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This study aims to elucidate the mechanism of velocity-dependent peak shear strength (PSS) for rock joints by examining the three-dimensional distribution characteristics of actual contact joint surface. Shear tests are conducted on saw-cut joints (SCJs) and natural rough rock joints under varying shear velocities (v) but a constant normal stress. The test results reveal that the PSS of SCJs decreases with increasing shear velocity, whereas the PSS of rough joints increases with v. Through analysis of the impact of v on the contact area, a PSS model is developed. The model demonstrates that for SCJs, the contact area decreases with increasing v, resulting in a reduction in the basic friction angle. Conversely, for rough joints, the contact area decreases with increasing v. However, it is observed that, considering the distribution characteristics of the apparent dip angle of asperities on the joint surface, the average roughness of joint surface increases as the contact area decreases. It is the increase in roughness that subsequently leads to an increase in the PSS of rock joints. The new model incorporates the influence of v on both the basic friction angle and peak dilation angle, and the interplay between these factors determines the PSS of rock joints. Highlights: Shear tests of rock joints are carried out under a constant normal stress with different shear velocities. A peak shear strength model considering shear velocity is obtained by analyzing the distribution of joint surface and shear velocity on the contact area. The influence of shear velocity on basic friction angle and dilation angle is taken into account in the new model. The competition between basic friction angle and dilation angle determines the increase or decrease of the peak shear strength of rock joints. [ABSTRACT FROM AUTHOR]

Numerical simulations of dust emission processes are essential for dust cycle modeling and dust‐atmosphere interactions. Models have coarse spatial resolutions which, without tackling sub‐grid scale heterogeneity, bias finely resolved dust emission. Soil surface wind friction velocity (us*) drives dust emission non‐linearly with increasing model resolution, due mainly to thresholds of sediment entrainment. Albedo is area‐integrated, scales linearly with resolution, is related to us* and hence represents its sub‐grid scale heterogeneity. Calibrated albedo‐based global dust emission estimates decreased by only 2 Tg y−1 (10.5%) upscaled from 0.5 to 111 km, largely independent of resolution. Without adjusting wind fields, this scaling uncertainty is within recent estimates of global dust emission model uncertainty (±14.9 Tg y−1). This intrinsic scaling capability of the albedo‐based approach offers considerable potential to reduce resolution dependency of dust cycle modeling and improve the representation of local dust emission in Earth system models and operational air quality forecasting. Plain Language Summary: Global computer models for atmospheric dust were developed to understand the global dust cycle. However, these coarse resolution global dust models cannot accurately represent small‐scale dust emission processes and partly cause different dust emission estimations between the models. Using our new approach based on satellite products, we show that global dust emission estimation is largely independent of model resolution. Key Points: Wind friction velocity calibrated to linearly upscaled albedo was largely independent of model resolution between 0.5 and 111 kmWithout modifying wind fields, global dust emissions decreased by only 10.5% (within model uncertainty) from 0.5 to 111 km model resolutionSub‐grid scale heterogeneity is necessary to accurately represent grain‐scale sediment supply and entrainment in large scale modeling [ABSTRACT FROM AUTHOR]

This paper studies particle motion and flow field characteristics with fly ash as an example and combine the experiments to analyze the separation process of rotary triboelectric separator in detail. Under boundary reference conditions of 22 kV plate voltage, 3 m/s feed flow, and 1815 r/min (189.95 rad/s) friction wheel speed, it simulated single-particle and particle group movement with different plate voltages and different feeding positions. Simulation of particle movement shows that particles may collide with a plate to cause serious particle mixing when plate voltage is higher. The initial velocity direction of particles is vertical downward and particles enter the separation chamber from the middle of the feeding port as much as possible to achieve better separation. The analysis of the flow field shows that the increase of the feeding airflow speed leads to a higher flow velocity inside the friction chamber and increases turbulence intensity; increases in friction wheel speed enhances flow field uniformity in the friction chamber and it is accompanied by an increase in velocity gradient; The co-flow speed mainly affects airflow field in the sorting room, the increase of co-flow speed within a certain range effectively stabilizes airflow field in the sorting room. [ABSTRACT FROM AUTHOR]

Although earthquakes are one of the most notorious natural disasters, a full understanding of the underlying mechanisms is still lacking. Here, nanoscale friction measurements were performed by atomic force microscopy (AFM) on calcite single crystals with an oxidized silicon tip to investigate the influence of roughness, contact aging, and dry vs. aqueous environment. In dry environments, smooth and rough calcite surfaces yielding single- and multiasperity contacts, respectively, exhibit velocity-weakening (βDIn V) or neutral friction at slow sliding velocities and velocity-strengthening friction (αDIn V) at higher velocities, while the transition shifts to slower velocities with an increase in roughness. The origin of the velocity-weakening friction is determined to be contact aging resulting from atomic attrition of the crystalline surface. Friction measurements in aqueous environment show evidence of pressure solution at sufficiently slow sliding velocities, which not only significantly reduces friction on single-and multiasperity contacts but also, eliminates atomic attrition and thereby, velocity-weakening friction. Importantly, the friction scaling law evolves from logarithmic (βDIn V) into linear (αPS V), deviating from commonly accepted rate-and-state friction (RSF) laws; this behavior extends over a wider range of velocities with higher roughness. Above a transition velocity, the scaling law remains logarithmic (αWIn V). The friction rate parameters αD, β D, αPS, and αW decrease with load and depend on roughness in a nonmonotonic fashion, like the adhesion, suggesting the relevance of the contact area. The results also reveal that parameters and memory distance differ in dry and aqueous environments, with implications for the understanding of mechanisms underlying RSF laws and fault stability. [ABSTRACT FROM AUTHOR]

How to directly relate frictions at the level of a single molecular chain to the viscoelasticity of soft materials is intriguing. Here, we choose to investigate classical elastomers, where molecular frictions are known to be generated when dangling chains move relatively to the surrounding polymer chain network. With explicit forms employed for the relationship between friction and velocity at the molecular scale, a constitutive theory is then developed for the coupling of molecular frictions and the macroscopic viscoelasticity of elastomers. With the utilization of this theory, viscoelastic behaviors of varied elastomeric materials are predicted, which agree well with existing experiments at both low and high strain rates under different loading conditions. The theory also reproduces the time-temperature equivalent principle of elastomers. We suggest that this work might have provided a modeling framework that directly couples frictions at the level of a single molecular chain to the viscoelasticity of soft materials. [ABSTRACT FROM AUTHOR]

Prior studies, which have been limited to horizontal ground shaking, have generally concluded that soil–structure interaction (SSI) has a negligible influence on the seismic response of isolated buildings. In this study, the effect of SSI on the response of a computational model of an isolated building using triple friction pendulum bearings is investigated. The analysis shows that SSI can amplify story drifts and accelerations when the model is subjected to 3-D shaking including a vertical component. Amplification occurs due to a vertical-horizontal coupling effect that is explained using the modal properties of the numerical model. Based on a parametric study using the key parameters of foundation system, it is concluded that amplification occurs when the effective shear wave velocity of the soil falls into a specific range. In this case, the response of the computational model is very sensitive to the hysteretic damping ratio of the soil, Poisson's ratio of the soil and the footing size. [ABSTRACT FROM AUTHOR]

