等径通道挤压过程三维有限元模拟

利用三维有限元模型对等径通道挤压过程中变形分布的不均匀性进行了模拟,通过对不同摩擦系数下截面等效塑性应变分布比较发现:沿三个方向截面上的变形分布都不是均匀的,这说明二维有限元模型不能真实模拟等径通道挤压过程中试样中的变形分布。此外,摩擦对等效塑性应变分布及挤压力的影响较大,截面变形不均匀参数和最大等效塑性应变出现的位置随摩擦系数的增大而变化。压力-位移曲线的变化可以结合试样的变形过程来解释。     

外包钢-砼组合梁纯扭作用下抗扭刚度的实验及数值模拟

为了研究新型外包钢-砼T形截面组合梁在纯扭作用下的变形性能,设计了5根不同配箍率的的足尺悬臂组合梁。通过对5根悬臂梁的抗扭性能的实验研究,得到了组合梁的扭矩-扭率关系曲线。利用有限元分析软件ANSYS,对组合梁的抗扭性能进行了非线性有限元分析,得到了混凝土与外包钢在极限阶段的应力云图。根据实验以及有限元结果分析了组合梁在整个加载过程中扭转刚度的变化。基于现行砼结构设计规范,提出了组合梁从开裂到极限阶段抗扭刚度的计算公式,可供组合梁受扭设计参考。把有限元模型和公式的计算结果与实验结果进行比较,三者吻合较好。     

GEOMETRICALLY NONLINEAR FINITE ELEMENT MODEL OF SPATIAL THIN-WALLED BEAMS WITH GENERAL OPEN CROSS SECTION

Based on the theory of Timoshenko and thin-walled beams, a new finite element model of spatial thin-walled beams with general open cross sections is presented in the paper, in which several factors are included such as lateral shear deformation, warp generated by nonuni- form torsion and second-order shear stress, coupling of flexure and torsion, and large displacement with small strain. With an additional internal node in the element, the element stiffness matrix is deduced by incremental virtual work in updated Lagrangian （UL） formulation. Numerical examples demonstrate that the presented model well describes the geometrically nonlinear property of spatial thin-walled beams.     

基于能量等效原理的应变局部化分析:Ⅱ.有限元解法

以非局部塑性理论为基础,应用状态空间理论,通过局部和非局部两个状态空间的塑性能量耗散率等效原理,提出了一种求解应变局部化问题的新方法,以得到与网格无关的数值解.针对二维问题的屈服函数和流动法则导出了求解非局部内变量的一般方程,并提出了在有限元环境中求解应变局部化问题的应力更新算法.为了验证所提出的方法,对1个一维拉杆和3个二维平面应变加载试件进行了有限元分析.数值结果表明,塑性应变的分布和载荷-位移曲线都随着网格的变小而稳定地收敛,应变局部化区域的尺寸只与材料内尺度有关,而对有限元网格的大小不敏感.对于一维问题,当有限元网格尺寸减小时,数值解收敛于解析解.对于二维剪切带局部化问题,数值解随着网格尺寸的减小而稳定地向唯一解收敛.当网格尺寸减小时,剪切带的宽度和方向基本上没有变化.而且得到的塑性应变分布和网格变形是平滑的.这说明,所提方法可以克服经典连续介质力学模型导致的网格相关性问题,从而获得具有物理意义的客观解.此模型只需要单元之间的位移插值函数具有C~0连续性,因而容易在现有的有限元程序中实现而无需对程序作大的修改.     

含有非圆形双孔的无限平板中应力的解析解研究

无限平板中含有任意形状单个孔的问题可以使用复变函数方法获得其应力解析解.对于无限平板中含有两个圆孔或两个椭圆孔的双连通域问题,也可以利用多种方法进行求解,比如双极坐标法、应力函数法、复变函数法以及施瓦茨交替法等.其中复变函数中的保角变换方法是获得应力解析解的一个重要方法.但目前尚未见到用此方法求解无限板中含有一个正方形孔和一个椭圆孔的问题.当板在无穷远处受有均布载荷和孔边作用垂直均布压力时,利用保角变换方法可以求解板中含有两个特定形状孔的问题.该方法将所讨论的区域映射成象平面里的一个圆环,其中最关键的一步是找出相应的映射函数.基于黎曼映射定理,提出了该映射函数一般形式,并利用最优化方法,找到了该问题的具体映射函数,然后通过孔边应力边界条件建立了求解两个解析函数的基本方程,获得了该问题的应力解析解.运用ANSYS有限单元法与结果进行了对比.研究了孔距、椭圆形孔大小和两孔布置方位对边界切向应力的影响,以及不同载荷下两孔中心线上应力分布规律.      

