The impact problem for two strips

This paper considers the dynamic problem of contact interaction of two infinite strips arranged in parallel with an initial gap. Contact interaction of strips occurs due to impulse loading.Equations of the 2-dimensional generalized theory of plates and the theory of elasticity are used to describe the dynamic behavior of strips. The contact problem is solved using the collocation method with iterative refining of the contact area sizes at each time step.     

Dynamic response of plates due to moving vehicles using finite strip method

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不同加载边界下混凝土巴西劈裂过程及强度的DIC实验分析

平板、垫条、圆弧压模及平台试样等不同加载方式下, 脆性材料巴西劈裂实验结果的差异一直是人们所讨论的问题. 本文设计了混凝土平板直接加载与采用垫条加载实验, 采用理论、高速相机与数字图像相关法(digital image correlation, DIC)相结合的方法, 对试样表面应变场演化、起裂位置及裂纹扩展过程进行分析, 探讨了不同的加载边界对试样应变集中演化及劈裂拉伸强度结果的影响. 结果显示: (1)混凝土拉伸非线性特性导致的应变集中演化对不同加载条件非常敏感, 平板加载时应变集中较早在加载端起始并向中心演化, 即使满足巴西实验"中心起裂"条件, 其强度仍低于垫条加载情况, 两者差别达17.9%; (2)如果软垫条加载接触角合理设计, 与平板直接加载相比, 其应力场更稳定, 有利于保证圆盘试样的应变集中及断裂均从中心起始, 更好满足巴西劈裂实验条件; (3)仅校验巴西实验 "中心起裂"有效性条件是不充分的, 设计时必须谨慎考虑. 研究结果对脆性材料巴西劈裂实验设计、测试分析具有重要参考意义.      

金属板条在圆柱形模中弯曲成形的大变形有限元分析

本文采用大变形弹塑性有限元法对金属板条在柱形模中的压弯成形过程进行了数值模拟,并与实验进行了比较。首先给出了纠正的拉格朗日有限元公式和基于弹塑性乘法分解的超弹性塑性本构关系。对接触摩擦问题的处理采用了罚函数法。通过对数值结果的分析得出了一些对弯曲工艺的设计有指导价值的结论。     

Process of debonding in RC beams shear-strengthened with FRP U-strips or side strips

Reinforced concrete (RC) beams may be strengthened for shear using externally bonded fiber reinforced polymer (FRP) composites in the form of side bonding, U-jacketing or complete wrapping. The shear failure of almost all RC beams shear-strengthened with side bonded FRP and the majority of those strengthened with FRP U-jackets, is due to debonding of the FRP. The bond behavior between the externally-bonded FRP reinforcement (referred to as FRP strips for simplicity) and the concrete substrate therefore plays a crucial role in the failure process of these beams. Despite extensive research in the past decade, there is still a lack of understanding of how debonding of FRP strips in such a beam propagates and how the debonding process affects its shear behavior. This paper presents an analytical study on the progressive debonding of FRP strips in such strengthened beams. The complete debonding process is modeled and the contribution of the FRP strips to the shear capacity of the beam is quantified. The validity of the analytical solution is verified by comparing its predictions with numerical results from a finite element analysis. This analytical treatment represents a significant step forward in understanding how interaction between FRP strips, steel stirrups and concrete affects the shear resistance of RC beams shear-strengthened with FRP strips.     

An Optimal Condition of Compactness for Elasticity Problems Involving one Directional Reinforcement

This paper deals with the homogenization of a homogeneous elastic medium reinforced by very stiff strips in dimension two. We give a general condition linked to the distribution and the stiffness of the strips, under which the nature of the elasticity problem is preserved in the homogenization process. This condition is sharper than the one used in Briane and Camar-Eddine (J. Math. Pures Appl. 88:483–505, 2007) and is shown to be optimal in the case where the strips are periodically arranged. Indeed, a fourth-order derivative term appears in the limit equation as soon as the condition is no more satisfied. In the periodic case the influence of oscillations in the medium surrounding the strips is also considered. The homogenization method is based both on a two-scale convergence for the strips and the use of suitable oscillating test functions. This allows us to obtain a distributional convergence of two of the three entries of the stress tensor contrary to the Γ-convergence approach of Briane and Camar-Eddine (J. Math. Pures Appl. 88:483–505, 2007).     

Solution of the plane Hertz problem

The paper considers the problem of onesided frictionless compression of plane elastic bodies that are initially in contact with each other at a point. The first terms of an asymptotic solution of the problem are constructed by the method of joined asymptotic expansions. Determination of the approach of the bodies as a function of the pressing force reduces to calculating socalled of local compliance. The problems of contact of an elastic ring and elastic circular disks with punches and an elastic disk compressed between two elastic strips are considered. An asymptotic model for the quasistatic collision of plane elastic bodies is proposed.     

