Simulation of jet-flow solid fraction during spray forming

A numerical model was developed to simulate the jet-flow solid fraction of W18Cr4V high-speed steel during spray forming. The whole model comprises two submodels:one is an individual droplet model, which describes the motion and thermal behaviors of individual droplets on the basis of Newton's laws of motion and the convection heat transfer mechanism; the other is a droplet distribution model, which is used to calculate the droplet size distribution. After being verified, the model was used to analyze the effects of parameters, including the initial gas velocity, deposition distance, superheat degree, and the ratio of gas-to-metal mass flow rates, on the jet-flow solid fraction. Finally, an equation to predict the jet-flow solid fraction directly and conveniently according to the parameters was presented. The values predicted by the equation show good agreement with those calculated by the numerical model.     

THE CONVERSION OF PAN-BASED STABILIZED FIBER TO CARBON FIBER

When the PAN-based stabilized fiber(PAN-SF) was converted to the carbon fiber, the effect of some of the carbonizing parameters on the structure and properties of the resulting carbon fibers, such as the molecular structure development, element contents, morphology and mechanical properties, was discussed. The results show that the carbonizing temperature, the purity of the inert gas and the de-oil pretreatment of the tiber have a great influence on them.     

Effect of thermal deformation parameters on the microstructure,texture, and microhardness of 5754 aluminum alloy

The evolution of the microstructure, texture, and microhardness of 5754 aluminum alloy subjected to high-temperature plastic deformation under different deformation conditions was studied on the basis of thermal simulations and electron-backscattered diffraction and Vickers microhardness experiments. The results of a misorientation angle study show that an increase in the deformation temperature and strain rate promoted the transformation of low-angle grain boundaries to high-angle grain boundaries, which contributed to dynamic recrystallization. The effect of the deformation parameters on the texture and its evolution during the recrystallization process was explored on the basis of the orientation distribution function. The results demonstrate that the deformed samples mainly exhibited the features of type A, B, and B textures. The formation and growth of the recrystallized grains clearly affected the texture evolution. The microhardness results show that the variation of the microhardness was closely related to the temperature, strain rate, and dynamic recrystallization.     

Modeling of microstructure evolution and mechanical properties during hot-strip rolling of Nb steels

An integrated metallurgical model was developed for Nb steels to predict the microstructure evolution and mechanical properties during the hot-strip rolling and cooling process. On the basis of the industrial data, the transformation kinetics, strength, and elongation rate were evaluated for different chemical compositions and processing parameters. The yield strength and tensile strength increase with increasing Nb content or decreasing finishing temperature. The bainite distributed in finer ferrite matrix, which is produced at relatively low coiling temperatures, can greatly increase the strength of steel, especially tensile strength, thereby decreasing the yield ratio. A reasonable agreement was found between the predicted and measured results. It indicates that the present models can be used to simulate the actual production process.     

Application of support vector machine in the prediction of mechanical property of steel materials

The investigation of the influences of important parameters including steel chemical composition and hot rolling parameters on the mechanical properties of steel is a key for the systems that are used to predict mechanical properties. To improve the prediction accuracy, support vector machine was used to predict the mechanical properties of hot-rolled plain carbon steel Q235B. Support vector machine is a novel machine learning method, which is a powerful tool used to solve the problem characterized by small sample, nonlinearity, and high dimension with a good generalization performance. On the basis of the data collected from the supervisor of hot-rolling process, the support vector regression algorithm was used to build prediction models, and the off-line simulation indicates that predicted and measured results are in good agreement.     

Structure instability forecasting and analysis of giant rock pillars in steeply dipping thick coal seams