双圆盾构隧道管片衬砌的拼装内力分析

讨论双圆盾构隧道（DOT：Double-O-Tube）施工时，管片衬砌拼装内力的有限元计算。包括整个拼装过程的有限元模拟方法，采用三维实体单元模拟隧道管片衬砌在工程中不同工况下的内力变化。结合双圆隧道的工程参数，进行实际问题的计算，为目前双圆隧道管片衬砌内力计算探索出一条计算模式。     

具有内域的双层加筋圆柱壳动响应特性简

采用外域双渐近法模拟外部流体域,内域双渐近法模拟内部流体域,采用非线性有限元软件ABAQUS 模拟结构,建立流固耦合数值模型并验证了其有效性. 利用建立的数值模型研究内部流体对双层加筋圆柱壳动响应的影响. 研究结果表明：一阶双渐近的解在中后期振荡周期变短、幅值变小,而二阶双渐近的解与解析解吻合良好;内部流体的存在减弱了双层加筋圆柱壳外壳的塑性变形,对其内壳塑性变形的影响较小;内部流体的存在减弱了双层加筋圆柱壳的速度响应.     

具有内域的双层加筋圆柱壳动响应特性

采用外域双渐近法模拟外部流体域,内域双渐近法模拟内部流体域,采用非线性有限元软件ABAQUS 模拟结构,建立流固耦合数值模型并验证了其有效性. 利用建立的数值模型研究内部流体对双层加筋圆柱壳动响应的影响. 研究结果表明:一阶双渐近的解在中后期振荡周期变短、幅值变小,而二阶双渐近的解与解析解吻合良好;内部流体的存在减弱了双层加筋圆柱壳外壳的塑性变形,对其内壳塑性变形的影响较小;内部流体的存在减弱了双层加筋圆柱壳的速度响应.      

An upper-bound analysis for frictionless TCAP process

Tubular channel angular pressing (TCAP) process was proposed recently as a novel severe plastic deformation technique for producing ultrafine grain and nanostructured tubular components. In this paper, an upper-bound approach was used to analyze the TCAP process. Deformation of the material during TCAP process is analyzed using upper-bound analysis to determine maximum required load. The effects of TCAP parameters such as channel and curvature angles, deformation ratio (R ₁/R ₂) and tube material on the process pressure were investigated. The results showed that an increase in the second channel angle and decrease in the ratio R ₁/R ₂ lead to lower process loads. In the first and third curvature angles ranging from 25 to 65°, the required load remains almost constant. The apparent punch load decrease when hardening exponent n is increased. To verify the theoretical results, the finite element (FE) modeling was employed. Good agreement was observed between the predicted pressure from upper-bound analysis and FE results.     

Tension-torsion behavior of single-crystal superalloys: Experiment and finite element analysis

This article investigates the macroscopic behavior of single-crystal superalloys under torsion or tension-torsion loadings. To this end, specific tests were performed at room temperature, on tubular specimens equipped with micro strain-gages for local deformation measurements. The comparison between the theory and the experiment concerns both the stress-strain hysteresis loop and the yield surface. The analysis of the experimental results requires a finite element analysis, due to the complex stress redistributions arising in the specimen. This is made with a crystalline model, which appears to be in good agreement with the experimental data.     

Necking evolution in dynamically stretched bars: New experimental and computational insights

This paper presents new results on dynamic neck evolution in steel bars of varying diameters. Dynamic tensile tests were carried out in a Kolsky apparatus using cylindrical steel specimens with various cross-section diameters ranging from 1.5 mm to 4 mm. A high speed digital camera was used to record the deformation of the specimen during the loading process. Video recording of the tests enabled accurate experimental measurements of the necking evolution, specifically its growth rate as a function of the diameter. The experiments show that increasing the specimen cross-section slows down the neck development. This behavior has been further investigated using two different kinds of numerical calculations: (1) axisymmetric finite element simulations and (2) one-dimensional finite difference computations. While the finite difference model only considers the normal stress along the longitudinal direction of the bar, the finite element model does not entail any simplification on the stress state of the specimen during the loading process. In agreement with the experiments, the finite element calculations show a decrease of the necking growth rate with the increase in the cross-section of the sample. On the contrary, the damping effect of the specimen cross-section on the necking evolution is not captured by the finite difference computations. We postulate that this result comes from the one-dimensional nature of the finite difference model. This work uncovers, by means of combined experiments and modelling, the key role played by stress multiaxiality in the growth rate of dynamic necks.     

Elastoplastic state of flexible cylindrical shells with two circular holes

The elastoplastic state of thin cylindrical shells with two equal circular holes is analyzed with allowance made for finite deflections. The shells are made of an isotropic homogeneous material. The load is internal pressure of given intensity. The distribution of stresses along the hole boundary and in the stress concentration zone (when holes are closely spaced) is analyzed by solving doubly nonlinear boundary-value problems. The results obtained are compared with the solutions that allow either for physical nonlinearity (plastic strains) or geometrical nonlinearity (finite deflections) and with the numerical solution of the linearly elastic problem. The stresses near the holes are analyzed for different distances between the holes and nonlinear factors.Translated from Prikladnaya Mekhanika, Vol. 40, No. 10, pp. 107–112, October 2004.     

