Development of a rheological model of stress relaxation in the structure of an oil film on the friction surface with fullerene additives

A rheological model of stress relaxation in a thin lubricant film, which is formed on the friction surface under the influence of the force field of the friction surface in the presence of fullerene compositions in lubricants, was developed. Analysis of the model made it possible to establish that the existence of elastic or viscous properties in surface structures depends on the ratio of two parameters. This is the time of stress relaxation in the structure on spots of actual contact and the duration of stress action on these spots, which is termed the lifetime of an actual contact spot. It was shown that an increase in the sliding rate reduces the time of relaxation of stresses in the surface structure. This is due to the destruction of aggregates in the structure of gel and the appearance of rotational movements of separate units ‒ flocs. An increase in the load on the tribosystem significantly increases the value of relaxation time. This is due to squeezing the viscous component out of the structure of a surface film. It was established that if the relaxation time exceeds the duration of actions of stresses on actual contact spots, the structure of a surface film behaves like an elastic solid. Conversely, if relaxation time becomes shorter than the duration of stress action, the film behaves like a viscous medium. Theoretically, it was shown that in the range of sliding and loading rates, when a film behaves like an elastic solid, a decrease in stresses on actual contact spots does not exceed the values of 1.1‒22.8 %. This property provides the bearing capacity of a film. The development of the model will make it possible to simulate elastic and viscous properties of "stitched" structures and substantiate the rational concentrations of additives to lubricants, as well as the ranges of their use.

A DISCRETE ELEMENT FORMALISM FOR MODELLING WEAR PARTICLE FORMATION IN CONTACT BETWEEN SLIDING METALS

The paper describes an advanced discrete-element based mechanical model, which allows modelling contact interaction of ductile materials with taking into account fracture and surface adhesion by the cold welding mechanism. The model describes these competitive processes from a unified standpoint and uses plastic work of deformation as a criterion of both local fracture and chemical bonding of surfaces in contact spots. Using this model, we carried out a preliminary study of the formation of wear particles and wedges during the friction of rough metal surfaces and the influence of the type of forming third body (interfacial) elements on the dynamics of the friction coefficient. The qualitative difference of friction dynamics in the areas of the contact zone characterized by different degrees of mechanical confinement is shown.

BASIS OF MATHEMATICAL INTERACTION MODELS OF WHEEL PROPELLERS OF FORESTRY MACHINES WITH SLIDING SURFACES

The purpose of the research, the results of which are presented in this article, is to analyze the scientific description of the properties of weak bearing movement surfaces of forest machines. The analysis has showed that universal mathematical models of the wheel propeller interaction with soil are based on the provisions of soil mechanics. This approach has been tested in the science of forestry production. It is successfully used by modern domestic and foreign researchers. However, with regard to the development and implementation of a mathematical description of interaction of ultra-low pressure wheeled mover (for example, in all-terrain wheeled vehicle) with supporting surfaces, it is necessary to take into account the ratio of the sides of the mover’s contact spot with the soil, since: mover pressure on the ground is defined as the partial load of a single mover and the area contact spots; the distribution of compressive stress over the depth of the soil mass depends on the ratio of the length and width of the contact spot; the bearing capacity characterizing the resistance to shear of the soil layers depends not only on its physical and mechanical properties, but also on the parameters of the contact spot, which is taken into account by special correction factors, the values of which depend on the aspect ratio of the contact spot. Soil rheology is considered to take into account the number of passes of a wheeled all-terrain vehicle along the route and its speed. One of the characteristics of the impact of the mover is exposure time. Value of the length of the contact spot is also used when determining the impact time of the mover on the soil.

A prediction method for wheel tread wear

Purpose The increasing demands of high-speed railway transportation aggravate the wheel and rail surface wear. It is of great significance to repair the worn wheel timely by predicting the wheel and rail surface wear, which will improve both the service life of the wheel and rail and the safe operation of the train. The purpose of this study is to propose a new prediction method of wheel tread wear, which can provide some reference for selecting proper re-profiling period of wheel. Design/methodology/approach The standard and worn wheel profiles were first matched with the standard 60N rail profile, and then the wheel/rail finite element models (FEMs) were established for elastic-plastic contact calculation. A calculation method of the friction work was proposed based on contact analysis. Afterwards, a simplified method for calculating wheel tread wear was presented and the wear with different running mileages was predicted. Findings The wheel tread wear increased the relative displacement and friction of contact spots. There was obvious fluctuation in the wheel tread friction work curve of the worn model. The wear patterns predicted in the present study were in accordance with the actual situation, especially in the worn model. Originality/value In summary, the simplified method based on FEM presented in this paper could effectively calculate wheel tread wear and predict the wear patterns. It would provide valuable clews for the wheel repair work.

