聚合物熔体双尺度模型和SIMPLER方法在收缩流中的应用

采用宏细观双尺度模型描述了聚合物熔体的流动行为,实现了聚合物熔体宏观流变和细观哑铃大分子尺度间信息的传递。采用基于交错网格的SIMPLER-FDMS对双尺度模型进行求解,成功地计算了粘弹性熔体在4∶1平板收缩流动中的速度、压力以及应力的分布,得到了与实验数据和Renardy理论渐近分析结果相一致的结果。从而说明宏细观双尺度模型和基于交错网格的SIMPLER-FDMS算法在粘弹性熔体流动行为的数值模拟中是合理的和可行的。     

均匀剪切流场中液晶聚合物微观结构的无网格模拟

采用基于Doi理论的微-宏观双尺度模型研究了液晶聚合物在均匀剪切流场中的微观结构。为保证模拟结果的可靠性,在模型求解中率先使用了高精度、高稳定性的无网格方法,并通过对均匀流场中均质液晶聚合物系统的数值模拟,验证了该方法的有效性。文中着重研究了De数对平板Couette流中液晶聚合物微观结构的影响。模拟得到了液晶聚合物分子四种典型的微观运动结构,即弹性稳态、周期翻转、周期摆动和随流取向。定量分析了De数对两种周期运动的影响,预测了各种结构下分子的有序程度和可能出现的缺陷,并通过对取向角的详细分析,发现了两种边界层效应。     

侧风环境下高速列车外部流场数值模拟方法研究

该文基于有限体积法的CFD 软件以及块结构化和四面体非结构化网格,采用三维常物性定常不可压缩流动N-S 方程的SIMPLE 算法,利用国外典型模型试验数据对ICE2 高速列车侧风环境下的空气绕流进行了数值模拟方法研究。结果表明:四面体非结构化和块结构化网格一样均能得到较好的结果,同时采用二次非线性高雷诺数k-ε模型和MARS差分格式的数值结果与试验数据吻合较好,从而验证了数值方法的正确性。在此基础上进行了四面体非结构化网格的独立性研究,得出适宜的网格总数和固壁法向第一层网格厚度合理的取值。该文确定的数值计算方法和四面体非结构化网格划分方法可用于复杂几何外形高速列车的数值模拟。     

简单流场中聚合物稀溶液的分子构象

介绍了研究聚合物分子模拟流变性质的Brown动力学模拟方法,提出利用FENE珠-簧链分子模型来确定聚合物稀溶液分子链在简单剪切流场下不同时刻位形的变化,并对分子链与流动方向的取向角和分子链在速度梯度方向上的构象厚度进行了计算。     

非定常不可压Navier-Stokes方程基于Crank-Nicolson格式的两水平变分多尺度方法

不可压缩粘性流是密度不发生变化的流体运动．它们被用来描述许多重要的物理现象,例如：天气、洋流、绕翼型流动和动脉内的血液流动．Navier-Stokes方程是不可压缩粘性流的基本方程．因此,求解Navier-Stokes方程的数值方法在近几十年得到了广泛的关注．本文主要给出非定常不可压Navier-Stokes方程基于Crank-Nicolson格式的两水平变分多尺度方法．该方法分为两步：第一步,在粗网格上求解稳定的非线性Navier-Stokes系统;第二步,在细网格上求解稳定的线性问题去校正粗网格上的解．通过该方法推导的速度的误差估计关于时间是二阶收敛的．数值实验验证了在粗细网格匹配合理的情形下,本文的方法与直接在细网格上使用单网格的变分多尺度方法相比,可以节约大量的计算时间．     

网格划分对叉排管束摩擦阻力系数的影响

应用流体计算软件FLUENT,对受限空间内烟气冲刷叉排管束(横竖均为5排,间距比均为1.25)的流动进行了数值模拟研究,主要研究了三角形和四边形非结构化网格及四边形结构化网格划分对流场流动摩擦阻力系数f的影响。结果表明:结构化网格相对于非结构化网格,虽然建模时间较长,但网格生成速度快,计算耗时短,易收敛且精度高;对于同类型网格,网格边界层底层厚度及圆周节点数的选取均对模拟结果存在较大影响。     