This work presents the numerical solution for different velocity profiles and friction factors on a rectangular porous microchannel fully saturated by the flow of a nanofluid introducing different viscosity models, including one nanofluid density model. The Darcy-Brinkman-Forchheimer equation was used to solve the momentum equation in the porous medium. The results show that the relative density of the fluid, the nanoparticle diameters and their volumetric concentration have a direct influence on the velocity profiles only when the inertial effects caused by the presence of the porous matrix are important. Finally, it was found that only viscosity models that depend on temperature and nanoparticle diameter reduce the friction factor by seventy percent compared to a base fluid without nanoparticles; furthermore, these models show a velocity reduction of even ten percent along the symmetry axis of the microchannel. [ABSTRACT FROM AUTHOR]

This study explored the evolution of friction-induced vibration (FIV) and wear in high-speed train friction braking through experiments, theoretical analysis, and numerical simulation. Experimental tests revealed that vibration behavior transitions from high-frequency modes at high speeds to low-frequency stick-slip at lower speeds. Theoretical analysis indicated that brake disc speed, coefficient of friction (COF), and contact stiffness affect system stability. Specifically, the system experiences high-frequency vibrations at high speeds due to mode coupling, and the friction block surface primarily undergoes abrasive wear mechanisms. At lower speeds, the system demonstrates low-frequency stick-slip vibrations induced by the combined effects of mode coupling and negative friction velocity slope (NFVS) characteristics, and the friction block surface mainly experiences adhesive wear mechanisms. • FIV in high-speed train braking exhibits diverse mechanisms with speed changes. • The system stability is affected by brake disc speed, COF, and contact stiffness. • FIV is influenced by mode coupling and NFVS from high to low speeds. • The wear mechanism changes from abrasive to adhesive wear during deceleration. • FIV characteristics correlate with wear mechanisms on the friction block. [ABSTRACT FROM AUTHOR]

While recent observational studies have suggested that the ice shell on Europa is undergoing active plate subduction, some theoretical studies have concluded that the high frictional strength of ice would prevent such icy plates from subducting. Constraining the frictional behavior of mixtures of ice and non-ice materials that exist on the surface is therefore key to understanding the frictional strength of plate boundaries on Europa. Here we perform friction experiments on 1 mm thick layers of gouge comprised of mixtures of ice-Ih and magnesium chloride with 0, 17, 29.5 and 47 wt% salt under low-temperature conditions (−40 to −10 °C). The experiments are conducted using a biaxial testing machine with constant normal stresses of either 2.5 or 5.0 MPa, and a constant shear velocity of 3 μm/s. Our experimental results for pure ice and ice–salt mixtures yield steady-state friction coefficients ranging from 0.72 ± 0.02 (−39.2 ± 0.7 °C) to 0.47 ± 0.01 (−16.4 ± 0.2 °C), and 0.51 ± 0.01 (−39.8 ± 1.4 °C) to 0.10 ± 0.02 (−16.9 ± 0.5 °C), respectively. These differences in friction coefficient indicate that non-ice materials can have a significant effect on the friction coefficient along the plate boundaries on Europa. Spatial variations in the frictional strength of plate boundaries are expected to be localized near the base of the conductive layer, as the friction coefficient of the ice–salt mixture exhibits a strong temperature dependence. The observed stick–slip behavior at low temperatures in our experiments indicates the potential for occurrence of intermittent stick–slip along the shallower parts of the plate boundaries, whereas steady-state sliding occurs along the deeper parts. • Friction experiments on ice–salt mixtures were conducted. • Below the eutectic temperature, MgCl 2 decreases the friction coefficient of ice. • Plate subduction on Europa may be explained by non-ice materials at the surface. [ABSTRACT FROM AUTHOR]

The paper concerns the problem of determining friction velocity in wallbounded flows affected by an adverse pressure gradient (APG). In the work of Niegodajew et al. [22] the corrected Clauser chart method (CCCM) for such flow conditions was proposed. This approach utilises the mean velocity profiles and turbulence intensity profiles to accurately estimate the friction velocity. In another work, Drózdz et al. [27] presented a modified version of the diagnostic-plot scaling (DPS) which allows for direct reconstruction of turbulence intensity profiles based on the local mean velocity profile, even when the flow is affected by a strong pressure gradient. This paper is aimed at verifying whether, when combining both of these methods (i.e. DPS and CCCM), the friction velocity can be accurately determined for APG flow conditions and one can possibly take advantage from both methods. The analysis revealed that the new approach is able to predict the friction velocity with uncertainty less than 5% for all the considered cases for the Clauser--Rotta parameter β < 17. Lastly, DPS-CCCM was also confronted with two empirical approaches (from available literature) allowing for estimation of the friction velocity under APG conditions. The performance of DPS-CCCM was found to be better than the ones of two other empirical approaches. [ABSTRACT FROM AUTHOR]

In the present study, the motion of a droplet in the boundary layer is investigated numerically. Volume of Fluid method is employed to solve the governing equations. It is found that the presence of the droplet leads to an increase in the pressure inside the boundary layer and on the wall. The results show that the droplets create a vortex on the bottom surface. The friction coefficient increases due to the presence of the droplets and is reduced before and after the droplet due to the formation of a vortex. It is concluded that increasing the viscosity and reducing the density at the same time will not affect the velocity and friction coefficient. It is shown that increasing the radius of the droplet increases the stress and thus decreases the boundary layer velocity. As the Reynolds number increases, the amount of surface friction coefficient decreases. By adding nanoparticles into the pure water, surface friction coefficient increases, especially in the region where the droplet is present. [ABSTRACT FROM AUTHOR]

While analysis of glacial seismicity continues to be a widely used method for interpreting glacial processes, the underlying mechanics controlling glacial stick‐slip seismicity remain speculative. Here, we report on laboratory shear experiments of debris‐laden ice slid over a bedrock asperity under carefully controlled conditions. By modifying the elastic loading stiffness, we generated the first laboratory icequakes. Our work represents the first comprehensive lab observations of unstable ice‐slip events and replicates several seismological field observations of glacier slip, such as slip velocity, stress drop, and the relationship between stress drop and recurrence interval. We also observe that stick‐slips initiate above a critical driving velocity and that stress drop magnitude decreases with further increases in velocity, consistent with friction theory and rock‐on‐rock friction laboratory experiments. Our results demonstrate that glacier slip behavior can be accurately predicted by the constitutive rate‐and‐state friction laws that were developed for rock friction. Plain Language Summary: Glacier beds and tectonic faults may at first appear to be quite different, but they share important characteristics. In both cases, motion may be smooth (aseismic creep) or earthquake‐producing "stick‐slip." A powerful physical constitutive relationship called rate‐and‐state friction has been developed to understand earthquakes and smooth slip on tectonic faults. Laboratory experiments reported here simulate glacier‐bed motion by sliding debris‐bearing ice over a rock plate under conditions that are typical for glacier beds. They produce the first laboratory icequakes. Transitions between steady and stick‐slip motions are generated by controlling shearing velocity and other conditions, as predicted by rate‐and‐state friction theory. Future studies can thus apply this physical framework to glacier slip, helping to understand ice motion and its potential to accelerate sea level rise in a warming world. Furthermore, because motion at the glacier bed is often much easier to study than tectonic faults, additional observations of glaciers may provide useful insights into earthquake behavior. Key Points: The first laboratory "icequakes" were generated and slip stability was predicted well by rate‐and‐state frictionLaboratory icequake attributes such as peak slip velocity, stress drop and healing agree well with other laboratory and field observationsThe laboratory icequake events suggest that debris‐bed contacts dominate the sudden slip mechanics [ABSTRACT FROM AUTHOR]

