Actual Contact Zone Research Articles

Моделирование контактного термического сопротивления при проектировании технологического оборудования

Analysis of the processing equipment structures when designing according to the temperature criterion is a necessary guarantee of ensuring the required performance characteristics. The presence of a significant number of parts in the processing equipment units and mechanisms requires, when designing, the prediction of the heat flow passage through the joints. When simulating contact thermal resistance, the variety of requirements for a joint can be taken into account by introducing a pseudolayer into the contact zone. The paper presents test results of the proposed regression dependence of the temperature change when the heat flow goes through the pseudolayer obtained considering four significant factors: the pseudolayer thickness, the nominal pressure, the material yield strength, and the actual contact zone location. The adequacy of the specified regression dependence was verified experimentally and applying numerical simulation using large-block finite elements. To describe the process of heat transfer in the thermal model elements, the authors determined contact thermal resistances for several conditions for the heat flow propagation: from one finite element to another within one part; from one finite element to another located in an adjacent part; heat flow passing through closed cavities; heat flow propagation into the environment for finite elements located on the outer (free) contour of the part. The experiments showed a good agreement between the experimental data and the simulation results. The application of large-block finite elements based on the proposed contact thermal resistance model allowed bringing the FE simulation technique to engineering use without complex software.

Tool wear and spring back analysis in orthogonal machining unidirectional CFRP with respect to tool geometry and fibre orientation

Machining abrasive carbon fibre reinforced polymers (CFRP) is characterised by extensive mechanical wear. In consequence, the cutting edge micro-geometry and thus the tool/material contact situation are continuously changing, which affects process forces and machining quality. As a conclusion, a fundamental understanding of the tool wear behaviour and its influencing factors is crucial in order to improve performance and lifetime of cutting tools. This paper focuses on a fundamental tool wear analysis of uncoated tungsten carbide cutting inserts with different combinations of fibre cutting angles and tool geometries. For this purpose, orthogonal machining experiments with unidirectional CFRP material are conducted, where the wear progression of the micro-geometry is investigated by means of five wear parameters lα, lγ, γ*, α*, and bc. For detecting the actual contact zone of the cutting edge and to measure the elastic spring back of the material, the flank face is marked via short pulsed laser processing. Furthermore, the process forces and the wear rate are measured. It is shown that the material loss due to wear clearly varies along the tool’s contact region and is highly dependent on the clearance angle and the fibre cutting angle Φ, while the influence of the tested rake angles is mostly negligible. Especially in machining Φ=30° and Φ=60°, a strong elastic spring back is identified, which is more intense for smaller clearance angles. For all tested configurations, the material’s elastic spring back increases in intensity as wear progresses which, in combination with the decreasing clearance angle, is the main reason for high thrust forces.

Simulation of a Contact Pseudo-Environment in Calculating Thermal Resistance

The use of the temperature criterion in the design of metal-cutting machines, determined on the basis of models that take into account the contact thermal resistances, is an objective necessity. These models should take into account to the maximum extent the actual conditions of contact of parts in the design under consideration, determined by the deviations of the mating surfaces from the ideal shape. The article presents the results of numerical modeling based on the finite element method of the formation of the contact thermal resistance and the evaluation of the influence of the parameters of the intermediate layer (pseudo-environment) that occurs in the contact zone of surfaces with macro-deviations on the passage of the heat flow. The obtained results allowed us to identify the most significant of the considered parameters. It is established that when modeling a pseudo-environment, it is necessary to take into account the coefficient of its thermal conductivity, the size, location and integrity of the actual contact zone.

Analysis of the Melt Erosion Patterns at Rail-Armature Contact of Rail Launcher in Current Range of 10–20 kA/mm

Sliding electrical contact between rail and armature in rail launchers is characterized by high speed and large current. Melt erosion is caused by the current concentration on the contact surface of armature. This current melt erosion (CME) has been experimentally studied in the current range of 10–20 kA/mm with a 20-mm caliber lab-scale rail launcher, and payload separated method was used to keep the recovered armatures intact. Thus, the onset behavior and erosion patterns of CME can be successfully observed in such relatively low current densities. In this paper, we will further our study to analyze the patterns of CME with the increasing current densities under the effect of different initial mechanical contact force. Results show that CME all start at the side edge, and then spreading longitudinally and transversely with the increase of current. It is found that the starting position of CME appears near the trailing edge at small mechanical contact force and moves to the leading edge with larger mechanical contact force. At the same current magnitude, erosion amounts are much more under small mechanical contact force. Stationary mechanical contact pressures and current distribution of armature surface are simulated to investigate the effect of initial mechanical contact force on starting position of CME. Current distributes at the edges of actual contact zone and the maximum of current density appears at the side edges of armature. As the initial mechanical contact force increases, actual contact zone and current density move toward to the leading edge from trailing edge. It can be confirmed that the erosion starting position is at the side edge along actual contact zone and will move toward to the leading edge from trailing edge with the increase of initial mechanical contact force. Smaller initial mechanical contact force will cause more erosion at the same current magnitude because of larger current density. Furthermore, an average friction coefficient calculation method is proposed to evaluate the melt lubrication effect of CME. The result shows that the lubrication effect becomes better with larger current densities.