Thermal contact stresses of bi-metal strip thermostat

The distribution of shearing and normal stresses on the contact surface of the two strips composing a thermostat is found in closed form. They are of local type and concentrated near the ends of the strip along a length almost equal to the thickness of the strip.     

水工圆形隧洞围岩衬砌摩擦滑动接触的新解法

在实际工程中,围岩和衬砌接触时,它们之间并非完全光滑,也并非可以承受任意大的摩擦力.如果围岩与衬砌之间的剪应力大于所能承受的最大静摩擦力,接触面间将发生切向滑动,定义接触面上产生最小滑动量的状态为衬砌的真实工作状态,这种接触即为摩擦滑动接触.以库仑摩擦模型模拟围岩和衬砌之间的摩擦滑动接触,在考虑支护滞后效应的前提下,利用平面弹性复变函数方法列出了应力边界条件、应力连续条件以及位移连续条件的方程, 再结合最优化理论,建立了具有一般性的摩擦滑动接触解法.在利用混合罚函数法求解最优化问题的过程中,减少了设计变量的个数,极大地简化了优化模型,提升了优化过程的迭代速度以及优化结果的精度.以此为基础,获得了围岩和衬砌相互作用下圆形水工隧洞的应力解析解.该方法可以求解光滑接触和完全接触两种极限情况,具有一般性.同时,利用一种精确的计算方法得到了不同情况下满足完全接触条件摩擦系数的阈值,还分析了衬砌和围岩边界上切向应力的变化规律.      

基于改进LBM的气液自发渗吸过程中动态润湿效应模拟

微通道内气液自发渗吸是广泛发生在自然界及诸多工业领域的物理现象,而动态接触角是影响整个渗吸过程的关键因素.针对该问题,本文使用改进的伪势多相流格子玻尔兹曼方法 (LBM),直接捕捉微通道内气液自发渗吸过程中的实时接触角,并分析接触角的动态变化特性及其对渗吸长度的影响.首先,本文在原始的伪势多相流LBM的基础上耦合Peng-Robinson (PR)状态方程,改进流体-流体作用力以及流-固作用力格式,并采用精确差分方法将外力添加至LBM框架中.然后,通过校准模型的热力学一致性,模拟测试界面张力,静态平衡接触角等界面现象验证了模型的准确性.最后,基于建立的模拟方法,在水平方向上模拟微通道内气液自发渗吸过程.结果表明:渗吸过程中的接触角呈现动态变化特征,在渗吸初期,因受到惯性力的影响存在较大波动;随着渗吸距离的增大,其逐渐减小并趋近于静态平衡接触角.渗吸过程中的接触角与微通道尺寸及静态接触角有关,随着微通道宽度增大,实时的动态接触角与静态接触角相差大;随着静态接触角增大,实时的动态接触角与静态接触角的相差增大.此外,忽略动态接触角的Lucas-Washburn (LW)方程所预测的弯液面位置...     

FE-analysis of sheet metal forming processes using continuous contact treatment

Discrete meshes cause stepwise propagation of the contact nodes of a sheet despite the fact that the contact region in the actual forming process is altered very smoothly. This can cause problems of convergence and accuracy in contact-sensitive processes, such as a bending process. In this study, a scheme for a continuous contact treatment is proposed in order to consider the more realistic behavior of the contact phenomena during the forming process. For verification of the proposed method, the contact pressures and forming load are evaluated during the compression forming of a tube. The analysis of a hemispherical dome formed without a blank holder is also presented in order to investigate the effects of the proposed algorithm. The results show that the precise deformation mode is predicted by the utilization of the proposed method.     

接触与摩擦对行走影响的仿真与实验研究

主要通过实验测量的方法研究被动行走中的足地接触过程,得到接触过程对行走稳定性的影响,对实验测量结果与仿真计算结果进行了对比,得到了较好的吻合效果.     

A modified discrete element method for concave granular materials based on energy-conserving contact model

The development of a general discrete element method for irregularly shaped particles is the core issue of the simulation of the dynamic behavior of granular materials. The general energy-conserving contact theory is used to establish a universal discrete element method suitable for particle contact of arbitrary shape. In this study, three dimentional (3D) modeling and scanning techniques are used to obtain a triangular mesh representation of the true particles containing typical concave particles. The contact volume-based energy-conserving model is used to realize the contact detection between irregularly shaped particles, and the contact force model is refined and modified to describe the contact under real conditions. The inelastic collision processes between the particles and boundaries are simulated to verify the robustness of the modified contact force model and its applicability to the multi-point contact mode. In addition, the packing process and the flow process of a large number of irregular particles are simulated with the modified discrete element method (DEM) to illustrate the applicability of the method of complex problems.     