Solution of the moiré hole drilling method using a finite-element-method-based approach

The moiré hole drilling method in a biaxially loaded infinite plate in plane stress is an inverse problem that exhibits a dual nature: the first problem results from first drilling the circular hole and then applying the biaxial loads, while the other problem arises from doing the opposite, i.e., first applying the biaxial load and then drilling the circular hole. The first problem is hardly ever addressed in the literature but implies that either separation of stresses or material property identification may be achieved from interpreting the moiré signature around the hole. The second is the well-known problem of determination of residual stresses from interpreting the moiré fringe orders around the hole. This paper addresses these inverse problem solutions using the finite element method as the means to model the plate with a hole, rather than the typical approach using the Kirsch solution, and a least-squares optimization approach to resolve for the quantities of interest. To test the viability of the proposed method three numerical simulations and one experimental result in a finite width plate are used to illustrate the techniques. The results are found to be in excellent agreement. The simulations employ noisy data to test the robustness of this approach. The finite-element-method-based inverse problem approach employed in this paper has the potential for use in applications where the specimen shape and boundary conditions do not conform to symmetric or well-used shapes. Also, it is a first step in testing similar procedures in three-dimensional samples to assess the residual stresses in materials.     

On the Measurement and Prediction of the Out-of-Plane Displacement Surrounding Cold-Expanded Holes

Experimental measurements of the out-of-plane displacement surrounding cold-expanded holes in a 6082-T6 aluminum alloy were made with a 3D optical scanner using the technique of the encoded light-pattern projection in white light. The measured surface profiles have shown the thickness discontinuity along the hole edge due to the effect of the split in the sleeve. An analytical–numerical solution of the out-of-plane displacement is presented based on existing analytical models. Thus, the results given by the analytical model were then compared with the experimental data and with a finite element (FE) model that simulates the cold-expansion process. The location of the elastic–plastic boundary was estimated as the point at which no change in thickness was observed; a good agreement was found in the comparison of measured, FE and analytical results. The measured surface profiles agreed with those predicted by the FE model and analytical solution. The proposed experimental approach can be used together with FE analysis for predicting the radial and circumferential residual stresses in cold-expanded hole. It is quite versatile and can also be used as quality-control technique in the manufacturing processes of cold-expanded holes.     

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.     

作大范围空间运动柔性梁的刚-柔耦合动力学

研究带中心刚体的作大范围空间运动梁的刚-柔耦合动力学问题．从精确的应变-位移关系式出发,在动力学变分方程中,考虑了横截面转动的惯性力偶和与扭转变形有关的弹性力的虚功率,用速度变分原理建立了考虑几何非线性的空间梁的刚-柔耦合动力学方程,用有限元法进行离散．通过对空间梁系统的数值仿真研究扭转变形和截面转动惯量对系统动力学性态的影响．     

Lüders bands propagation of 1045 steel under multiaxial stress state

Inhomogeneous plastic deformation of 1045 steel under monotonic loading was experimentally studied. Thin-walled tubular specimens were used in the experiments and custom-made small strain gages were bonded on the specimen surface to characterize the local deformation. Experiments were conducted under tension, torsion, and combined tension–torsion. During the propagation of Lüders bands, the local deformation experienced two-stage deformation: an abrupt plastic deformation stage followed by a slower deformation process. In some area of the gage section of the specimen, a small amount of initial plastic deformation occurred before the Lüders front reached. During the propagation of Lüders bands, multiple Lüders fronts can be formed. Under tension, torsion, and combined tension–torsion with a constant axial load, the Lüders front was approximately parallel to the material plane of maximum shear stress. When the combined axial-torsion followed a proportional fashion, the stress–extensometer strain responses were dependent on the axial/torsional loading ratio, and the Lüders fronts were oriented differently and propagated along the specimen axis at a different velocity. The local strain was inhomogeneous even at the work-hardening stage. The relationships between the equivalent stress and the equivalent plastic strain were found to be practically identical for all the loading cases studied.     

粉末冶金涡轮盘简单模拟件低周疲劳寿命研究

采用光滑圆棒试样和带孔平板试样,对不同温度下的镍基粉末高温合金(FGH95)的低周疲劳(LCF)寿命进行了试验研究和有限元分析。在详细分析试验和有限元计算结果的基础上,提出了复杂应力状态下的低周疲劳寿命模型。模型寿命表达为真实应力幅的函数,模型参数由不同应力水平加载作用下的光滑圆棒试样试验结果给定,进一步采用涡轮盘简单模拟件即带孔平板试样对比验证LCF寿命模型的有效性。有限元计算结果显示,理论预测寿命与试验结果能很好地吻合。