延性材料损伤演化及材料软化的孔洞尺寸影响

应用有限元方法，对延性材料孔洞型细观损伤演化进行探讨。考察具有相同初始孔洞体积分数fo、不同相对尺寸孔洞长大和对材料软化的影响。三种体胞分别为简单立方体胞、体心立方体胞以及非均匀孔洞群状分布体胞。体胞基体为Mises等向强化材料。采用大变形有限元方法，简单加载，应力三维度保持定值。分析结果发现，孔洞群状分布体胞模型所代表小尺寸孔洞的长大速度较慢，且在高应力状态下具有较强的承载力和较低的裂纹敏感性。     

High-resolution gas–oil two-phase flow visualization with a capacitance wire-mesh sensor

The application of a novel wire-mesh sensor based on electrical capacitance (permittivity) measurements for the investigation of gas–oil two-phase flow in a vertical pipe of 67 mm diameter under industrial operating conditions is reported in this article. The wire-mesh sensor employed can be operated at up to 5000 frames per second acquisition speed and at a spatial resolution of 2.8 mm. By varying the gas and liquid flow rates, different flow patterns, such as bubbly, slug and churn flow, were produced and investigated. From the images of gas void fraction distribution, quantitative flow structure information, such as time series of cross-sectional void fraction, radial void fraction profiles and bubble size distributions, was extracted by special image-processing algorithms.     

飞行时间质谱的离子峰形模拟分析方法和应用

飞行时间质谱的离子峰形中包含了离子产生时的速度分布和空间分布信息，了解速度和空间分布对离子峰形的影响，可以根据峰形推测离子的产生机理。运用概率密度理论建立了一套飞行时间质谱的峰形模拟的方法和程序，并通过模拟分析考察了不同的空间分布和速度分布对离子峰形的影响。在偏离空间聚焦条件下，离子峰形显示了空间分布的信息。     

切口试样破坏过程试验与细观数值分析

对切口圆棒试样与切口板试样进行单向拉伸试验和有限元数值分析，以试样最小横截面上各点变化的应力应变场为体胞演化的载荷控制条件，采用三维含球形微孔洞体胞模型分别对各点微孔洞长大规律进行数值模拟。结果表明，在试样得到的断口韧性区域内，微孔洞的生长速度较快；试样失稳前微孔洞相对体积百分数最大的区域与试验时试样的起裂位置一致，因而证实微孔洞的演化是导致试样材料在拉伸条件下破坏的主要原因，同时也证实体胞模型用于局部破坏分析的有效性。     

线接触弹流问题中可压缩性的影响

采用复合直接迭代法求解Reynolds方程,变形矩阵法求解表面变形,针对线接触弹流问题中压缩性的影响进行计算研究,讨论不同速度、不同负载条件下润滑剂可压缩性对压力和油膜厚度的影响。计算结果表明,可压缩性虽然对膜厚的影响相对小一些,但是对二次压力峰的位置与大小的影响很大;速度较高时,可压缩性是必须考虑的要素;考虑润滑剂可压缩性的影响,可以提高弹流润滑问题求解的稳定性。     

Study on forming defects in the rolling process of large aluminum alloy ring via adaptive controlled simulation

According to the distribution characteristics of equivalent plastic strain (PEEQ) in radial?Caxial ring rolling, the plastic deformation zones in cross section were established. A 3D rigid-viscoplastic finite element model (FEM) which was controlled adaptively was applied to investigate defects that occurred during ring rolling under ABAQUS software. PEEQ, stress, and temperature distributions in different deformation zones have been analyzed in this study. Strain peaks were found in the cross-section corners. Moreover, it was investigated that non-uniform strain, stress, and temperature distributions in the ring tend to cause non-uniform microstructure and properties. Therefore, forming defects and microstructure damage would appear in cross-section corners due to the high-strain deformation. Based on the new developed FEM of the radial?Caxial ring-rolling process and comprehensive numerical simulations, the size effects of feed rate and lubrication conditions on strain and temperature distributions and their uniformity were investigated by 3D coupled thermomechanical FE simulation. The results have good agreement with experiment. The achievements of this study can provide basis for quality control and technical guidance.     

Simulation and quantitative assessment of homogeneous and inhomogeneous particle distributions in particulate metal matrix composites

Reinforcement distributions play an important role in various aspects of the processing and final mechanical behaviour of particulate metal matrix composites (PMMCs). Methods for quantifying spatial distribution in such materials are, however, poorly developed, particularly in relation to the range of particle size, shape and orientation that may be present in any one system. The present work investigates via computer simulations the influences of particle morphology, homogeneity and inhomogeneity on spatial distribution measurements obtained by finite-body tessellation. Distribution inhomogeneity was simulated both by the segregation of particles away from specified regions within a microstructure and by generating point density peaks at random locations within a microstructure. Both isotropic and anisotropic inhomogeneous distributions were considered to simulate distribution patterns in PMMCs before and after mechanical working. It was found that the coefficient of variation of the mean near-neighbour distance (COV( d _mean )), derived from particle interfaces using finite-body tessellation, was essentially independent of particle shape, size distribution, orientation and area fraction in homogeneous (random) distributions, but showed great sensitivity to inhomogeneity. Increased values of COV( d _mean ) were seen for both forms of inhomogeneous distributions considered here, with little influence of particle morphology. The COV( d _mean ) was also seen to be sensitive to anisotropic clustering, the presence of which was identified via nearest-neighbour angles and cell orientations. Although generally formulated for PMMCs, the present results may be generalized to other systems containing low aspect ratio finite bodies of low to moderate area fraction.     