The impact between a rotating link and a solid flat surface is considered. For the impact, we consider three distinct periods: elastic period, elastoplastic period, and restitution period. A Hertzian contact force is considered for the elastic period. Nonlinear contact forces developed from finite element analysis are used for the remaining two phases. The tangential effect is taken into account considering a friction force that combines the Coulomb dry friction model and a viscous friction function of velocity. Simulations results are obtained for different friction parameters. An experimental setup was designed to measure the contact time during impact. The experimental and simulation results are compared for different lengths of the link. [ABSTRACT FROM AUTHOR]

The strength properties of fault rocks at shearing rates spanning the transition from crystal–plastic flow to frictional slip play a central role in determining the distribution of crustal stress, strain, and seismicity in tectonically active regions. We review experimental and microphysical modelling work, which is aimed at elucidating the processes that control the transition from pervasive ductile flow of fault rock to rate-and-state-dependent frictional (RSF) slip and to runaway rupture, carried out at Utrecht University in the past 2 decades or so. We address shear experiments on simulated gouges composed of calcite, halite–phyllosilicate mixtures, and phyllosilicate–quartz mixtures performed under laboratory conditions spanning the brittle–ductile transition. With increasing shear rate (or decreasing temperature), the results consistently show transitions from (1) stable velocity-strengthening (v -strengthening) behaviour, to potentially unstable v -weakening behaviour, and (2) back to v strengthening. Sample microstructures show that the first transition seen at low shear rates and/or high temperatures represents a switch from pervasive, fully ductile deformation to frictional sliding involving dilatant granular flow in localized shear bands where intergranular slip is incompletely accommodated by creep of individual mineral grains. A recent microphysical model, which treats fault rock deformation as controlled by competition between rate-sensitive (diffusional or crystal–plastic) deformation of individual grains and rate-insensitive sliding interactions between grains (granular flow), predicts both transitions well. Unlike classical RSF approaches, this model quantitatively reproduces a wide range of (transient) frictional behaviours using input parameters with direct physical meaning, with the latest progress focusing on incorporation of dynamic weakening processes characterizing co-seismic fault rupture. When implemented in numerical codes for crustal fault slip, the model offers a single unified framework for understanding slip patch nucleation and growth to critical (seismogenic) dimensions, as well as for simulating the entire seismic cycle. [ABSTRACT FROM AUTHOR]

Resistance to flow because of the presence of bed forms over armored riverbeds is of paramount importance, leading to the effective design of water-resources-related projects. Based on the findings over bed armored surfaces, it is shown that the controlling roughness (ks) can be taken as equal to the median diameter of the armor layer. Analytical methodologies for total and grain friction factors have been proposed here that take flow non-uniformity into account using the velocity distribution and friction slope. The percentage composition of form friction factor in the total friction factor was estimated to be 40%. The results were explained in light of the coupling of the sediment threshold problem with the friction factor and coarse-grain rearrangement in armor layer. The computed form friction factor by proposed method was compared with Keulegan's method and is found to give satisfactory results, showing 80% agreement of all field data sets. [ABSTRACT FROM AUTHOR]

Discrete Element Method (DEM) simulations coupled with shear cell experimental results have been used to investigate the flow behaviour of a dry particle assembly of glass beads in the quasi-static regime. Experimental studies have been undertaken using an FT4 powder shear cell apparatus, in parallel with extensive DEM simulations of both homogeneous simple shear and the FT4 shear cell itself. The findings show that it is not possible to accurately predict the bulk friction coefficient with homogeneous simple shear simulations unless both rolling and sliding friction are considered. There are, however, multiple pairs of sliding and rolling friction coefficients which can reproduce the experimental bulk friction coefficient. Sliding test experiments were conducted to yield the coefficient of sliding friction, and hence minimise the set of potentially correct pairs. Simulations of the full FT4 shear cell with two different calibration pairs, along with a pair without rolling friction, were then undertaken to understand the effect of their selection on realistic wall-bounded shearing conditions. Discrepancies were mainly found in the obtained radial contact number and velocity profiles, with increasing friction coefficients - particularly sliding friction - found to inhibit packing and particle velocity in the shear deformation zone. Comparison between homogeneous simple shear and shear cell simulation results showed a significant effect of the wall on the obtained force network, with almost a complete absence of the weakest structures which were seen supporting the strong structures in the simple shear scenario. Unlabelled Image • FT4 shear cell experiments and sliding tests were carried out. • DEM shows sliding and rolling friction coefficient pairs of correct bulk friction. • Many friction pairs exist, sliding tests allow the correct pair to be determined. • Simple shear simulations show the importance of rolling friction on flow behaviour. • Shear cell simulations showed a significant effect of wall on the force network. [ABSTRACT FROM AUTHOR]

Frictional motion between contacting bodies is governed by propagating rupture fronts that are essentially earthquakes. These fronts break the contacts composing the interface separating the bodies to enable their relative motion. The most general type of frictional motion takes place when the two bodies are not identical. Within these so-called bimaterial interfaces, the onset of frictional motion is often mediated by highly localized rupture fronts, called slip pulses. Here, we show how this unique rupture mode develops, evolves, and changes the character of the interface's behavior. Bimaterial slip pulses initiate as "subshear" cracks (slower than shear waves) that transition to developed slip pulses where normal stresses almost vanish at their leading edge. The observed slip pulses propagate solely within a narrow range of "transonic" velocities, bounded between the shear wave velocity of the softer material and a limiting velocity. We derive analytic solutions for both subshear cracks and the leading edge of slip pulses. These solutions both provide an excellent description of our experimental measurements and quantitatively explain slip pulses' limiting velocities. We furthermore find that frictional coupling between local normal stress variations and frictional resistance actually promotes the interface separation that is critical for slip-pulse localization. These results provide a full picture of slip-pulse formation and structure that is important for our fundamental understanding of both earthquake motion and the most general types of frictional processes. [ABSTRACT FROM AUTHOR]

