Non-local and numerical formulations for dry sliding friction and wear at high velocities

Severe contact stress problems generate high temperature and create thermomechanical gouging and wear due to high velocity sliding between two materials staying in contact. In order to improve the facilitation of the design of particular components and improve performance of these engineering applications, it is necessary to better understand the physical behavior of high speed environment. As presented here this environment is made up of two components in contact. Therefore, basing on the experimental approach ( [Lodygowski, 2010] and [Lodygowski et al., submitted for publication]) the major consideration of this paper is aimed to develop an experimental/theoretical model for the material constitutive behavior in order to better characterize and predict the internal failure surrounding the gouging and wear events.This research is to be carried out in two stages. First, by investigating the phenomenon of wear and later it will be extended to incorporate gauging problems. The principle of virtual power is used by introducing the contributions from damage and its corresponding gradients as a measure of micro motion of damage within the bulk. In addition two internal state variables are introduced on the frictional contact interface, one measuring the tangential slip and another measuring the wear. By using these internal state variables together with displacement and temperature, the constitutive model is formulated with state laws based on the free energies and the complimentary laws based on the dissipation potentials. The proposed theoretical model is implemented as user defined subroutine VUMAT in the explicit finite element code ABAQUS to analyze the structural response of the ultra high speed sliding experiment between Steel and VascoMax steel at Ecole de’Nationale Institut der Mechanic, at Metz France.This model provides a potential feature for enabling one to relate the non-local continuum plasticity and damage of the bulk material to friction and wear at the contact interfaces. The findings of this research effort is invaluable in providing a multiscale material model and numerical procedure that will be used within a hydrocode to better facilitate the design components of the severe contact stress applications.     

Structure and properties of (AlCrMnMoNiZrB0.1)Nx coatings prepared by reactive DC sputtering

The microstructure and properties of AlCrMnMoNiZrB_0.1 nitride films prepared by reactive direct current sputtering at various N₂-to-Ar flow ratios (R_N) were investigated. The films had an amorphous structure at low R_N and a face-centered cubic structure at a high R_N. As the R_N increased, the decrease in clusters and defects resulted in a dense columnar structure and low surface roughness. The peak hardness and modulus of the nitride films were 10.3 and 180 GPa, respectively. The enhanced hardness is ascribed to the increased metal-nitrogen bonding, solid solution strengthening of several metallic nitrides, and lattice strain. The nitride films deposited at R_N = 0.2, 0.5, and 0.8 had friction coefficients of 0.16, 0.12 and 0.15, respectively. Wear-out failure occurred within 400 s when R_N = 0 and 1.0. Adhesive wear was the dominant wear mechanism.     

A mechanism leading to q-power law distribution due to the inhomogeneity of phase space in the self-gravitational system

One mechanism leading to the q-power law distribution has been proposed in the self-gravitational system. The friction force in the stochastic process may be nonlinearly relevant to the momentum of particle, whose friction coefficient can be expressed as the function of the kinetic energy of the particle. The correlation strength of noises is inhomogeneous in the such system, maybe due to the strength of gravitational field. This implies that the gradient of correlation strength is the function of gravitational force. With the Taylor?s low order approximation, the stationary solution of the two-vector variable Fokker–Planck equation is the power law form in self-gravitational system. This Letter also verifies that the neutrino flux deviation from the theory value in the solar core is due to the same mechanism.     

A new approach to describe the skin surface physical properties in vivo

In the present paper, we describe a new mechanical method characterising the physico-chemical properties of human skin and their variations along with liquid exposure scenario to the skin surface. A specific bio-tribometer has been developed to study the physical properties of the skin in vivo by measuring the maximum adhesion force between the skin and the bio-tribometer. We showed that the lipidic film present on skin surface was responsible for skin adhesion due to capillary phenomena. The measure of pull-off force between skin and bio-tribometer has permitted to estimate the liquid/vapour surface tension of the lipidic film (γ_LV ≈ 6.3 mJ/m² in 30-year-old volunteer). The kinetic of sorption/desorption (sorption means indifferently adsorption and absorption process) of distilled water from the skin has been observed through the variation of the indenter/skin pull-off force versus time after distilled water application to the skin surface. This permits to follow in real time the variation of the skin physico-chemical properties after liquid application onto the skin surface. Finally, the increasing of skin friction coefficient after distilled water application onto skin surface was explained by the capillary adhesion force between the probe and the skin.     

