Apparent Friction Coefficient Research Articles

Estimation of Contact Interface between Tool and Workpiece in Cold Forming Using FEM Analysis

The effect of the flow stress of solid lubricant for cold forging on the tribological conditions was investigated using a rigid-plastic finite element method. The thickness of lubricant film decreases with decreasing flow stress of the solid lubricant and then decreases rapidly. The apparent friction coefficient also decreases with the decrease in the flow stress of the solid lubricant. The thickness of lubricant film tends to decrease with decreasing friction shear factor. When flow stress of solid lubricant is low, the thickness of lubricant film decreases remarkably with increasing tool stroke. We can observe a good correlation between the flow stress of solid lubricant, friction shear factor, minimum film thickness and apparent friction coefficient.

Multiscale Modelling of Stick-Slip Transition of Rough (Fractal) Surfaces

The apparent shear strength of rock discontinuities is lower than that of small scale samples. At the same time, the sliding behavior is characterized, in situ, by marked instabilities. Numerical algorithms permit to calculate contact forces at any point, and to describe the stick-slip transition. On the other hand, the critical aspects are not captured by classical theories. Multiscale simulations show that the contact domain between rough surfaces is a lacunar set. This explains the size-dependence of the apparent friction coefficient. By applying an increasing tangential force, the regime of partial-slip comes into play. However, the continuous and smooth transition to fullsliding predicted by the Cattaneo-Mindlin theory is not occurring in real situations. We implement a numerical renormalization group technique, taking into account the redistribution of stress consequent to partial-slip. This permits the critical value of the tangential force to be found. The critical force is less than the one predicted by Coulomb’s theory, and depends on the specimen size and on the topology of the interface.

Development of a friction model for the tool-chip-workpiece interfaces during dry machining of AISI4142 steel with TiN coated carbide cutting tools

This work deals with the identification of a new friction model for high speed dry cutting of AISI4142 steel with TiN coated carbide tools. Both experimental and numerical studies were used to find the relationship between friction coefficient, sliding velocity, contact pressure and contact temperature. A tribometer, based on a plane-sphere contact configuration, has been developed to reach extreme contact conditions similar to the ones occurring during metal cutting. The apparent friction coefficient, provided by the ratio between the tangential and normal forces, is measured. It is assumed that this coefficient can be divided into two components: an adhesive coefficient and a plastic deformation coefficient. The proportion of each component is investigated by means of a numerical model simulating the friction of a sphere with similar contact conditions.

Contact Analysis of Tire Tread Rubber on Flat Surface with Microscopic Roughness

Abstract This paper describes experimental and analytical studies of the dependence of tire friction on the surface roughness of pavement. Abrasive papers were adopted as representative of the microscopic surface roughness of pavement surfaces. The rolling∕sliding friction of tire tread rubber against these abrasive papers were measured at low slip velocities. Experimental results indicated that rolling∕sliding frictional characteristics depended on the surface roughness. In order to examine the interfacial phenomena between rubber and the abrasive papers, real contact length, partial slip, and apparent friction coefficient under vertical load and tangential force were analyzed with two-dimensional explicit finite element analysis in which slip-velocity-dependent frictional coefficients were considered. Finite element method results indicated that the sum of real contact area and local partial slip were larger for finer surfaces under the same normal and tangential forces. In addition, the velocity-dependent friction enhanced local slip, where the dependence of local slip on surface roughness was pronounced. It proved that rolling∕sliding friction at low slip ratio was affected by local frictional behavior at microslip regions at asperity contacts.

Analysis of the apparent friction of polymeric surfaces

The apparent friction coefficient is the ratio between the tangential force and the normal load applied to moving body in contact with the surface of a material. This coefficient includes a so-called “true local friction” at the interface and a “geometrical friction” which is the ploughing effect. The material underneath a moving tip may display various types of behaviour: elastic, elastic–plastic where elastic and plastic strain are present in the contact area, or fully plastic. As is usual in polymers, the material behaviour is time and temperature dependent and may exhibit strain hardening. A surface flow line model of a scratching tip which links the apparent friction to the local friction and contact geometry was recently proposed. An inverse analysis is used in the present work to estimate the local friction from the measured apparent friction and a knowledge of the contact area and tip shape. The polymer true friction coefficient displays temperature and sliding speed dependency, which may be attributed to the surface thermodynamics. It is shown that the local friction depends on the level of strain in the polymer at the contact interface.