The strength properties of fault rocks at shearing rates spanning the transition from crystal-plastic flow to frictional slip play a central role in determining the distribution of crustal stress, strain and seismicity in tectonically-active regions. We review experimental and microphysical modelling work aimed at elucidating the processes that control the transition from pervasive ductile flow of fault rock to rate-and-state dependent frictional (RSF) slip and to runaway rupture, carried out at Utrecht University in the past two or so decades. We address shear experiments on simulated gouges composed of calcite, halite-phyllosilicate mixtures, and phyllosilicate-quartz mixtures, performed under laboratory conditions spanning the brittle-ductile transition. With increasing shear rate (or decreasing temperature), the results consistently show transitions from (1) stable, velocity(v)-strengthening to potentially unstable, v-weakening behavior, and (2) back to v-strengthening. Sample microstructures show that the first transition, seen at low shear rates and/or high temperatures, represents a switch from pervasive, fully ductile deformation to frictional sliding, involving dilatant granular flow in localized shear bands, where intergranular slip is incompletely accommodated by creep of individual mineral grains. A recent microphysical model, treating fault rock deformation as controlled by a competition between rate-sensitive (diffusional or crystal-plastic) deformation of individual grains and rate-insensitive sliding interactions between grains (granular flow), predicts both transitions well. Unlike classical RSF approaches, this model quantitatively reproduces a wide range of (transient) frictional behaviors using input parameters with direct physical meaning. When implemented in numerical codes for crustal fault-slip, it offers a single, unified framework for understanding slip patch nucleation and growth to critical (seismogenic) dimensions, and for simulating the entire seismic cycle. [ABSTRACT FROM AUTHOR]

Variations in friction velocity with wind speed and height are studied under moderate (≥9 m s−1)-to-strong onshore wind conditions caused by three landfalling typhoons. Wind data are from a coastal 100-m tower equipped with 20-Hz ultrasonic anemometers at three heights. Results show that wind direction affects variations in friction velocity with wind speed. A leveling off or decrease in friction velocity occurs at a critical wind speed of ~20 m s−1 under strong onshore wind conditions. Friction velocity does not always decrease with height in the surface layer under typhoon conditions. Thus, height-based corrections on friction velocities using the model from Anctil and Donelan may not be reliable. Surface-layer heights predicted by the model that are based on Ekman dynamics are verified by comparing with those determined by a proposed method that is based on the idea of mean boundary layer using wind-profile data from one of the landfalling typhoons. Friction velocity at the top of the surface layer is then estimated. Results show that friction velocity decreases by about 20% from its surface value and agrees well with previous results of Tennekes. [ABSTRACT FROM AUTHOR]

Model-based condition monitoring is an increasingly important area for rail transportation. The key elements of such condition monitoring methodologies are low-cost vehicle sensors and intelligent algorithms. In this study, a swarm intelligence-based multiple models approach is proposed to detect different friction conditions by using velocity measurements of a railway vehicle. In this case of application, estimated parameter is the maximum friction coefficient. Additionally, proposed methodology is tested experimentally by using the measurements taken from a tram wheel test stand. Multiple mathematical models of the test stand are created with different maximum friction coefficients, whereas all initial conditions and other system parameters are same for each model. Therefore, comparison of the output of each model with measurements is considered to interpret the parameter value of the model, which best represents the system, is selected as parameter estimate. Unlike the traditional multiple models approach, a swarm intelligence-based evolution of the models is proposed. Experiments carried out on the test stand reveal that the proposed methodology is promising to be used as an on-board friction condition monitoring tool for railway vehicles with traction. Furthermore, it can be considered to detect weather conditions since friction conditions change due to the weather events such as rain, ice, snowfall, condensation of water droplets, and leaves on the line and it can be used as an auxiliary system for intelligent traction and high adhesion control systems. [ABSTRACT FROM AUTHOR]

Steel silos with corrugated walls are widely used because they allow an increase of the wall buckling resistance, but corrugations affect the material flow and the forces applied over the walls. Silo design codes propose a method to estimate the friction on corrugated walls, but important aspects of the problem are not considered. This paper presents DEM simulations for a flat-bottom silo with different corrugation depths. Results show the differences in the friction of particles stored in a silo against a corrugated wall from a smooth wall, thus producing changes in many aspects such as flow, velocity of particles or friction coefficient. Flow rates and velocity of particles were higher for models with larger corrugation depths, especially for the initial period of discharge. Greater mobilization of particle-to-particle friction coefficient and higher values for contact normal forces were also detected as the value of the depth of the corrugation increased. Unlabelled Image • A silo with corrugated walls was modeled by using DEM. • DEM predicts changes in some parameters with regards to a smooth wall. • Flow rates and velocities were higher in models with larger corrugation depth. • Greater mobilization of friction was detected as the corrugation depth increases. [ABSTRACT FROM AUTHOR]

In order to suppress the effect of friction on electric linear load simulator(ELLS),the simulation analysis and experimental methods are used to analyze the effect of friction on ELLS,the genetic algorithm is used to identify the friction model in ELLS,and the relationship between friction and velocity is obtained. A hybrid control method of friction feedforward and variable gain PID is proposed,the friction feedforward compensation is used to eliminate the effect of friction,and the variable gain PID is used to further suppress the phase lag of ELLS at low velocity. Comparative simulation and experiment were made. The simulated analysis and the experimental results show that,compared with traditional PID control,the loading wave distortion of ELLS using the friction feedforward and variable gain PID is suppressed,and the loading error and phase lag are significantly reduced. The proposed control method can better suppress the effect of friction on ELLS and improve the loading accuracy. [ABSTRACT FROM AUTHOR]

The elastic strain energy release rate and seismic waves emitted during earthquakes are controlled by the on-fault temporal evolution of the shear stress during rupture propagation. High velocity friction experiments highlighted that shear stress on the fault surface evolves rapidly during seismic slip pulses. This temporal evolution of shear stress is controlled by both fault weakening at seismic slip initiation and re-strengthening rate towards the end of slip. While numerous studies focused on fault weakening, less attention was given to co-seismic re-strengthening processes. Here we performed 53 friction experiments (normal stress ≤30 MPa, slip-rate ≤6.5 m s−1) imposing constant slip acceleration and deceleration (7.8 m s−2), on cohesive Carrara marble (99% calcite) and micro-gabbro (silicate-built rock) under dry, vacuum and water pressurized conditions. Microstructural observations showed that micro-gabbro accommodated seismic slip by bulk melting of the sliding surfaces, whereas Carrara marble by coupled decarbonation and grain-size dependent crystal plastic processes. Under room humidity conditions and low imposed power density (i.e., product of normal stress per slip rate), re-strengthening rate during the deceleration stage was up to ∼ 17 times faster in marble than in micrograbbro. In the latter, the re-strengthening rate increased slightly with the power density. The presence of water enhanced further this trend. On the contrary, in marbles the re-strengthening rate decreased drastically with power density and in the presence of water. Our experimental observations highlighted the first order importance of the mineralogy and rheology of the slip zone materials and, to a second order, of the presence of water in controlling co-seismic re-strengthening of faults during seismic slip deceleration. • Effect of rock composition and water content on fault re-strengthening during earthquakes. • Re-strengthening rate is faster in carbonate-than silicate-bearing rocks at low power density. • In silicate rocks, re-strengthening rate increases with power density and in presence of water. • In carbonate rocks, re-strengthening rate decreases with power density and in presence of water. • Silicate rocks promote intense high frequency radiation during deceleration of the seismic slip. [ABSTRACT FROM AUTHOR]

