Modeling of Transient Thermal Contact Resistance out of Conjugate Gradient Method

Temperature profile at contacting pairs of any material combination is crucial in many engineering applications. A new experimental approach is suggested to estimate the contact heat transfer coefficient as well as the thermal contact resistance. This method is based on temperature infrared measurements of contact bodies and then solve the inverse problem with conjugate gradient method. Different affecting parameters are studied namely; surface roughness, presence of interlayer, applied pressure and bodies temperature difference. The time dependant heat transfer coefficients resulting from the proposed method need to be validated by a computer model. Close results are shown with slight deviation by the increase of the applied pressure and the temperature difference.     

车轮原地打滑时轮轨接触界面摩擦温升分析

简要介绍了轮轨滚滑接触摩擦温升的研究历史和现状。基于有限元法和非稳态传热方程,建立了车轮在钢轨上原地打滑时轮轨摩擦热分析模型,分析车轮打滑速度对钢轨接触斑附近温度场的影响。模型考虑了轮轨与环境间的热辐射和热对流边界条件、轮轨接触区的相互换热以及轮轨材料热物性参数随温度的变化。分析结果显示:车轮原地打滑时,钢轨表面温升率在打滑初期非常高,随后温升率逐渐趋于零;表面温升达到稳定状态后,钢轨表面温升与轮轨接触区相对滑动速度呈线性关系;当车轮打滑转动的角位移相同而其他条件不变时,钢轨表面热影响层厚度基本不随相对滑动速度的大小而变化。     

高速轮轨水介质存在下的黏着特性数值研究

为了获得高速轮轨在水介质存在时的黏着特性,该文建立了三维轮轨间存在水介质时考虑表面粗糙度的轮轨黏着计算模型。该模型以部分弹流理论及弹性微观固体接触模型为基础,借助于多重网格作为数值计算工具。研究了水介质存在时列车运行速度、轴重及轮轨表面粗糙度对轮轨黏着系数的影响,并从数值角度揭示了其影响机理。计算结果表明,随着速度的提高,黏着系数逐渐降低;随着轴重的增大,黏着系数逐渐降低;适当地增加轮轨表面粗糙度有利于增加轮轨黏着特性。     

考虑基体变形的结合面连续光滑接触刚度模型

结合面的接触刚度直接影响着机床整机的静、动态力学性能和精度保持性水平。在弹性、弹塑性以及完全塑性接触变形过程中,延续微凸体接触状态变量具有连续且光滑特性的思想,利用Hermite多项式插值函数,建立单个微凸体法向接触刚度模型。考虑到基体变形的影响,在微凸体的3个变形阶段分别引入了基体的弹性接触变形,通过统计学方法,建立了一种考虑基体变形且连续光滑的结合面刚度模型。对比分析了GW、ZMC、KE、Brake模型与该文模型之间的差异,并揭示了表面粗糙度对接触刚度的影响规律。研究表明:考虑基体变形时所获得的接触刚度和接触载荷比忽略基体变形时的要小,且随着表面粗糙度的减小,基体变形的影响快速增加;接触载荷与表面粗糙度是影响结合面接触刚度的2个主要因素,随着载荷的增大或表面粗糙度的减小,接触刚度随之递增。     

Effect of Current Crowding on Microstructural Evolution at Rail-Armature Contacts in Railguns

During firing of a railgun, the armature typically undergoes melting at the rail-armature contact and deposits melt on the rail, thereby reducing rail life. This melting starts at the trailing edge of the armature, and can be attributed to current crowding and localized Joule heating. This paper reports on dual laboratory-scale armature tests, one with current and one without, and by microstructural analysis of the two armatures, demonstrates that localized Joule heating due to current crowding is the principal source of melting, with frictional heating playing a much smaller role. The paper reports on microstructural changes in the armature due to Joule heating, along with associated degradation of the rail surface. It evaluates the impact of skin effect in the rail and the electrical contact conductance at the rail-armature interface on current crowding at the points of entry and exit of the current into and from the armature. In general, reduction of current crowding decreases the peak temperature at the rail-armature interface. But while reducing interfacial contact conductance reduces current crowding, it enhances the peak temperature and causes more melting.     