结构化流道近壁区软性磨粒流流动仿真及实验研究

为了揭示结构化流道近壁区软性磨粒流的压力场及速度场的分布规律,有效预测其软性磨粒流的材料去除特性,采用VOF多相流模型和RNG 湍流模型,通过对结构化流道的结构进行合理的网格划分和特殊边界条件设置,对结构化流道内部的软性磨粒流流动进行数值模拟。仿真结果表明：不同的软性磨粒流入口位置对工件加工会产生影响;结构化流道中软性磨粒流的压力场、速度场和软性磨粒流的去除率随着加工区域位置的不同而不同;加工区域入口区域的压力、速度值存在突变,为此在实际加工用引入了引流模块。流场数值模拟和实验研究结果趋势是一致的,数值模拟为深入研究软性磨粒流的基本规律提供一种理论工具     

微-细双尺度单胞下织物预成型体渗透率预测

建立了描述双尺度单胞内树脂流动的数学模型,基于最小势能原理建立了织物单胞逼近真实的几何模型。研究了由纱线卷曲和相互挤压引起的微观渗透率分布的不均匀性及其在流动求解上的处理方法; 基于有限差分法建立了树脂流动控制方程数值求解方法,并开发了相应软件包,求得了单胞内树脂流动压力和速度场,进而获得了渗透率预测值; 通过与文献预测数据和实验值对比,证明了本文中预测模型和求解方法的正确性。     

Cartesian网格结合无网格法处理的二维柱体

本文发展了基于四叉树数据结构的网格生成和二维流动的N-S方程数值求解器及动边界问题的Euler方程求解方法。采用压力梯度或者密度梯度的绝对值作为网格自适应的控制参量,同时采用基于最小二乘法的无网格方法处理对于一般Cartesian网格难于处理的物面边界条件。文中采取了绕方柱流动和绕圆柱流动的经典算例对所发展的方法进行了验证。计算的结果验证了所发展的方法在处理绕流流动时的合理性和有效性。采用Naca0012翼型的几种工况验证了所发展的动网格技术在处理无粘流动的合理性和可行性。从而为数值模拟具有复杂几何外形的流动提供了一种网格布局合理、高效,边界处理简单易行的新思路。     

振荡压气机叶栅叶片表面非定常响应以及气弹稳定性分析

发展并验证了一种适用于叶轮机内部非定常跨音流动诱导的叶片气弹问题的高效、准确的数值模拟方法。采用有限体积的多块结构化网格形式,多重网格方法加速收敛,隐式的双时间步时间推进,Spalart-Allmaras（S-A）湍流模型求解非定常雷诺平均Navier-Stokes方程。通过气动弹性标准算例10,叶片在高亚音和跨音流动下做弯曲振动,分析了流动状态、折合频率以及叶片间相位角对叶片表面非定常气动力响应以及叶栅气弹稳定性的影响。分析结果表明激波在此跨音振荡压气机叶栅中起失稳作用,叶片间相位角对气弹稳定性的影响在高折合频率下被加强。     

An analysis of hot deformation of an Al–Cu–Mg alloy produced by powder metallurgy

The high-temperature plasticity of a 2014 aluminium alloy produced by powder metallurgy was investigated in a wide range of temperatures and strain rates. When the strain rate was plotted as a function of stress (either peak flow stress in torsion, or applied stress in tensile creep), the alloy exhibited the same threshold-like behaviour observed in similar materials. The microstructure of representative torsioned samples was analysed in a transmission electron microscope (TEM) and the characteristics of particles and precipitate distribution were estimated. The dependence on stress and temperature was analysed by means of the conventional constitutive equations used for describing the hot-working behaviour and by means of a modified form of the sinh-equation, where the stress was substituted by an effective stress i.e. by the difference between the actual stress and a threshold stress. This temperature-dependent threshold stress was found to be a constant fraction (15%) of the Orowan stress generated by the dispersion of alumina particles and of precipitated intermetallic phases.     