This paper presents a study on the effects of shearing velocity, surface roughness and overconsolidation ratio on the strength and deformation behaviour of fine-grained soil–structure interfaces. The results of shear box direct interface shear tests, augmented with soil deformation measurements from particle image velocimetry analyses, indicate that shearing velocity (i.e. velocity at which the continuum surface is displaced) has a controlling effect on the drainage conditions mobilised during shearing. As shearing velocity was increased, undrained conditions were progressively mobilised, characterised by smaller interface strength and volumetric changes for tests on normally consolidated specimens. Increases in shearing velocity resulted in increases in interface strength for overconsolidated specimens. The results presented herein indicate that, as the magnitude of surface roughness was increased, drained shearing conditions were favoured for a given shearing velocity. Soil deformation measurements indicate that the shear zone where localisation occurs becomes thicker as the surface roughness increases, and the magnitude of soil deformations decreases as shearing velocity is increased. The findings presented herein are relevant for prediction and interpretation of soil–structure interface behaviour, as well as for development of constitutive models that capture the combined effect of shear rate, surface roughness and stress history. [ABSTRACT FROM AUTHOR]

The calculations of friction pressure drop is the most complicated in multiphase flow. Understanding of the friction pressure drop in pipes are of great importance for many industry problems. The existing friction pressure drop models are generally based on low gas and liquid velocities in horizontal and vertical pipes. In this paper, a two-phase flow experiment in inclined pipe was performed. We report the effect of superficial gas and liquid velocities on friction pressure drop, the results indicate that the pressure drop increases with the increase in superficial gas velocity and superficial liquid velocity. According to the experimental analysis and assuming that the fluid in contact with the pipe wall at high velocity is in a liquid phase, we developed a new friction pressure drop model of different flow pattern. This model based on the correlation between the fraction factor and the superficial gas and superficial liquid Reynold numbers. Compared with the previous models, the predictive performance of the present model is good and remains basically stable. [ABSTRACT FROM AUTHOR]

Conformally symplectic systems include mechanical systems with a friction proportional to the velocity. Geometrically, these systems transform a symplectic form into a multiple of itself making the systems dissipative or expanding. In the present work we consider the limit of small dissipation. The example we study is a family of conformally symplectic standard maps of the cylinder for which the conformal factor, b (ε) , is a function of a small complex parameter, ε. We assume that for ε = 0 the map preserves the symplectic form and the dependence on ε is cubic, i.e., b (ε) = 1 − ε 3. We compute perturbative expansions formally in ε and use them to estimate the shape of the domains of analyticity of invariant circles as functions of ε. We also give evidence that the functions might belong to a Gevrey class at ε = 0. We also perform numerical continuation of the solutions as they pass through the boundary of the domain to illustrate that the monodromy of the solutions is trivial. The numerical computations we perform support conjectures on the shape of the domains of analyticity. • We study numerically the domain of analyticity of invariant circles. • Computations suggest that the domain's boundary may have a self-similar structure. • We conjecture Gevrey regularity with respect to the small dissipation parameter. [ABSTRACT FROM AUTHOR]

Newly found evidence on the negative slope of the friction-velocity curve is described in terms of the contact area on the surface of brake friction materials. The tribotests were performed using two commercial friction materials with and without steel fibers. The results showed that velocity weakening friction was pronounced in the friction materials containing steel fibers, which facilitated high pressure contact areas by the agglomeration of wear debris. The size of the high pressure contact area increased as the sliding velocity decreased, which supported the correlation among velocity weakening friction, friction instability, and high pressure contact areas. The pronounced velocity dependence of the friction coefficient due to transient contact area was highly relevant to noise propensity of the friction material. • The sliding surface of a brake friction material shows wide pressure distribution. • The size of the high-pressure contact area changes with the sliding velocity. • μ-v character of the friction material is affected by high-pressure contact areas. • Large stick-slip amplitudes are germane to velocity weakening friction phenomena. • Velocity dependence of the contact area affects the friction and noise propensity. [ABSTRACT FROM AUTHOR]

Fault normal stress changes dynamically during earthquake rupture; however, the impact of these changes on dynamic frictional strength is poorly understood. Here we report on a laboratory study to investigate the effect of normal stress perturbations on the friction of westerly granite surfaces sheared under normal stresses of 1‐25 MPa. We measure changes in surface friction and elastic properties, using acoustic waves, for step changes in normal stress of 1–50% and shearing velocities of 1‐100 μm/s. We demonstrate that transmitted elastic wave amplitude is a reliable proxy for the real contact area at the fault interface at steady state. For step increases in normal stress, wave amplitude increases immediately and then continues to increase during elastic shear loading to a peak value from which it decreases as fault slip rate increases. Friction changes in a similar fashion, showing an inelastic increase over a characteristic shear displacement that is independent of loading rate. Perturbations in normal stress during shear cause excursions in the frictional slip rate that must be accounted for in order to accurately predict the evolution of fault strength and elastic properties. Our work improves understanding of induced seismicity and triggered earthquakes with particular focus on simulating static triggering and stress transfer phenomena using rate‐and‐state frictional formulations in earthquake rupture models. Key Points: Shear stress evolution following normal stress change follows a three‐stage responseSlip rate reduction due to normal stress increase is tracked by ultrasonic amplitudesUltrasonic amplitudes increase linearly with normal stress at steady state [ABSTRACT FROM AUTHOR]

Understanding what controls the travelling distance of large landslides has been the topic of considerable debate. By combining observation and experimental data with depth-averaged continuum modelling of landslides and generated seismic waves, it was empirically observed that lower effective friction had to be taken into account in the models to reproduce the dynamics and runout distance of larger volume landslides. Moreover, such simulation and observation results are compatible with a friction weakening with velocity as observed in earthquake mechanics. We investigate here as to whether similar empirical reduced friction should be put into discrete element models (DEM) to reproduce observed runout of large landslides on Earth and on Mars. First we show that, in the investigated parameter range and for a given volume, the runout distance simulated by 3D DEM is not much affected by the number (i.e. size) of grains once this number attains ~ 8000. We then calibrate the model on laboratory experiments and simulate other experiments of granular flows on inclined planes, making it possible for the first time to reproduce the observed effect of initial volume and aspect ratio on runout distances. In particular, the normalised runout distance starts to depend on the volume involved only above a critical slope angle > 16–19°, as observed experimentally. Finally, based on field data (volume, topography, deposit), we simulate a series of landslides on simplified inclined topography. The empirical friction coefficient, calibrated to reproduce the observed runout for each landslide, is shown to decrease with increasing landslide volume (or velocity), going down to values as low as 0.1–0.2. No distinguishable difference is observed between the behaviour of terrestrial and Martian landslides. [ABSTRACT FROM AUTHOR]