CONTACT OF MULTI-LEVEL PERIODIC SYSTEM OF INDENTERS WITH COATED ELASTIC HALF-SPACE

The contact of a periodic system of spherical indenters of different heights and radii of curvature with two-layered elastic half-space is considered. Numerical-analytical method is developed to determine contact pressure distribution and internal stresses taking into account mutual effect of contact spots. The results for relatively hard and soft coatings are analyzed for different values of input parameters: nominal pressure, contact density, coating thickness.

Contact of rough surfaces: Modeling adhesion in advanced multiasperity models

Surface roughness affects several tribological phenomena and in particular adhesion. For many years, multiasperity models have been the most used in the study of rough contacts notwithstanding their evident limitations. In this work, we propose a fair assessment of improved asperity models with adhesion modeled according to the Derjaguin, Muller and Toporov theory, which assumes attractive forces do not deform the surface profiles. Results are given for three enhanced asperity models: the discrete Greenwood and Williamson model, where the effective heights and curvatures of the surface asperities are used rather than a statistical description; the interacting Hertzian asperities model, where the elastic coupling effects are included; the interacting and coalescing Hertzian asperities model, where the coalescence of contact spots is also conveniently considered. A comparison with advanced contact mechanics theories shows that only the interacting and coalescing Hertzian asperities model correctly captures the physics of the problem at all roughness scales.

Influence of Copper-Sided Tin Coating on the Weldability and Formation of Friction Stir Welded Aluminum-Copper-Joints

Although the joining of aluminum and copper is a difficult task, several studies have shown that friction stir welding (FSW) is capable of producing aluminum-copper-joints with excellent performance. Therefore, it is desirable to use this joining technique for the production of cost- and weight-reduced conductors for the automotive sector. The exposed copper contact spots in automobiles are usually coated with tin for design reasons and in order to improve their corrosion resistance. In this context, it is possible to perform the weld at first and to coat afterwards, or to weld already coated copper workpieces. Taking this into account, this paper presents results on the influence of copper-sided tinning on the joint formation as well as the achievable mechanical and electrical properties of friction stir butt welded aluminum-copper joints. Two variants were considered. The first variant included copper blanks with a tinned surface. For the second variant the surface and the abutting edge of the copper were coated. The best welds achieved excellent electrical properties and their tensile strength was only slightly reduced compared to the aluminum base material. Thus, it was shown that if these tensile strength losses are acceptable, FSW of aluminum to tin coated copper is applicable.

Development of an improved wheel profile for a freight car. Theoretical justification

With the increasing loads on the axle and speeds, the increase in wear resistance and resistance to contact fatigue of the wheels is of particular importance. In part, this result was achieved using a standard profile according to GOST 10791–2011 with the use of more efficient spring suspension. A further reduction in the wear rate of both the rolling surface and the flange can be achieved by developing a new wheel profile.The authors have developed a technique that allows, based on theoretical calculations and experimental observations, to make the choice of the shape of the wheel profile, consistent with the profiles of the rails. The results of measurements of the wheels of cars of the model 12-9853 installed on bogies 18-9855 were used as the initial data, which were under controlled operation. The profile development consisted of four main stages: the choice of baseline data, the choice of the curvature of the rolling surface, the choice of the curvature of the transition section to the throttle, and the choice of the flange thickness and radius of the throttle in the transition zone to the rolling surface.The technique was used to develop a new wheel profile that differs from the profile according to GOST 10791–2011 in that the rolling surface is made with three conjugate radii, respectively from the wheel to the flange: 500, 325 and 87.5 mm. The flange throttle radius of the developed profile is 17 mm, the angle of inclination of the flange to the horizontal is 68°, and the thickness of the flange is 32.5 mm. A part of the wheel surface from the wheel to the outer part of the rim fully complies with GOST 10791–2011. Calculations showed that the contact zone of the wheel with the VNITsTT profile relative to the rail is shifted from the center to the flange and the contact area is larger. It was found that for the VNITsTT wheel profile the ratio of the semi-axes of the ellipse of contact spots is lower than for the profile according to GOST 10791–2011.

Reinforcement Corrosion Characteristics with Periodical Profile

The theoretical analysis of corrosion processes on the periodical profile reinforcement in low potential corrosion environment has been conducted using a mathematical model. The boundary between liquid and metal phases in the model was set by a sinusoidal laxly concaved profile. In this research, it’s been suggested the hypothesis that mass transfer of cathodic and anodic depolarizers on the surface of corroborated reinforcement is defined by diffusion such that current density is constant on long distance from the boundary between the phases. The Laplacian equation has been solved by perturbation method representing concentrations as mathematical row by a small parameter. It has been analyzed the possibility of occurring a liquid coating under the concrete protective layer on the reinforcement surface in the contact spots between the ribs and reinforcement body. It’s been discovered how the profile geometry impacts the distribution of corrosion current.