A study of the hot formability of an Al-Cu-Mg-Zr alloy

The high-temperature plasticity of a Zr-stabilised 2014 aluminium alloy was investigated in a wide range of temperatures and strain rates. The microstructure of representative torsioned samples was analysed by transmission electron microscopy, and the characteristics of particle and precipitate distribution were quantitatively estimated. The strain-rate dependence on stress and temperature was analysed by means of the conventional constitutive equations used for describing hot-working behaviour as well as a modified form of the sinh equation, where the stress was substituted by an effective stress, i.e., by the difference between the stress and a threshold stress. This temperature-dependent threshold stress was found to be a fraction of the Orowan stress generated by precipitates increasing from 62 to 94% as temperature decreased from 773 to 573 K.     

STRESS-RATIO EFFECT OF FATIGUE CRACK PROPAGATION IN A BIAXIAL STRESS FIELD

Fatigue crack propagation tests were conducted under conditions of equibiaxial, uniaxial and shear loading by using a cruciform specimen in a servo hydraulic testing machine. The effect of non-singular stress cycling on the fatigue crack propagation rate was examined based on the observation of crack opening behavior. The crack propagation rate was significantly influenced by the non-singular stress parallel to the crack when it was correlated to the stress intensity range. The crack closure behavior was greatly affected by the non-singular stress. The crack propagation rate was uniquely correlated to the effective range of the stress intensity factor except for the case of completely reversed shear loading where significant plasticity was detected. The crack opening displacement range was concluded to be a parameter controlling the crack propagation rate for all the stress conditions examined in the present experiments. Some discussion is made on the effect of material anisotropy on fatigue crack propagation in a biaxial stress field.     

ANALYSIS OF CAST STEEL FRACTURE MECHANISMS FOR DIFFERENT STATES OF STRESS

The process of fracture in a low-carbon cast steel was studied for different states of stress. As a result of heat treatment, two different microstructures have been obtained: ferritic-pearlitic and bainitic. The triaxial states of stress were realised by tensile tests on specimens with various notch configurations and on smooth specimens subjected to different hydrostatic pressures.
During tensile tests carried out under triaxial stress states, the following quantities at fracture were determined: the effective strain, effective stress, stress state components, mean stress and stress triaxiality factor. Fractography of the specimens was carried out to observe the fracture mechanisms and relate them to the state of stress. The fracture mechanism depended on the state of stress and microstructure. With a decreasing stress triaxiality factor, the failure mechanism changed from ductile to shear. The fracture mechanism changed across the diameter of the sample and also depended on the microstructure. The small, smooth samples fractured at a higher stress than the larger samples. Ductile fracture in the ferritic-pearlitic microstructure was controlled by cracking of the matrix–precipitate boundary. Samples with the bainitic microstructure fractured by shear, and fracture depended mainly on the effective stress, although void growth (which is controlled by stress triaxiality) reduced the critical effective stress at positive values of mean stress.     

初始应力对三维光学轮廓法测试焊接接头残余应力的影响

目的研究不同初始应力状态下,三维光学轮廓法测试焊接接头残余应力的变化规律。方法采用MIG焊分别对供货态与去应力退火态试板进行多层多道焊,焊后试板经慢走丝切割,经三维光学测量技术扫描切割面轮廓,将所得轮廓数据经所建立的数据处理平台处理,将其结果作为有限元计算的边界条件,经应力反算得到残余应力分布。最后再进行有限元模拟,计算焊接接头残余应力。结果含初始应力、去应力退火和数值模拟的焊缝中心均为拉应力区,最大拉应力分别为480, 450, 523 MPa,且都位于焊缝根部区域。三者试板两侧为压应力区域,最大压应力分别为380, 280, 157 MPa,三者数值相差较大。结论将含有初始残余应力试板、退火处理试板与数值模拟结果的残余应力分布进行对比,可以发现三者在焊缝中心处的残余应力分布较为一致,但沿着焊缝向两侧的区域内,应力差别逐渐变大。主要原因为焊接热循环温度高于金属再结晶温度时可以消除部分残余应力,而温度循环较低时对应力消除不明显,导致实验结果相差较大。     

