小尺寸低碳钢试件吕德斯效应的三维数字图像相关测量

吕德斯效应是多种金属和合金材料由于屈服阶段的不均匀变形而在材料表面产生条带状褶皱的现象,它会使冲压件表面质量降低.为了防止它的出现,对吕德斯效应进行研究变得非常重要.采用小视场(15 mm×15 mm)下三维数字图像相关方法对小尺寸低碳钢试件在单轴拉伸载荷作用下的变形场进行测量,实际观测了小尺寸试件的吕德斯效应,结合理论模型解释了其形成机理,并分析了吕德斯带传播过程中应变及应变率的变化规律.实验研究表明,运用三维数字图像相关方法测量试件表面变形场,实现了对小尺寸低碳钢试件的吕德斯带演化过程以及颈缩、断裂等细观力学行为的观测,该方法是研究材料变形细观机理的一种有效测量手段.     

MEASUREMENT OF LÜDERS BAND IN SMALL SIZE LOW CARBON STEEL SPECIMEN BY 3D DIGITAL IMAGE CORRELATION METHOD

吕德斯效应是多种金属和合金材料由于屈服阶段的不均匀变形而在材料表面产生条带状褶皱的现象,它会使冲压件表面质量降低. 为了防止它的出现,对吕德斯效应进行研究变得非常重要. 采用小视场(15mm×15 mm)下三维数字图像相关方法对小尺寸低碳钢试件在单轴拉伸载荷作用下的变形场进行测量,实际观测了小尺寸试件的吕德斯效应,结合理论模型解释了其形成机理,并分析了吕德斯带传播过程中应变及应变率的变化规律.实验研究表明,运用三维数字图像相关方法测量试件表面变形场,实现了对小尺寸低碳钢试件的吕德斯带演化过程以及颈缩、断裂等细观力学行为的观测,该方法是研究材料变形细观机理的一种有效测量手段.     

古水水电站工程区域堆积体边坡工程地质分析

基于弹性有限变形理论和电弹性体偏场理论,对半无限压电体及其表面电极层间存在 穿透脱层的屈曲问题进行了分析. 采用平面应变模型,在脱层远处作用有平行于脱层的应变 载荷. 使用Fourier积分变换,应用脱层界面的连续条件和电极表面的边界条件将问题归为 第2类Cauchy型奇异积分方程组. 利用Gauss-Chebyshev积分公式将奇异积分方程组变为 齐次线性代数方程组,以确定临界应变载荷. 通过数值算例,给出了底层为PZT-4材料、 电极为金属Pt在不同的脱层长厚比时的临界应变载荷和屈曲形状,分析了压电体的压 电、介电效应对屈曲载荷的影响. 另外给出了脱层屈曲时,脱层尖端奇异性振荡因子随不同 脱层长厚比的关系曲线.     

非稳态载荷下轮轨滚动接触及其钢轨波磨研究

利用有限元法,考虑材料反复滚压条件下棘轮效应和局部滑动的影响,研究了非稳态机车和车辆车轮载荷作用下轮轨滚动接触的弹塑性应力、应变和变形,进而分析了塑性流动型钢轨波浪形磨损的形成和发展过程以及波谷和波峰处材料的力学行为.结果表明:在非稳态载荷作用下,钢轨接触表面产生不均匀塑性变形引起的波磨,波磨发展速率呈衰减趋势,最终趋于稳定状态;在相同载荷下,与车辆车轮相比,机车车轮对钢轨波磨影响较大;波谷处的残余应力、应变和变形大于波峰处.     

疲劳文献的初步分析

疲劳是力学上的一个分支.主要是研究材料和构件在承受交变循环载荷作用下的强度、应力状态与寿命关系问题.材料在循环加载下,某部可发生损伤递增过程.经一定的应力或应变循环后,损伤累积可使材料产生裂纹,进一步扩展后可完全断裂.这种情况叫作疲劳破坏.疲劳问题是一个涉面很广的学术课题.据统计, ...     

