碳纤维-泡沫铝夹芯板低速冲击响应

为研究夹芯结构的低速冲击响应,以碳纤维(T700)/环氧树脂复合材料层合板为上下面板,以闭孔泡沫铝为芯层,模拟夹芯板落锤冲击时的损伤演化过程。复合材料层合板采用三维实体单元建模,基于有限元软件ABAQUS中的用户子程序VUMAT,引入三维Hashin失效准则模拟复合材料的损伤破坏;采用二次应力准则,Cohesive单元模拟黏结层的层间失效;闭孔泡沫铝芯层采用3D Voronoi细观模型建模。分析复合材料夹芯结构在落锤冲击下的损伤起始、损伤扩展和最终破坏模式,通过锤头的接触力、位移、夹芯板的内能、后面板的最大位移研究夹层结构的能量吸收情况及抗冲击特性,得出了在质量保持不变的情况下,5种芯层相对密度和厚度的耦合关系中的最优设计是芯层相对密度15.0%,厚度为10 mm,为满足实际工程中的需求提供了设计依据。     

水平滚筒内二元颗粒体系径向分离模式的数值模拟研究

发展了考虑法向接触力、切向接触力和力矩、以及滚动摩擦力矩的三维三方程线性弹性-阻尼离散单元模型及计算程序,对薄滚筒内二元S型颗粒体系进行了数值模拟,发现采用本文的数学模型可以准确地预测出滚筒内二元S型颗粒流的分层现象.分析了影响滚筒内颗粒分层的因素,讨论了滚筒转速、颗粒装载率等参数对分层的影响,当转速较高时,滚筒内形成大颗粒在外、小颗粒在内、具有圆形界面的月亮模式,当转速较低时形成具有波浪形界面的花瓣模式,并且随着滚筒转速的逐渐降低,花瓣的数量逐渐增加,数值模拟结果与实验完全符合.模拟还得到了花瓣模式的形 关键词： 分层 滚筒 模式形成 离散单元方法     

涡轮动叶冷却结构设计方法I:参数化设计

基于冷却结构参数化设计、传统的管网计算方法与新兴的气热耦合数值模拟技术,提出了一套涡轮冷却结构设计方法,并编写了相关的设计程序与计算程序.利用参数化设计方法可以快速而精确地设计冷却结构,管网计算用于指导方案设计,气热耦合数值模拟用于指导详细设计.参数化设计中利用"单元设计法"程序实现动叶冷却通道快速设计;借助管网计算模...     

圆柱界面不稳定性的多组分气体动理学数值计算

在假定单元内各组分同温同速的条件下,采用气体动理学格式(Gas-Kinetic Scheme,GKS)对空气/He和空气/R22圆柱界面不稳定性进行了数值计算,得到不同时刻的密度分布以及界面上特征位置的位移历史和平均速度。当激波穿过界面后,界面上特征位置的位移随时间逐渐增大,特征位置的平均速度与前人的实验结果和数值模拟结果吻合很好。对比结果表明,从微观气体运动角度出发的GKS方法对于界面不稳定性问题具有良好的模拟能力。     

水平滚筒内二元颗粒体系径向分离模式的数值模拟研究

发展了考虑法向接触力、切向接触力和力矩、以及滚动摩擦力矩的三维三方程线性弹性-阻尼离散单元模型及计算程序，对薄滚筒内二元S型颗粒体系进行了数值模拟，发现采用本文的数学模型可以准确地预测出滚筒内二元S型颗粒流的分层现象.分析了影响滚筒内颗粒分层的因素，讨论了滚筒转速、颗粒装载率等参数对分层的影响，当转速较高时，滚筒内形成大颗粒在外、小颗粒在内、具有圆形界面的月亮模式，当转速较低时形成具有波浪形界面的花瓣模式，并且随着滚筒转速的逐渐降低，花瓣的数量逐渐增加，数值模拟结果与实验完全符合.模拟还得到了花瓣模式的形     

ρ-VOF:一种可清晰模拟自由界面的两相流方法

文章通过对EFM(effective field modeling)模型进行简化, 消除了原模型的非守恒性项和非双曲性特性项, 发展了一种基于密度的气液两相流模拟方法: ρ-VOF方法.利用体积分数信息对控制单元内的自由界面进行重构, 得到了控制单元内流体的空间分布, 并采用AUSM+-up格式获得考虑气液流体接触间断信息的对流通量.新方法可统一处理激波间断和接触间断的相互作用, 保持自由界面的尖锐性, 并且其计算量与自由界面的空间复杂度无关.最后, 数值模拟了液体激波管气液激波管和气体激波跨二维液滴传播等问题, 并与文献结果进行对比, 验证了本方法在气液两相流模拟中的准确性.      