Improvement of anticorrosion ability of epoxy matrix in simulate marine environment by filled with superhydrophobic POSS-GO nanosheets.

Super-hydrophobicity polyhedral oligomeric silsesquioxane-modified graphene oxide (POSS-GO) was synthesized by one-step reaction between graphene oxide (GO) and aminopropylisobutyl polyhedral oligomeric silsesquioxane (POSS-NH2). Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra indicated that the POSS were successfully connected to the edge and surface of GO nanosheets. Scanning probe microscope (SPM) and transmission electron microscope (TEM) images demonstrated that the POSS-GO with a thickness of 1.58 nm presented a stably dispersion in anhydrous ethanol. Results from electrochemical behaviors showed that the proper incorporation of POSS-GO could availably improve the anticorrosion ability of epoxy based coatings in simulate marine environment (3.5 wt% NaCl solution). The good dispersion of POSS-GO helped to fill the original defect and enhanced the complication of penetration path. The super-hydrophobicity of POSS-GO was beneficial to the reduction of actual contact zone. The synergistic effect of good dispersion and super-hydrophobicity promoted the enhancement of anticorrosion ability for composite coatings.

A reliable, efficient and localized error estimator for a discontinuous Galerkin method for the Signorini problem

We present a new residual-type a posteriori error estimator for the discontinuous finite element solution of contact problems. The theoretical results are derived for two and three-dimensional domains and arbitrary gap functions. The estimator yields upper and lower bounds to a suitable error notion which measures the error in the displacements and in a quantity related to the contact stresses and the actual contact zone. In the derivation of the error estimator the local properties of the discontinuous solution are exploited appropriately so that, on the one hand, the error estimator has no contributions related to the non-linearity in the interior of the actual contact zone and, on the other hand, the critical region between the actual and non-actual contact zone can be well refined.

Asymmetric indentation of an elastic beam by a rigid cylinder

A simple method is proposed to analyze asymmetric indentation of a supported elastic beam by a rigid cylinder placed at an arbitrary position on the beam. A key advantage of our method over existing methods is that a single governing Kerr-type differential equation, which gives the relation between contact pressure and the normal deflection of the pressured surface of the beam, is applied both inside and outside the contact zone. With the present method, the two ends of contact zone will be determined as part of the solution, in contrast to the existing related studies which were all based on a simplifying assumption that the contact zone is geometrically symmetric about the tip of the indenter. Indeed, our results confirm that the contact zone is generally geometrically non-symmetric about the indenter tip, and even the whole contact zone can locate on one side of the indenter tip in some cases. Asymmetric indentation behaviors are demonstrated with numerical examples, with an emphasis on asymmetric contact pressure distribution inside the contact zone. Explicit formulas for the contact-zone width–displacement, force–displacement and moment–displacement relations are derived and illustrated with numerical examples. In particular, the present model predicts that as the indentation displacement exceeds a critical value, the indenter will lose contact with the beam inside the contact zone, which implies that the actual contact zone becomes two separate contact strips. Validity and accuracy of the present method are demonstrated by comparing its predictions with known results for some special cases. New features of asymmetric indentation as compared to symmetric indentation are summarized at the end of the paper.

Luminal decoration of blood vessels by activated perivasal mast cells in allergic rhinitis

In allergic diseases, like in rhinitis, antigen challenge induces rapid degranulation of tissue resident mast cells and subsequent recruitment of leukocytes in response to soluble immunmodulators. The fate of mast cell-derived, membrane associated factors in inflamed tissue remained however unresolved. Components of the mast cell granular membrane, including the unique marker CD63var, were examined by FACS and by confocal laser scanning microscopy in cell culture and in diseased human tissue. We discovered that selected mast cell membrane components appeared on the surface of distinct bystander cells. Acceptor cells did not acquire these molecules simply by uptake of soluble material or in the form of exosomes. Instead, physically stable cell-to-cell contact was required for transfer, in which a Notch2-Jagged1 interaction played a decisive role. This process is activation-dependent, unidirectional, and involves a unique membrane topology. Endothelial cells were particularly efficient acceptors. In organotypic 3D in vitro cultures we found that transferred mast cell molecules traversed an endothelial monolayer, and reappeared focally compacted on its distal surface, away from the actual contact zone. Moreover, we observed that such mast cell-derived membrane patches decorate microcapillaries in the nasal mucosa of allergic rhinitis patients. Direct membrane transfer from perivasal mast cells into nearby blood vessels constitutes a novel mechanism to modulate endothelial surface features with apparent significance in allergic diseases.