Conformational and adsorptive characteristics of albumin affect interfacial protein boundary lubrication: From experimental to molecular dynamics simulation approaches

The lifetime of artificial joints is mainly determined by their biotribological properties. Synovial fluid which consists of various biological molecules acts as the lubricant. Among the compositions of synovial fluid, albumin is the most abundant protein. Under high load and low sliding speed articulation of artificial joint, it is believed the lubricants form protective layers on the sliding surfaces under the boundary lubrication mechanism. The protective molecular layer keeps two surfaces from direct collision and thus decreases the possibility of wear damage. However, the lubricating ability of the molecular layer may vary due to the conformational change of albumin in the process. In this study, we investigated the influence of albumin conformation on the adsorption behaviors on the articulating surfaces and discuss the relationship between adsorbed albumin and its tribological behaviors. We performed the friction tests to study the effects of albumin unfolding on the frictional behaviors. The novelty of this research is to further carry out molecular dynamics simulation, and protein adsorption experiments to investigate the mechanisms of the albumin-mediated boundary lubrication of arthroplastic materials. It was observed that the thermal processes induce the loss of secondary structure of albumin. The compactness of the unfolded structure leads to a higher adsorption rate onto the articulating material surface and results in the increase of friction coefficient.     

低温蓄冷器流动特性的理论分析和实验研究 第三部分:压降因子和摩擦系数

交变流动蓄冷器的压降因子和阻力系数对实际的工程计算、数值模拟及制冷机的设计有重要的意义 ,然而对低温交变流动蓄冷器的阻力特性一直缺乏系统的实验结果。文中给出了低温交变流动蓄冷器的压降因子及液氮温区交变流动蓄冷器摩擦系数的拟合公式 ,并为常温下的实验对比 ,对低温蓄冷器的设计提供了参考     

Tyre rubber friction on a rough road

One of the most challenging aspects of vehicle dynamics is accurate modelling of the tyre-road interface. Forces between the tyre and road need to be accurately represented in simulation. This is challenging over rough roads since the friction changes along the road due to large surface asperities.The Heinrich/Klüppel friction coefficient estimation model has been implemented on smooth roads in the past. However, this study investigates the applicability of using this model over a rough but hard terrain, such as Belgian paving or cobblestones. The model is based on physical properties that can be determined mathematically or experimentally. The study includes detailed terrain topography and the difference between the top and bottom topography is used to determine the radially averaged PSD. Emphasis is placed on finding and implementing the flash temperature in a practical manner that could also be used in further studies.An experimental setup is built to validate the model. The experimental friction coefficient is compared to the friction coefficient calculated using the Heinrich/Klüppel model. The relative percentage error difference between experimental and friction model results is found to be less than 10% on a smooth concrete road and 20% on a rough road (concrete Belgian paving).     

On impacts with friction

For modeling contacts or more specifically impacts we have two possible concepts. Either we discretize the contact zone by applying some continuum mechanical approach, or we use a rigid body model, where the impact behavior follows from complementarity rules. It depends on the problem itself, which approach might be more convenient, more contact details in the first and more system aspects in the second case. In the following we shall consider the second method and apply it to large industrial problems.     

Rolling friction of a hard cylinder on a viscous plane

The resistance against rolling of a rigid cylinder on a flat viscous surface is investigated. We found that the rolling-friction coefficient reveals strongly non-linear dependence on the cylinder's velocity. For low velocity the rolling-friction coefficient rises with velocity due to increasing deformation rate of the surface. For larger velocity, however, it decreases with velocity according to decreasing contact area and deformation of the surface. Received 24 December 1998 and Received in final form 14 January 1999     

Forward and backward motion control of a vibro-impact capsule system

A capsule system driven by a harmonic force applied to its inner mass is considered in this study. Four various friction models are employed to describe motion of the capsule in different environments taking into account Coulomb friction, viscous damping, Stribeck effect, pre-sliding, and frictional memory. The non-linear dynamics analysis has been conducted to identify the optimal amplitude and frequency of the applied force in order to achieve the motion in the required direction and to maximize its speed. In addition, a position feedback control method suitable for dealing with chaos control and coexisting attractors is applied for enhancing the desirable forward and backward capsule motion. The evolution of basins of attraction under control gain variation is presented and it is shown that the basin of the desired attractors could be significantly enlarged by slight adjustment of the control gain improving the probability of reaching such an attractor.