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Surface roughness has a significant influence on mineral flotation. The assisting effect of surface roughness on minerals flotation is extensively investigated from its physical properties (e.g., the existing form of asperity and its size), however, the associated effect on mineral flotation based on the differences in surface chemical property caused by surface roughness has been rarely touched. With such a question in mind, in this study, we investigated the flotation recoveries of two batches of magnesite particles with varying degree of surface roughness produced by two different mills, and associated the flotation performances to their surface chemical properties (amount of adsorption sites for the collector) via a series of detections, including Scanning Electron Microscope-Energy Dispersive Spectrometry (SEM-EDS) observations, X-ray photoelectron spectroscopy (XPS) analysis, adsorption capacity tests, and contact angle measurements. Finally, we concluded that rougher magnesite particles could provide more active sites (Mg2+) for a larger capacity of sodium oleate (NaOL), thereby improving the hydrophobicity and floatability.

Pantograph-catenary system is a key component of high-speed railway. Under high speed and strong current operation conditions, the surface roughness of the sliding electrical contact between contact wire and slide changes continuously. It will cause the fluctuation of contact resistance and lower the electrical contact property of the system. Therefore, it is of great significance to study the relationships between the contact resistance and surface roughness characteristics. In this paper, lots of current-carrying friction experiments were carried out under different contact pressure, contact current and sliding speed conditions. A roughness tester was used to measure the roughness of the contact surface after experiment. Four characteristic parameters were proposed to describe the roughness. They are the root mean square and the maximum peak height of the rough surface, the peak count and the curvature radius of asperity respectively. The function relationships between these characteristic parameters and experimental conditions were obtained and discussed. On the basis of Greenwood-Williamson contact model, a new mathematical model of contact resistance in pantograph-catenary system considering rough surface characteristics were established and verified. It can be used to further study the current conduction mechanism of sliding electrical contact in pantograph-catenary system and evaluate the electrical contact performance.

A series of large scale direct shear experiments is used to investigate the effect of the geomembrane (GMB) surface roughness, geotextile (GTX) properties, and GTX ageing, on the GMB-GTX interface shear behaviour. Interfaces involving smooth, coextruded textured, and structured surface GMBs underlying four different nonwoven needle-punched staple fibres (GTXs) with mass per unit areas between 200 and 2400 g/m2, and a geocomposite drain (GCD) are examined at normal stresses between 250 and 1000 kPa. The results showed that the interlocking between the GMB and GTX increased with increasing the GMB asperity height and/or decreasing the mass per unit area of the GTX. For the interfaces that involved GTXs preaged prior to the shear box experiments for up to 2 years at 85 °C, it was found that the 2400 g/m2 heat bonded two-layered GTX exhibited internal shear failure at low shear displacements. However, all the highly aged single layered GTXs showed an increase in the peak interface friction angles with the increase in their ageing. For these single layered GTX, the results suggest that assessing the interface friction angles using unaged GTXs for the stability analysis is conservative as long as the GTX remains intact in the field.

Fully resolved numerical simulations of a micron-sized spherical particle residing on a surface with large-scale roughness are performed by using the Lattice–Boltzmann method. The aim is to investigate the influence of surface roughness on the detachment of fine drug particles from larger carrier particles for transporting fine drug particles in a DPI (dry powder inhaler). Often the carrier surface is modified by mechanical treatments for modifying the surface roughness in order to reduce the adhesion force of drug particles. Therefore, drug particle removal from the carrier surface is equivalent to the detachment of a sphere from a rough plane surface. Here a sphere with a diameter of 5 μm at a particle Reynolds number of 1.0, 3.5 and 10 are considered. The surface roughness is described as regularly spaced semi-cylindrical asperities (with the axes oriented normal to the flow direction) on a smooth surface. The influence of asperity distance and size ratio (i.e. the radius of the semi-cylinder to the particle radius, Rc/Rd) on particle adhesion and detachment are studied. The asperity distance is varied in the range 1.2

This study evaluates and quantifies fracture roughness effects on proppant particle settling in fractures utilizing coupled, resolved computational fluid dynamics (i.e., simulation of carrier fluid behavior) and the discrete element method (i.e., simulation of proppant particle behavior). This work focuses specifically on “slickwater” based proppant slurries. Rough fracture geometries are generated using a spectral-based algorithm with surficial roughness varied by adjusting fractal dimension and root-mean-square asperity heights descriptive parameters. This work also considers particle volumetric concentration and influence from flowing conditions on particle settling. Results indicate that the attenuation of quiescent settling rates is more pronounced in rough fractures as the aperture narrows. The degree of quiescent slurry settling attenuation corresponds to fractal dimension and root-mean-square asperity height values. A formulation to describe the average settling rate of quiescently settling slurries in rough fractures is proposed and evaluated. Fracture roughness causes erratic settling velocities of flowing slurries, with some cases having upward proppant velocity, specifically at narrow apertures at or below two times the particle size for the condtions considered in this work. The onset of this behavior is dependent on degree of roughness, with erratic velocities occurring at narrower relative apertures for smoother fractures compared to rougher fractures. Preferential flow pathways formed in rough fractures at narrower apertures, are noted to cause these erratic settling behaviors. The overall findings provide a greater understanding of particle settling behavior in rough rock fractures, aiding in the design process for fracture enhancement projects.

This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.

Pressure solution of carbonate-based rocks participates in many geophysical and geochemical processes, but fundamental knowledge of the interfacial processes is still lacking. By concurrently pressing and sliding two single calcite crystals past each other, the pressure solution rate and the friction force between the crystals were concurrently measured in calcium-carbonate saturated water with an extended surface forces apparatus. These studies reveal that both a decrease and an increase in frictional strength can originate from the pressure-solution of calcite single crystals. By conducting nanoscale force measurements with an atomic force microscope, ion specific effects were unveiled at the level of a single asperity. Pressure solution is promoted when the interfacial water layers of calcite remain undisturbed under stress (e.g. with Ca(II)) and the dissolved ions and water lubricate the interface – a phenomenon called pressure-solution facilitated slip. The mechanically induced disruption of the hydration layers of the calcite surface (e.g. with Mg(II) and low Ni(II) concentration) correlates with the more fluid-like and lubricious behavior of the confined fluid in the absence of pressure solution. Charge neutralization of the calcite surface leads to an abrupt change of calcite’s hydration layers, which promotes pressure-solution facilitated slip. This work advances the fundamental understanding of physicochemical interactions occurring at confined surfaces of stressed calcite.

