含缺陷钢丝多道次拉拔模拟及工艺参数优化

利用有限元软件ABAQUS建立了含缺陷热轧盘条多道次拉拔的分析模型, 研究了道次压缩率、模具顶角和裂纹长度对拉丝安全性的影响,以减少断丝率为目标探讨 道次压缩率和模具顶角的最佳配合方式. 研究结果表明：模具顶角相同时,COD值和$J$积分 值均随着裂纹尺寸的增加而增大;裂纹长度一定时,最佳入线直径随着模具顶角的增加而增 大;最佳模具顶角应在4°- 8°之间. 利用有限元方法和断裂力学理论模拟含缺陷的热轧盘条经过多道次拉丝成型的过程,建立了钢丝经过多道次拉拔模具发生拉拔变形的分析模型,用模拟方法重点研究了道次压缩率、模具顶角以及裂纹长度对拉丝安全性的影响,以减少断丝率为目标探讨道次压缩率和模具顶角的最佳配合方式;研究结果表明：模具顶角相同时,随着裂纹尺寸的增加,J积分值随之增大;裂纹长度一定时,随着模具顶角的增加,入线直径的条件最佳值也在增大;最佳模具顶角的范围应在4°－8°之间。在拉拔前应对线材进行无损探伤从而控制裂纹长度,提高线材使用率和成品质量。研究结果对进一步优化拉丝工艺参数和改进成品PC钢绞线质量提供了有价值的理论指导。     

筒形件液压拉深过程最优压边力的研究

结合筒形件液压拉深新装置的基本工作原理，提出了筒形件拉深法兰部分切向应变更为精确的简化计算公式，进一步获得了该法兰区域板料径向及切向应力的新的数学表达式。应用能量法重新建立了筒形件拉深时法兰区板料不起皱的最小压边力的数学表达式。明确指出液压拉深时拉深中的危险断面在板料凹模圆角区和筒壁区相交处，建立了相应的拉深力的计算公式，并获得了拉深破裂临界状态时极限拉深力及径向应力表达式。推导出了拉深破裂最大压边力的公式．指出实际拉深时最佳压边力应在防皱最小压边力和拉破最大压边力之间选取，同时根据该结果也能计算液体压力与最小拉深系数的关系。计算结果表明，该液压拉深方法可大幅度提高拉深变形程度，值得在工业实际中推广应用。     

Analysis of tube hydroforming in a square cross-sectional die

A mathematical model considering sliding friction between the tube and die is proposed to explore the plastic deformation behavior of the tubes during a hydraulic expansion process in a square cross-sectional die. This model is used to predict the forming pressure needed to hydroform a circular tube into a square cross-section and the thickness distribution of the product. FE simulations on tube expansion have also been carried out to compare the analytical results with those by the proposed mathematical model. The effects of the friction coefficient between the die and tube, upon the forming pressures needed and thickness distributions after the expansion process are discussed. Experiments of tube expansion in a square die using a self-designed apparatus are also conducted. The analytical results of forming pressure and thickness distributions of the formed parts are compared with the experimental measurements to verify the validity of this newly proposed model.     

Nonisothermal model of glass fiber drawing stability

Draw resonance is caused by a constant speed winder that leads to non-constant axial forces (Schultz, 1984). The well studied isothermal Newtonian fiber drawing predicts very modest critical draw ratios (around 20, much less than the typical production draw ratios for glass fibers of 10³ – 10⁵). The nonisothermal fiber drawing model presented here shows that cooling along the spin line strongly stabilizes the process. However, we show that the conclusion of Shah and Pearson (1972a,b) that non-isothermal Newtonian fiber spinning is unconditionally stable is based on non-converged numerical results. The choice of viscosity-temperature correlation function has a strong influence in determining the stability of the process. While viscoelasticity generally has an adverse effect on the stability, low viscoelasticity in the presence of extensional thinning helps to slightly improve the maximum critical draw ratio.Dedicated to the memory of Professor Tasos C. Papanastasiou     

