Phase transitions in adsorbed layers VII: A model for mixed gas adsorption on solid surfaces

A simple model for the adsorption of two-component gaseous mixtures on crystalline solids is developed. This theory is based on the assumption that the adsorbed layer is neither completely localized nor completely mobile. It is shown that the model predicts various phase transitions involving two-dimensional gas, two-dimensional liquid as well as two-dimensional solid phases. The conditions for the appearance of a solid phase are discussed.     

The adsorbate-adsorbate association model in mixed gas adsorption on homogeneous solid surfaces

We discuss the adsorbate-adsorbate association model which assumes the formation of higher associates on the surface. Association both parallel and normal to the surface is considered. The normal association model together with Brunauer-Emmett-Teller and Gonzalez-Holland assumptions lead to interesting isotherms that describe multilayer adsorption from binary gas mixtures onto homogeneous surfaces.     

A mixed finite element model for the elastic contact problem

The static elastic contact problem is approached using Lagrange multipliers, leading to a mixed finite element problem. A non-linear friction law is introduced explicitly and the non-local character of the friction phenomena is implicitly assumed. In order to avoid stress oscillations near singular points, a perturbed Lagrangian functional is considered. The algorithms herein proposed do not impose nodal dependencies over the contact surfaces, allowing for the independent discretization of both bodies. The method is able to model simultaneous contact over different regions of any geometrical shape. Computer code, examples and results presented here are restricted to axisymmetrical and bidimensional cases.     

混合润滑状态下结合面法向动态接触刚度与阻尼模型

机械结合面的动态接触特性对评估机床整机性能有着重要的意义.针对混合润滑状态下固定结合面复杂的接触特性,提出了一种结合面的法向接触刚度与阻尼模型.采用三维Weierstrass?Mandelbrot函数获得粗糙表面形貌,并基于分形理论建立了结合面固体部分的接触刚度与接触阻尼模型;根据平均流动的广义雷诺方程建立了液体油膜接...     

A boundary element model for near surface contact stresses of rough surfaces

This study proposes a boundary element based near surface elastic stress prediction model for rough surface point contacts. The contact body is considered as a half space, whose boundary is divided into the rough finite field and the smooth infinite field. The integral kernel singularities in the displacement boundary integral equation are eliminated through coordinate transformation and the approach of rigid body motion. For the integral of the nearly singular stress kernels, a numerical scheme that combines the distance transformation and a subdivision technique is devised to improve the numerical accuracy and efficiency. The computational results are compared to closed-form solutions to assess the model accuracy. Using a three-dimensional sinusoidal roughness profile, the near surface stress concentrations induced by surface asperities are demonstrated. It is shown the exclusion of roughness profile in stress computation leads to significant errors in both the locations and the magnitudes of near surface stress concentrations.     

A cohesive element model for mixed mode loading with frictional contact capability

We present a model that combines interface debonding and frictional contact. The onset of fracture is explicitly modeled using the well‐known cohesive approach. Whereas the debonding process is controlled by a new extrinsic traction separation law, which accounts for mode mixity, and yields two separate values for energy dissipation in mode I and mode II loading, the impenetrability condition is enforced with a contact algorithm. We resort to the classical law of unilateral contact and Coulomb friction. The contact algorithm is coupled together to the cohesive approach in order to have a continuous transition from crack nucleation to the pure frictional state after complete decohesion. We validate our model by simulating a shear test on a masonry wallette and by reproducing an experimental test on a masonry wall loaded in compression and shear. Copyright © 2012 John Wiley & Sons, Ltd.     

结合面静态接触参数的统计模型研究

基于对粗糙表面形貌统计分析的基础上,综合考虑微凸体的完全弹性、弹-塑性和完全塑性三种变形机制,建立了结合面的接触面积、接触载荷及接触刚度的统计模型。该模型揭示了结合面接触参数与材料性能参数及粗糙表面形貌参数之间复杂的非线性关系。在不同的微凸体高度随机分布及塑性指数条件下,对接触参数进行预估和对比研究。结果表明,修正的指数分布对高斯分布有着较好的近似,而简单的指数分布与高斯分布之间的误差较大,且相差1至3个数量级;接触表面间距减小时,接触参数值均呈现增大的趋势;塑性指数增大时,接触载荷和接触刚度都随之增大,而接触面积的变化较小。     

Solidification contact angles of molten droplets deposited on solid surfaces

Droplet impact and equilibrium contact angle have been extensively studied. However, solidification contact angle, which is the final contact angle formed by molten droplets impacting on cold surfaces, has never been a study focus. The formation of this type of contact angle was investigated by experimentally studying the deposition of micro-size droplets (∼39 μm in diameter) of molten wax ink on cold solid surfaces. Scanning Electron Microscope (SEM) was used to visualize dots formed by droplets impacted under various impact conditions, and parameters varied included droplet initial temperature, substrate temperature, flight distance of droplet, and type of substrate surface. It was found that the solidification contact angle was not single-valued for given droplet and substrate materials and substrate temperature, but was strongly dependent on the impact history of droplet. The angle decreased with increasing substrate and droplet temperatures. Smaller angles were formed on the surface with high wettability, and this wetting effect increased with increasing substrate temperature. Applying oil lubricant to solid surfaces could change solidification contact angle by affecting the local fluid dynamics near the contact line of spreading droplets. Assuming final shape as hemispheres did not give correct data of contact angles, since the final shape of deposited droplets significantly differs from a hemispherical shape.     

