纳米压痕过程的三维有限元数值试验研究

采用有限元方法模拟了纳米压痕仪的加、卸载过程，三维有限元模型考虑了纳米压痕仪的标准Berkovich压头．介绍了有限元模型的几何参数、边界条件、材料特性与加载方式，讨论了摩擦、滑动机制、试件模型的大小对计算结果的影响，进行了计算结果与标准试样实验结果的比较，证实了模拟的可靠性．在此基础上，重点研究了压头尖端曲率半径对纳米压痕实验数据的影响．对比分析了尖端曲率半径r＝0与r＝100nm两种压头的材料压痕载荷—位移曲线．结果表明，当压头尖端曲率半径r≠0时，基于经典的均匀连续介质力学本构理论、传统的实验手段与数据处理方法，压痕硬度值会随着压痕深度的减小而升高．     

Extraction-condensation method for solving large sets of equations in FE analysis

In static and dynamic analysis of a large complex structure, one may merely be interested in the stresses and displacements at some important locations. A new solution technique for structural analysis with FEM is proposed in this paper by which those important unknowns are separated from the rest. In static analysis the important unknows can be found directly. In dynamic analysis all mass and exciting forces can be condensed onto a small number of predetermined points in order to make the dynamic analysis much simplified. A large scale problem can be solved with a small capacity computer quickly.     

多晶金属弹粘塑性的取向元模型

基于“三维组集式本构模型”［１～３］，运用功共轭原理和场平均方法，发展了一种“取向元”概念——多晶聚集体内具有相同取向的滑移系集的一个体积平均意义上的代表性元素；由一维的率（粘性）敏感“取向元”，通过在取向空间里的积分，获得了三维的弹粘塑性本构方程式，并模拟了蠕变和松弛现象．与以往的粘塑性模型相比，本文模型不仅能同时考虑多晶金属材料的率相关性和路径相关性，而且由于引入了物理机制，因而具有较强的预测能力．数值算例也展示了该模型的合理性     

一种岩石概率型本构及其应用

在岩石材料非均匀特性的基础上,建立了弹塑性概率材料本构．认为材料单元的切线模量与破坏强度都是服从二参数Weibull分布的随机量,通过用户材料子程序将该本构导入到LS—DYNA3D中,推导了程序流程,分析了导入过程中的关键技术．算例验证表明,应用该本构模拟得出的现象与实验观测完全一致,这是确定性本构无法实现的．     

构元组集损伤断裂模型及其与内聚区模型的比较

结构的响应实质上是材料的响应,宏观结构损伤至断裂的发展过程也是材料性质不断演化的结果.构元组集模型从材料的微观物理变形机制出发,基于对泛函势理论和Cauchy-Born准则,抽象出两种构元:弹簧束构元和体积构元.在微观层次上,结构损伤和断裂的实质都是原子间键合力减弱和丧失的结果,而弹簧束构元是同一方向上的原子键的抽象,因此损伤可以通过弹簧束构元的响应曲线来反映.组集两种构元的响应,建立了材料的弹性损伤本构关系,从而能一致描述材料从弹性到损伤、破坏的发展过程.将构元组集模型的本构关系嵌入ABAQUS的用户材料单元子程序UMAT,实现对结构响应的数值模拟.论文模拟了包含中心预制裂纹三点弯曲梁的裂纹扩展过程,并与内聚区模型比较,给出了内聚区模型所假设的应力-位移关系曲线,并从材料损伤演化的角度对材料裂纹扩展过程做出了物理解释.     

航行体垂直出水载荷与空泡溃灭机理分析

针对航行体水下垂直发射出水载荷机理问题, 首先开展了典型工况全过程的数值 模拟, 得到了航行 体肩、尾空泡及表面压力的演化过程, 并与试验结果进行了对比验证. 在此基础上揭示了航 行体肩部空泡溃灭的过程和机制, 进而提出了出水溃灭压力的物理模型, 探索了空泡厚度、 水层厚度、声速等重要参数的影响, 讨论了试验溃灭压力相似律等相关问题.     