Physics of the formation of fiber lightguides

A fiber lightguide is a fine continuous or tubular transparent filament. Fiber lightguides are formed from the liquid mass exuded through a dye or drawn from a suitable blank. Both of these processes can be considered using the equations of the hydrodynamics of an incompressible Newtonian liquid. (Polymers, which are not Newtonian liquids, are not considered here.) The drawing of a continuous glass fiber from a dye is considered in [1]. The drawing of a microcapillary from a dye is considered in [2], where a qualitative consideration is given which is insufficient for an understanding of the effect of different parameters of the process on the dimensions of the drawn microcapillary. In this paper we consider the formation of a microcapillary from a tubular blank using the approximation of an incompressible Newtonian liquid with a variable viscosity determined by the given temperature distribution. The effect of surface tension and of the excess pressure produced in the channel to counteract the surface tension are taken into account. It is assumed that the drawing process is steady, the blank has thin walls and is axisymmetrical, and the transition to a microcapillary occurs smoothly. With these assumptions the problem of obtaining the shape of the transition and the dimensions of the microcapillary obtained is reduced to a system of ordinary differential equations. The dependence of the dimensions of the microcapillary on the dimensions of the blank and the parameters of the process is established, thereby enabling the process to be optimized.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 2, pp. 167–174, March–April, 1976.The authors thank B. Z. Katsenelenbaum and A. D. Shatrov for useful discussions, A. I. Leonov for a number of observations, and I. V. Aleksandrov, T. V. Bukhtiarov, and A. A. Dyachenko for discussing the results at various stages.     

Stability analysis of a polymer film casting problem

The polymer cast film process consists of stretching a molten polymer film between a flat die and a drawing roll. Drawing instabilities are often encountered and represent a drastic limitation to the process. Newtonian fluid film stretching stability is investigated using two numerical strategies. The first one is a ‘tracking’ method, which consists of solving Stokes equations in the whole fluid area (extrusion die and stretching path) by finite elements. The interface is determined to satisfy a kinematic equation. A domain decomposition meshing technique is used in order to account for a flow singularity resulting from the change in the boundary conditions between the die flow region and the stretching path region. A linear stability method is then applied to this transient kinematic equation in order to investigate the stability of the stationary solution. The second method is a direct finite element simulation in an extended area including the fluid and the surrounding air. The time‐dependent interface is captured by solving an appropriate level‐set function. The agreement between the two methods is fair. The influence of the stretching parameters (Draw ratio and drawing length) is investigated. For a long stretching distance, a critical Draw ratio around 20 delimitating stable and unstable drawing conditions is obtained, and this agrees well with the standard membrane models, which have been developed 40 years ago. When decreasing the stretching distance, the membrane model is no longer valid. The 2D models presented here point out a significant increase of the critical Draw ratio, and this is consistent with experimental results. Copyright © 2015 John Wiley & Sons, Ltd.     

Analysis of blank thickening in deep drawing processes using the theory of a Cosserat generalized membrane

This paper analyzes a transient, nonlinear deep drawing process where a circular blank of a rigid-plastic material is forced by a rigid circular punch to deform into a cylindrical cup. Attention is focused on the plastic flow beneath the blank-holder. Using the Cosserat theory of a generalized membrane it is possible to obtain analytical solutions which examine the following two major effects: (a) the importance of added “rim pressure” acting on the outer edge of the blank; and (b) the importance of a controlled moveable blank-holder to allow blank thickening during the drawing process. Guided by these analytical results, a new deep drawing machine was built to exploit these effects and increase the limit drawing ratio (LDR) of the drawing process. Specifically, the LDR (in one stroke) reached the value of 3.16 compared with the value of about 2.0 in the conventional process. Moreover, the analytical prediction of the punch force versus the punch stroke is in good agreement with the experimental data and with simulations using the computer code DYTRAN.     

Upper bound analysis of thick wall tubes extrusion process through rotating curved dies

In this study, extrusion process of thick wall tubes through rotating curved dies is investigated by the method of upper bound. Total deformation region is divided into four deformation zones and a velocity field is developed for each deformation zone. The twist moments generated on container and mandrel surfaces are calculated and by equating them with the twist moment exerted by rotating die, the twisting length of tube inside the container is determined. Then, the internal powers, the powers dissipated on frictional and velocity discontinuity surfaces for a rigid-perfectly plastic material are evaluated and they are used in upper bound model. By optimizing the total power with respect to the slippage parameter between die and the tube material, the required relative extrusion pressure for a given process conditions and die angular velocity is determined. The results of finite element simulations are also presented and satisfactory agreement between the calculated and FEM results are demonstrated.     