Frictional strength heterogeneity and surface heat flow: Implications for the strength of the creeping San Andreas fault

Heat flow measurements along much of the San Andreas fault (SAF) constrain the apparent coefficient of friction (μapp) of the fault to <0.2, much lower than laboratory‐derived friction values for most geologic materials. However, heat flow data are sparse near the creeping section of the SAF, a frictional “asperity” where the fault slips almost exclusively by aseismic creep. We test the hypothesis that the creeping section has a substantially higher or lower μapp than adjacent sections of the SAF. We use numerical models to explore the effects of faults with spatially and temporally heterogeneous frictional strength on the spatial distribution of surface heat flow. Heat flow from finite length asperities is uniformly lower than predicted by assuming an infinitely long fault. Over geologic time, lateral offset from strike‐slip faulting produces heat flow patterns that are asymmetric across the fault and along strike. We explore a range of asperity sizes, slip rates, and displacement histories for comparing predicted spatial patterns of heat flow with existing measurements. Models with μapp ∼ 0.1 fit the data best. For most scenarios, heat flow anomalies from a frictional asperity with μapp > 0.2 should be detectable even with the sparse existing observations, implying that μapp for the creeping section is as low as the surrounding SAF. Because the creeping section does not slip in large earthquakes, the mechanism controlling its weakness is not related to dynamic processes resulting from high slip rate earthquake ruptures.

Rapid block glides: slide-surface fragmentation in New Zealand's Waikaremoana landslide

Physical modelling of part of prehistoric Waikaremoana landslide shows that the blockslide must have hit the valley wall at c . 40 m/s, after sliding 2 km on a 5.5–8° slope, in order to form the 150-m high mound of debris known as Raekahu. Both the blockslide and a distal rock avalanche were in simultaneous motion when the impact occurred. Finely ground rock on the slide plane suggests that a mechanism of dynamic rock fragmentation may explain the low friction necessary for acceleration to 40 m/s. When a rock particle fractures in a confined space, an isotropic dispersive pressure equal to the rock's Hugoniot elastic limit (in the GPa range) at the ambient pressure and strain rate may be exerted on its surroundings. Beneath the 275-m thick block, about one particle in 15–30 or so fragmenting at any instant (with lower density for higher rock strength at higher strain rate), could completely support the weight of the block by fragmentation pressure; but then there would be no frictional resistance (and hence no further fragmentation). Self-regulation of the process may explain the apparent coefficient of friction of c . 0.1 in the blockslide. Low friction through dynamic fragmentation may apply widely to blockslides with a basal layer of comminuted rock.

Factors affecting the pullout behaviour of extruded geogrids embedded in a compacted granular soil

In order to study the factors affecting the behaviour of reinforcement geogrids embedded in granular compacted soils, a large-scale pullout test apparatus has been designed. More than 40 pullout tests have been performed, at constant displacement rate, on three different HDPE extruded geogrids embedded in a compacted granular soil by varying the specimen lengths and the applied vertical effective pressures. The different geogrids used in the research have been tested using unconfined tensile tests performed at different speeds, and, in particular, at the same speed of the pullout tests; granular soil have been characterized through classification, Proctor and shear tests. The pullout test results showed the influence of the different parameters studied on pullout behaviour. Moreover, on the basis of the test results it was possible to evaluate the peak and the residual pullout resistance and the apparent coefficient of friction mobilized in the same conditions.