Environmental legislation continues to drive optimisation of internal combustion engines in the automotive sector in an effort to reduce harmful emissions and promote fuel efficiency. Significant frictional losses occur at the ring pack, prompting research on reducing sliding friction at the ring – liner interface through the use of surface texturing. This study assesses the influence of contact area on the friction and wear behaviour of light-weight monolithic hyper-eutectic Al-Si alloy with a textured surface generated by electrochemical jet texturing. Flat rectangular samples were prepared from a cast cylinder liner via electro-discharge machining and flat lapped to 6 µm diamond finish prior to immersion in a 1 mol solution of NaOH for 120 s to simulate the mechanical honing process. Surfaces were electrochemically jet textured using a 2.3 mol solution of NaCl and current density of 220 A/cm2. An offset array of 1.3 × 0.3 mm rectangular features at 1.2 mm spacing was created with an average depth of 39 µm. Lubricated reciprocating sliding was carried out at a stroke length of 25 mm in a bath of PAO (4 cSt) at 100 °C against a 6 mm diameter, 13 mm long 52100 steel cylinder with three different contact areas corresponding to initial contact pressures of 157, 12 and 4 MPa. Sliding frequencies were varied between 1 and 15 Hz at a load of 50 N in order to establish the influence of both the contact pressure and velocity on the friction behaviour as a function of the lubrication regime. Textured features were observed to reduce the average coefficient of friction by up to 37% via a lubricant reservoir mechanism if the counter-surface contact width was greater than the feature dimension. Increases in contact pressure prevented a micro-EHL effect due to adhesive wear rather than pressurization of textured features. • Electrochemical Jet Texturing (EJT) can create complex surface features on Al-Si surfaces. • The coefficient of friction can be reduced by up to 37% in mixed to fluid film lubrication. • Reduction in friction was due to film enhancement from a lubricant reservoir effect. • MicroEHL was prevented by adhesive transfer of aluminium to the steel counter surface. [ABSTRACT FROM AUTHOR]

The dependence of rolling friction on velocity for various contact conditions is discussed. The principal difference between rolling and other types of relative motion (sliding and spinning) is that the points of the body in contact with the support change over time. Due to deformations, there is a small contact area and, entering into contact, the body points have a normal velocity proportional to the diameter of this area. For describing the dependence of the friction coefficient on the angular velocity in the case of "pure" rolling, a linear dependence is proposed that admits a logical explanation and experimental verification. Under the combined motion, the rolling friction retains its properties, the sliding and spinning friction acquiring the properties of viscous friction. [ABSTRACT FROM AUTHOR]

Abstract—A spectral problem of the Orr–Sommerfeld type for describing stable and unstable perturbations of oceanic geostrophic flows with linear vertical velocity shear is considered (parabolic vertical velocity profile). Calculations of eigenvalues, increments of growth rate of unstable modes, and eigenfunctions of the fastest growing perturbations are presented. It is found that the instability of the geostrophic flow is observed over a wide range of horizontal scales: in addition to longwave perturbations with a phase velocity exceeding the maximum velocity of the geostrophic current and perturbations with the scales of the Rossby radius, shortwave modes exist with scales much smaller than the Rossby radius (submesoscale structures). The results of the model are used to describe intrusions in the Arctic basin, which are observed under conditions of absolutely stable stratification. [ABSTRACT FROM AUTHOR]

This paper studies the nonlinear behavior of the friction-induced vibration by using spring-mass model subject to the smooth frictionvelocity curve. The nonlinearity and instability of the friction may produce the chaotic vibration depending on the friction curve. In order to show this, the Lyapunov exponents are calculated for a variety of the slope and magnitude in the smooth friction curve. In turn, the dependency of the friction curve on the chaotic attractor is illustrated. [ABSTRACT FROM AUTHOR]

In this paper, we investigated the significance of intergranular friction weakening on the dynamics of the Jiweishan rock avalanche. First, high-velocity-friction tests simulating the shearing between the surfaces of adjacent grains were conducted to obtain a model of friction weakening of grains inside granular debris during transport. Then, the model was incorporated into an improved discrete element method that could consider intergranular friction weakening to simulate the dynamic course of the Jiweishan rock avalanche. Finally, we compared the results from the numerical models with and without granular friction weakening to investigate its influence on the dynamics of the rock avalanche. The results of the high-velocity-friction tests revealed that intergranular shearing could significantly reduce the friction of granular surfaces; this was determined by the overall results for normal stress, shear velocity and shear displacement. Our modeling results indicated that the rapid increase in normal stress and shear velocity were the main reasons for the rapid decrease in intergranular friction coefficients in the falling and collision stages. However, the friction coefficients further decreased due to increasing shear displacement between rock debris in the flowing stage. Furthermore, obvious intergranular friction coefficient distribution characteristics in rock avalanche deposits could be observed from our modeling; they tended to decrease with increased depth in the deposit and increasing distance from the source. With intergranular friction weakening, a faster velocity (∼10.0–15.5 m/s) and a longer transport time (∼12–44 s) of the rock debris were modeled due to the lower frictional energy consumption caused by intergranular friction weakening. Finally, the transport distance considering intergranular weakening was ∼ 395–711 m longer than that without weakening. Further analysis showed that intergranular friction weakening may be applied to explain the high mobility of rock avalanches. The results of our study may improve predictions of the speed and travel distance, and consequently the risk assessment, of widely distributed rock avalanches worldwide. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synergistic lubrication of a combination of GML and N888 OFMs are performed by MD simulations. • The number ratio of 3:1 of GML to N888 shows the best synergistic lubrication. • High shear velocity damages adsorption film and results in high friction. • Synergistic effect is attributed to the interface slip rather than interlaminar shear. The synergistic effect of organic friction modifiers (OFMs) combining glycerol monolaurate (GML) and tri-n-octylamine (N888) in base oil is studied by molecular dynamics simulations. The adsorption and shear behaviors indicate that the triple-chains N888 molecules intersperse among the long-chain GLM molecules to form an intercoordination adsorption film. The system with a number ratio of 3:1 of GML to N888 shows a better synergistic lubrication with the lowest friction coefficient. The chemisorption of GML molecules on Fe 2 O 3 surface is verified by density function theory. High shear velocity accompanying friction heat causes adsorption film breakage and high friction, while contact pressure conduces to friction reduction. The synergistic lubrication is attributed to the interface slip between base oil and OFM films, rather than the interlaminar shear caused by an imbalanced combination or high shear velocity. [ABSTRACT FROM AUTHOR]

Two polymers and five metal pairs were tested at six low temperature points and three constant load speeds. The tribological properties were determined by joint iterative coupling of the temperature, load and friction velocity value. Molecular dynamics simulation was used to study the micro-tribological properties of friction pairs at the atomic scale. The macro-tribological performance test results were similar to the results of the molecular dynamics simulation and exhibited the same trend. Two profound hypothermia points were predicted. The evolution of the tribological mechanism and properties of the polymer interface under different working conditions at low temperature were revealed on the macroscopic and atomic scales. [Display omitted] • A friction mechanism researching method based on a combination of macro and micro research was developed. • The macroscopic and microscopic tribological evolution rules of a polymer interface at low temperature were found. • The simulation was applicable to predicting polymer materials' tribological properties and explaining the test phenomena. • This method can be used for researches under harsher conditions and more extreme environments. [ABSTRACT FROM AUTHOR]

