Development and assessment of atomistic models for predicting static friction coefficients

2016 ◽  
Vol 94 (7) ◽  
Author(s):  
Soran Jahangiri ◽  
Gavin S. Heverly-Coulson ◽  
Nicholas J. Mosey
Keyword(s):  
Static Friction ◽  
Friction Coefficients ◽  
Atomistic Models

Friction Coefficients Measured at Lubricated Planar Contacts During Start-Up

1988 ◽  
Vol 110 (3) ◽  
pp. 533-538 ◽  
Author(s):  
E. H. Gassenfeit ◽  
A. Soom
Keyword(s):  
Static Friction ◽  
Transient Behavior ◽  
Molybdenum Disulphide ◽  
Friction Behavior ◽  
Friction Characteristic ◽  
Friction Coefficients ◽  
Motion Data ◽  
Start Up ◽  
Planar Contact ◽  
Lubrication Conditions

Measurements of instantaneous coefficients of friction and associated motions during start-up at a planar contact are presented for four different lubrication conditions. The various patterns of transient behavior are discussed. Difficulties in interpreting static friction coefficients during rapidly applied tangential loads are described in relation to the motion data. It is shown that a molybdenum disulphide grease yields a friction characteristic that is quite different from either dry or boundary lubricated conditions in the presence of liquid lubricants. Transition distances from a static or maximum initial friction to kinetic conditions are examined and found to be considerably longer than had been previously found for concentrated contacts. Some suggestions regarding future studies of unsteady friction behavior are made.

Novel Formulation of the Tightening and Breakaway Torque Components in Threaded Fasteners

2006 ◽  
Author(s):  
Sayed A. Nassar ◽  
Xianjie Yang
Keyword(s):  
Contact Pressure ◽  
Static Friction ◽  
Bolted Connections ◽  
Friction Coefficients ◽  
Difference Analysis ◽  
Threaded Fasteners ◽  
Thread Profile ◽  
Percent Difference ◽  
Breakaway Torque ◽  
Torque Values

New formulas are developed for the torque-tension relationship, various torque components, and for the break-away torque values in threaded fastener applications. The 3-D aspects of the lead helix and thread profile angles, the kinetic and static friction coefficients are all taken into account. Two scenarios of the contact pressure between threads and under the turning fastener head are considered; namely, uniform distributed and linearly distributed contact pressure scenarios. The effect of thread pitch, lead helix and thread profile angles, friction coefficients, and the fastener geometry is discussed. Results from the new formulas are compared with the approximate torque-tension relationship provided in the literature. A percent difference analysis indicates that the new formulas provide significant improvement that would enhance the reliability and safety of bolted connections, especially in critical applications.

Lubricant Performance in Magnetic Thin Film Disks With Carbon Overcoat—Part I: Dynamic and Static Friction

1991 ◽  
Vol 113 (1) ◽  
pp. 22-31 ◽  
Author(s):  
J. L. Streator ◽  
B. Bhushan ◽  
D. B. Bogy
Keyword(s):  
Thin Film ◽  
Film Thickness ◽  
Static Friction ◽  
Disk Surface ◽  
Stick Slip ◽  
Sliding Speed ◽  
Friction Coefficients ◽  
Friction Forces ◽  
Strong Adhesion ◽  
Lubricant Viscosity

Static and dynamic friction coefficients are presented for an Al2O3·TiC slider in contact with 130 mm carbon-coated rigid thin film disks lubricated with several different perfluoropolyether lubricants. The lubricants tested include three nonpolar liquid lubricants and one polar liquid lubricant with dihydroxyl end groups. The effects of lubricant film thickness, disk surface topography, sliding speed and lubricant viscosity are investigated. In many cases, the interfaces exhibited a sharp increase in the dynamic and static friction coefficients after a certain film thickness was reached, due to strong adhesion in the interface. In most cases, the lubricant thickness for the onset of high friction forces was found to increase with increasing disk surface roughness, lubricant viscosity and sliding speed. Under certain conditions stick/slip of the slider occurred during which the static friction increased with time of contact. The various data suggest that the rate at which strong adhesion develops depends on the lubricant viscosity.

Static Friction and Initiation of Slip at Magnetic Head-Disk Interfaces

1999 ◽  
Vol 122 (1) ◽  
pp. 246-256 ◽  
Author(s):  
S. Wang ◽  
K. Komvopoulos
Keyword(s):  
Surface Roughness ◽  
Friction Coefficient ◽  
Film Thickness ◽  
Contact Resistance ◽  
Friction Force ◽  
Static Friction ◽  
Electric Contact ◽  
Magnetic Head ◽  
Friction Coefficients ◽  
Lubricant Films

The apparent friction force and electric contact resistance at the magnetic head-disk interface were measured simultaneously for textured and untextured disks lubricated with perfluoropolyether films of different thicknesses. The initial stick time, representing the time between the application of a driving torque and the initiation of interfacial slip, was determined based on the initial rise of the apparent friction force and the abrupt increase of the electric contact resistance. Relatively thin lubricant films yielded very short initial stick times and low static friction coefficients. However, for a film thickness comparable to the equivalent surface roughness, relatively long initial stick times and high static friction coefficients were observed. The peak value of the apparent friction coefficient was low for thin lubricant films and increased gradually with the film thickness. The variations of the initial stick time, static friction coefficient, and peak friction coefficient with the lubricant film thickness and surface roughness are interpreted in the context of a new physical model of the lubricated interface. The model accounts for the lubricant coverage, effective shear area, saturation of interfacial cavities, limited meniscus effects, and the increase of the critical shear stress of thin liquid films due to the solid-like behavior exhibited at a state of increased molecular ordering. [S0742-4787(00)03101-5]