Effect of shape factor on the electrokinetic response of slot-like capillaries

The effect of the shape factor on the electrokinetic response of pressure-driven liquid flow through slot-like capillaries is analyzed in this work. The electrokinetic response is found by first solving for the electrical potential using the Poisson–Boltzmann equation, and then using it as an input to construct an external force term in the Navier–Stokes equation. It is found that flow properties are significantly modified for small pore sizes with hydraulic radius in the range of microns and submicrons, according to the properties of the electrolyte solution. The modified flow rate in the presence of such electrokinetic effects can be less than 50% of the predictions expected with conventional flow models such as Darcy's equation. Apparent viscosities larger than expected are calculated using the reduced flow. Actual values are dependent on the capillary dimensions, pressure gradient, solid-surface conductivity, and properties of the electrolyte fluid. For a rectangular capillary cross-sectional geometry the apparent viscosity and friction coefficients values decrease with the shape factor in a nontrivial way. These results are relevant for the modelling of certain porous formations of interest to the oil industry.

On the friction behaviour in nanoscratch testing

In nanoscratch testing, it is generally observed that the lateral force does not linearly increase with the applied load. In other words, the apparent friction coefficient, defined as the ratio between the lateral force and the applied load, does not have a constant value. This one may be split into two components, the adhesion coefficient and the ploughing coefficient due to the surface deformation. In the present paper, simulations of the evolution of the ploughing friction coefficient estimated from the model of a conical indenter with a blunted spherical tip taking into account the elastic recovery is compared to the experimental values of the apparent friction coefficient. The comparison of simulations and experiments shows that the non-linear behaviour of the lateral force with the applied load may be mainly explained by the fact that the indenter is not perfectly sharp but has a rounded extremity.

Study on Indentation-Sliding Contact Conditions between Semiconductor Terminal and Electrically Testing Probes

Contact conditions of the testing probes for the testing of IC electrical conductivity were investigated. A method was proposed that could clarify the conditions for designing and selecting both the optimum probe geometry and the mechanical properties of pads suitable for effectively decreasing the contact electrical resistance. The elastic deformation of contact probes and the indenting plastic deformation of the pad surface were analyzed theoretically as a coupled problem. Deformations of the pad were mainly divided into two modes depending on the relative sliding velocity: a scratch type and an indentation divergence type. When the radius at the tip of a probe was larger than the thickness of the aluminum-deposited film, the deformation tends to be an indentation divergence. The expansion of the surface area generated at the contact region (surface expansion) and the amount of swelling by bulge deformation (bulge height) were found as important factors in the design of probes. The surface expansion, which indicates the probability of the appearance of a newly generated surface due to the breaking of the insulating oxidized film, affected strongly electrical conductivity. The bulge height, together with the frictional shear factor, influenced the apparent coefficient of friction between the probe and pad. These values can provide design criteria of contact probes for achieving stable electrical conduction.

Mechanical analysis of the scratching of metals and polymers with conical indenters at moderate and large strains

Scratch test provides a convenient mean to study the surface mechanical properties and the tribological performances of materials. The representative strain of the material in this test increases with the attack angle β of the indenter and so for a conical indenter increases as its apical angle 2 θ decreases. But the mechanical analysis of this test by analytic models is very intricate. First we perform a preliminary discussion of the various aspects of the problem by considering the plane strain scratching of materials by wedges. After we present the conditions of the numerical simulations of the scratch test with conical indenters with a three-dimensional (3D) finite element code. These simulations provide the scratch geometry (contact surface, elastic recovery), the plastic strain map and the volume average plastic strain, the scratch hardness and the force ratio, the apparent friction coefficient μ 0= F t/ W. So we compare the behaviour of polymeric and metallic materials in scratch test at low and large strain and relate their difference in scratching resistance to their rheological properties. Polymers develop more higher elastic strains than metals a phenomenon which is characterised at low strain by the yield stress to Young's modulus ratio ε e= σ y/ E. For θ=70.3° where pure ploughing occurs we study the scratching of elastic perfectly plastic solids with various values of ε e under zero friction. Some comparisons with the behaviour in indentation are performed and we study the influence of friction in the scratching of workhardened steel with the same cone. At high strain the main rheological difference is the workhardening behaviour: it is described by a power law for metals and an exponential law for polymers. For θ decreasing from 70.3 to 20° we compare the behaviour of a cold worked steel to the behavour of polycarbonate, a thermoplastic polymer: a transition from ploughing to ploughing–cutting occurs only for steel.