A method is proposed for estimating the surface-layer depth $$(z_s)$$ and the friction velocity $$(u_*)$$ as a function of stability (here quantified by the Obukhov length, L) over the complete range of unstable flow regimes. This method extends that developed previously for stable conditions by Argaín et al. (Boundary-Layer Meteorol 130:15-28, 2009), but uses a qualitatively different approach. The method is specifically used to calculate the fractional speed-up $$(\varDelta S)$$ in flow over a ridge, although it is suitable for more general boundary-layer applications. The behaviour of $$z_s \left( L\right) $$ and $$u_*\left( L\right) $$ as a function of L is indirectly assessed via calculation of $$\varDelta S\left( L\right) $$ using the linear model of Hunt et al. (Q J R Meteorol Soc 29:16-26, 1988) and its comparison with the field measurements reported in Coppin et al. (Boundary-Layer Meteorol 69:173-199, 1994) and with numerical simulations carried out using a non-linear numerical model, FLEX. The behaviour of $$\varDelta S$$ estimated from the linear model is clearly improved when $$u_*$$ is calculated using the method proposed here, confirming the importance of accounting for the dependences of $$z_s\left( L \right) $$ and $$u_*\left( L \right) $$ on L to better represent processes in the unstable boundary layer. [ABSTRACT FROM AUTHOR]

Hydrogels have attracted enormous frictional investigation owing to their great potential in hydration lubrication. Researchers have revealed that the lubrication behavior of hydrogel depends on a number of parameters such as contact area, sliding velocity, and solution condition. However, less effort has been devoted to understanding the time-dependent friction behavior of polymeric hydrogels. In this work, the time-dependent behavior of hydrogel friction was systematically investigated through three typical polyacrylamide hydrogels (PAM, PAM-PEI, and PAM-PAA) on a rheometer. Interestingly, the friction coefficients (μ) of the hydrogels varied with time, and two distinct regimes (viz., the friction-decrease regime and friction-constant/increase regime) were observed. In the friction-decrease regime, all the friction coefficients for the three PAM-based hydrogels dramatically decreased (e.g. μ decreased from ∼ 0.107 to ∼ 0.034 for PAM, μ decreased from ∼ 0.059 to ∼ 0.021 for PAM-PAA). The decrease of friction coefficients can be explained by frictional relaxation mechanism, where the deformation of polymers in the shear direction and the desorption of polymer chains from solid surface play dominant roles. Tribological results show that the increase in friction is determined by the charge behavior and polymer network of the PAM-based hydrogels. Besides, shear velocity and pH of the solution also significantly affected the friction behavior of hydrogels on the glass substrate. The understanding of the time-dependent friction behavior of PAM hydrogels provides insights to develop water-based lubricants with engineering applications. [Display omitted] [ABSTRACT FROM AUTHOR]

The optimal design of two-phase flow pipes depends on various variables. Pressure drop and liquid hold-up are essential parameters in the appropriate design of pipes. These parameters can be estimated by numerical simulation of pipes. However, the simulation strongly depends on both phase frictions. Frictions are commonly calculated using friction factors, and various empirical correlations have been proposed to calculate friction factors, but most of these correlations are dependent on the experimental conditions. The present study proposes a semi-empirical friction factor model by modifying an existing model in the literature for stratified two-phase flows. Experimental data from various laboratories are taken to validate the interface shear stress, velocity profile and pressure gradient. Compared with previous models, the predicted velocity profiles and shear stresses are in better agreement with the experimental data. • An improved semi-empirical friction model is proposed. • The interfacial turbulent parameters are derived using the mean velocity. • The pressure gradient is validated against experimental data. • The velocity profile is improved. [ABSTRACT FROM AUTHOR]

We studied the frictional behaviour and deformation products of large (20 cm × 20 cm bare surfaces) experimental limestone faults. We sheared samples in a direct shear configuration, with an imposed normal force of 40–200 kN and shear velocity of 10 μm/s. The steady-state shearing of these surfaces yielded a coefficient of friction 0.7<μ<1 (average μ∼0.9), significantly higher than gouge friction of the same material, μ∼0.6. Frictional healing, studied via slide-hold-slide tests, is null (Δμ≤0 upon re-shear). Moreover, sliding of these surfaces is accompanied by dilatation and production of grooves, gouge striations and fault mirrors. These products are entirely analogous to slip surface phenomena found on natural limestone-bearing faults at both the macroscale and at the microscale. We infer that high friction, accompanied by dilatant deformation, and null frictional healing are the macroscopic effect of brittle damage on the sliding surface, constrained by the strength of the rock and by fast healing processes in the gouge. Simultaneously to brittle failure, plastic deformation occurs on the sliding surface and inside the intact rock via nanoparticle formation (mirrors) and twinning at the micron scale. Because of the similarity between experimental and natural structures, we suggest that sliding of carbonate-bearing faults in the uppermost crust could be characterized by high friction, fast healing and strongly dilatant deformation, which would help to explain shallow seismicity frequently documented in carbonatic terrains such as the Northern Apennines of Italy. [ABSTRACT FROM AUTHOR]

This study investigated the friction performance of polyimide-matrix composites with various silica powder contents of different sizes. The friction tests were performed on a pin-on-disk tester at various velocities, ranging from 30 to 160 km/h. The experimental results indicate a silica powder size of 0.25±0.05 mm, a sample of 4 wt.% SiO2 demonstrated a high friction coefficient and better wear performance as compared to a sample without silica. The influence of silica content and powder size on the friction coefficient is directly related to the friction velocity. At low friction velocity (30-90 km/h), the friction coefficient increases as the SiO2 content increases, but decreases with increasing SiO2 particle size. At high friction velocity (90-160 km/h), the content and particle size of SiO2 have little influence on the friction coefficient. Experimental results indicate that large SiO2 particles can effectively hinder the motion of third body, which is advantageous to the formation of a compact third body and to the protection of the friction surface. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lubricants intends to prevent the perforation and demonstrated a striking 'slippery but bulletproof' behavior. • Special designed dynamic sliding experiment indicates the lubricated surface act as an enhancement factor under the lower normal load. • Friction as a function of velocity was incorporated in subroutine for simulating the strengthening perforation behavior of aluminum laminar. • Lubricant strengthening and toughening should be taken into consideration as the thickness of anti-bullet protective structure has been decreasing. Lubricant has been well-recognized in reducing friction/wear resistance and subsequently facilitating the movement of objects. However, the so-called lubricant inclines to increase the transverse dynamic piercing resistant properties of laminar aluminum over a wide range of high-velocity from 60 m/s to 230 m/s. The underlying mechanism in the high-speed perforation process of laminar structural raises confusion about the traditional concept that the lubricant did not act in reducing the frictional effect. Lubricant, such as high vacuum grease, smeared on the thinner metal laminar show the 'slippery but bulletproof' behavior. A specially designed dynamic sliding experiment provides further evidence that the lubricants act as a strengthening factor under the lower normal load state, but have no significant effect under a heavy normal load during the sliding initiation stage. The lubricant film avoids direct metallic contacting, reducing the stress concentration, postponing the failure displacement, and increasing the dissipated energy. This abnormal lubricant phenomenon in terms of strengthening and toughening should be taken into consideration as the thickness of the anti-bullet protective structure has been decreasing. These findings provide physical insights into the evolution of friction and fracture for developing membrane-like protective structures. [ABSTRACT FROM AUTHOR]