循环加载条件下混凝土Ⅰ型裂缝扩展模拟的接触算法

沿裂缝可能开展路径设置接触点对,把求解接触问题的有限元混合法进行扩展,以实现循环加载条件下混凝土Ⅰ型裂缝扩展的数值模拟.根据荷载过程状态和缝面接触状态的不同,将循环荷载作用下的混凝土本构关系抽象为力学模型中的六种不同接触状态,不同的接触状态对应着不同的位移-应力曲线关系.以缝面张开位移和接触应力作为接触状态转变的判断参量,并给出了各状态的转变关系和数值判断条件,以接触算法实现了循环加载条件下混凝土Ⅰ型断裂扩展的数值模拟.首先给出了有限元混合法求解接触问题的基本思路,然后引入循环荷载下的混凝土本构关系,再对Ⅰ型裂缝扩展的数值实现方法进行了阐述,最后通过数值算例说明了数值实现方法的正确性和有效性.     

Photoviscoplastic investigations of slip-line fields and principal-stress trajectories in viscoelastoplastic strip for drawing with back tension

Photoviscoplasticity is applied to the drawing with back tension through rollers and wedge-shaped dies in model tests of plane-strain state. The effects of the back tension on the slip-line fields and on the principal-stress trajectories for the strips are discussed in the light of experimental results obtained from the model tests. Actual strips are considered as viscoelastoplastic media in the actual drawing processes. The model specimens used are softened celluloid as the viscoelastoplastic medium for the strip and Araldite as the glassy elastic medium for the dies. At a suitable experimental temperature, the viscoelastoplastic strip is drawn viscoplastically through the gap between the dies mentioned above. The slip-line fields, the principal-stress trajectories in the strip and the normal pressure on the contact surface are greatly influenced by the effect of the back tension.     

板材冲压成形有限元数值模拟界面摩擦约束处理方法

基于弹塑性大变形板材冲压成形增量有限元法,采用线性弹簧单元提出一种空间三维板材冲压成形等效界面摩擦力的处理方法.这种方法的优点是可以反映界面摩擦力的同步、被动产生效果,同时它还可以保证约束后刚度矩阵的对称性.将提出的摩擦约束处理算法引入自主开发的板材成形模拟软件FASTAMP,计算了汽车油底壳的成形过程,计算结果的厚度分布与实际冲压件吻合比较好,验证了等效界面摩擦力约束处理方法的有效性.     

基于分子动力学-格林函数法的微凸体接触数值分析

表面接触是摩擦的先决条件,其真实接触面积、压应力大小、空间分布等一直是接触力学关注的核心问题.采用分子动力学-格林函数法(GFMD)模拟粗糙面的接触过程,验证了其在大规模接触分析中的高效及准确性,同时探讨了由微球体组成的粗糙面的接触力学特性,并分析了分子尺度下的结果和传统力学模型计算结果的差异.结果表明,单个微凸体接触结果和分子动力学-格林函数法模拟所得非常接近,误差在5%以内.数值模拟发现,在微凸体高度符合高斯分布的情况下,接触面积和接触力成线性关系;在相同接触面积下,微凸体模型得出的接触力偏高,是上限值.微凸体模型没有考虑微凸体间的相互影响,实际是高估了弹性体的刚度;实际接触过程中微凸体相互影响,微凸体对临域形变影响尤其大,使接触区域更加离散.GFMD模型可以准确计算数十亿量级别分子、原子接触过程中真实接触面积及分布,为后续摩擦、滑移等分析提供可靠的参考.     

Elastic–plastic behavior of a semicircular frame being pressed against a rigid plane

As a simplified structural model, a semicircular frame is used to study the crashworthiness behavior of an aircraft fuselage. The quasi-static large elastic-plastic deformation of a semicircular frame in the process of its being pressed against a rigid ground is analyzed. First, based on the linear elastic assumption, the quasi-static large deformation contact process of the frame can be divided into three phases, i.e., point contact, line contact and post-buckling. By means of a shooting method, the relations between the displacement and contact force as well as the distribution of bending moment in the three phases are obtained. Then, by assuming an elastic, perfectly-plastic moment-curvature relationship for the semi-circular frame, the contact process is analyzed in detail to reveal the plastic collapse mechanism, the traveling of plastic hinge and the force-displacement relationship. In order to verify the analysis, a preliminary experiment was conducted, in which two types of half rings with clamped ends were pressed by a rigid plate. In addition, a numerical simulation is also conducted by employing ABAQUS to analyze both rectangular cross-sectional beam and I-beam. Finally, the theoretical predictions are compared with the experimental results and numerical solutions, showing that the elastic-plastic analysis can predict the contact process very well.     

考虑动静摩擦影响的可接触型裂纹动态响应的隐式算法

提出一种考虑裂纹面摩擦影响的可接触型裂纹动力反应的时域隐式计算方法。这一方法不但可以较好地描述裂纹面间的碰撞过程及反映动接触时裂纹面间动、静摩擦力的影响，而且使动接触问题分析中的系统矩阵成为对称型式，在弹塑性有限元迭代过程中，无需增加额外的迭代即可求得较好的结果。数值结果表明本文方法具有良好的稳定性和较高的精度。