Study of void closure in hot radial forging process using 3D nonlinear finite element analysis

Hot radial forging is used to reduce porosity and increase strength for large-diameter billets. The goal of this research is to study void closure behavior in the hot radial forging process. A nonlinear coupled finite element model is developed to investigate the deformation mechanism of internal void defects during the hot radial forging process. The model is formulated in a three-dimensional frame and a viscoplastic material model has been used to describe the material behavior subjected to large deformation and high temperature. A global–local technique is employed to obtain accurate solutions around the void region. The effects of void location, mandrel, die shape, and the reduction of the tube thickness on the final void reduction are systematically investigated. The predicted reductions for central longitudinal voids in hot upsetting and hot rolling processes are in good agreement with experimental findings. The simulation results provide a valuable procedure for the design of porosity reduction during the hot radial forging process.     

大型锻件内部空洞缺陷修复的力学条件

介绍了大型锻件内部缺陷修复条件研究的现状,针对钢锭内部空洞缺陷的尺寸远远小于本身尺寸的特点,依据圣维南原理,将各种空洞缺陷的形状假设为数学上可处理的椭球形。由于大型锻件的锻造是在高温下进行的,材料具有粘性流动的特征。在上述分析假设的基础上,对于线性粘性材料模型,利用损伤力学中远场应力与物体内部微观损伤的力学关系,得到了锻造过程中外载荷与内部空洞体积变化的解析式,进而得到了空洞缺陷闭合的修复条件。条件表明应力状态影响空洞的闭合方式,三向压应力是修复空洞缺陷的最佳应力状态。该条件可直接根据锻造水压机的在线载荷与压下量计算锻件内部空洞缺陷的修复情况。不同边界应力状态下的试验表明,计算结果与试验结果接近。     

Numerical analysis of the impact effect of foreign bodies on a steam strainer in a steam turbine valve

An in-service steam strainer was found to have failed in a steam turbine valve due to the impact of a foreign body. Thus, a Finite element (FE) model was built to study the impact effect of a foreign body on a steam strainer. A Johnson–Cook strength model and its failure model were used to describe the impact-induced failure of the film and deformation of the support in the strainer. The effects of the impact location, shape, mass and speed of the foreign body on the failure of the strainer were investigated. The possible locations of micro- cracks in the film were determined. The FE results indicated that impact by an exfoliated metal part (with small mass, sharp edges and high speed hitting the film close to the rib) is likely to cause failure of the film and large deformation of the support.

     

循环加载下热处理对NiTi记忆合金丝超弹性变形行为和寿命的影响

通过循环加载试验,研究了不同热处理工艺下NiTi形状记忆合金丝超弹性的变形行为和寿命特征,得到了试样应力和寿命间的疲劳曲线。结果表明,不同的退火温度和退火时间对合金的超弹性性能具有不同程度的影响。     

截面形状对材料微孔洞损伤破坏的影响

为探讨截面形状对金属材料微孔洞损伤破坏的影响,针对圆形、矩形横截面缺口试样进行了拉伸破坏、预拉伸低温冲断试验,断口扫描电镜观察及有限元计算;用细观力学模型计算了缺口试样断裂时微孔洞长大体积率沿断口的分布和断口的起裂位置,并模拟了断口上起裂处微孔洞长大。结果表明：截面形状对材料微孔洞损伤演化有较大的影响,圆形、矩形横截面缺口试样断裂时起裂处微孔洞长大体积率相差较大,微孔洞长大的临界值不是材料常数;用R-T模型、GTN模型模拟非轴对称三轴应力状态下材料微孔洞损伤演化有较大的误差。     

基于狭缝光阑的剪切散斑干涉动态测量

为了精确、实时地测量物体表面的动态形变,提出了基于狭缝光阑的空间载波剪切散斑干涉系统。该系统通过倾斜迈克尔逊干涉仪的一个平面镜来产生剪切量和载波频率,实现空间频谱的移动;采用一个可调节的狭缝光阑控制散斑大小和空间频谱宽度。基于傅里叶变换与反变换在空间频率域上提取所需的频谱并计算相位图,最后通过一幅干涉条纹图得到相位分布信息。采用该系统对一个中心加载、四周固支的薄铝板进行了动态测量,分析了光学系统参数对测量结果的影响。结果表明,采用像素尺寸为4.65μm×4.65μm的高分辨率相机,焦距为8mm的成像镜头,设置剪切量为25mm,狭缝光阑X方向的尺寸为1mm时,可得到高质量的剪切散斑相位图。该方法可以在25frame/s的采集速度下,以43.6°的视场角实现动态形变的测量,可测形变峰值为0.5~30μm。     