We examined experimentally the tribological behavior of smooth silica particles interacting with a polymer-based viscous fluid, which was used as a stabilizing agent. The results demonstrated enhanced frictional resistance of the silica interfaces when immersed into the polymer, however, this trend was observed for relatively lower shearing velocities, implying potential rate effects. Thus, examination and modeling of larger-scale problems involving particles and viscous fluids should take into account rate effects to provide inferences on potential stabilization. Additional inferences were provided on the tribological behavior of interfaces and the role of surface textural and elastic characteristics by synthesizing literature data. • The interface behavior of silica particles immersed in polymer viscous fluid was examined. • Methocel cellulose of a given viscosity was studied. • The tribological data suggested increased frictional resistance of the particle-polymer system. • Velocity effects must be considered in modeling the response of particle-polymer interfaces. • Abrasion influences were also considered based on repeating sliding experiments. [ABSTRACT FROM AUTHOR]

We present a comprehensive investigation of the adhesive rubber-solid contact problem by analytical and numerical methods. Theories of rubber friction are reviewed and a new contact theory is developed. The adhesive contact problem is solved using the boundary element method. The introduction of adhesion leads to full coverage below a certain length scale. Friction coefficients are calculated from the spectral density of the rubber surface by splitting the numerical problem into two length scales. Adhesion is shown to increase friction at low velocities. The influence of filler is modeled by assuming that their size corresponds to a linear cross-over dimension separating the bulk elastomer from the composite. Finally, we discuss open problems and describe a simplified picture of rubber friction. • Adhesion increases low-velocity friction by increasing low-wavelength deformation amplitude. • Rubber has to be viewed as composite material at length scales relevant for friction. • Rubber's contour generally has a higher Hurst exponent than the substrate. • (Adhesion-induced) elastic instabilities probably determine friction at lowest velocities. [ABSTRACT FROM AUTHOR]

Medical-grade titanium alloys used for orthopaedic implants are at risk from infections and complications such as wear and tear. We have recently shown that hydrothermally etched (HTE) nanostructures (NS) formed on the Ti6AlV4 alloy surfaces impart enhanced anti-bacterial activity which results in inhibited formation of bacterial biofilm. Although these titanium alloy nanostructures may resist bacterial colonisation, their frictional properties are yet to be understood. Orthopaedic devices are encapsulated by bone and muscle tissue. Contact friction between orthopaedic implant surfaces and these host tissues may trigger inflammation, osteolysis and wear. To address these challenges, we performed simulation of the contact behaviour between a smooth control Ti6Al4V alloy and HTE surfaces against a hardwearing SiO 2 sphere using Atomic Force Microscopy (AFM) in Lateral Force Microscopy mode. The friction study was evaluated in both air and liquid environments at high (5 Hz) and low (0.5 Hz) scan velocities. Lower scan velocities demonstrated opposing friction force changes between the two mediums, with friction stabilising at higher velocities. The friction measured on the NS alloy surfaces was reduced by ~20% in air and ~80% in phosphate buffered saline, in comparison to the smooth control surface, displaying a non-linear behaviour of the force applied by the AFM tip. Changes in friction values and cantilever scan velocities on different substrates are discussed with respect to the Stribeck curve. Reduced friction on nanostructured surfaces may improve wear resistance and aid osseointegration. [Display omitted] • Friction was reduced on titanium alloy nanostructures compared to a flat control surface. • Lower friction on titanium alloy nanostructures in liquid fits the Stribeck curve. • Low velocity saw lower kinetic friction in air on nanostructures, the opposite in liquid. [ABSTRACT FROM AUTHOR]

More and more supershear rupture earthquakes, with rupture propagation velocity larger than the local shear wave speed, have been identified in recent years. However, the number of supershear events is scarce compared with large strike-slip earthquakes. This problem remains unresolved although several previous workers have offered some explanations. In this study, we utilize finite element method to simulate spontaneous dynamic rupture propagations in three dimensional half-space model with and without velocity strengthening friction layer (VSFL) near the Earth's free surface employing different frictional constitutive relations, including linear slip weakening law and rate- and state-dependent relations with aging form and slip form. The simulated results showed that the VSFL suppresses supershear rupture transition at the Earth's free surface occurring in the absence of a VSFL. In particular, we found that the minimum thickness of VSFL to prevent supershear rupture transition is ~1.4-km. Meanwhile, we reviewed a great deal of previous work on thickness of VSFL along mature faults in nature, and its thickness generally exceeds ~3-km, which is enough to suppress the transition to supershear rupture in the real-world. The presence of VSFL may be the primary reason why the supershear rupture earthquakes in nature are so scarce. In addition, the modelling results indicated that spatial heterogeneity of VSFL on the surface along fault strike affects the supershear rupture transition to some extent. Therefore, this study contributes to understanding the earthquake source dynamics and seismic hazard mitigation. • The minimum thickness of velocity-strengthening friction layer is ~1.4-km to suppress supershear rupture transition. • Heterogeneous distribution of velocity-strengthening layers has a blocking effect on supershear rupture transition. • Scarcity of supershear earthquakes may be due to the existence of shallow velocity-strengthening layer near the surface. [ABSTRACT FROM AUTHOR]

This paper presents results from an extensive experimental study on the rubbing behavior of labyrinth seal fins (SFs) and a honeycomb liner. The objective of the present work is to improve the understanding of the rub behavior of labyrinth seals by quantifying the effects and interactions of sliding speed, incursion rate, seal geometry, and SF rub position on the honeycomb liner. In order to reduce the complexity of the friction system studied, this work focuses on the contact between a single SF and a single metal foil. The metal foil is positioned in parallel to the SF to represent contact between the SF and the honeycomb double foil section. A special test rig was set up enabling the radial incursion of a metal foil into a rotating labyrinth SF at a defined incursion rate of up to 0.65 mm/s and friction velocities up to 165 m/s. Contact forces, friction temperatures, and wear were measured during or after the rub event. In total, 88 rub tests including several repetitions of each rub scenario have been conducted to obtain a solid data base. The results show that rub forces are mainly a function of the rub parameters incursion rate and friction velocity. Overall, the results demonstrate a strong interaction between contact forces, friction temperature, and wear behavior of the rub system. The presented tests confirm basic qualitative observations regarding blade rubbing provided in literature. [ABSTRACT FROM AUTHOR]

In this paper, we study stability of the rate, state and temperature friction (RSTF) model. The Segall and Rice approach is used to model heat transfer at the sliding interface with its surroundings. The effect of pore pressure is not considered in the model to avoid the complex expression for critical stiffness. Linear stability analysis of the spring-mass sliding system is carried out with the ageing law under the quasistatic conditions in order to determine the critical stiffness above which sliding behaviour changes from unstable to stable or vice versa. Our numerical simulations establish that critical stiffness of the heated surface may increase or decrease from corresponding to the critical stiffness of the unheated surface depending on the relative values of two contradictory parameters related with velocity effect and temperature effect. Parametric studies are also carried out to understand shear velocity and temperature of the sliding surface dependence of steady friction. The RSTF model is also used to study the gravity induced failure of a creeping rock slope and the results are justified. [ABSTRACT FROM AUTHOR]