Granular materials are often subjected to certain degrees of coating in natural conditions due to weathering and soil–environment interactions and the current literature has suggested that the development of surface coatings has an important influence on the macroscopic response of geological materials. In the present study, a new method was developed to artificially coat sand grains with iron oxides and a series of experiments were performed for surface morphological and mechanical characterization as well as quantification of the tribological behavior of iron oxides coated grains. Through the material characterization tests, it was observed that the artificial iron oxides coating notably altered the roughness and hardness of the grains, which in turn resulted in higher inter-particle friction and lower normal contact stiffness. Clear dependency of the coefficient of friction on the magnitude of normal load and the loading history was observed for the coated specimens, which is speculated to be attributed to asperity breakage, plowing damage and debris build-up of the surfaces as also supported by microscopic images. The test results could also provide, qualitatively, some multi-scale insights into the influence of iron oxide coating on the bulk behavior of natural soils as the increase in inter-particle friction angle can be linked with the observed trend of increased critical state strength.

Understanding dual-mode wear behavior, including fatigue and abrasive wear, is essential for determination of surface damage under the sliding-impact condition, which can induce early motion-pair f...

We present a novel meso-scale model for computing anisotropic and asymmetric friction for contacts in rigid body simulations that is based on surface facet orientations. The main idea behind our approach is to compute a direction dependent friction coefficient that is determined by an object's roughness. Specifically, where the friction is dependent on asperity interlocking, but at a scale where surface roughness is also a visual characteristic of the surface. A GPU rendering pipeline is employed to rasterize surfaces using a shallow depth orthographic projection at each contact point in order to sample facet normal information from both surfaces, which we then combine to produce direction dependent friction coefficients that can be directly used in typical LCP contact solvers, such as the projected Gauss-Seidel method. We demonstrate our approach with a variety of rough textures, where the roughness is both visible in the rendering and in the motion produced by the physical simulation.

Tamusu mudstone, located in Inner Mongolia, is being recognized to be the potential host formation in deep HLW disposal across China. This paper describes diverse laboratory experiments performed using the fractured Tamusu mudstone, with concern to the study of the combined mechanical and thermal effects on its hydraulic properties, as well as its self-sealing effect. A single rough fracture is generated down the sample diameter direction before the sample’s installation. Considering a heating-cooling cycle, the evolution of the samples’ hydraulic properties under different stress conditions has been monitored. The hydraulic conductivity is proved to be not only related to the temperature-induced water viscosity and density variation but also the change of stress state. At the same thermal conditions, the increase of confining pressure (or deviatoric stress) may weaken the flow capacity of the fractures and thus reduce the permeability. The non-coincidence for the curve of permeability versus temperature at the heating and cooling stage indicates that the permeability alterations in the fractures are irreversible. Fracture zone changes were observed by X-ray computer tomography (CT) before and after the tests. The self-sealing behavior, which may be beneficial from high confining pressure, was detected for fractured Tamusu mudstone. Morphologies of fracture surfaces were detected by the 3D scanning technique, which shows that the fracture surface roughness, due to the alteration of asperity contacts and void spaces under high stress, may take an important part of the responsibility to the change of permeability.

Taking advantage of the magnetic field inside transmission electron microscope (TEM), a unique Lorentz-force-actuated method for quantitative friction tests was developed via a commercial electromechanical holder. With this approach, a submicron-sized silver asperity sliding on a tungsten flat punch was actuated by Lorentz force due to electrical current through the punch, with the normal force imposed by the built-in transducer of the holder. The friction force was determined by tracking the elastic deflection of the fabricated cantilever from in situ video. Through correlating the friction behavior with the microstructural evolution near the silver-tungsten interface, we revealed that even when the relative motion commenced with the plastic deformation of the silver asperity, the interface can still sustain the further increasing static friction force. Exactly following the arrival of the maximum static friction force, the sliding occurred at the interface, indicating the transition from static to dynamic friction. This work enriches our understanding of the underlying physics of the dynamic friction process for metallic friction behavior.

In this paper, scanning electron microscope (SEM) tests and 3D scanning technologies were adopted to investigate the creep failure mechanism of sandy mudstone from a micromesoscopic view. The SEM test results showed that the fracture surface micromorphology of the specimens that suffered creep loading was more fractured and rougher. It was also found by the fractal analysis of the SEM microscopic images that the fractal dimensions of the creep failure specimens were larger than those of the uniaxial compression failure, indicating that the creep damage increased the irregularity and a larger degree of roughness fluctuation. The 3D scanning technologies combining with the 3D reconstruction methods proved that the crack expansion path of crept specimens was more complicated, showing a more prominent asperity height and slope angle. Finally, a mesostrain dilating microelement model of the sandy mudstone specimen’s fracture surface was proposed to prove that the dilatancy effect would be more pronounced in the creep process.

Considering the eccentricity and deformation of the plunger, a lubrication model of laser-textured plunger pair was established to research friction-and-wear performance of typical surface texture on the plunger under diesel and methanol lubrication. The calculation results show that for the non-textured plunger, the oil film thickness and fluid friction under methanol lubrication are lower than diesel lubrication, and the asperity friction was greater than diesel lubrication, indicating that methanol may deteriorate the lubrication performance of the plunger surface. After texturing the plunger, the oil film thickness and fluid friction during methanol lubrication are greater than the non-textured scheme, and the asperity friction is lower than the non-textured scheme, which means that the surface texture has the potential to improve the lubrication performance of the plunger. The friction and wear tests results indicate that the average friction coefficient of the textured plunger is reduced by 3.4 ∼ 6.2 % compared with the non-textured plunger in methanol lubrication. The lubrication performance of the textured plunger during methanol lubrication reaches the level of the non-textured plunger in diesel lubrication. After the wear test, the surface wear depth of the textured plunger is reduced by 16.2 % compared with the non-textured plunger in methanol lubrication condition.