平面应变问题广义有限元法

结合广义有限元和理性有限元,针对平面应变问题提出一种广义四边形单元.该单元考虑泊松效应,以节点位移自由度约束弹性力学平面应变方程的半解析解,构造单元位移模式的附加项,较准确地反映真实位移场,提高了单元的计算精度.首先推导广义单元及其等参单元的形函数,之后设计分片试验和数值算例来验证单元的精度,数值算例表明,在规则和非规则网格下新单元计算精度均优于传统有限元和广义有限元.新单元精度高且易于程序实现,可应用于实际工程的结构分析.     

阻抗梯度飞片加载下的超高速发射二维数值模拟方法

阻抗梯度飞片准等熵加载和超高速发射的二维数值模拟,在计算方法上集中体现了多介质、多界面、大变形、高密度比等特点.采用多介质流体高精度PPM计算方法,以VOF为基础研制MFPPM2二维计算程序,数值模拟Sandia实验室的超高速发射实验模型,获得了与Sandia实验室数值计算一致的结果.     

应用多介质PPM方法计算斜激波与物质交界面的相互作用

利用多介质PPM方法研究斜激波与物质交界面的相互作用.采用与体积分数耦合的Euler方程组作为计算模型,用双波近似来求解一般刚性气体状态方程Riemann问题.通过体积分数的计算来获得界面的位置,在整个流场采用统一的高阶PPM格式进行计算.文中对斜激波与不同物质界面相互作用进行了数值模拟,并给出了交界面上由于斜压效应产生的涡列的演化过程,特别是强斜激波与不同物质界面的相互作用的情况.     

相场方法模拟铝合金三维枝晶生长

以相场模型为基础，采用宏微观耦合方法和界面捕获液态方法对铝合金枝晶生长进行模拟计算.为解决试样全场微观计算的困难，采取宏微观耦合的计算方法，试样整体计算温度场，而微观组织计算只在一个确定的宏观单元内进行，宏观微观计算交替耦合进行.在不改变相场模型的条件下，提出界面捕获液态计算方法.赋值计算单元界面标志，只对界面处的单元求解相场变量，当枝晶生长时，捕获液态单元为界面从而推进界面，并对捕获到的单元校正相场变量.通过界面捕获液态方法加速相场模型的计算，实现了铝合金试件局部三维单晶粒和多晶粒的模拟.对模拟结果与实 关键词： 相场方法 液态捕获 微观组织 枝晶     

Theoretical calculation for the equation of state and phase transition of lithium hydride crystal

Abstract

From the point of view of overlapping interactions between the nearest neighbours, while considering the compression effect of each ion, an ionic overlap-compression model is founded and applied to lithium hydride. The repulsive potential and cohesive energy curves of the crystal are calculated by a one-parameter variational method. The obtained equilibrium lattice constant (3.865 a₀), cohesive energy (? 218.82 kcal/mol), and bulk modulus (353 kbar) agree with experimental values surprisingly well. The calculated values of the equation of state also reach a good agreement with the experimental ones available below 40 kbar. A phase transition from NaCl to CsCl structure is predicted to occur around 0.85 Mbar, with a volume jump of about 6%.     