FE-investigations of micro-polar boundary conditions along interface between soil and structure

The interface between granular bodies and structures is analysed with the finite element method and a micro-polar hypoplastic constitutive model. Quasi-static shearing of an infinitely long and narrow granular strip between two rigid walls of different roughness under conditions of free dilatancy and constant vertical pressure is investigated. The constitutive model can reproduce the essential features of granular bodies during shear localization. To model the different roughness of the interface, micro-polar boundary conditions are proposed taking into account the asperity of the wall roughness and grain diameter. Some emphasis is given to the influence of the wall roughness on the thickness of shear zone and the mobilization of wall friction.     

The effect of loading on surface roughness at the atomistic level

One of the key points to better understand the origins of friction is to know how two surfaces in contact adhere to one another. In this paper we present molecular dynamics (MD) simulations of two aluminium bodies in contact, exposed to a range of normal loads. The contact surfaces of both aluminium bodies have a self-affine fractal roughness, but the exact roughness varies from simulation to simulation. Both bodies are allowed to have an adhesive interaction and are fully deformable. Tracking important contact parameters (such as contact area, number of contact clusters, and contact pressure) during a simulation is challenging. We propose an algorithm (embedded within a parallel MD code) which is capable of accessing these contact statistics. As expected, our results show that contact area is increasing in proportion with applied load, and that a higher roughness reduces contact area. Contact pressure distributions are compared to theoretical models, and we show that they are shifted into the tensile regime due to the inclusion of adhesion in our model.     

Effective mechanical properties of layered rough surfaces

In contact mechanics of layered rough surfaces, found in various systems such as magnetic storage devices and micro/nanoelectromechanical systems, it is of interest to calculate effective elastic modulus and hardness so as to obtain contact parameters using simple analyses for homogeneous surfaces. In this study, effective elastic modulus and hardness of layered rough surfaces are defined on the basis of real area of contact. A numerical model developed by the first author to simulate the contact of layered rough surfaces is used to derive these equations. Completely nondimensionalized empirical equations for these effective mechanical properties are presented. Separate equations are developed for the contact of a single conical asperity on a flat surface, a single spherical asperity on a flat surface, and for multiple asperity contact between two rough surfaces. These equations establish the dependence of effective mechanical properties on indentation depth, layer thickness, hardness and elastic modulus ratios of layer and substrate and surface roughness/asperity geometry. Comparisons of values predicted by the equations with experimentally obtained results are presented. Contact stress contours obtained from this model are analyzed to get a better understanding of the mechanics of contact.     

Performance of truncated surface asperities in elastohydrodynamic lubrication

The elastohydrodynamic lubrication in a sliding line contact between a flattened rough plane surface and a smooth plane surface is analytically studied. Both surfaces are assumed as elastic and parallel to one another. The flattened rough surface is treated as equivalent to a rough plane surface with uniformly distributed cylindrical asperities evenly truncated on the top. For understanding the elastohydrodynamic lubrication performance between these two plane surfaces, an inlet zone analysis is taken for the elastohydrodynamic lubrication formed between a truncated cylindrical asperity and the smooth plane surface. It is found from the obtained results that the asperity truncation reduces the elastohydrodynamic load-carrying capacity, and this effect is significant for low sliding speeds or/and heavy loads, while it is negligible for high sliding speeds and moderate loads. The asperity truncation increases the friction coefficient of the asperity contact especially at relatively light loads, while it only slightly increases the friction coefficient of the asperity contact at heavy loads. For heavy loads, the asperity truncation can have a significant effect on the reduction in the maximum surface temperature rise. It is recommended by the present study that the surface asperity be truncated in a certain degree in an elastohydrodynamic contact with high sliding speeds and relatively heavy loads because of giving the benefits of considerably reducing the maximum surface temperature rise while maintaining the elastohydrodynamic load-carrying capacity.     

Effect of asperity damage on shear behavior of single fracture

Experimental studies have shown that fractures often exhibit shear resistance softening and dilatation under shearing loads. From a mechanistic viewpoint, these phenomena are a consequence of the fracture surface roughness and the material mechanical properties. Consequently, this paper utilizes a micromechanical model of fractures that explicitly considers asperity interactions on fracture surfaces. Elastic deformations and inelastic frictional sliding are considered at inclined asperity contacts. A modified spherical harmonic expansion is used to model the orientation distribution of asperity contacts. Evolution laws for asperity heights and asperity contact orientations are introduced to account for the change in surface roughness resulting from asperity damage under shear. Results obtained from the model show that the asperity contact orientation evolution law is essential for correctly modeling the softening and dilatation behavior of fractures. The model results are compared with experimental data culled from the literature.     