Springback compensation in deep drawing applications

This work deals with the problem of springback compensation in sheet metal forming. Satisfactory results can be achieved by performing “die compensation”: the die is modified pretending to obtain a different configuration at the end of the punch stroke, but in order that the final piece coincides with the desired one after the deformation due to springback. Empirical die compensation has nowadays been replaced by numerical simulation, but the inverse problem that needs to be solved is non-trivial since the transformation from the modified geometry of the die and the final piece obtained from it implies a very complex FEM simulation. In this work we set the whole process of springback compensation on solid physical and mathematical grounds. An optimization algorithm based on the Gauss-Newton method is proposed to deliver automatic die compensation and its performance is investigated on some test cases.     

A new die profile with high process efficiency

A new technique is used to design die profiles which yield high process efficiency during axi-symmetric extrusion and wire drawing. These profiles have a convex shape. The upper bound of the average ram pressure is calculated for the practical range of reductions and optimal die lengths. For m=0,1 where m is the constant friction factor, and up to 55 per cent reduction the reduced extrusion pressure is the same for this convex die and the optimal length cosine die. Above 55 per cent reduction the pressure is slightly higher than that for the cosine die. Up to a reduction of 55 per cent the optimal lengths for this one are slightly shorter than that of the cosine die, while those values for higher reductions are a little higher for this die. The efficiency of the proposed die exceeds those of conventional conical dies.     

On longitudinal impact III

Summary The graphodynamical method which was introduced and applied to elastic longitudinal impact in the first two articles of this series is here used to study the transmission of a rectangular compressive pulse along a system consisting of a bar which is joined by friction to a stiff anvil or to a co-axial elastic tube. Some of these cases will in the next article be used as models of pile driving. The influence of the frictional force, of the dimensions of the pulse (the dimensions and impact velocity of the hammer) and of the area and length of the tube on the sign and magnitude of the slide at a concentrated friction joint and on the associated energy loss is determined, and the results are illustrated by diagrams. The results of a few simple experiments agree reasonably well with the theory. The theory is extended to a model of a pile with constant friction per length unit, and it is found that a light hammer with great impact velocity gives better penetration than a heavy hammer with the same energy and thus smaller velocity.     

The curvilinear integral problems to velocity field for drawing through parabolic dies

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A finite element analysis of the flange earrings of strong anisotropic sheet metals in deep-drawing processes

Flange earrings of strong anisotropic sheet metals in deep-drawing process are numerically analyzed by the elastic-plastic large deformation finite element formulation based on a discrete Kirchhoff triangle plate shell element model. A Barlat-Lian anisotropic yield function and a quasi-flow corner theory are used in the present formulation. The numerical results are compared with the experimental ones of cylindrical cup drawing process. The focus of the present researches is on the numerical analysis and the constraining scheme of the flange earring of circular sheets with strong anisotropy in square cup drawing process. The project supported by the National Natural Science Foundation of China (19832020) and Provincial Natural Science Foundation of Jilin, China (200000519)     

双金属板拉延成形的有限元分析

双金属板拉延成形同时包含几何非线性、塑性、接触、摩擦以及双金属板中组元金属力学性能差异等多种复杂因素。组元金属之间不均匀的塑性变形是导致组元金属在拉延过程中产生缺陷的主要根源。本文基于刚塑性有限元方法，对双金属板的拉延成形过程进行了有限元分析，揭示了组元金属的等效应变分布规律和厚向应变分布规律。本文的研究方法和结论可以为工艺参数的合理选择和优化提供参考和依据。     

Combined effects of vortex flow and the Shchelkin spiral dimensions on characteristics of deflagration-to-detonation transition

