Experimental investigation of thermal contact conductance for nominally flat metallic contact

A unique experimental set-up was fabricated to carry out axial heat flow steady state experiments for the assessment of thermal contact conductance (TCC) at the interface of two materials. Three different materials (copper, brass and stainless steel) were selected for the experiments considering their mechanical and thermal properties. Heat transfer experiments were performed in vacuum environment (0.045 torr) to find out solid spot contact conductance for nominally flat surfaces with different surface roughness (1–5 μm) for each specimen under several load conditions (0.6–15 MPa). A precise estimation of TCC for the interface of sets of similar materials was one of the most important results of this research. The effects of the surface roughness, the material properties and the load conditions (nominal interface pressure) have been studied and documented. Furthermore, the experimental results of solid spot contact conductance were compared with four theoretical models, showing their limitations to make a precise estimation of the TCC in the range of the used parameters.     

Effects of partial crack–face contact for the bending of thin shell structures

The physical occurrence that crack surfaces are in contact at the compressive edges when a flat or a shell is subjected to a bending load has been recognized. This article presents a theoretical analysis of crack–face contact effect on the stress intensity factor of various shell structures such as spherical shell, cylindrical shell containing an axial crack, cylindrical shell containing a circumferential crack and shell with two non-zero curvatures, under a bending load. The formulation of the problem is based on the shear deformation theory, incorporating crack–face contact by introducing distributed force at the compressive edge. Material orthotropy is concerned in this analysis. Three-dimensional finite element analysis (FEA) is conduced to compare with the theoretical solution. It is found that due to curvature effect crack–face contact behavior in shells differs from that in flat plates, in that partial contact of crack surfaces may occur in shells, depending on the shell curvature and the nature of the bending load. Crack–face contact has significant influence on the stress intensity factor and it increases the membrane component but decreases the bending component.     

Physics-based modeling for fretting behavior of nominally flat rough surfaces

A physics-based modeling approach for fretting behavior of nominally flat rough contact is proposed. This approach employs physics-based models for partial slip of spherical contacts to formulate the contact forces at asperity tips. The individual asperity forces are added by a statistical method to obtain the fretting response of a flat rough contact. This approach suggests the plasticity index as an important parameter for studying the surface roughness effects on fretting. Fretting responses obtained by one of the models favorably compare with experimental results obtained from bolted steel lap joints. Tangential stiffness and energy loss per cycle obtained from the experiments and the model predictions deviate at higher preloads. This discrepancy is due to limitations of the modeling approach in accounting for plastic response to tangential loading.     

原子尺度摩擦行为的分子动力学模拟

应用大规模分子动力学方法,模拟了具有原子级光滑和原子级粗糙形貌的刚性球形探头与弹性平面基体的干摩擦行为,研究了无/有粘附条件下的载荷与摩擦力、载荷与真实接触面积,以及摩擦力与真实接触面积之间的关系,对纳米尺度下的摩擦行为规律进行了分析.几种系统的真实接触面积-载荷关系都与相应的连续力学接触模型定性的一致,它们分别是Hertz光滑表面接触模型、Greenwood-Williamson粗糙表面接触模型和Mau-gis Dugdale粘着接触模型.无论是由光滑表面还是粗糙表面构成的摩擦系统,在无粘附条件下摩擦力与载荷成正比,而摩擦力与真实接触面积之间没有一个简单的关系;在粘附条件下摩擦力与真实接触面积成正比,而摩擦力与载荷之间表现为Maugis Dugdale模型预测的亚线性关系.研究表明,当表面作用从无粘附到粘附时,控制摩擦力的决定因素从载荷转变为接触面积,摩擦行为从载荷控制摩擦转变为粘着控制摩擦.     

Dynamics of a Semi-Infinite Cylindrical Shell with a Liquid Containing a Vibrating Spherical Segment

A semi-infinite cylindrical shell filled with a perfect incompressible liquid is considered. A vibrating rigid spherical segment placed on the shell axis excites the shell. The Laplace equation is solved under appropriate boundary conditions on the spherical, cylindrical, and flat surfaces bounding the liquid. Possibility is used to reexpand a spherical harmonic function in terms of a system of cylindrical harmonic functions and vice versa. The potential constructed is used to compute the shell deflections and the liquid pressure and velocity.     