厚壁圆筒的载荷特征与变形规律研究

为了研究厚壁圆筒在受到相同冲量但载荷特征不同时材料的变形规律,分别采用解析解、LS-DYNA软件和有限元计算程序对厚壁圆筒在受到内压作用下的力学响应进行了计算比较,在得到基本一致的结果后,利用有限元计算程序对冲量相同时四种加载方式下厚壁圆筒的动力学响应进行了计算,计算结果表明冲量相同时,不同载荷特征的载荷作用下的应变是不同的,应变与载荷作用时间和大小相关.     

静动组合载荷下混凝土率效应机理及强度准则

混凝土结构在受到动载荷作用之前,通常已承受着初始静载荷的作用.大量关于混凝土应变率效应的研究均没有考虑初始静载荷对动强度的影响,会导致过高地估计混凝土的动强度,使混凝土结构设计偏于危险.本文通过分析混凝土材料在静动组合载荷下的率效应机理,给出了初始静载荷的定义.在此基础上,推导了混凝土材料参数与初始静载荷和应变率的表达式,提出了建立静动组合强度准则的一般方法.通过材料参数反映初始静载荷与应变率的联合影响,给出了由初始有效静载荷、动态黏聚强度和摩擦强度共同组成的混凝土动态强度,将广义非线性强度准则发展为静动组合多轴强度准则.建立的强度面在相同初始静载荷下随应变率的增大向外扩张,在相同应变率下随初始静载荷的增大向里收缩,即混凝土的强度在相同初始静载荷下随应变率的增大而增大,在相同应变率下随初始静载荷的增大而减小.此外,当初始静载荷和应变率不变时,加载路径对混凝土材料的应变率效应无影响,但会影响混凝土材料的静水压力效应,即当初始静载荷和应变率固定不变时,静动组合强度面的位置和大小即可确定,不同加载路径下强度的不同是由于静水压力效应导致的.最后利用多组混凝土材料静动组合强度试验对建立的静动组合强度准则进行了验证.     

固体氧化物燃料电池宏观力学效应研究进展

固体氧化物燃料电池(SOFC)是一种清洁高效且具有广泛应用前景的绿色发电装置. SOFC采用了陶瓷材料电极和电解质并在高温下工作, 力学损伤是造成其性能和寿命衰减的主要因素之一. 由于实验测试的局限性, 基于宏观力学模型的数值模拟是优化SOFC电池和电堆结构、提高其性能和耐久性的重要手段. 本文综述并评价了SOFC宏观力学效应的研究进展, 介绍了SOFC在制造、正常运行和长期工作的不同阶段受到的残余应变、阳极氧化应变、化学膨胀、工作热应变以及蠕变等力学效应. 总结了各种力学效应以及目前关注较少的电化力耦合效应的理论和数值模拟研究现状, 最后展望了SOFC宏观力学性能研究的发展前景.      

压电体表面金属电极脱层的屈曲分析

基于弹性有限变形理论和电弹性体偏场理论,对半无限压电体及其表面电极层间存在穿透脱层的屈曲问题进行了分析. 采用平面应变模型,在脱层远处作用有平行于脱层的应变载荷. 使用Fourier积分变换,应用脱层界面的连续条件和电极表面的边界条件将问题归为第2类Cauchy型奇异积分方程组. 利用Gauss-Chebyshev积分公式将奇异积分方程组变为齐次线性代数方程组,以确定临界应变载荷. 通过数值算例,给出了底层为PZT-4材料、电极为金属Pt在不同的脱层长厚比时的临界应变载荷和屈曲形状,分析了压电体的压电、介电效应对屈曲载荷的影响. 另外给出了脱层屈曲时,脱层尖端奇异性振荡因子随不同脱层长厚比的关系曲线.      