Structure stability analysis of rock masses is essential for forecasting catastrophic structure failure in coal seam mining. Steeply dipping thick coal seams (SDTCS) are common in the Urumqi coalfield, and some dynamical hazards such as roof collapse and min-ing-induced seismicity occur frequently in the coal mines. The cause of these events is mainly structure instability in giant rock pillars sand-wiched between SDTCS. Developing methods to predict these events is important for safe mining in such a complex environment. This study focuses on understanding the structural mechanics model of a giant rock pillar and presents a viewpoint of the stability of a trend sphenoid fractured beam (TSFB). Some stability index parameters such as failure surface dips were measured, and most dips were observed to be between 46° and 51°. We used a digital panoramic borehole monitoring system to measure the TSFB’s height (ΔH), which varied from 56.37 to 60.50 m. Next, FLAC3D was used to model the distribution and evolution of vertical displacement in the giant rock pillars;the re-sults confirmed the existence of a TSFB structure. Finally, we investigated the acoustic emission (AE) energy accumulation rate and ob-served that the rate commonly ranged from 20 to 40 kJ/min. The AE energy accumulation rate could be used to anticipate impeding seismic events related to structure failure. The results presented provide a useful approach for forecasting catastrophic events related to structure in-stability and for developing hazard prevention technology for mining in SDTCS.     

Thermomechanical Behavior in Continuous Bloom Casting with Different Mold Tapers

A two-dimensional finite element model was used to analyze the thermal and mechanical behavior during solidification of the strand in a continuous bloom casting mold. The coupled heat transfer and deformation were analyzed to simulate the formation of the air gap between the mold and the strand. The model was used to investigate the influence of mold taper on the temperature and stress distributions in the strand. The results show that the air gap mainly forms around the strand corner, causing a hotter and thinner solidifying shell in this region. The mold taper partially compensates for the strand shell shrinkage and reduces the influence of the air gap on the heat transfer. The mold taper compresses the shell and changes the stress state around the strand corner region. As the strand moves down into the mold, the mold constraint causes compressive stress beneath the corner surface, which reduces the hot tear that forms on the strand.     

Transitional phenomenon of particle dispersion in gas-solid two-phase flows

Without using any turbulent model, direct numerical simulation of a three-dimensional gas-solid two-phase turbulent jet was performed by finite volume method. The effects on dispersion of particles with different Stokes numbers by the transitional behavior of turbulent structures were investigated. To produce high-resolution results and reduce the computation and storage, the fractional-step projection algorithm was used to solve the governing equations of gas phase fluid. The low-storage, three-order Runge-Kutta scheme was used for time integration. The governing equations of particles were solved in the Lagrangian framework. These numerical schemes were validated by the good agreement be-tween the statistical results of flow field and the related experimental data. In the study of particle dis-persion, it was found that the effects on particle dispersion by the spanwise vortex structures were prominent. The new behaviors of particle dispersion were also observed during the evolution of the flow field, i.e. the transitional phenomenon of particle dispersion occurs for the particles with small and intermediate Stokes numbers.     

Evolution of the texture and mechanical properties of 2060 alloy during bending

In this study, we examined the evolution of the texture and mechanical properties of 2060 (T8) alloy during bending. A pixel rotation method (PRM) was proposed and used to characterize the textural evolution during bending determined by electron backscatter diffraction. The results showed that the textural components changed insignificantly, with the exception of a decrease in the cube texture. The tensile and yielding properties of the alloy were evaluated at three different orientations with respect to the rolling direction. The mechanical strength was found to increase in three directions with decreasing bending radius; thus, it was concluded that the 2060 (T8) alloy sheet satisfies the usage requirement after bending deformation.     

Structural Optimization of Damping Groove Based on Valve Plate Dynamic Cavitation Response Model

To reduce cavitation occurring on valve plate of piston pump,an optimization design method was introduced to quantitively analyze the accurate relationship between structural jet groove parameters and cavitation. Using the computational fluid dynamics（ CFD） method,the absorbing and discharging processes in piston pump were simulated dynamically. The damping groove＇s structure effects on both jet angle and pressure shock were analyzed visually with a series of different parametrical grooves. By establishing parametrical damping groove model,the piston pumps＇ dynamic analysis was integrated with CFD analysis,experimental design and approximation model, etc. The mathematical model of plunger pressure during oil back period,jet angle and structural parameters of damping groove were established in the form of second-order response surface method（ RSM） model. The damping groove structure of valve plate was optimized on the basis of the RSM model. Test data shows that the anti-cavitation performance of optimized valve plate was obviously improved. And this method provides theoretical foundation for the structure design of damping groove.     