延性材料损伤孔洞尺寸效应探讨

应用有限元方法 ,对延性材料孔洞型损伤中的尺寸效应问题进行尝试性探讨。考察具有相同初始孔洞体积分数 f0 、不同孔洞尺寸 (半径比为 2 :1.6 :0 .96 )的延性材料中 ,孔洞尺寸对表征材料损伤程度的孔洞体积分数 f演化过程的影响。结果发现 :在较低应力状态下 ,孔洞尺寸的差异 ,对孔洞增长的快慢影响较小 ;但在裂纹试件所对应的高应力状态下 ,这种影响很突出 :大尺寸孔洞加速了材料中孔洞体积分数 f的增长 ,加剧了材料损伤。所以降低材料中孔洞或颗粒尺寸有助于其延性、尤其是断裂韧性的提高     

Lubrication Analysis of a Connecting-Rod Bearing in a High-Speed Engine. Part I: Rod and Bearing Deformation

Analysis was performed on a connecting rod assembly operating at 6,500 rpm. The analysis was based on the finite element method, which includes effects of interference fitted bearings and pre-loaded bolts. The rod inertia force associated with the gas pressure was found to have a significant effect on the variation of the rod bearing shape. The shape variation of a connecting rod assembly is commonly ignored in rod bearing analyses. Investigation of bearing deformation was conducted in order to evaluate the lubrication characteristics.     

Microstructure sensitivity of fretting fatigue based on computational crystal plasticity

Three-dimensional finite element simulations are conducted to study the effects of microstructure on the fretting fatigue behavior of duplex Ti-6Al-4V. These fretting simulations involve a rigid cylindrical indenter pressed on the half space of Ti-6Al-4V with different realizations of microstructure. The deformation behaviors of the primary α and α/β lamellar phases at room temperature are described by three-dimensional crystal plasticity constitutive relations. Microstructure attributes considered in this sensitivity study include crystallographic texture, grain size, and grain size distribution. Voronoi tessellation is used to construct the three-dimensional finite element models with various grain size distributions. The plastic strain behaviors and the distribution of the average maximum plastic shear strain among grains are analyzed and contrasted. The relative susceptibility for crack formation, including effects of various microstructure features, is determined using the Fatemi-Socie parameter. The results suggest that both average grain size and especially crystallographic texture have more influence on the plastic deformation and fretting fatigue behavior than grain size distribution for the fretting condition considered.     

热态轴类含空洞锻件内部温度场分布

热态轴类锻件内部温度场分布特征对锻件内部空洞检测起着重大作用。针对该问题,在传热学理论的基础上建立了热态轴类含空洞锻件的二维非稳态传热模型。首先利用集总参数法获得该模型的内部边界条件。然后通过红外热像仪采集得到该模型的初始温度条件,结合分离变量法求解该模型,从而获得含空洞锻件的内部温度场分布。最后通过实验分析了不同空洞尺寸条件下锻件内部温度场的分布特征。     

Parametric study of two-phase flow by integral analysis based on power law distribution

To understand the fluid dynamic forces acting on a structure subjected to two-phase flow, it is essential to obtain detail information on the characteristics of that flow. The distributions of flow parameters across a pipe, such as gas velocity, liquid velocity and void fraction, may be assumed to follow a power law (Cheng 1998; Serizawa et al. 1975). The void fraction profile is, for example, uniform for bubbly flow, whereas for slug flow it is more or less parabolic. In the present work, the average values of momentum flux, slip ratio and other parameters were derived by integral analysis, based on approximate power law distributions. A parametric study with various distributions was performed. The existing empirical formulations for average void fraction, proposed by Wallis (1969), Zuber et al. (1967) and Ishii (1976), were considered in the derivation of the present results. Notably, the unsteady momentum flux for slug flow was approximated.     

供暖系统混水离心泵叶轮正反问题计算

基于流线迭代法和逐点积分法,用Fortran语言编程实现了供暖系统混水离心泵叶轮的水力设计,讨论了轴面流道形状、叶轮包角和滑移系数对设计计算结果的影响.基于Navier-Stokes方程,在贴体坐标系中,采用交错网格技术和SIMPLEC算法,对依据不同设计参数设计出的两个叶轮内部流场进行了数值模拟,并将计算得到的流场压力分布和轴面速度分布进行了对比,同时对两个叶轮进行性能预估.研究表明:轴面流道形状和叶轮包角影响叶轮内部压力场和速度场的均匀性,导致两个叶轮的水力效率值存在约2%的差别.     

The residual stress distribution in a plastically deformed model asperity

When rough metallic surfaces come into contact, plastic deformation may occur locally, even at the lightest loads. This plastic deformation is thought to be an important element in a wide range of contact failure mechanisms, including fatigue and nearly all forms of wear.In this paper a simple model of asperity plastic deformation is presented. The model is based on slip line field theory and is used to calculate residual and full-load stress distributions at fully plastic asperity contacts for normal and moderate tangential loads.Measurements of surface residual stress were carried out using two different techniques on a range of plastic contacts of various materials and geometries. The results show agreement with the main predictions of the theory.