The effects of axial profile parameters on the main bearing performance of the engine were investigated through the numerical method based on elasto-hydrodynamic lubrication theory, average flow, and asperity contact model. Results show that quadratic profile significantly improves the bearing performance, and the influence of profile varies with its width-to-height ratio. The performance is most improved when the ratio is between 0.8 and 2. An artificial neural network fitting model was developed to predict bearing performance, and multiobjective optimum analyses were performed using genetic algorithm and particle swarm optimization. The optimization goals are average peak asperity contact pressure and average total friction loss. The obtained Pareto front roughly includes three groups, and solutions in group 1 achieve a balance of the two goals, with a width-to-height ratio of 1.5–2. Finally, bearing friction tests were conducted on four profiled bearings to verify the simulation model and optimization results.

The general discrete scheme of time-varying Reynolds equation loses the information of the previous step, which makes it unreasonable. A discretization formula of the Reynolds equation, which is based on the Crank-Nicolson method, is proposed considering the physical message of the previous step. Gauss-Seidel relaxation and distribution relaxation are adopted for the linear operators of pressure during the numerical solution procedure. In addition to the convergent criteria of pressure distribution and load, an estimation framework is developed to investigate the relative error of the most important term in the Reynolds equation. Smooth surface with full contacts and mixed elastohydrodynamic lubrication is tested for validation. The asperity contact and sinusoidal wavy surface are examined by the proposed discrete scheme. Results show the precipitous decline in the boundary of the contact area. The relative error suggests that the pressure distribution is reliable and reflects the accuracy and effectiveness of the developed method.

In this study, a series of artificial rock joint specimens were subjected to a pre-peak cyclic loading test. The effects of the initial joint roughness parameters (Z2(initial)), joint wall compressive strength (JCS), normal stress during cycle (σnc), cycle frequency (f), cycle amplitude (A), and cycle number (N) on the mechanical behaviors and asperity degradation were studied extensively. The results revealed that the joint surface degradation coefficient Rd and the joint surface degradation thickness Td were positively correlated with f, A, and N and negatively correlated with the JCS. As Z2(initial) and σnc increased, $${\varvec{T}}_{\varvec{d}}$$ increased, and Rd decreased. It was concluded that specimens with larger Z2(initial) and σnc were dominated by vertical degradation, whereas specimens with smaller Z2(initial) and σnc were dominated by horizontal degradation. The joint roughness parameter Z2(N) decreased with increasing f, A, Z2(initial), σnc, and N but decreased with decreasing JCS. As N increased, the rate of decrease in Z2(N) transitioned from fast to slow, and the degradation process was divided into a rapidly declining stage and a slowly declining stage. Based on regression analyses, a joint asperity degradation model was presented to calculate the joint roughness parameters Z2(N) of rock joints under various influential factors in the pre-peak cyclic loading stage. Additionally, based on the constant normal load (CNL) direct shear test on the specimens after cycling, it was found that with an increase in N, the peak stress, residual stress, and peak dilation decreased, while the peak shear displacement increased. Finally, a critical condition was presented to assess the joint pre-peak cyclic loading stage, and the critical cycle number Nc of the joint from the pre-peak cyclic loading stage that was transmitted to the peak shear stress stage was proposed.

When two anisotropic rough surfaces contact, the interface is shown as the contact anisotropy or contact isotropy, which closely relates to the asperity contact azimuthal angles on the microscopic scale. The interfacial contact anisotropy/isotropy will influence the normal contact stiffness and damping of mechanical joints. A normal contact stiffness and damping model between two anisotropic rough surfaces is proposed, which includes the effects of the asperity shoulder-to-shoulder contact and adjacent asperity interaction. And the influence laws of the contacting match degree and the contact anisotropy on the normal contact stiffness and contact damping are revealed by a series of simulation experiments. Thereby, the normal contact stiffness and damping can be controlled by adjusting the surface topographic features and the contacting orientation between mechanical parts.

The contact stiffness of the joint surface directly affects the static and dynamic mechanical behavior, and accuracy of the machine tool. A new elastoplastic contact stiffness model is proposed by considering continuous and smooth contact characteristics and substrate deformation. First, the interpolation interval of cubic Hermite polynomials is improved to meet the continuous and smooth change of contact parameters during asperity deformation. Then, the micro-contact mechanism considering substrate deformation is explored by establishing an asperity-substrate system model. Furthermore, combined with statistical principles, a new contact stiffness model of the joint surface is established. Finally, the correctness of the built model is verified by comparing with experimental data and different contact models. The simulation results show that the model changes continuously and smoothly in the three deformation regions. The substrate deformation mainly affects the asperities in the elastic contact stage. The smoother is the surface, the more significant is the influence of substrate deformation.

Bio-particles are usually considered as an elastic material, while it has been proven that these particles such as viruses and cells behave more like viscoelastic materials because of the liquid cytoplasm in cells and also the protein capsid around viruses; thus, this property is not ignorable because it leads to a non-precise prediction in simulations. On the other hand, the surface of these particles is usually considered smooth, while the topography shows asperities on them. In this paper, the viscoelastic contact model along with the asperities’ distribution of the bio-particles is considered near reality. Simulation results reveal that the prediction of the viscoelastic contact models is more precise and closer to the experimental data than the elastic state. Using experimental results and applying their effect, asperities’ radii are obtained and comparison shows that the rough viscoelastic theory is more accurate and closer to the experimental data in comparison with viscoelastic models without asperity. The simulation of the first phase of the manipulation, applying three states of the elastic, the viscoelastic, and the rough viscoelastic in addition to a comparison with similar modes, shows that in the particle’s sliding on the substrate, tip sliding on the particle, and the particle rolling modes, the critical force has the highest magnitudes in the rough viscoelastic, the elastic, and the viscoelastic, respectively. However, results for the critical time are different, i.e., the highest critical times are related to the elastic, the viscoelastic, and the rough viscoelastic states, respectively.