Solving dynamic contact problems with local refinement in space and time

Frictional dynamic contact problems with complex geometries are a challenging task – from the computational as well as from the analytical point of view – since they generally involve space and time multi-scale aspects. To be able to reduce the complexity of this kind of contact problem, we employ a non-conforming domain decomposition method in space, consisting of a coarse global mesh not resolving the local structure and an overlapping fine patch for the contact computation. This leads to several benefits: First, we resolve the details of the surface only where it is needed, i.e., in the vicinity of the actual contact zone. Second, the subproblems can be discretized independently of each other which enables us to choose a much finer time scale on the contact zone than on the coarse domain. Here, we propose a set of interface conditions that yield optimal a priori error estimates on the fine-meshed subdomain without any artificial dissipation. Further, we develop an efficient iterative solution scheme for the coupled problem that is robust with respect to jumps in the material parameters. Several complex numerical examples illustrate the performance of the new scheme.

Prediction of contact surface temperature between rough sliding bodies – numerical analysis and experiments

PurposeThe paper's aim is to predict numerically the contact temperatures between two rough sliding bodies and to compare with the experimental results.Design/methodology/approachAn elastic contact algorithm is used to analyze the normal contact between two nominally smooth surfaces. The algorithm evaluates real contact area using digitized roughness data and the corresponding contact pressure distribution. Using finite element method a steady state 3D temperature distribution at the interface between the sliding bodies is obtained. Using infrared (IR) imaging technique, experiments were carried out to measure the contact temperature distribution between rough rubbing bodies with a systematic variation of surface roughness and operating variables.FindingsContact temperature distributions over a wide range of normal load, sliding velocity and surface roughness have been obtained. It was seen that the maximum contact temperature expectedly increases with surface roughness (Sa values), normal load and sliding velocity. The results also indicate that the “hot spots” are located exactly at the positions where the contact pressures are extremely high. Temperatures can be seen to fall drastically at areas where no asperity contacts were established. The temperature contours at different depths were also plotted and it was observed that the temperatures fall away from the actual contact zone and relatively high temperatures persist at the “hot spot” zones much below the contact surface. Finally it is encouraging to find a good correlation between the numerical and experimental results and this indicates the strength of the present analysis.Research limitations/implicationsExperimental accuracy can be improved by using a thermal imaging camera that measures emissivity in situ and uses it to find the contact temperature. The spatial resolution and the response time of the camera also need to be improved. This can improve the correlation between numerical and experimental results.Practical implicationsOne of the major factors attributed to the failure of sliding components is the frictional heating and the resulting flash temperatures at the sliding interface. However, it is not easy to measure such temperatures owing to the inherent difficulties in accessing the contact zone. Besides, thermal imaging techniques can be applied only with such tribo‐pairs where at least one of the contacting materials is transparent to IR radiation. In practice, such cases are a rarity. However, the good correlation observed between the numerical and experimental results in this work would give the practicing engineer a confidence to apply the numerical model directly and calculate contact temperatures for any tribo‐material pairs that are generally seen around.Originality/valueA good correlation between the numerical and experimental results gives credence to the fact that the numerical model can be used to predict contact temperatures between any sliding tribo‐pairs.

A primal–dual active set strategy for non-linear multibody contact problems

Non-conforming domain decomposition methods provide a powerful tool for the numerical approximation of partial differential equations. For the discretization of a non-linear multibody contact problem, we use the mortar approach with a dual Lagrange multiplier space. To handle the non-linearity of the contact conditions, we apply a primal–dual active set strategy to find the actual contact zone. The algorithm can be easily generalized to multibody contact problems. A suitable basis transformation guarantees the same algebraic structure in the multibody situation as in the one body case. Using an inexact primal–dual active set strategy in combination with a multigrid method yields an efficient iterative solver. Different numerical examples for one and multibody contact problems illustrate the performance of the method.

Contact model of cracks in orthotropic materials

We consider a plane strain problem for an orthotropic half plane loaded at infinity and containing a crack along its fixed edge. To remove a singularity near the right crack tip, we introduce an artificial contact zone. The problem under consideration is reduced to the mixed Dirichlet-Riemann boundaryvalue problem. We present the exact solution of this problem and deduce formulas for stresses in the contact zone and on the continuation of the crack and for the stress intensity factors. By using both analytic and numerical methods, we prove that the energy-release rate is quasiinvariant in the process of crack propagation relative to the size of the contact zone. On the basis of these results, we propose an algorithm for the evaluation of the paramenters of fracture of composites of finite dimensions with interface cracks. As a special case, we develop a model of interface cracks with actual contact zone and establish the dependences of the length of this zone on the external load and elasticity moduli of the material.