基于比容测量对材料三维残余应力及其分布的测试分析

目的 判断比容差法是否可应用于材料内部残余应力分布状态的定量研究,旨在提供一种新的残余应力的测试方法。方法 通过对材料非切应变时的弹性力学进行讨论,推导体积应变,得出残余应力的计算公式。应用比容差法对一种工业纯铝的残余应力状态进行定量测量与表征,并进行钻孔法的定量验证。在此基础上,应用比容差法对纯铝镦粗变形后的三维平均残余应力的分布进行了测量研究。结果 比容差法对纯铝镦粗变形后的体积变化测量,可精确、定量表征材料的残余应力状态,与钻孔法实验结果相比,两者的误差在20%以内;镦粗变形后,残余压应力在坯料中心处取得最大值,且对称于中心沿高度方向逐渐减小,呈对称递减分布。结论 比容差法可用于材料三维残余应力分布的研究,为残余应力分布规律,提供了一种新的有效且精确的测试方法,具有实际应用的价值。     

Fatigue strength of small-notched specimens under variable amplitude loading within the fatigue limit diagram

Although the fatigue limit diagram is defined in principle for constant stress amplitude, it is often considered that fatigue failure would not occur, even in varying loading, if applied stresses were kept within the fatigue limit diagram. However, it was shown in the case of small‐notched specimens that fatigue failure occurred in some special cases of variable amplitude loading, even when all stress amplitudes were kept within the fatigue limit diagram. The cause of this phenomenon was examined using two‐step stress and repeated two‐step stress patterns in which the first step stress was chosen to be equal to the fatigue limit with zero mean stress and a mean stress was superposed on the second step stress. A non‐propagating crack was formed by the first step stress. This crack functioned as a pre‐crack for the second step stress with high mean stress. Consequently, fatigue failure occurred even when all stress amplitudes were kept within the fatigue limit diagram. It was an unexpected fracture caused by the interference effect of a non‐propagating crack and a mean stress change.     

Expansion of a cavity in a rubber block under unequal stresses

Elastic expansion of a small spherical void in the interior of a rubber block has been investigated by finite element analysis (FEA). The block was subjected to a far-field tensile or compressive stress while an internal pressure was applied to the cavity. The rubber was assumed to be virtually incompressible in volume and neo-Hookean in elastic behavior. Critical stress states were determined at which the void would become indefinitely large-a form of elastic instability. An applied tensile stress was found to lower the critical inflation pressure for instability, in agreement with experimental observations [1] whereas a compressive stress increased it. The critical mean stress was lowest for an isotropic stress system (when the applied tension was zero). It was higher when either compressive or tensile stresses were applied, i.e. under non-isotropic stresses, in agreement with the analysis of Hou and Abeyaratne [2]. However, the present results show a considerably smaller effect of tensile stress than predicted by Hou and Abeyaratne.     

Evaluation of Residual Stress in Flip Chip Using 3-D Optical Interferometry/FEM Hybrid Technique

Abstract:  The residual stress in flip chips was investigated by a hybrid technique of 3-D finite element method (FEM) and 3-D optical interferometry. The residual stress consists of two parts: the chemical shrinkage stress caused by underfill curing and the thermal stress caused by coefficient of thermal expansion (CTE) mismatch and cooling. Warpage and in-plane deformation of the flip chip during the underfill curing and cooling procedure was real-time characterised with an integrated 3-D optical interferometry system. The measurement results were used to evaluate the chemical shrinkage stress and to verify/modify the FEM model for the analysis of thermal stress. It was found that the chemical shrinkage stress under isothermal curing condition is very small in average and negligible. As the difference between the simulated and measured thermal deformation falls within a limited scope, the accuracy of the simulated thermal stress can be guaranteed.     

采用应力场强法对某型航空发动机压气机轮盘的疲劳寿命评估及试验研究

采用应力场强法对某型航空发动机压气机轮盘疲劳寿命进行了评估,对如何确定场径的大小、应力场拟合,以及应力场强因子计算等步骤进行了详细的讨论。基于该轮盘的全尺寸件开展了疲劳寿命试验。试验结果显示,采用应力场强法进行疲劳寿命评估的结果具有较高的精度,相对于传统的名义应力法具有显著的优越性。