Effect of curing time on the mesoscopic parameters of cemented paste backfill simulated using the particle flow code technique

Several special mechanical properties, such as dilatancy and compressibility, of cemented paste backfill(CPB) are controlled by its internal microstructure and evolution.The mesoscopic structure changes of CPB during the development process were investigated.On the basis of the scanning electron microscopy(SEM) and mechanical test results of CPB, the particle size information of CPB was extracted, and a two-dimensional particle flow code(PFC) model of CPB was established to analyze the evolution rule of mesoscopic parameters during CPB development.The embedded FISH language in PFC was used to develop a program for establishing a PFC model on the basis of the SEM results.The mesoscopic parameters of CPB samples at different curing times, such as coordination number(C_n), contact force chain, and rose diagram, were obtained by recording and loading and used to analyze the intrinsic relationship between mesoscopic parameter variations and macroscopic mechanical response during CPB development.It is of considerable significance to establish the physical model of CPB using the PFC to reveal the mesoscopic structure of CPB.     

基于平行黏结模型的类岩石材料宏细观参数影响

为了研究材料宏观力学特性与细观参数的关系并进行细观参数标定,将平行黏结模型作为颗粒的接触模型,对颗粒流程序中的细观参数进行正交试验设计,结合室内试验确定类岩石材料的细观参数。结果表明:类岩石材料宏观弹性模量主要由颗粒模量、平行黏结模量以及半径乘子影响;法向黏结强度与切向黏结强度共同影响抗压强度,且不同黏结强度比对裂纹的分布有影响;泊松比主要由颗粒刚度控制,且颗粒刚度越大,剪切裂纹占比越大。物理试验与数值模拟的峰值应力接近,应力-应变曲线基本吻合,裂纹扩展形态一致,表明细观参数结果可靠,为其他岩石材料颗粒流模拟提供借鉴。     

基于颗粒流方法的碎石渗透系数分析

渗透系数是级配碎石渗流特性的重要参数,通过颗粒流数值方法建立碎石土渗流计算模型,利用流动方程和水压方程以及碎石之间的应力变形响应进行耦合计算,得到相应级配碎石的渗流系数,数值计算结果表明:颗粒流数值分析方法(PFC)适用于模拟任意形状、大小的单元集合体,允许单元在受力过程中发生开裂和运动.颗粒流不需要给出材料的宏观参数,只需给出材料的细观参数,通过细观结果的累加,PFC可以模拟材料在宏观状态下的表象.给出的9种级配碎石的渗透性远大于规范要求的级配集料的渗透系数,渗透系数均满足工程要求,但综合比较,孔隙率较大的碎石3类较优.     

复合土钉墙支护基坑颗粒流数值模拟研究

利用基于离散元理论的颗粒流软件PFC3d建立基坑开挖土钉支护的三维颗粒流模型,通过分析有复合土钉墙支护基坑和无复合土钉墙支护基坑数值模型的位移场和应力场,研究复合土钉支护基坑开挖过程的规律和土钉支护的机理。由于复合土钉的作用,复合土钉墙支护基坑和无复合土钉支护基坑在变形程度和变形模式等方面存在较大差异。复合土钉墙支护基坑挡墙变形较小,出现“两头小、中间大”的大肚状变形模式,坑底隆起量和墙后土体沉降量相对于无土钉墙支护基坑均较小。利用土拱效应来分析复合土钉支护机理,认为土钉支护作用不仅仅是土钉和土体之间相互拉拔作用,土拱效应才是复合土钉墙支护作用机理的关键。细观机理研究和颗粒流数值模拟为复合土钉墙支护基坑的机理研究提供新的思路和方法。     

三维颗粒流数值模型的胶结砂岩力学特性

为了更好地反映砂岩胶结性状对力学特性的影响,以油藏储层砂岩为研究对象,建立基于平行粘结接触的三维颗粒流数值模型(PFC3D)用以模拟剪切试验中砂岩结构的破坏机理.进行PFC3D的二次开发描述平行粘结分布和力学性质,分析剪切过程中砂岩体应力比、体应变、配位数和粘结破坏与应变之间的关系,通过接触网络的演化说明力链的重要作用,验证了数值模型的可行性.对比研究了不同胶结半径比、平行粘结刚度和胶结量变化对胶结砂岩力学特性的影响,阐明了胶结性状对砂岩结构受力的控制作用,为建立特定条件的胶结砂岩破坏机理提供了科学依据.     