Lubrication effect of liquid nitrogen in cryogenic machining friction on the tool-chip interface

The liquid nitrogen as an environmentally safe coolant has been widely recognized in cryogenic machining, its function as a lubricant is plausible due to its chemical inertness, physical volatility and low viscosity. Since a reduced friction is a direct witness of the lubrication effect from a tribological viewpoint, this paper presents an evaluation of the apparent friction coefficient on the tool-chip interface in cryogenic cutting operations to prove and characterize the lubricity of LN2 in cryogenic machining. The cryogenic cutting technology used in this study is based on a cooling approach and liquid nitrogen delivery system which are intended to apply liquid nitrogen in well-controlled fine jets to selectively localized cutting zones and to penetrate liquid nitrogen to the tool-chip interface. It has been found that the apparent friction coefficient can be significantly reduced in cryogenic machining, depending on the approach of liquid nitrogen delivery.

千鳥配置高カボルト摩擦接合部のすべり耐力に関する研究

This paper examines the slip strength of high strength zigzag bolted connections with base plate and splice plate material plasticity by monotonic loading. The loading experiment which made the number of bolts, base plate thickness, splice plate thickness is conducted. Slip behavior of zigzag bolted connections is compared with slip behavior of one sequence bolted connections. As a result, it is clear that, using zigzag arrangement, bolt axial force is early drawn out and an apparent friction coefficient is decreased. The slip strength model of one sequence bolted connection is applied at the slip strength of zigzag bolted connections.

A surface flow line model of a scratching tip: apparent and true local friction coefficients

The apparent friction coefficient is the ratio between the tangential force and the normal load applied to a moving scratching tip. It includes a so-called “true local” friction coefficient, which is the scission at the interface between the tip and the surface being scratched, and a “geometrical” friction coefficient, which is the plough effect due to the wave front created ahead of the moving tip and depends on the shape of the tip. Like in any mechanical test, three basic types of behaviour of the material at the interface are observed: purely elastic, elastic–plastic and fully plastic. As is usual in polymers, the material behaviour is time and temperature dependent and may exhibit strain hardening. A surface flow line model is developed here to deduce the geometrical and true friction coefficients at the interface between a moving scratching tip and a surface from the apparent friction coefficient. Using this model, several situations may be simulated to predict the influence of the geometry of the tip on the scratch resistance of the material.

Friction and orthodontic mechanics: clinical studies of moment and ligation effects

<h2>Abstract</h2> The efficiency of tooth movement associated with orthodontic sliding mechanics can be compromised by friction between arch wire and tube or bracket slot. The described studies measured the apparent coefficient of static friction (μ_a) during sliding along an arch wire. Known moments were applied to simulate tooth tipping. Measurements tested whether intraoral vibration during gum chewing reduced friction. The first experiments measured intraoral friction associated with the sliding of 4 and 8 mm stainless steel (SS) tubes along a SS auxiliary arch wire, in 10 subjects. Mean intraoral μ_a for 4 and 8 mm tubes were 0.12 (SD = 0.04) and 0.17 (SD = 0.05), respectively. Analysis of variance (ANOVA) showed that μ_a was significantly higher (<i>P</i> < 0.001) for the longer tubes. Although intraoral vibration decreased μ_a compared with bench-top tests (<i>P</i> < 0.001), frictional resistance was never completely eliminated. The second experiments studied the effects of ligation. Ten professionals performed exercises to characterize average tight and loose SS ligation forces. These ligation forces (F_{N Ligation}), and those associated with elastic ligation, were reproduced by a calibrated operator in a modified intraoral device, where SS orthodontic brackets slid along a SS auxiliary wire. Ten subjects chewed gum with the device in place. Nested ANOVA and Tukey Honest Significant Difference tests determined the effects of ligation type and environment. No significant differences (<i>P</i> > 0.01) were found between ex vivo and intraoral μ_a values for tight and loose SS ligation. Intraoral values for μ_a were significantly greater than ex vivo values (<i>P</i> < 0.001) for elastic ligation. Overall, the results suggested that vibration introduced by gum chewing did not eliminate friction. Tipping moments and ligation forces were equally significant in determining frictional forces. As well, there was considerable intraoperator variation in F_{N Ligation} for SS ligatures. Variations in clinical ligation forces are likely to be equal or greater than these experimental data and have potential to affect treatment efficiency during orthodontic sliding mechanics.