两种弹性损伤模型的基本方程与色散关系讨论

分析了传统连续介质损伤理论的控制方程在损伤过程中的变化特点,方程类型的改变导致错误预测波不能在损伤区域传播.二阶隐式应变梯度损伤理论是对传统理论的一种改进,严格证明了其控制方程在损伤过程中类型不变,这意味着损伤区域能传播波,同时有助于克服病态的网格相关性.色散分析结果表明:传统连续介质损伤理论不能反映色散现象,隐式梯度模型可以并对波长有上限截断作用.     

An elasto-plastic damage constitutive theory and its prediction of evolution of subsequent yield surfaces and elastic constants

Based on pair functional potentials, Cauchy-Born rule and slip mechanism, a material model assembling with spring-bundle components, a cubage component and slip components is established to describe the elasto-plastic damage constitutive relation under finite deformation. The expansion/shrink, translation and distortion of yield surfaces can be calculated based on the hardening rule and Bauschinger effect defined on the slip component level. Both kinematic and isotropic hardening are included. Numerical simulations and predictions under tension, torsion, and combined tension-torsion proportional/non-proportional loading are performed to obtain the evolution of subsequent yield surfaces and elastic constants and compare with two sets of experimental data in literature, one for a very low work hardening aluminum alloy Al 6061-T6511, and another for a very high work hardening aluminum alloy annealed 1100 Al. The feature of the yield surface in shape change, which presents a sharp front accompanied by a blunt rear under proportional loading, is described by the latent hardening and Bauschinger effect of slip components. Further, the evolution law of subsequent yield surfaces under different proportional loading paths is investigated in terms of their equivalence. The numerical simulations under non-proportional loading conditions for annealed 1100 Al are performed, and the subsequent yield surfaces exhibit mixed cross effect because the kinematic hardening and isotropic hardening follow different evolution tendency when loading path changes. The results of non-proportional loading demonstrate that the present model has the ability to address the issue of complex loading due to the introduction of state variables on slip components. Moreover, as an elasto-plastic damage constitutive model, the present model can also reflect the variation of elastic constants through damage defined on the spring-bundle components.     

走进地震预测——————加卸载响应比方法

 概括介绍了加卸载响应比的基本思路、基本概念、物理基础、加卸载 响应比在地震预测中的应用以及取得的一些成果. 加卸载响应比方法物理基础扎实, 只需利 用地球内部的某些实测数据, 为地震预测提供了一条新的途径.     

Effective elastic moduli of triangular lattice material with defects

This paper presents an attempt to extend homogenization analysis for the effective elastic moduli of triangular lattice materials with microstructural defects. The proposed homogenization method adopts a process based on homogeneous strain boundary conditions, the micro-scale constitutive law and the micro-to-macro static operator to establish the relationship between the macroscopic properties of a given lattice material to its micro-discrete behaviors and structures. Further, the idea behind Eshelby’s equivalent eigenstrain principle is introduced to replace a defect distribution by an imagining displacement field (eigendisplacement) with the equivalent mechanical effect, and the triangular lattice Green's function technique is developed to solve the eigendisplacement field. The proposed method therefore allows handling of different types of microstructural defects as well as its arbitrary spatial distribution within a general and compact framework. Analytical closed-form estimations are derived, in the case of the dilute limit, for all the effective elastic moduli of stretch-dominated triangular lattices containing fractured cell walls and missing cells, respectively. Comparison with numerical results, the Hashin–Shtrikman upper bounds and uniform strain upper bounds are also presented to illustrate the predictive capability of the proposed method for lattice materials. Based on this work, we propose that not only the effective Young’s and shear moduli but also the effective Poisson’s ratio of triangular lattice materials depend on the number density of fractured cell walls and their spatial arrangements.