实验力学的任务

实验力学是用实验手段对工程结构物的强度、刚度、稳定性和振动以及对工程材料的力学性质进行量测和研究的学科。材料的力学性质,主要是指材料在承受载荷后所产生的应力和应变间的关系及一些主要力学特征值。对工程结构物的强度,刚度和稳定性的量测研究主要     

A slip-line plasticity analysis of abrasive wear of a smooth and soft surface sliding against a rough (fractal) and hard surface

Abrasive wear of a soft and smooth surface sliding against a rough (fractal) and hard surface was analyzed by the slip-line theory of plasticity. The analysis is based on a slip-line model of a rigid spherical asperity (wear particle) plowing through a soft surface and removing material by microcutting. Integration of this single-contact model into a three-dimensional contact mechanics analysis of an abrasive surface exhibiting multi-scale roughness described by fractal geometry yielded relationships of the abrasive wear rate and wear coefficient in terms of the interfacial shear strength (adhesion), topography (fractal) parameters of the hard/rough surface, elastic–plastic material properties of the soft/smooth surface, and total normal load. Analytical results from the single-contact analysis provide insight into the deformation of a perfectly plastic material caused by the abrasive action of a rigid asperity/wear particle under different normal load and interfacial friction (adhesion) conditions. The dependence of the abrasive wear rate and wear coefficient on normal load (global interference), roughness of the abrasive surface, elastic–plastic material properties of the abraded surface, and interfacial shear strength (lubrication effect) is interpreted in the context of numerical results obtained for representative ceramic/ceramic, ceramic/metallic, and metal/metal sliding systems.     

Geometrically nonlinear finite-element models for thin shells with geometric imperfections

A finite-element method to analyze the stress–strain state and stability of thin shells with geometric imperfections is proposed. An arbitrary curvilinear finite element with vector approximation of the displacement function is used. To solve the systems of nonlinear algebraic equations by iteration methods, linearized stiffness matrices of finite elements and residual and load vectors are formed. The stress–strain state of a thin-walled shell with real geometric imperfections under surface pressure and axial compression is analyzed. The effect of geometric imperfections on the critical combination of loads is evaluated     

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木材声发射信号与其内外部损伤情况存在一定的定量关系. 依据损伤理论和含有裂 纹以及缺陷的木材不同阶段的声发射特征,分别探讨并建立了受交变载荷和单向静载荷情况下以声发 射特征参数表示的木材损伤演变模型. 分析结果与木材的实际情况相一致,表明以声发射特 征参量建立的木材损伤演变模型对木材的损伤预测具有提前性与敏感性,而且木材的不均匀 性是其提前破坏的主要原因之一.     

Thickness effects in birefringent coatings with radial symmetry

One method of determining the strains on the surface of a metal part is by the use of a birefringent coating. Because of the thickness of the coating, however, the observed fringe order depends not only on the surface strains but also on two other factors, namely, the curvature that the surface undergoes and the variation of strain acrose the thickness of the coating. These thickness effects have already been studied in a paper dealing with one dimensional variations in strain. In the present paper, the solution is extended to a two-dimensional deformation with radial symmetry. An experiment is described in which the surface strains produced by the impact of a bullet on a flat plate are determined from the birefringence of a surface coating taking into account the effects of strain gradients and curvature. The values of surface strain are compared with measurements made by means of a square grid inscribed on the metal surface. A procedure is suggested for determining an unknown surface strain having radial symmetry from the total birefringence of the surface coating when the change in curvature of the metal surface under load is known or can be determined separately.     

针尖的化学物理力学研究

扫描探针显微镜的发明, 使人们了解纳米、分子和原子尺度的超微结构, 探测原子、分子间的力、电、磁及其复杂的物理和化学性质, 以及进行单原子、单分子操纵 成为现实. 本文从探针技术与表面化学物理力学耦合的角度出发, 首先对针尖的化学物理力 学研究领域进行了概述, 接着介绍了针尖的几种重要的化学修饰方法, 包括金属薄膜、自组 装单分子膜、胶体粒子及碳纳米管修饰; 然后以理论与实验结合的方式介绍了几个研究活跃 的领域: 表面力及分子间力(主要包括Van der Waals力, 双电层力, 憎水亲合力, Casimir力和单键力等)的理论描述与测量, 针尖的化 学物理力学在化学力滴定和表面化学识别等研究中的应用. 讨论了针尖与基底材料对测量力 的影响. 而这些复杂的原子、分子相互作用和物理、化学、力学及生物特性的实现均发生于 小小针尖上, 由此我们提出了``针尖力学'的概念. 并且指出多场(如电场、磁场、超声、 微波等)作用下, 针尖的化学物理力学研究将成为力学交叉学科研究的重点和热点.     