A hexahedron element formulation with a new multi-resolution analysis

A multiresolution hexahedron element is presented with a new multiresolution analysis(MRA)framework.The MRA framework is formulated out of a mutually nesting displacement subspace sequence,whose basis functions are constructed of scaling and shifting on element domain of a basic node shape function.The basic node shape function is constructed from shifting to other seven quadrants around a specific node of a basic isoparametric element in one quadrant and joining the corresponding node shape functions of eight elements at the specific node.The MRA endows the proposed element with the resolution level(RL)to adjust structural analysis accuracy.As a result,the traditional 8-node hexahedron element is a monoresolution one and also a special case of the proposed element.The meshing for the monoresolution finite element model is based on the empiricism while the RL adjusting for the multiresolution is laid on the solid mathematical basis.The simplicity and clarity of shape function construction with the Kronecker delta property and the rational MRA enable the proposed element method to be more rational,easier and efficient in its implementation than the conventional mono-resolution solid element method or other MRA methods.The multiresolution hexahedron element method is more adapted to dealing with the accurate computation of structural problems.     

Nonlinear constitutive behavior of ferroelectric materials

The ferroelectric specimen is considered as an aggregation of many randomly oriented domains. According to this mechanism, a multi-domain mechanical model is developed in this paper. Each domain is represented by one element. The applied stress and electric field are taken to be the stress and electric field in the formula of the driving force of domain switching for each element in the specimen. It means that the macroscopic switching criterion is used for calculating the volume fraction of domain switching for each element. By using the hardening relation between the driving force of domain switching and the volume fraction of domain switching calibrated, the volume fraction of domain switching for each element is calculated. Substituting the stress and electric field and the volume fraction of domain switching into the constitutive equation of ferroelectric material, one can easily get the strain and electric displacement for each element. The macroscopic behavior of the ferroelectric specimen is then directly calculated by volume averaging. Meanwhile, the nonlinear finite element analysis for the ferroelectric specimen is carried out. In the finite element simulation, the volume fraction of domain switching for each element is calculated by using the same method mentioned above. The interaction between different elements is taken into account in the finite element simulation and the local stress and electric field for each element is obtained. The macroscopic behavior of the specimen is then calculated by volume averaging. The computation results involve the electric butterfly shaped curves of axial strain versus the axial electric field and the hysteresis loops of electric displacement versus the electric field for ferroelectric specimens under the uniaxial coupled stress and electric field loading. The present theoretical prediction agrees reasonably with the experimental results. Supported by the National Natural Science Foundation of China (Grant No. 10572138)     

Crack Tip Opening Angle as a Fracture Resistance Parameter to Describe Ductile Crack Extension and Arrest in Steel Pipes under Service Pressure

The angle between two element sides representing the crack tip is defined as the crack tip opening angle (CTOA). Its critical value is used as a criterion of fracture resistance for characterizing stable tearing in thin metallic materials. Various methods are used for determination of the CTOA. Optical microscopy is one of the most common methods as well as fitting of experimental load-displacement diagrams by the finite element method (DIC). Additionally, analytical analysis using the experimental load-displacement curve method (SSM) derived from the plastic hinge model of deflection in three-point bending of a ductile specimen is applied. This approach assumes a constant rotation centre distance. Values of CTOA for API 5L X65 pipe steel found by three methods—DIC, CNM, and SSM—are given. Values of CTOA given by these three methods are similar and close to 20°. A discussion on the different parameters used to characterize the fracture resistance of running cracks in a pipe under service pressure is presented. The energy of fracture at impact determined by Charpy or drop-weight tear test (DWTT) tests and the critical J energy parameter are considered as well as the yield locus after damage, cohesive zone energy, and CTOA is another approach. One notes that CTOA is assumed to be constant during stable crack extension and decreases linearly with crack length during the instable and primary phase. A numerical technique to describe a ductile running crack using the node release technique and using CTOA as the fracture resistance criterion is presented. This method is compared with three different two-curve methods (TCMs): the Battelle, high strength line pipe (HLP), and HLP-Sumitomo methods. The Batelle TCM, as the oldest method, based on Charpy energy, gives a strongly conservative prediction. Predictions by the CTOA method are close to those obtained by the HLP-Sumitomo one.     