Purpose This paper aims to present the mixed elastohydrodynamic lubrication (EHL) model and obtain the leakage characteristics for the skeleton reciprocating oil seal. Design/methodology/approach The model consists of a finite element analysis of the contact pressure, a fluid mechanics analysis of the fluid film, a contact analysis of the asperity contact pressure, a deformation analysis of the seal lip and an iterative numerical simulation process. Findings Simulation results show that the leakage is in direct proportion to the seal roughness and speed, and in inverse proportion to the fluid viscosity. Comparisons with the experimental results verify the validity of the mixed EHL model. Originality/value This study provides a helpful method to calculate the leakage of the skeleton reciprocating oil seal, which shortens its development cycles.

In the Three Gorges Reservoir Area (TGRA) has frequently occurred a large number of landslides in the stratified rock slopes with typical saw-tooth rock discontinuities since its impoundment in 2003, in which the shear mechanical behaviors of the saw-tooth rock discontinuities have been explored by few scholars. In this study, the macro-meso shear mechanical behaviors of the rock discontinuities with multiscale triangular-shaped asperities under two different lithologic combination types, when subjected to a static constant normal load (CNL), were investigated using experimental and numerical approaches. The hard layer and soft layer are, respectively, limestone and clayey soil taken from the TGRA, and the normal loading rate for the combination of hard layers and the combination of soft–hard layers is 0.5 kN/min and 0.05 kN/min, respectively, while the shear loading rate of the two different lithologic combination types is 0.5 kN/min. The laboratory direct shear tests on the rock discontinuities with different first-order asperity angles, i.e., 30°, 45° and 60°, and the same second-order asperity angle of 45°, were first conducted under different normal stresses and moisture contents. Four various types of shear-tensile failure modes, i.e., climbing failure, climbing-gnawing failure, gnawing failure and sliding failure, were observed for the combination of hard layers; while three diverse types of shear-tensile failure modes, i.e., disengaging failure, dislocating failure and gnawing failure, were observed for the combination of soft–hard layers. Subsequently, the Two Dimensional Particle Flow Code discrete element method (PFC 2D DEM) was used for the numerical simulations, in which the meso-damage evolution process of the rock discontinuities was simulated, and the variation laws of the meso-damage crack number and energy were obtained. Based on the good agreement between the experimental observations and numerical results, a shear strength equation for the rock discontinuities was proposed, and its rationality was validated by comparing the factor of safety (FOS) of a slope calculated by the limit equilibrium method (LEM) and the strength reduction method (SRM).

Modeling the real contact area plays a key role in every tribological process, such as friction, adhesion, and wear. Contact between two solids does not necessarily occur everywhere within the apparent contact area. Considering the multiscale nature of roughness, Persson proposed a theory of contact mechanics for a soft and smooth solid in contact with a rigid rough surface. In this theory, he assumed that the vertical displacement on the soft surface could be approximated by the height profile of the substrate surface. Although this assumption gives an accurate pressure distribution at the interface for complete contact, when no gap exists between two surfaces, it results in an overestimation of elastic energy stored in the material for partial contact, which typically occurs in many practical applications. This issue was later addressed by Persson by including a correction factor obtained from the comparison of the theoretical results with molecular dynamics simulation. This paper proposes a different approach to correct the overestimation of vertical displacement in Persson’s contact theory for rough surfaces with self-affine fractal properties. The results are compared with the correction factor proposed by Persson. The main advantage of the proposed method is that it uses physical parameters such as the surface roughness characteristics, material properties, sliding velocity, and normal load to correct the model. This method is also implemented in the theory of rubber friction. The results of the corrected friction model are compared with experiments. The results confirm that the modified model predicts the friction coefficient as a function of sliding velocity more accurately than the original model.

There exist many well-known analytical models for adhesive contact for spherical asperities. However, in many situations, the asperities are not spherical and may be better described by a power-law...

Wear and scuffing failures often occur in marine transmission gears due to high friction and flash temperature at the interface between the meshing-teeth. In this paper, a numerical solution procedure was developed for the predictions of transient friction and flash temperature in the marine timing gears during one meshing circle based on the 3D line contact mixed lubrication simulation, which had been verified by comparing the flash temperature with those from Blok’s theory. The effect of machined surface roughness on the mixed lubrication characteristics is studied. The obtained results for several typical gear pairs indicate that gear pair 4–6 exhibits the largest friction and the highest interfacial temperature increase due to severe rough surface asperity contacts, while the polished gear surfaces yield the smallest friction and the lowest interfacial temperature. In addition, the influences of the operating conditions and the gear design parameters on the friction-temperature behaviors are discussed. It is observed that the conditions of heavy load and low rotational velocity usually lead to significantly increased friction and temperature. In the meantime, by optimizing the gear design parameters, such as the modulus and the pressure angle, the performance of interfacial friction and temperature can be significantly improved.

The paper presents experimental analysis of relation between friction coefficient and contact pressure of $$\hbox {MoS}_2$$ MoS 2 film deposited on $$\hbox {Ti}_6\hbox {Al}_4\hbox {V}$$ Ti 6 Al 4 V substrate in contact with sapphire ball during reciprocating sliding motion. It is shown that the value of friction coefficient decreases with increasing contact pressure. A microscale modeling approach is next developed to mimic the experimental observations. Representative volume element is defined based on the actual topography of outer surface of $$\hbox {MoS}_2$$ MoS 2 film. Assuming thermo-elastic material properties, the calculations on the asperity level are performed in two steps. Firstly, the mechanical contact between two surfaces is calculated. As a result, the relation between the global load and micro-stress distribution is obtained. Secondly, for a given stress load, thermal analysis is performed providing temperature fluctuation within simplified conical asperity. By assuming relation between friction coefficient and temperature on the microscale, it is possible to obtain macroscopic friction coefficient as a function of contact pressure. In the end, model results are compared with experimental data. The novel aspects of presented approach lie in the selection of three main factors on a micro-level defining macroscopic friction. They are actual surface topography, microscopic temperature and microscopic friction-temperature relation.