The objective of this investigation was to achieve the shortest possible distance for the deflagration-to-detonation transition (DDT) for a pulse detonation engine without losing the engine’s simplicity. The effects of rapid flame propagation, rotating velocity, and Shchelkin spiral dimensions in the vortex flow (VF) on DDT characteristics were examined. A VF field was established in the ignition and transition domains of a detonation tube using a VF-type injector. The flame propagation in the ignition domain was observed by a high-speed video camera and the detonation propagation process was observed by measuring the pressure and ionization current in the detonation tube. The DDT distance in the VF could be shortened by 50–57 % of that in the counterflow by optimizing the VF and the Shchelkin spiral dimensions. The shortening effect became remarkable as the rotating velocity increased. The governing factors for this effect are considered to be the formation of an area of higher energy density in the ignition domain of the tube, as well as flame acceleration due to rapid flame propagation in the VF and turbulence promotion near the tube wall by the rotating velocity and the Shchelkin spiral in the transition domain of the tube. However, the combined effects of VF and the Shchelkin spiral on the shortening of the DDT distance were deteriorated in very strong turbulence. It is necessary to optimize the rotating velocity and dimensions of the Shchelkin spiral to achieve the shortest possible distance of DDT in the VF.     

Axisymmetric irrotational potential flows of an ideal fluid in doubly connected domains

Amethod of constructing the model of ideal incompressible unbounded flow past an axisymmetric tube is developed; the tube shape and dimensions are determined in the process of the problem solution. The problem is solved using the method of spatial annular sources and sinks. It is shown that under certain conditions the resulting flow corresponds to that past a round finite-length tube, whose wall has a finite thickness. It is established that near the tunnel entry the flow is considerably restructured.     

板材冲压翻边的解析理论模型

基于全量塑性理论及膜应变假设,给出两种新的冲压板材内曲拉伸／外曲收缩翻边坯料尺寸预示解析数学模型。引入板材冲压成形性分析软件KMAS系统中,对铁路客车牵引架实际冲压件的翻边成形坯料尺寸进行了预示,并与其它解析模型的预示结果以及实验及实测数据进行了对比。讨论了板材的面内各向异性对翻边高度的影响。     

Effects of Geometry and Dimension of Specimen and Rig on Small Punch Creep Property

The purpose of this study was to examine the effects of the geometries and dimensions of the rig and specimen of a small punch (SP) test on the determined creep property. The SP creep property of a specimen was evaluated using finite element analysis by varying the specimen thickness and diameter, diameter of the spherical and hemispherical indentation punches, inner diameter of the lower die, and dimensions of the chamfer of the lower die. We observed that the rupture time decreased with decreasing specimen thickness and punch diameter and with increasing chamfer size and inner diameter of the lower die. Under similar analytical conditions of static average equivalent stress in steady state, the SP creep curves reasonably agreed even in the case where the specimens or rigs have different geometries, which implied the possibility of direct comparison of the test results obtained from specimens and rigs with different dimensions and geometries.     

Theoretical analysis and experimental study of Coriolis mass flow sensor sensitivity

The measuring tube is the core sensitive unit of the Coriolis mass flow sensor. Its design parameters directly influence natural frequency and sensitivity, such as shape and structure dimensions. In this study, we obtained under concentrated force the equivalent elastic coefficient of the measuring tube by adopting static analysis and calculating static deflection curves, including the respective U-shape, slightly curved, and straight tubes. We then obtained the resonant frequency from the second-order vibration equation. Additionally, the maximum sensitivity and position coordinates were obtained by calculating the torsional displacement curve of the measuring tube under the distribution of Coriolis force during a rated flow. Sensor models with different measuring tube shapes were designed by applying this theoretical analysis. Calibration tests for sensors were performed using a static gravimetric method. Theoretical analysis and test results show that the resonant frequency and sensitivity of the sensors calculated by applying static mechanical analysis and Coriolis distributing force align with the experimental results, thereby proving the validity of the theoretical method. Furthermore, the proposed method simultaneously obtained the relation curve of the measuring tube structure dimensions and natural frequency and sensitivity. It therefore provides theoretical evidence for the sensor design and detector installation position.     

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为了研究爆炸及燃烧产生的“蘑菇状”烟云,采用中尺度气象模式RAMS(Regional Atmospheric Modeling System)模拟计算了冲击波过后的爆炸烟云运动过程。对于稳定层结大气条件,模拟结果显示爆炸烟云造成的流动是一个多层次涡-波运动的过程。爆炸烟云在浮力作用下会形成浮力涡环结构,在到达最大高度以后,其烟云外侧边缘部分涡度变号,形成了环套环的多涡结构。随时间增加,涡能量的逐步衰减,涡运动转变为重力内波运动。通过模拟不同爆炸当量和不同稳定度烟云的上升最大高度,得到了与目前常用的爆炸烟云上升公式一致的形式。