Construction of the velocity potential of a semiinfinite liquid column excited by a spherical calotte oscillating inside a cylindrical cavity

A semiinfinite liquid column excited by a spherical calotte oscillating inside a rigid cylindrical cavity is considered. The solution of the Laplace equation is searched for under boundary conditions on spherical, cylindrical, and flat surfaces bounding the liquid. The possibility of decomposing a spherical harmonic function in terms of cylindrical harmonic functions and vice versa is considered. The potential constructed is used to evaluate the pressure and the velocity of the liquid. S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 36, No. 3, pp. 81–89, March, 2000.     

Retraction and Adhesion of a Single Droplet Normal Impact on the Solid Surface

The impact behavior of individual biomass oil droplets was investigated on solid surfaces having different structures (flat, cylindrical, and spherical) using the high-speed video technique. This makes it possible to compare the evolution of the droplet impact on various surface structures. The impact behaviors of retraction–oscillation and adhesion are analyzed for different hydrophobic surfaces. The influence of the Weber number (We), the surface structure, and the surface curvature is further examined by focusing on the retraction and stable adhesion (thickness, adhesion, and contact angle) for different biomass oil droplets. The results show that the retraction factor gradually increases as We increases to some critical value, beyond which the increase rate slows down or the retraction factor begins to decrease. The largest retraction factor is observed on the flat surface and the smallest one appears on the spherical surface. The adhesion thickness of the liquid film oscillates periodically over time, and its oscillation amplitude gradually decreases with a constant frequency, which is smaller for the more hydrophobic surfaces. The curvatures of the cylinder and sphere have little influence on the stable adhesion behavior. For the different droplet types, the adhesion diameter on the flat surface gradually increases as We rises, whereas the adhesion thickness gradually decreases with increase in We. These results are helpful for understanding the impact behaviors of biomass oil droplets with high viscosity and small surface tension on solid surfaces.

     

A discrete particle approach to simulate the combined effect of blast and sand impact loading of steel plates

The structural response of a stainless steel plate subjected to the combined blast and sand impact loading from a buried charge has been investigated using a fully coupled approach in which a discrete particle method is used to determine the load due to the high explosive detonation products, the air shock and the sand, and a finite element method predicts the plate deflection. The discrete particle method is based on rigid, spherical particles that transfer forces between each other during collisions. This method, which is based on a Lagrangian formulation, has several advantages over coupled Lagrangian-Eulerian approaches as both advection errors and severe contact problems are avoided. The method has been validated against experimental tests where spherical 150 g C-4 charges were detonated at various stand-off distances from square, edge-clamped 3.4 mm thick AL-6XN stainless steel plates. The experiments were carried out for a bare charge, a charge enclosed in dry sand and a charge enclosed in fully saturated wet sand. The particle-based method is able to describe the physical interactions between the explosive reaction products and soil particles leading to a realistic prediction of the sand ejecta speed and momentum. Good quantitative agreement between the experimental and predicted deformation response of the plates is also obtained.     

外加极化电位对316L不锈钢微动磨蚀行为的影响

采用球－平面接触微动磨损试验机考察了轧制固溶316L不锈钢在不同极化状态下的微动磨蚀行为。结果表明：在阳极极化状态下,随着极化电位的升高,腐蚀疲劳微断裂作用增强,促进了微动损伤过程的发展;在阴极保护状态下,摩擦系数随微动过程的变化规律及微动损伤形貌与阳极极化态下的存在显著差异,在阴极极化态下,微动磨擦副之间的粘着导致较高的微动摩擦应力状态,但与阳极极化态相比并未产生严重损伤。     

正畸弓丝与托槽间摩擦力建模及试验研究

正畸矫治过程中,正畸弓丝与托槽间的相对滑动趋势将产生摩擦力,进而降低有效矫治力,影响矫治的性能和效率. 针对目前正畸摩擦力预测方法量化预测精度低的问题,依据正畸弓丝与托槽间的几何关系、力学关系及物理参数,提出一种基于分力叠加原理的计及接触角度的正畸摩擦力预测模型建立方法. 探究影响正畸摩擦力的主要因素以及变化规律,提出采用有限滑动法测量正畸摩擦力,搭建了基于六维力传感器的正畸摩擦力测量系统,进行了不同弓丝-托槽组合和不同接触角度的摩擦力测量,试验数据与预测模型的理论数据间误差率处于0.55%~9.65%之间,证明该预测模型可为医师明确正畸矫治器参数-摩擦力-矫治力的关系提供理论依据,为实现数字化正畸提供理论支撑,保证个性化正畸方案的高效、高可靠性和高舒适度,最终达到轻力矫治的效果.      