硅单粒子位移损伤多尺度模拟研究

本文结合分子动力学方法和动力学蒙特卡罗方法, 研究了单个粒子入射硅引起的位移损伤缺陷的产生和演化过程; 基于Shockley-Read-Hall理论计算了单个粒子入射引起的位移损伤缺陷导致的泄漏电流增加及其演化过程, 比较了缺陷退火因子与泄漏电流退火因子之间的差异, 并将计算结果与实验值进行了对比. 结果表明, 计算泄漏电流时, 仅考虑一种缺陷的情况下缺陷退火因子与泄漏电流退火因子相同, 考虑两种缺陷类型情况下二者在数值上有所区别, 但缺陷退火因子仍能在一定程度上反映泄漏电流的退火行为. 分子动力学模拟中采用Stillinger-Weber势函数和Tersoff势函数时缺陷退火因子和泄漏电流退火因子与实验结果一致, 基于Stillinger-Weber势函数的计算结果与实验值更为接近.     

A theoretical study on vertical finite cracks detection using pulsed laser spot thermography (PLST)

A theoretical study on vertical finite cracks detection in low carbon steel (A36) blocks using pulsed laser spot thermography (PLST) was carried out in this paper. The basic principle of PLST was described. The 3D heat conduction model of flux transfer under pulsed laser spot excitation was established and calculated using finite element method (FEM). The effect of laser-crack distance to temperature abrupt jump across the vertical finite cracks was analyzed, and results show that the optimum laser-crack distance is the radius of laser spot. The relationship between geometric parameters of cracks and temperature abrupt jump was studied, and results show larger geometric size of cracks leads to greater peak values of temperature abrupt jump, and tends to stable values at certain size.     

Micro-crack detection of nonlinear Lamb wave propagation in three-dimensional plates with mixed-frequency excitation

We propose a nonlinear ultrasonic technique by using the mixed-frequency signals excited Lamb waves to conduct micro-crack detection in thin plate structures. Simulation models of three-dimensional(3D) aluminum plates and composite laminates are established by ABAQUS software, where the aluminum plate contains buried crack and composite laminates comprises cohesive element whose thickness is zero to simulate delamination damage. The interactions between the S_0 mode Lamb wave and the buried micro-cracks of various dimensions are simulated by using the finite element method.Fourier frequency spectrum analysis is applied to the received time domain signal and fundamental frequency amplitudes,and sum and difference frequencies are extracted and simulated. Simulation results indicate that nonlinear Lamb waves have different sensitivities to various crack sizes. There is a positive correlation among crack length, height, and sum and difference frequency amplitudes for an aluminum plate, with both amplitudes decreasing as crack thickness increased, i.e.,nonlinear effect weakens as the micro-crack becomes thicker. The amplitudes of sum and difference frequency are positively correlated with the length and width of the zero-thickness cohesive element in the composite laminates. Furthermore,amplitude ratio change is investigated and it can be used as an effective tool to detect inner defects in thin 3D plates.     

主动光学薄镜支撑及驱动研究

随着光学遥感器分辨能力的不断提高和口径的不断增大,导致遥感器的重量越来越重,使得载体无法承受.针对此问题,提出了对薄反射镜进行主动支撑的方式,以解决口径大、重量重的问题.利用有限元分析和数学理论相结合的方法在对反射镜施加垂直镜面1g加速度(g=9.80665m/s2为重力加速度)和10℃的温升载荷情况下,对薄反射镜的位...     

稀土元素的价电子结构和熔点、结合能的关联性

稀土是重要的战略物资资源,有一些已形成产业,在永磁、发光、催化和储氢等方面有着重要的作用. 对稀土及其化合物的研究一直是国内外研究的热点.对稀土原子结构和价电子结构的研究有助于对其的更深刻的理解. 依据固体与分子经验电子理论,对镧系稀土和Sc, Y的价电子结构进行系统地研究, 并以此为基础,对它们的熔点和结合能做进一步分析,分析结果和实验值相符. 研究结果表明:稀土金属的结构与晶格电子和共价电子密切相关,随着晶格电子向共价电子的转换, 稀土金属的熔点趋于增加.稀土的价态变化也是影响电子分布和性能的主因, 如: Sm和Eu的熔点与电子结构的关系不同于其他稀土,外壳层的共价电子之间的转化是其熔点与其他元素之间差别较大的主要原因.稀土的4f电子对结合能的影响大,这是源于其4f电子引发的收缩效应. 此结果揭示了决定稀土熔点和结合能的主要因素是稀土的价电子结构的变化.