Purpose The purpose of this study is to investigate the influence of dimple radius, depth and density on the lubrication performance of the plunger. Design/methodology/approach A lubrication model was adopted to consider eccentricity and deformation during the working process of the plunger, and a rig test was performed to confirm the simulation results. The texture was fabricated using laser surface texturing. Findings The simulation results suggested that when dimple radius or depth increases, oil film thickness of the plunger increases before decreasing, and asperity friction displays an opposite trend. Therefore, appropriate microdimple texture could facilitate lubrication performance improvement and reduce the wear. Microdimples were then lased on the plunger surface, and a basic tribological test was conducted to validate the simulation results. The experimental results suggested that the average friction coefficient decreased from 0.18 to 0.13, a reduction of 27.8%. Social implications The introduction of microdimple on a plunger couple to reduce friction and improve lubrication is expected to provide a new approach to developing high-performance plunger couple and improve the performance of the internal combustion engine. If applied, the surface texture could help reduce friction by around 27% and cap the cost relative to the plugger friction. Originality/value The microdimple texture was introduced into the plunger couple of a vehicle to reduce the friction and improve the performance. Findings suggested that surface texture could be used in the automotive industry to improve oil efficiency and lubrication performance. Peer review The peer review history for this article is available at: http://dx.doi.org/10.1108/ILT-07-2020-0259.

Purpose In this study, the dynamic characteristics of the water-lubricated rubber bearing considering asperity contact are numerically studied, including water-film stiffness and damping coefficients and plastic-elastic contact stiffness coefficient. Design/methodology/approach The Kogut-Etsion elastic-plastic contact model is applied to calculate the contact stiffness coefficient at the bearing-bush interface and the perturbed method is used to calculate the stiffness and damping coefficients of water-film. In addition, the rubber deformation is determined by the finite element method (FEM) during the simulation. Parametric studies are conducted to assess the effects of the radial clearance, rubber thickness and elastic modulus on the dynamic characteristic of water-lubricated rubber bearing. Findings Numerical results indicate that stiffness and damping coefficients of water film and the contact stiffness of asperity are increased with the decreasing of the radial clearance and the dynamic coefficients are less sensitive to the rubber thickness compared with the elastic modulus of rubber. Furthermore, due to the existed groove, a sudden change of the water film direct stiffness and damping coefficients is observed when the eccentricity ratio ranges from 0.6 to 1.0. Originality/value It is expected this study can provide more information to establish a dynamic equation of water-lubricated rubber bearings exposed to mixed lubrication conditions.

Purpose This paper aims to research the effect of the different structures of dimpled textures on the rod bearing surfaces on improving the engine’s lubrication efficiency and friction power loss (LE-FPL). Design/methodology/approach Based on the hydrodynamic model of the rod bearing, the effect of different structures of dimpled surfaces including circular dimples (CD), square dimples (SD), wedge-shaped dimples (WSD), circular-square dimples (CSD) and square-wedge-shaped dimples (SWSD) on ameliorating the LE-FPL is analyzed under the different operating conditions of the engine. The oil film pressure (p), asperity contact force (Wac), friction force (Ff) and coefficient of friction (COF) of the rod bearing are chosen to evaluate the LE-FPL. Findings The SD’s performance on improving the LE-FPL is better than all other structures of the CD, WSD, CSD and SWSD. Particularly, the average values of Wac, Ff and COF with the SD is significantly reduced by 14.5%, 28.5% and 33.3% compared to the optimal dimensions of the rod bearing; and by 26.4%, 34.5% and 43.7% compared to the optimal CD (n = m = 6). Originality/value The generated friction between surfaces of rod bearings of the engine not only reduces the engine power but also affects the durability of the structures. Thus, the optimal design of the SD to further improve the LE-FPL is very necessary.

The rotation superstructure construction method is a widespread technique in bridge engineering. The critical issue for the successful application of this technique is the contact interface analysis and design for the rotating mechanism. A semi-analytical method predicated upon obtaining a uniform distribution of pressure on the slide plates within the interface is proposed. The surface design typically generates a nonlinear stress distribution. It leads to local damage and local asperity interlocking, which increase the contact friction dramatically during the rotation. In contrast, the proposed approach provides a surface that avoids stress concentrations and is expected to reduce the material cost of the slide plates. The proposed method is verified by the Finite Element Model. It can be used in a broad area involving contacting surface design, especially in the rotating mechanism design for bridge construction.

Studying and understanding the characteristic ground motions of destructive earthquakes is very important in the prevention of earthquake disasters. In this paper, two 2018 Ms 5.0 earthquakes are used as an empirical Green’s function to reproduce the ground motion strength of the 1970 Ms 7.8 Tonghai earthquake. The present paper considers the uncertainties in the numerical simulation, the source model, the asperities, and the initial rupture positions. The obtained peak ground acceleration (PGA), the time history of the acceleration, and the acceleration response spectrum at each station are compared with the ground motion strength at corresponding stations for the Ms 8.0 Wenchuan earthquake to test the reliability of the ground motion simulation. The results show that the PGA of the one-asperity source model is the largest. With an increasing number of asperities, the ground motion strength decreases, and the earthquake duration increases, indicating that an increase in the number of asperities disperses seismic energy. The two-asperity source model is consistent with the real source model of the Tonghai earthquake. The acceleration time history and acceleration response spectrum at each station reflect the characteristics of the ground motion strength of the destructive Tonghai Ms 7.8 earthquake. Thus, reproducing the main ground motion strength characteristics of the Tonghai earthquake by combining the empirical Green's function method with two Ms 5.0 earthquakes is feasible.