两圆柱体结合面的接触热导分形模型研究

基于三维分形理论,在考虑微凸体的弹性变形、弹塑性变形和塑性变形的基础上,建立了两圆柱体结合面接触热导分形模型。通过数值仿真,分析了分形维数,分形尺度参数、圆柱体曲率半径和接触类型对接触热导的影响。研究结果表明：接触热导随着分形维数的增大而增大,随着分形尺度参数的增大而减小;相同参数下,内接触比外接触的接触热导要大;此外,当固定其中一个圆柱体的曲率半径时,随着另一个圆柱体曲率半径的增大,接触热导增大。该模型为开展齿轮等曲面接触热导的研究提供了理论基础。     

Roughness and surface material effects on nucleate boiling heat transfer from cylindrical surfaces to refrigerants R-134a and R-123

This paper presents results of an experimental investigation carried out to determine the effects of the surface roughness of different materials on nucleate boiling heat transfer of refrigerants R-134a and R-123. Experiments have been performed over cylindrical surfaces of copper, brass and stainless steel. Surfaces have been treated by different methods in order to obtain an average roughness, Ra, varying from 0.03 μm to 10.5 μm. Boiling curves at different reduced pressures have been raised as part of the investigation. The obtained results have shown significant effects of the surface material, with brass being the best performing and stainless steel the worst. Polished surfaces seem to present slightly better performance than the sand paper roughened. Boiling on very rough surfaces presents a peculiar behavior characterized by good thermal performance at low heat fluxes, the performance deteriorating at high heat fluxes with respect to smoother surfaces.     

690合金传热管在不同摩擦副条件下的微动磨损性能研究

采用MFF-3000电磁振动微动疲劳与磨损试验机,研究两种抗振条材料(退火405不锈钢和淬火回火06Cr13)对690合金传热管的微动磨损性能的影响,试验采用块/管线接触方式,即线接触.试验结束后,采用OM、SEM、EDX和EPMA和三维光学显微镜对磨痕微观形貌和成分等进行分析,对比分析两种摩擦副的微动摩擦行为.结果表明:当配副材料为405不锈钢时,690合金的磨损相对严重;工况相同时,06Cr13的磨损比405不锈钢严重,且405不锈钢的磨屑尺寸较小,06Cr13磨痕表面存在大量的剥层裂纹.随着温度增加,磨屑增加,氧化程度加剧,磨损加剧,主要的磨损机制为磨粒磨损和剥层.     

Effect of micropillar surface texturing on friction under elastic dry reciprocating contact

Surface texturing is considered to be a promising method to improve the tribological properties. Depending upon the experimental conditions, the effect of texturing varies from favourable to unnoticeable to detrimental. In this work, surfaces with micropillars are studied under elastic dry reciprocating contact. An array of micropillars with different pillar heights are generated on stainless steel using wire-cut electrical discharge machining. The effect of stiffness of the micropillars on friction is investigated, keeping the number of micropillars in contact with a flat aluminium alloy (Al6061) slider and contact geometry constant. Reciprocating experiments are carried out against a flat surface such that about 81 micropillars are in contact. From the experimental results, it is found that the coefficient of friction is independent of the stiffness of the texture elements. However, work done per cycle significantly varied with the stiffness of texture element and applied normal load. A lumped system model with Coulomb friction shows that the work done per cycle varies quadratically with the normal load. The experimental results agree with this simplified model except in the incipient sliding regime. These results show how the work done per cycle varies, for different contact stiffness under elastic contact even though the coefficient of friction remains constant. The implication of this study for a macroscopic measured coefficient of friction as a function of microscopic asperity level friction is discussed.     