Experimental investigation of nonuniform heating and heat loss from a specimen for the measurement of thermal diffusivity by the laser pulse heating method

Nonuniform heating effect and heat loss effect from the specimen in the measurement of thermal diffusivity by the laser pulse heating method have been experimentally investigated using an axially symmetric Gaussian laser beam and a laser beam homogenized with an optical filter. The degree of error is theoretically estimated based on the solution of the two-dimensional heat conduction equation under the boundary condition of heat loss from the surface of the specimen in the axial direction and the initial conditions of axially symmetric nonuniform and uniform heating. A correction factor, which is determined by comparison of the entire experimental and the theoretical history curves, is introduced to correct the values obtained by the conventionalt _1,2 method. The applicability of this modified curve-fitting method has been experimentally tested using materials in the thermal diffusivity range 10⁻³ to 1 cm²·s⁻¹. The experimental error due to the nonuniform heating and heat loss was reduced to approximately 3%.     

氧化物陶瓷闪烧机理及其应用研究进展

闪烧是近些年广受关注的一种电场辅助烧结技术。本文介绍了闪烧的起源与发展, 并对闪烧的基本特征进行了分析。在闪烧孕育与引发过程的研究方面, 发现了孕育阶段的非线性电导特征和电化学黑化现象, 提出了氧空位主导的缺陷机制; 在闪烧阶段的快速致密化研究方面, 提出了电场作用导致的缺陷产生和运动会在粉体颗粒间产生库仑力, 有利于烧结前期的致密化过程, 同时发现闪烧致密化过程中还伴随着金属阳离子的快速运动; 在闪烧阶段的晶粒生长和微结构演变方面, 发现了试样温度沿电流方向呈非对称分布, 试样中间位置的晶界迁移率明显提高, 提出电化学缺陷对微观结构有重大影响。基于上述研究成果, 本团队利用电场作用下出现的低温快速传质现象, 发展了陶瓷闪焊技术, 实现了同种陶瓷/陶瓷、陶瓷/金属, 甚至异种陶瓷/陶瓷之间的快速连接; 发展了陶瓷闪烧合成技术, 不仅实现了典型氧化物陶瓷的快速合成, 而且实现了高熵陶瓷和具有共晶形貌的氧化物陶瓷的快速合成; 发展了氧化物陶瓷的电塑性成形技术, 初步实现了氧化锆陶瓷低温低应力下的快速拉伸和弯曲变形。本文最后总结了闪烧机理研究面临的挑战, 并从焦耳热效应和非焦耳热效应两方面展望了闪烧的发展方向, 期望对闪烧技术在国内的发展有所裨益。     

A contact formulation based on localized Lagrange multipliers: formulation and application to two‐dimensional problems

The non‐penetration condition in contact problems is traditionally based on the classical Lagrange multiplier method. This method makes extensive use of modelling details of the contacting bodies for contact enforcement as the contact surface meshes are in general non‐matching. To deal with this problem we introduce a novel element in the Lagrange multiplier approach of contact modelling, namely, a contact frame placed in between contacting bodies. It acts as a medium through which contact forces are transferred without violating equilibrium in the contact domain for discrete contact models. Only nodal information of the contacting bodies is required which makes the proposed contact enforcement generic. The contact frame has its own independent freedoms, which allows the formulation to pass contact patch tests by design. Copyright © 2002 John Wiley & Sons, Ltd.     

空气源热泵自然冷源过冷制热性能实验研究

冬季我国北方室外环境蕴含大量天然冷源,热力学分析表明热泵工质过冷释放的热量可以在蒸发器的等温吸热过程中获得补偿。为了研究大气自然冷源对热泵制热性能的影响,增设室外过冷器,搭建利用自然冷源过冷的空气源热泵实验装置。实验结果表明：当室外环境温度大于0 ℃,冷凝温度小于45 ℃的条件下,自然冷源过冷对热泵制热量与制热COP影响均较小,系统制热量维持在6.22 ~ 6.70 kW,制热COP维持在3.03,压缩机排气温度维持在103 ℃以下;当室外环境温度小于 -10 ℃,冷凝温度大于50 ℃时,随过冷度的增加,压缩机功率增加、排气温度显著增高,系统制热量呈先缓慢增加后减小趋势,制热COP降至2.3。基于上述研究提出一种空气源热泵过冷融霜新型除霜方式,融霜同时不停止制热。     

Thermoelastic Contact of a Rotating Inertial Cylinder with a Bush under the Conditions of Friction Self-Oscillations

We construct the solution of the problem of thermoelastic contact of an inertial cylinder with a rigid bush under the conditions of friction heating and wear. It is assumed that the friction coefficient depends on the relative velocity of contacting bodies. We analyze the stability of stationary solutions, determine contact parameters in the process of nonuniform rotation of the cylinder, and demonstrate the possibility of friction self-oscillations.     