Levels of topography in mechanics of precision joints

Characterizing even very smooth surfaces it is necessary to take into account multilevel structure of topography. By methods of stylus profilometry and atomic force microscopy (AFM) roughness of hard magnetic disk substrates textured under different regimes was measured. It is shown that the methods reflect various levels of roughness with overlapping ranges of wave lengths. Quantitative parameters for each roughness level suitable for the subsequent modeling of an actual contact zone were determined. To reduce influence of the roughness' long-wave components on the relief parameters at a small-scale level, computer processing of the AFM-images was performed. For the description of the actual contact area hybrid two-level model was offered. The Greenwood-Williamson scheme using the profilometry data and complemented by the account of superficial forces influence was used at a microlevel. Computer simulation of a contact on a database of AFM-image of a surface was used at submicrolevel. Influence of the contact area discreteization at a microlevel on a degree of submicroasperities deformation was taken into account. Visualization of the real contact area was performed. Distinctions in the operational characteristics of surfaces depending on their texturing modes was shown.

The Effect of Nonuniform Interfacial Pressures on the Heat Transfer in Bolted and Riveted Joints

In bolted or riveted joints where the interfacial pressure is not uniform, the total resistance to heat flow in a vacuum is the result of two separate components: the microscopic resistance, which arises due to the constraint of the heat flow through the actual microscopic contact spots, and the macroscopic resistance, which exists because the contact zone, over which these microscopic contact spots are located, is only a fraction of the total interfacial area. Presented here is a review of the recent literature addressing the interfacial pressure distribution and the size of the contact zone, in so far as they affect the heat transfer at these interfaces. A survey of the experimental work on contact pressure and the associated heat transfer in bolted joints is presented, along with the size of the actual contact zone which was identified as an important parameter affecting both the microscopic and the macroscopic resistances. An analysis is performed in which it is formally shown that the exact form of the stress distribution within the contact zone is immaterial for the computation of the total microscopic conductance if the available theoretical results for local solid spot conductance are used. If experimental correlations for local solid spot conductance are used, however, the computed total microscopic conductances may differ about 5 to 10 percent, depending on the type of stress distribution chosen. It is also shown that, for a given load, the total microscopic conductance may be increased by increasing the loading radius and/or the plate thickness.

Kinetics and Transport at AMTEC Electrodes: II . Temperature Dependence of the Interfacial Impedance of Na(g)/Porous Mo/Na‐Beta′ Alumina

The exchange current, transfer coefficient, mass‐transport parameters, and electrode capacitance at the Na(g)/porous Mo/BASE phase boundary have been evaluated from 740 to 1220 K. The transfer coefficient exhibits a value close to 0.5 and the exchange current is dominated by collision frequency, with no significant activation energy. Since the porous Mo electrode adopts a fairly regular microstructure on the beta′ alumina solid electrolyte (BASE) surface, the magnitude of the exchange current of mature electrodes directly depends on the actual contact zone (the region where reaction occurs close to the porous metal/gas/electrolyte three‐phase contact) of the porous metal film with the BASE ceramic, and decreases slightly as grain growth occurs. The exchange currents and the mass‐transport parameters derived for very porous thin Mo electrodes indicate that the charge‐transfer reaction occurs at a small fraction of the interface. The capacitance calculated from the admittance of mature, clean, thin, porous Mo electrodes shows significant frequency dependence, suggesting a contribution from a Warburg impedance associated with surface diffusion. High‐frequency limiting capacitance and resistance values due to the interface show potential dependence and a value on the order of 1 F/m2 and 0.1–1.0 Ω‐cm2. If and are present, the interfacial capacitance is unstable with time as well as greatly increased, but after the electrodes have matured, with loss of these species, it exhibits very reproducible behavior with respect to potential, pressure, and frequency. Since the reaction area increases due to in the pores, when they are present the apparent exchange current is also increased. Grain growth in very thin, mature, porous Mo electrodes can have a small effect on the electrode's exchange current.

Investigation into the mechanism of sealing in shaft stuffing boxes

A study of the mechanism of sealing in shaft stuffing boxes is performed using the finite element method. The degree of stress and deformation of the packing is considered for a conventional design and for a modified unit, together with the values of the contact pressure for different values of the fluid pressure. The actual contact zone between the packing and the shaft is analysed. The tightness of the seal for different fluid and contact pressures is evaluated. The theoretical and experimental values are compared.