Heat exchange of an immovable filtered layer with immersed surfaces in a two-component model of heat transfer

Solutions for the problem of steady-state heat transfer in an immovable filtered layer with immersed surfaces are presented. Use is made of versions of a two-component homogeneous model differing in the ways of considering heat exchange between the layer components and the immersed surfaces. The results predicted by both versions are compared to one another and to experimental data, and the range of applicability is identified for each of them. Relations describing heat exchange of the layer components with a staggered tube bundle are given.Odessa Institute of Low-Temperature Technology and Power Engineering. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 63, No. 1, pp. 63–68, July, 1992.     

混合润滑状态下结合面的法向接触刚度研究

针对润滑条件下机械结合面的接触特性受油膜影响的问题,基于结合面接触刚度由油膜接触刚度和固体表面接触刚度组成的思想,建立混合润滑状态下结合面的法向接触刚度模型。采用三维的Weierstrass-Mandelbrot函数获得具有分形特征的粗糙表面,并基于统计学方法建立干摩擦条件下结合面的法向接触刚度模型,考虑了微凸体的完全弹性变形、弹塑性变形以及完全塑性变形过程。在此基础上,求解了油膜的等效厚度并建立油膜的接触刚度模型。结果表明:结合面的法向接触刚度随法向载荷的增加而增加,且混合润滑状态下结合面的接触刚度大于干摩擦条件下结合面的接触刚度;该模型避免了油膜厚度测量难的问题,为机械结构的润滑状态预测提供了帮助。     

A model of a liquid droplet impinging on a high-temperature solid surfaces

The paper deals with a model of a liquid droplet vertically impinging on a heated solid surface. The model uses the following assumptions. The value of the wall temperature is taken to be such that the droplet-wall interaction would proceed via gas-vapor interlayer (T 400 °C). The droplet liquid is incompressible and nonviscous. The droplet surface is taken to be free, with its deformation caused by the effect of external pressure distributed over the droplet surface. The pressure is made up by two components, of which the first one is the surface tension pressure due to the curvature of the droplet surface; the second component is the pressure of vapor between the droplet and wall, which is determined by analyzing the process of vapor escape from the vapor interlayer. The motion of liquid within the droplet is taken to be potential and axisymmetric. The equations of droplet motion are solved relative to the potential of the vector field of velocity. The suggested model is used to perform numerical calculations of the droplet collision process, and the obtained results are compared with the data of other authors.Translated from Teplofizika Vysokikh Temperatur, Vol. 42, No. 6, 2004, pp. 921–927.Original Russian Text Copyright © 2004 by A. V. Gulikov, I. I. Berlin, and A. V. Karpyshev.     

Heating of solid surfaces by an electric arc

We have developed a calorimetric probe for measuring unsteady heat fluxes with a resolving time t < 10^– sec. We have determined the flux to solid surfaces from an electric arc stabilized by a rotating cylinder.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 46, No. 3, pp. 461–466, March, 1984.     

A mixed mode crack growth model taking account of fracture surface contact and friction

Fracture surface interactions, of whatever origin, can significantly affect the stress intensity factor, and consequently, can also be relevant to fatigue crack propagation. In the occurrence of interaction between fracture surfaces, the effective loading cycle experienced by material near the crack tip may be very different from that evaluated on the basis of the external loadings only. The purpose of the work described in this paper is to obtain the effective mode II stress intensity factor, k _IIeff, in a surface cracked elasto-plastic plate with a factory roof fracture surface subjected to an in-plane shear (mode II) loading. A new model estimating the magnitude of the frictional mode II stress intensity factor, k _f, arising from the mismatch of the fracture surface roughness during in-plane shear, is developed. Furthermore, the results of this study are employed in modeling the fatigue response of the surface cracked plates subjected to mixed mode loading.     

Determining thermal resistance of contact between finished wavy metallic surfaces

A design formula for determining the thermal resistance of a contact is obtained using functions describing the relief of real wavy surfaces.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 32, No. 5, pp. 779–785, May, 1977.     

Jumplike variation of the contact area between randomly rough surfaces

A numerical model of the contact between two solid surfaces with statistically random roughness has been considered. As the two surfaces are brought in contact and pressed against each other, the real contact area initially increases in proportion to the normal force, with a proportionality factor dependent on the spectral density of the surface profile. As the pressing force grows further, the contact area exhibits two sequential jumps. This behavior is universal, being manifested for various spectral densities of the surface roughness in both two-and three-dimensional cases. The physical reasons for the observed features and their role in the mechanics of contact between soft materials (rubber, biological tissues) are discussed. The effect can be used for creating new adhesive systems capable of exhibiting strong adhesion upon application of a critical pressing force.     

Heat transfer between a fluidized bed and an immersed cylinder

An analytical formula is given for the heat-transfer coefficient as a function of the frequency of density fluctuations in a fluidized bed and of the radius of curvature of the heat-transfer surface. Calculation coincides with the experimental data.     

A perturbation analysis on solid polymer surfaces

A linear stability model was formulated to analyze the perturbation of solid polymer surfaces. Surface energy and thermal stress were considered as the main variables. The surface tends to more unstable as the temperature increase. This is interpreted as the dominancy of the lattice vacancy diffusion over surface mass diffusion and the increase in thermal stress.