Rolling–sliding contact fatigue of surfaces with sinusoidal roughness

Surfaces of mechanical components under combined rolling and sliding motions may be subjected to accelerated contact fatigue failure due to increased number of microscopic stress cycles and pressure peak heights caused by rough-surface asperity contacts. Available rolling contact fatigue (RCF) theories were developed mainly for rolling element bearings, for which the effect of sliding is usually insignificant. In various types of gears, however, considerable sliding exist in the critical tooth contact area below the pitch line, where excessive wear and severe pitting failures originate. Ignorance of sliding is most likely the reason why the conventional RCF models often overestimate gear fatigue life. This paper studies the effect of sliding motion on the contact fatigue life of surfaces with sinusoidal roughness that mimicks the topography from certain manufacturing processes. A set of simple equations for stress cycle counting is derived. Mixed elastohydrodynamic lubrication simulations are executed with the considerations of normal loading and frictional shear. Relative fatigue life evaluations based on a subsurface stress analysis is conducted, taking into account the two sliding-induced mechanisms, which are the greatly increased number of stress cycles and the pressure peak heights due to surface interactions. Obtained results indicate that sliding leads to a significant reduction of contact fatigue life, and rough surface asperity contacts result in accelerated pitting failure that needs to be considered in life predictions for various mechanical components.     

Analytical Solution of Two‐Dimensional Contact Problems of Unsteady Heat Conduction in the Presence of Mixed Boundary Conditions in the Contact Plane

A method for solution of systems of parabolic differential equations of heat conduction on the model of thermal contact between two bodies with different thermophysical characteristics in the presence of mixed boundary conditions in the plane of their contact has been suggested for the first time. The case of contact of two semibounded bodies has been considered. In this case, a heat source of low heat capacity acts in a circular region of finite radius on the contact surface, and beyond this region the initial temperature is maintained during the whole period of heat transfer.     

The impact of a thermoelastic rod against a rigid heated barrier

The impact of a thermoelastic rod with a heat-insulated lateral surface against a rigid heated barrier is considered. The heat exchange between the rod and the wall occurs at one of its ends contacting with the wall, while the other end is heat-insulated and free from external forces. The behaviour of the rod during the impact process is described by the Green-Naghdy theory which allows one to take finite speed of heat propagation into account, neglecting therewith thermal relaxation. The Laplace integral transform with the subsequent expansion of the found images in terms of the natural functions of the problem is used as a method of solution, which is found in explicit exact closed form. The analytical time-dependence of displacements, stresses, and temperature at each rod particle is obtained. The emphasis is on the analysis of the contact stress, the temperature of the colliding bodies during their contact interaction, and on the detection of the duration of contact of the rod with the rigid wall. It is shown that the contact time essentially depends on the relationship between the mechanical and thermal values.     

燃气轮机拉杆转子考虑接触效应的扭转振动模态分析

燃气轮机转子一般是由多个层叠的轮盘通过拉杆组合而成,各轮盘接触面由磨削加工形成。粗糙度测试结果表明磨削加工的实验拉杆转子轮盘面具有两个不同分形结构的区域。利用结构函数法计算了这种双重分形面的轮廓曲线的分形维数D1、D2和分形粗糙度参数G1、G2 。采用双重分形几何描述接触表面的拓扑结构,并根据赫兹接触理论导出接触微凸体的切向接触刚度。弹塑性双重分形面的切向接触刚度等于所有微观弹性接触点的切向接触刚度的总和。粗糙层是由相接触的微凸体所构成的,其抗扭刚度模化为接触转子轮盘间的一个抗扭弹簧。通过三维有限元模态分析和实验模态分析得到了拉杆转子在不同预紧力下的扭振模态频率。通过上述计算和实验结果识别了粗糙层的抗扭刚度,实验测试结果和理论计算结果相一致,这表明上述接触层抗扭刚度的双重分形模型实合理的,可以有效地考虑接触效应对拉杆转子扭转振动模态频率的影响。