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从城市垃圾填埋场沉降的 机理出发,以经典力学中应力-应变之间的关系为基础,建立起城市垃圾填埋场在载荷、蠕变 和生物降解耦合作用下沉降的力学模型,并分别确定在载荷、蠕变和生物降解作用下屈服表 面应力增量的表达式. 通过填埋场沉降的实例计算,该模型得到了较为满意的预测结果.     

A combined necking and shear localization analysis for aluminum sheets under biaxial stretching conditions

A combined necking and shear localization analysis is adopted to model the failures of two aluminum sheets, AA5754 and AA6111, under biaxial stretching conditions. The approach is based on the assumption that the reduction of thickness or the necking mode is modeled by a plane stress formulation and the final failure mode of shear localization is modeled by a generalized plane strain formulation. The sheet material is modeled by an elastic-viscoplastic constitutive relation that accounts for the potential surface curvature, material plastic anisotropy, material rate sensitivity, and the softening due to the nucleation, growth, and coalescence of microvoids. Specifically, the necking/shear failure of the aluminum sheets is modeled under uniaxial tension, plane strain tension and equal biaxial tension. The results based on the mechanics model presented in this paper are in agreement with those based on the forming limit diagrams (FLDs) and tensile tests. When the necking mode is suppressed, the failure strains are also determined under plane strain conditions. These failure strains can be used as guidances for estimation of the surface failure strains on the stretching sides of the aluminum sheets under plane strain bending conditions. The estimated surface failure strains are higher than the failure strains of the forming limit diagrams under plane strain stretching conditions. The results are consistent with experimental observations where the surface failure strains of the aluminum sheets increase significantly on the stretching sides of the sheets under bending conditions. The results also indicate that when a considerable amount of necking is observed for a sheet metal under stretching conditions, the surface failure strains on the stretching sides of the sheet metal under bending conditions can be significantly higher.     

Charging of a low-conductivity liquid flowing through a cylindrical pipe with allowance for the finite rate of a surface electrochemical reaction

A study is made of the charging of a low-conductivity liquid flowing through a cylindrical metal pipe with allowance for the finite rate of an electrochemical reaction that takes place on the walls of the pipe and is responsible for the charging. The solution of the problem uses the system of equations and boundary conditions derived by Gogosov, Tolmachev, and Nikiforovich [1]. The source of the space charge in the liquid is assumed to be a surface electrochemical reaction of ions of a contaminating impurity dissolved in the liquid [2–4]. The resulting current is found as a function of the flow velocity, the geometrical dimensions of the pipe, and the rate of the surface electrochemical reaction. It is shown that in the initial section of the pipe a kinetic regime is always realized in which the rate of the charging process is limited by the rate of the chemical reaction. Far from the initial section there is a diffusion regime, in which the rate of the charging process is determined by the rate of supply of ions to the surface of the pipe. With increasing velocity of the liquid the current tends to a limiting value, which depends only on the rate of the surface reaction, i.e., the current is saturated. The results given in the paper of experiments on the charging of hexane flowing through small-diameter pipes confirms the theory. The current was observed to be saturated in the experiments. A method is proposed for the experimental determination of the rate of a surface electrochemical reaction and the concentration of an ionic impurity.     

低碳钢试件弹塑性边界的白光相关检测

金属试件的弹塑性边界检测对判断该试件是否失效有着重要意义,因此一直是研究人员较感兴趣的研究课题。利用激光和白光作为照明光源的方法都曾被提出以检测金属试件的弹塑性边界以及塑性变形。为避免激光测量的诸多缺点,本文提出一种用白光作为照明光源并结合数字图像处理来检测低碳钢试件弹塑性边界的方法。该方法将数字图像相关和低碳钢试件表面弹塑性变形前后对照明光强的反射特性结合起来以判断试件是否进入塑性屈服。文中通过对加载过程中低碳钢试件表面序列图像的处理,准确地判断了该试件是否进入塑性屈服,验证了该方法的可行性。