Various analogies for the equilibrium shapes of an elastic thread on two-dimensional surfaces

In accordance with the Kirchhoff analogy, the equilibrium equations of an elastic thread on a plane are equivalent to the equations of motion of a simple pendulum. This analogy is generalized to the case when the thread is situated on a smooth curved surface. The equilibrium equations for the threads in the general case and in the particular cases of flat, cylindrical, and spherical surfaces are derived. For these surfaces the Kirchhoff analogy is generalized to the case of a simple pendulum in an additional force field. There are also considered the electromagnetic and nonholonomic analogies for the equilibrium equations of an elastic thread.     

Heterogeneous nucleation on ultra smooth surfaces

An experimental investigation is presented with heterogeneous nucleate boiling on ultra smooth metallic surfaces (30-365 nm RMS roughness), including brass, unpolished stainless steel, and electropolished stainless steel. The fluids used for the investigation are highly wetting pentane and butane. It is observed that the incipient superheat is low for all cases considered, despite the fact that no vapor trapping cavities are available for incipience at low superheat. These data provide further evidence that in addition to vapor trapping, another mechanism must be available for heterogeneous nucleation in boiling systems. The boiling curves are presented for the different surface/fluid combinations. It is found that the heat transfer rate on the brass surface is considerably better than that on the stainless steel surfaces due to the ease with which nucleation sites are formed. In contrast, nucleation site formation on stainless steel is considerably more sparse than that on brass.     

丝径对316L不锈钢丝摩擦磨损行为的影响

为探究金属橡胶微丝的最适直径,研究了不同载荷和速度条件下,金属橡胶不锈钢丝丝径对其小位移摩擦磨损行为影响的规律及机理,建立了磨损深度与丝径之间的定量关系来评定丝径对不锈钢丝摩擦磨损行为的影响. 结果表明:相同载荷、速度条件下,不同丝径实际接触面积的不同导致不锈钢丝的磨损深度随其丝径的增大而减小,且磨损深度随丝径的变化规律呈多项式曲线规律;而摩擦系数与其实际接触形貌和磨屑运动状态有关,不同的磨损状态导致了摩擦系数随丝径的增大而增大;探究表明改变载荷和速度并不影响丝径对不锈钢丝摩擦磨损行为的影响规律;但由于粗丝径试件间实际接触面积的稳定性,使得载荷和速度对粗丝试件的磨损深度、摩擦系数的影响要明显小于对细丝试件的影响.      

Curvature effect on droplet impacting onto hydrophobic and superhydrophobic spheres

Droplet impact on hydrophobic and superhydrophobic solid surfaces finds numerous applications, while the wide range of the parameters affecting its outcome necessitate a thorough study to reveal the underlying physics. Specific applications are related to the drop impact upon curved surfaces, such as micro-encapsulation in fluidized beds. Three-dimensional numerical simulations by applying Level-Set Method have been performed to investigate the water droplet impact on curved and flat hydrophobic and superhydrophobic substrates. Parameters such as the impact Weber number, the surface curvature and the equilibrium contact angle have been varied in order to assess their effects on the dynamics of the impact process. After providing a strong validation, it is found that impact on spherical surfaces generally presents a higher area of liquid to be in contact with the substrate with respect to the case of flat surfaces, when all other impact conditions are the same.     

Stress state of two collinear stamps over the surface of orthotropic materials

An exact contact analysis of two collinear stamps over the surface of orthotropic materials is performed. Eigenvalue analyses related to the governing equations are conducted, and real fundamental solutions are obtained for each eigenvalue distribution. Using Fourier sine or cosine transforms, a singular integral equation of Cauchy type is obtained. In particular, the exact solution of the reduced singular integral equation is obtained in the case of two collinear flat or cylindrical stamps. Explicit expressions for various stresses are obtained in terms of elementary functions. Numerical results are presented, and interesting observations are found. For two collinear flat stamps, detailed calculations are provided to show influences of distance between two flat stamps on the contact stress and in-plane stress. For a cylindrical stamp, influences of distance between two cylindrical stamps, mechanical loading P, and the cylindrical stamp radius R on the surface normal stress and surface in-plane stress are detailed.