step.     

考虑土-结构相互作用效应的三维桩基结构动力有限元分析

结合简化阻抗法建立了三维相互作用结构的动力有限元分析模型,导出了考虑群桩刚体、惯性效应作用时的结构相互作用时程积分方程式,以近乎纯结构有限元的建模途径合理地反映出桩-土-上部结构在水平地震作用下的动力相互作用特性。模型中引入具有桩(筏)-土阻抗特性的弹阻单元来描述不同群桩布置、土层状况因素对体系反应的参与作用,而在动力方程中竖向SV剪切波经桩土刚体相互作用产生的水平、摇摆分量对体系的影响亦得以体现。20层桩承刚框架结构的动力分析表明:较柔的桩、土基础使得体系的SSI效应增强,但其参与程度与群桩效应相关,而结构构件内力较不考虑相互作用时有较大折减;应用该法可精确、快速地进行复杂上、下部结构时程反应分析。     

An Application of the Kane and Mindlin Theory to Crack Problems in Plates of Arbitrary Thickness

Classical plane solutions of the theory of elasticity, which are sometimes more than 100 years old, are still used today and provide a framework for the analysis of many practical problems. But, strictly speaking, these analytical solutions are only applicable to plates with vanishing thickness or infinite thickness, where the stress state could be classified as plane stress or plane strain, respectively. However, the through-the-thickness stresses that exist in a plate of given thickness have a significant impact in a number of practical applications; and these stresses are often inevitably ignored due to the lack of analytical tools. This paper presents new analytical results for crack tip opening displacement (CTOD) for the through-the-thickness crack in infinite plates with various thicknesses. These results are based on the solution for an edge dislocation in infinite plate of arbitrary thickness and an application of the distributed dislocation technique. The analytical predictions of the CTOD and the constraint factor are compared with the three-dimensional elasto-plastic finite element (FE) results. It is shown that both analytical and numerical results are in good agreement when the numerical calculations are not affected by the size of the FE mesh and by the boundaries of the FE model.