Existing studies primarily focus on stiffness and damping under full-film lubrication or dry contact conditions. However, most lubricated transmission components operate in the mixed lubrication region, indicating that both the asperity contact and film lubrication exist on the rubbing surfaces. Herein, a novel method is proposed to evaluate the time-varying contact stiffness and damping of spiral bevel gears under transient mixed lubrication conditions. This method is sufficiently robust for addressing any mixed lubrication state regardless of the severity of the asperity contact. Based on this method, the transient mixed contact stiffness and damping of spiral bevel gears are investigated systematically. The results show a significant difference between the transient mixed contact stiffness and damping and the results from Hertz (dry) contact. In addition, the roughness significantly changes the contact stiffness and damping, indicating the importance of film lubrication and asperity contact. The transient mixed contact stiffness and damping change significantly along the meshing path from an engaging-in to an engaging-out point, and both of them are affected by the applied torque and rotational speed. In addition, the middle contact path is recommended because of its comprehensive high stiffness and damping, which maintained the stability of spiral bevel gear transmission.

This study comprehensively investigates the source mechanisms associated with the mainshock and aftershocks of the Mw = 6.3 Yogyakarta earthquake which occurred on May 27, 2006. The process involved using moment tensor inversion to determine the fault plane parameters and joint inversion which were further applied to understand the spatial and temporal slip distributions during the earthquake. Moreover, coseismal slip distribution was overlaid with the relocated aftershock distribution to determine the stress field variations around the tectonic area. Meanwhile, the moment tensor inversion made use of near-field data and its Green’s function was calculated using the extended reflectivity method while the joint inversion used near-field and teleseismic body wave data which were computed using the Kikuchi and Kanamori methods. These data were filtered through a trial-and-error method using a bandpass filter with frequency pairs and velocity models from several previous studies. Furthermore, the Akaike Bayesian Information Criterion (ABIC) method was applied to obtain more stable inversion results and different fault types were discovered. Strike–slip and dip-normal were recorded for the mainshock and similar types were recorded for the 8th aftershock while the 9th and 16th June were strike slips. However, the fault slip distribution from the joint inversion showed two asperities. The maximum slip was 0.78 m with the first asperity observed at 10 km south/north of the mainshock hypocenter. The source parameters discovered include total seismic moment M0 = 0.4311E + 19 (Nm) or Mw = 6.4 with a depth of 12 km and a duration of 28 s. The slip distribution overlaid with the aftershock distribution showed the tendency of the aftershock to occur around the asperities zone while a normal oblique focus mechanism was found using the joint inversion.

The Weihe Plain is at high risk of disastrous earthquakes, such as the Huaxian Ms8.5 earthquake, which caused enormous casualties and economic losses. Therefore, reproducing the strong ground motion characteristics of the Huaxian earthquake is important for seismic design and disaster mitigation. The empirical Green’s function method is used to predict the ground motion characteristics of the Huaxian Ms8.5 earthquake by considering the uncertainty factors of asperities, considering the limited observations in the Weihe Plain and the inaccessibility of the Earth’s interior. The uncertainty asperity factors can be studied by the logic tree method, and source models considering these factors are constructed here. The uncertainty factors include the seismic moment M0, number of subfaults N, stress drop of an asperity Δσ, and asperity area Sa. The asperity area is assigned values of 19%, 22%, and 25% of the fault rupture area; the M0 values of the corresponding asperities account for 40%, 44%, and 48% of the entire M0; and there are two to four asperities. The permutations are combined into multiple-asperity source models. The two-asperity source models reflect the Huaxian earthquake characteristics better than the three- or four-asperity models. The asperity area accounts for approximately 22–25% of the fault rupture area, while the M0 of the corresponding asperities accounts for 44% and 48% of the entire M0. The long-axis direction of the ground motion distribution is the same as that of the mountains southeast of the Weihe Plain. The azimuth angle of the elliptical long axis is approximately 40–60° northeast. The greater the magnitude is, the more complex the number, location, and area of the asperities. This research extends the application of the empirical Green’s function method and can numerically simulate seismic intensity characteristics in areas with destructive earthquakes but few seismic records.

Monitoring the hydraulic properties within subsurface fractures is vitally important in the contexts of geoengineering developments and seismicity. Geophysical observations are promising tools for remote determination of subsurface hydraulic properties; however, quantitative interpretations are hampered by the paucity of relevant geophysical data for fractured rock masses. This study explores simultaneous changes in hydraulic and geophysical properties of natural rock fractures with increasing normal stress and correlates these property changes through coupling experiments and digital fracture simulations. Our lattice Boltzmann simulation reveals transitions in three-dimensional flow paths, and finite-element modeling enables us to investigate the corresponding evolution of geophysical properties. We show that electrical resistivity is linked with permeability and flow area regardless of fracture roughness, whereas elastic wave velocity is roughness-dependent. This discrepancy arises from the different sensitivities of these quantities to microstructure: velocity is sensitive to the spatial distribution of asperity contacts, whereas permeability and resistivity are insensitive to contact distribution, but instead are controlled by fluid connectivity. We also are able to categorize fracture flow patterns as aperture-dependent, aperture-independent, or disconnected flows, with transitions at specific stress levels. Elastic wave velocity offers potential for detecting the transition between aperture-dependent flow and aperture-independent flow, and resistivity is sensitive to the state of connection of the fracture flow. The hydraulic-electrical-elastic relationships reported here may be beneficial for improving geophysical interpretations and may find applications in studies of seismogenic zones and geothermal reservoirs.

At present, there are many researches on the loading between contact surfaces, but it is also very important to predict the unloading process between rough surfaces. In this paper, based on the three-dimensional fractal loading model we built earlier and the unloading finite element results of the single asperity by Etsion et al., the unloading fractal prediction model between rough contact surfaces is established. The model takes into account the friction factor between surfaces, the three-dimensional fractal characteristics of rough surfaces and the elastic–plastic deformation mechanism of asperities. The model is compared with the unloading model based on the two-dimensional fractal curve established by Miao et al. and the finite element results. The results show that the unloading process between rough surfaces depends on the final loading state of the surface; the loading and unloading curves form a hysteresis loop, which represents the energy dissipation in a contact cycle; both the friction factor and fractal dimension of the contact surface have important effects on the unloading model.