Assessment of shoe-floor slipperiness with respect to contact-time-related variation in friction during heel strike

2003 ◽  
Vol 3 (4) ◽  
pp. 197-208
Author(s):  
Raoul Grönqvist ◽  
Simon Matz ◽  
Mikko Hirvonen
Keyword(s):  
Risk Assessment ◽  
Contact Time ◽  
Static Friction ◽  
Heel Strike ◽  
Time Intervals ◽  
Friction Coefficients ◽  
Low Viscosity ◽  
Steady State Kinetic ◽  
Heel Contact

The variation in utilized and available friction over shoe-floor contact time was determined in the presence of high- and low-viscosity contaminants. The objectives were to improve the validity of slipperiness evaluations and to find better criteria for safe friction during heel strike. The utilized friction coefficients for six shoe types were determined during gait-trials with male participants. The available friction coefficients of these shoes were measured with a test rig simulating heel slipping. The experiments were performed on a stainless steel floor with concentrated glycerol ('oily' condition) and diluted glycerol (1:10 in water) as contaminants. It was hypothesized that any single friction measurement criterion would be an insufficient predictor for safe gait with no slip or with slip recovery, not leading to a fall. The results showed that both transitional friction (time-intervals from zero to about 250 ms of heel contact) and steady state kinetic friction (time-intervals from about 250 ms to 450 ms) properties in the shoe and floor interface play an important role in slipperiness measurement and slip/fall risk assessment. The role of static friction in the risk assessment remained unclear.

INFLUENCE OF SURFACE LASER TEXTURING ON FRICTION AND WEAR OF PTFE DURING SLIDING AGAINST STEEL

Tribologia ◽  
2018 ◽  
Vol 281 (5) ◽  
pp. 27-31
Author(s):  
Piotr KOWALEWSKI ◽  
Tomasz GAŁEK
Keyword(s):  
Microscopic Observation ◽  
Friction And Wear ◽  
Static Friction ◽  
Linear Wear ◽  
Apparent Effect ◽  
Laser Texturing ◽  
Friction Coefficients ◽  
Plate Test ◽  
Surface Laser ◽  
Friction Surfaces

The paper presents research and analysis of PTFE wear during cooperation with steel. The surface of steel elements was subjected to laser texture. Three different patterns of texture were applied to the surface of steel elements. For comparative purposes, an element without a structure was also tested. The experiment was carried out in a roller-plate test on the author’s research stand. As part of the conducted tests, the linear wear of PTFE plates was determined, and the values of the kinetic and static friction coefficients of the tested friction pairs were determined. Steel friction surfaces were also subjected to SEM microscopic observation. The obtained results showed an apparent effect of laser texture on changes in friction and wear of PTFE. This influence is different and depends on the pattern of the texture.

Bridging Long- and Short-term Behavior Shows Fault Strength as a Strain-average

2020 ◽  
Author(s):  
Ylona van Dinther
Keyword(s):  
Subduction Zones ◽  
Large Scale ◽  
Dominant Role ◽  
Mechanical Energy ◽  
Fluid Pressure ◽  
Static Friction ◽  
Short Term ◽  
Friction Coefficients ◽  
Fault Strength

<p><span>The strength of faults is subject of an important debate throughout various Earth Scientific disciplines. Different scientific communities have different perspectives with respect to appropriate values for friction coefficients μ. Geodynamicists with a long-term perspective require very low effective strengths (μ<0.05), while at the same time realizing mountains need to be sustained as well. Geologists and seismologists typically start from Byerlee friction coefficients of 0.6<μ<0.85, whereas rock mechanics experiments at high seismic slip rates show short-term low dynamic friction values of 0.03<μ<0.3. Here I show that both long- and short-term approaches can be made more compatible through considering that a regional or global frictional strength should be approached as a strain-averaged quantity. Doing this accounts for large variations of strain in both time and space. What matters for large-scale models is that most deformation occurs over a very small space and time during which friction is exceptionally low, thus making the representative long-term strength low. This is supported by seismo-thermo-mechanical models that self-consistently simulate the dynamics of both long-term subduction and short-term seismogenesis. The latter sustain mountain building, while representative earthquake-like events occur on faults with pore fluid pressure-effective static friction coefficients between 0.125 and 0.005 (or 0.75<Pf/Ps<0.99). These low friction values suggest faults are weak and suggest the dominant role of fluid pressures in weakening faults in subduction zones. This is confirmed in analytical considerations based on mechanical energy dissipation, which provide an equation to calculate the long-term fault strength as a strain-average quantity. Constraining the four parameters in this equation by observations confirms that fluid weakening is more important for long-term weakening than dynamic frictional weakening and low static friction coefficients. From the short-term perspective of modeling earthquake rupture dynamics it is now also becoming evident that fluid overpressured faults are preferable. They namely facilitate the incorporation of laboratory-observed dynamic weakening (70-90%) by limiting the stress drop to reasonable values. In summary, this cross-scale perspective supports long-term effective friction values in the range of about 0.03 to 0.2.</span></p>

Static friction coefficients for mechanical and structural joints

Wear ◽  
1974 ◽  
Vol 28 (1) ◽  
pp. 1-13 ◽  
Author(s):  
H. Nolle ◽  
R.S.H. Richardson
Keyword(s):  
Static Friction ◽  
Friction Coefficients ◽  
Structural Joints
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