Landslide basal friction as measured by seismic waves

Dynamical predictions of landslide runout require measurements of the basal friction. Here we present the first seismically determined bounds on the frictional coefficients for three large volcanic landslides. The three landslides (Bezymianny, Russia 1956, Sheveluch, Russia 1964 and Mount St. Helens, USA 1980) have masses that vary by a factor of 5 and were all followed immediately by eruptions. We use teleseismic and regional seismic data to show that all three landslides are consistent with an apparent coefficient of friction of 0.2 which corresponds to an actual areally‐averaged frictional coefficient of 0.2–0.6. The apparent friction is independent of the quantity of hot gas subsequently released.

Pullout resistance of single and double nails in a model sandbox

Pullout tests on single and double soil nails were conducted in a model sandbox. The test parameters included variations in the surface roughness, the ratio of nail length to nail diameter, the overburden pressure, and the distance between two nails. The characteristic of a single asperity, the asperity number per unit length, and the ratio of the thread depth to the soil particle size were used to define the surface roughness factor. The test results showed that the apparent friction coefficients at the soil–nail interface were dependent upon the surface roughness of the nail. Group efficiency was used to evaluate the effectiveness of a nail when installed within a group. The test results showed that the group efficiency of a double-nail system was dependent upon the surface roughness factor and has a linear relationship with the nail distance until 100% efficiency is reached. The minimum required distance for 100% efficiency also varied with the surface roughness factor.Key words: apparent friction coefficient, group efficiency, pullout test, surface roughness factor.

A Dynamic Friction Model for Unlubricated Rough Planar Surfaces

Modeling dynamic or kinetic friction for realistic engineering surfaces continues to be a challenge, partly due to the coupling between system dynamics and interfacial forces. In this paper, a dynamic friction coefficient model for realistic rough surfaces under external normal vibrations is developed. From the system dynamic model, the instantaneous time varying normal separation at the interface is obtained under normal harmonic excitation. Subsequently, the instantaneous dynamic contact and tangential (friction) forces are calculated as a function of the instantaneous normal separation. The dynamic friction coefficient defined as the ratio of the time varying friction to the interfacial normal forces that explicitly includes interfacial damping, is also calculated. The results show that a mean increase in the instantaneous normal separation may or may not lead to a decrease of the mean friction force and the mean friction coefficient, which is supported by published data. For unlubricated elastic sliding contact conditions considered in this paper, the effect of damping on the dynamic friction coefficient is found to be negligible, whereas loss of contact causes significant apparent dynamic friction force and dynamic friction coefficient reductions. Several different interpretations of the time varying dynamic friction coefficient are presented and the implications of using a simple constant value to represent the time varying dynamic friction coefficient are discussed.

Poly(caprolactone)/silica organic-inorganic hybrids as protective coatings for poly(methyl methacrylate) substrates

Organic-inorganic hybrid materials (also known as ceramers or phase-interconnected nanocomposites) were prepared starting with tetraethyoxysilane and α,θ-triethoxysilane-terminated poly(e-caprolactone) (PCL-Si) using the sol-gel process. In all cases the formation of nanocomposites with a high level of interpenetration between PCL and SiO2 phases was noted. Poly(methyl methacrylate) (PMMA) slabs were dip-coated with PCL-Si/silica hybrids and a preferential segregation of silica on to the outer surface was found, together with a PCL-rich coating PMMA interface. In all cases, a marked reduction in both the average roughness and the apparent friction coefficient was noted with respect to the uncoated PMMA. All the PCL-Si/SiO2-coated samples showed a significant increase in flame resistance before and after UV irradiation, while improvements in the anti-scratch properties were noted only in the case of the PMMA coated with silica-rich ceramers.