The contact of nominally flat surfaces can be treated as a bilateral periodic contact problem considering the stochastic surface similarity to the asperity distribution in a representative region. This similarity treatment method can be extended to material inhomogeneities. A novel numerical model for simulating the elastoplastic contact between nominally flat surfaces of materials containing inhomogeneities or coatings is developed via extending the concept of the discrete convolution and FFT (DC–FFT) algorithm with double superpositions of influence coefficients, which is named the DCSS–FFT algorithm. Several cases are analyzed with this new algorithm to examine its convenience, efficiency, and accuracy in dealing with complicated nominally flat–flat contact problems. The effects of surface roughness and material inhomogeneity are explored, and the mechanisms of contact surface failure are discussed.

Three-dimensional asperity model of rough surfaces is proposed using valley–peak ratio (VPR) of the maximum peak. Asperities of rough surfaces are reconstructed according to paraboloid asperity function, which is derived using least square method and asperity projection plane determined via VPR. The minimum profile deviation between reconstructed asperities and rough surface is calculated using simulated annealing method to optimize VPR. Effects of simulated mutation surfaces, measured surfaces, and different sampling intervals (SIs) on profile deviation are investigated. Compared with nine–point method, the proposed model demonstrates a smaller deviation of mutational rough surface, the absence of interference among asperities, and more stable surface parameters and contact mechanics performance at different SIs.

The Garhwal–Kumaun region (29°–31.5°N; 77°–81°E) in NW Himalaya lies in the Central Seismic Gap where the accumulated strain energy is sufficient for a future great earthquake. A broadband seismic network is continuously monitoring the seismic activity in this region and recorded a total of 3634 well located seismic events justified within the acceptable error bars (ERZ, ERH

This paper examines the quasi-static behaviour of a non-mated rock fracture during the first and second loading cycles. Asperity deformation, substrate deformation, asperity interaction and gouge production are included in the modelling. The composite topography assumption is revisited, and the irregular fracture topography is idealized by representing the asperities as ellipsoidal surfaces. It is shown that the modelling can capture the closure behaviour and hysteresis response of rock fractures on both the first and second loading–unloading cycles with an acceptable accuracy. The model is then used to examine sensitivity of deformation behaviours to mechanical properties. It is shown that Young’s modulus of the rock is the most sensitive parameter with regard to the estimation of the closure behaviour of joints, followed by the compressive strength of the material, while the deformation is independent of the Poisson’s ratio variations in the range of ± 0.1. Moreover, while the closure behaviour is sensitive to variations in the joint roughness coefficient on the first loading cycle, this sensitivity reduces considerably on the second cycle.

The contact calculation of three-dimensional real rough surfaces is the frontier field of tribology and surface science. In this study, we consider the interaction and elastic-plastic deformation characteristics of asperities and further, propose an analytical contact calculation method for rough surfaces considering the interaction of asperities. Based on the watershed algorithm, the rough surface is segmented and the asperities are reconstructed into ellipsoids. According to the height relationship between the asperities, the definition of the deformation reference height of the matrix between each couple of asperities is provided. Subsequently, the calculation formula of the substrate deformation is provided according to the local contact pressure considering the elastic-plastic deformation of the asperity, and the contact state under a specific load is determined using the iterative correction method. The results correspond with those of finite element numerical calculation and the study reveals the following: (1) compared with the results obtained without considering the asperity interaction, contact area, distance, and stiffness will be reduced by 6.6%, 19.6%, and 49.5%, respectively, when the influence of asperity interaction is considered; (2) the interaction of the asperities has the greatest influence on the surface contact distance and stiffness. Under the same load, the existence of asperity interaction will reduce the contact distance, area, and stiffness; (3) considering the interaction of the asperities, the higher asperity will bear more load, but it will simultaneously reduce the contact of the surrounding area and increase that of the distant area. The calculation method proposed in this study has the advantages of high calculation efficiency and accuracy, thus, providing the calculation basis and method for subsequent studies on service performance of rough surfaces, such as the calculation of contact stiffness and fatigue performance analysis of rough surfaces.

A network of vertical static pendulums (tiltmeters) has been in operation in Central Europe since July 2007. Hundred and eighty three seismic events of magnitude 7 and greater have occurred worldwide (EMSC) during the ten-year period. Several kinds of tilt anomalies were recognised within days up to months before the mainshocks. The most typical anomaly was a sudden tilting, parallel with the geologic structure, where the pendulum was installed. Based on the observations, we are proposing an asperity model, explaining the generation of so called “stress waves” before the mainshocks. Such stress waves of very low frequency (i.e. periods of days up to the first months) could be detected anywhere on the globe, especially on active geological structures, parallel with the fault, where a mainshock can happen. The observations suggest that we could estimate that part of the global fault system, which generates the detected stress wave, and which reaches the critical state. The estimation can be performed according to the amplitude of the stress wave, compared with stress waves detected on other structures of different orientations. According to the length of the stress wave (its period) and the tilt amplitude, it could be possible to estimate the magnitude of the mainshock.

Pad asperities in chemical-mechanical polishing (CMP) provide necessary forces for mechanical abrasion. This article investigates the abrasive behaviour of polishing pads at the asperity contact scale. A contact mechanics model predicts that compliant and soft asperities or rigid and hard asperities may solely achieve either large contact area or high indentation depth respectively, whereas bi-layered asperities can enable both the enlarged contact and deep abrasion. Hemispherical pad micro-asperities with precise dimensions, including the new bi-layered design, were fabricated using thermal reflow and micro-replica molding techniques and their polishing behaviours were experimentally compared using a pin-on-disk polishing setup.