基坑开挖变形的颗粒流数值模拟

采用基于散体介质特性建立的颗粒流细观力学数值方法,通过二次开发对重力式搅拌桩围护基坑的开挖过程和宏细观力学响应进行了研究.结合具体工程,基于相似理论建立了基坑的数值模型,通过双轴试验确定了土的细观力学参数,将土体细观结构特征和宏观力学响应联系起来,重点分析了开挖过程中基坑外土体沉降、基坑内土体隆起及围护桩水平位移等变化规律,模拟结果与实测结果具有较好的一致性,初步验证了用颗粒流方法模拟基坑开挖的可行性,也为从微细观角度研究围护结构和土相互作用机理提供了新的途径.     

心墙沥青混凝土压实PFC模拟细观参数反演

为了从细观角度研究沥青混凝土心墙坝料的压实机理,构建了包含粗骨料、沥青砂浆和空隙三相的离散元模型;基于支持向量机模型(SVM)和自适应差分算法(ADE)给出了沥青混凝土心墙颗粒流PFC模型细观参数的反演方法;通过不同温度、不同压力下的多组室内静压试验,对前述建立模型与方法进行了验证。结果表明:基于SVM建立的回归模型可以较好地反映输入样本(待反演参数)与输出样本(模拟沉降量)的对应关系;采用反演的细观参数进行PFC模拟沉降量,其结果与物理试验沉降量匹配较好。反演所得细观参数可为进一步研究沥青混凝土心墙压实质量细观机理提供条件。     

船运铁精矿流态化模型试验的离散元数值模拟

采用离散元程序PFC3D对散装铁精矿在动力荷载作用下发生流态化的室内振动台模型试验进行数值模拟,从细观的角度研究铁精矿流态化的演化规律及内在机理。考虑流态化过程中的非饱和特性,利用微小颗粒模拟水团,通过设置黏结模型模拟矿粉颗粒间的基质吸力,实时观察流态化过程中铁精矿位移场、水颗粒迁移情况以及细观组构的变化和发展,并将数值模拟结果与室内模型试验进行对比分析。结果表明:铁精矿发生流态化的主要原因是水液面的上升;其细观机理为在动力荷载作用下,铁精矿细颗粒沿着粗颗粒孔隙向下运动和颗粒间水团汇集形成水膜后的向上迁移。     

基于骨架网络模型的岩石粒径分析方法

应用扫描电子显微镜,获取岩石的二维灰度图像,并进一步分割得到岩石的骨架和孔喉结构。利用马尔可夫链蒙特卡洛法构建三维数字岩心,提取相应的三维孔隙网络模型。根据其逆过程提出骨架网络模型的提取方法,可对岩石粒径进行定量分析。最后对三块低渗透岩心样品的颗粒半径、孔隙半径和喉道半径等结构参数进行对比分析,得到各样品结构参数之间的定量关系。研究表明,基于网络模型的岩石粒径分析法能够快速获取岩石的结构参数,对油藏岩石结构参数进行定量表征,大大缩短实验数据获取周期,为岩石结构参数分析提供了一套有效的研究思路。     

耐热不锈钢X8CrNi25-21介观尺度材料本构建模与仿真

针对耐热不锈钢X8CrNi25-21的材料特性,基于弹塑性力学理论和位错动力学对经典的Johnson-Cook（JC）本构模型进行修正,引入位错塞积理论,并在本构中考虑晶粒尺寸和晶体取向等微观参数,建立了基于尺度效应的耐热不锈钢X8CrNi25-21介观尺度材料响应行为本构模型.通过采用介观尺度非标准件,并利用热处理工艺改变材料的晶粒尺寸,开展准静态拉伸实验、动态霍普金森压杆实验进行介观尺度材料测试,建立了晶体尺寸与介观尺度材料塑性变形与断裂行为特征的变化规律,并最终确定了耐热不锈钢X8CrNi25-21介观尺度材料本构模型各参数值.