The 2D nanomaterials have achieved the superlubrication property whatever in solid or liquid lubrication in recent years. However, whether or not the nanosheets can stably disperse in oils and smoothly enter into the asperity of friction pairs is crucial for exerting the function of antifriction. The structure of 2D QDs is desirable for addressing these issues due to its smaller 3D size. In this study, we developed a facile preparation process for WS2 QDs with uniform 2 nm size from nanosheets via hydrothermal-assisted grinding approach. The structure of the as-obtained WS2 QDs was determined by a series of characterizations. The results showed that the as-obtained WS2 QDs exhibited the typical spectrum features of nanosized quantum dot. The results of the tribological performance in grease verified that the average friction coefficient (ACOFs) and wear volume (AWVs) were decreased by 7.89% and 63.90% relative to grease, respectively, exhibiting a preferable friction reducing and wear resistance.

The branching crack model for earthquakes was developed by Vere-Jones and Kagan in the 1970s and the 1980s, respectively. With some simple and rational assumptions, its simulation results explain the Gutenberg-Richter magnitude-frequency relationship and the Omori-Utsu aftershock decay formula. By introducing the concept of the critical zone, this model can be connected with the asperity model, the barrier model, and the nucleation model through a parameter – criticality. Particularly, the size of the critical zone determines the maximum magnitude of potential earthquakes and the source of their anomalies. The key to earthquake forecasting is to determine whether the concerned area is in a critical state and how large the critical zone is. We discuss what kinds of anomalies are meaningful as candidates of earthquake precursors. Finally, we outline modelling strategies for earthquake precursors with low probability gains that are due to the inherent randomness of earthquake source processes.

The influence of surface topography on the mechanical behavior of soil-structure interfaces over a range of particle sizes and shapes is systematically investigated in this research. 3D printed interfaces with different topographies and uniformly graded 3D printed soil particles and two types of natural sands (Ottawa sand 20/30 and Dolomite sand #1, #2 and #3) were employed and tested. Laboratory investigations showed that the shear strength and volume change responses of the 3D printed interfaces are positively influenced with the increase of the inclination of asperities. The proposed wedge friction model successfully explained the distinct mechanical behaviors of soil-structure interfaces under shearing. A “turning point” was found for the interface shear resistance with the increase of the inclination of asperities. When the surface topography produces passive resistance to the soil, the change of the surface topography has little effect on the interface mechanical behavior. The findings from this research will provide insights for soil-structure interface design and discrete element method (DEM) simulations in considering the mechanical behavior of soil-structure interfaces.

In the long history of study on friction and wear phenomena, the single asperity system (SAS) is found to be the primary unit to understanding the macroscopic mechanisms. Despite of the importance,...

The contact fatigue of aviation gears has become more prominent with greater demands for heavy-duty and high-power density gears. Meanwhile, the coexistence of tooth contact fatigue damage and tooth profile wear leads to a complicated competitive mechanism between surface-initiated failure and subsurface-initiated contact fatigue failures. To address this issue, a fatigue-wear coupling model of an aviation gear pair was developed based on the elastic-plastic finite element method. The tooth profile surface roughness was considered, and its evolution during repeated meshing was simulated using the Archard wear formula. The fatigue damage accumulation of material points on and underneath the contact surface was captured using the Brown-Miller-Morrow multiaxial fatigue criterion. The elastic-plastic constitutive behavior of damaged material points was updated by incorporating the damage variable. Variations in the wear depth and fatigue damage around the pitch point are described, and the effect of surface roughness on the fatigue life is addressed. The results reveal that whether fatigue failure occurs initially on the surface or sub-surface depends on the level of surface roughness. Mild wear on the asperity level alleviates the local stress concentration and leads to a longer surface fatigue life compared with the result without wear.

Tactile perception and friction can be modified by producing a deterministic surface topography. Change of surface feature arrangement and texture symmetry can produce an anisotropic frictional behaviour. It is generally achieved through skin hysteresis by promoting its deformation. This work investigates whether a bidirectional friction can be created with microscale ellipsoidal asperity textures, thus relying on the adhesive component of friction. For this purpose, four textured samples with various asperity dimensions were moulded with a silicone rubber having an elastic modulus comparable to that of the skin. Coefficient of friction measurements were conducted in-vivo in two sliding directions with a range of normal loads up to 4 N. Finite element method (FEM) was used to study elastic deformation effects, explain the observed friction difference, and predict surface material influence. Measurements performed perpendicular to the asperity major radii showed consistently higher friction coefficients than that during parallel sliding. For the larger asperity dimensions, a change of the sliding direction increased friction up to a factor of 2. The numerical analysis showed that this effect is mostly related to elastic asperity deflection. Bidirectional friction differences can be further controlled by asperity dimensions, spacing, and material properties.

Processing (grinding, polishing) of phosphate laser (PL) glass involves material removal at two vastly different (spatial) scales. In this study, the nano- and macro-tribological properties of PL glass are investigated by rubbing the glass against a SiO2 counter-surface in both dry and humid conditions. The results indicate that the friction of the PL glass/SiO2 pair has opposing trends at the nano- and macro-scales. At the nanoscale, the friction coefficient (COF) in humid air is much higher than in dry air, which is attributed to the capillary effect of the absorbed water-film at the interface. At the macroscale, on the other hand, the COF in humid air is lower than in dry air, because the water-related mechanochemical wear makes the worn surface less susceptible to cracking. Material removal for PL glass is better facilitated by humid air than by dry air at both scales, because the stress-enhanced hydrolysis accelerates the material-removal process in glass. Moreover, the material-removal is more sensitive to contact pressure at the macroscale, because stronger mechanical-interaction occurs during material removal at the macroscale with the multi asperity contact mode. At the macroscale, the material removal is more sensitive to contact pressure in humid air compared to dry air. Because almost all mechanical energy is used to remove material in humid air, and most of the mechanical energy is used to produce cracks in PL glass in dry air. The results of this study can help optimize the multi-scale surface processing of optical glasses.