Nominal Contact Area Research Articles

An Investigation Into the Thermal Behavior of the Grooved Dry Friction Clutch

The heat generated during the sliding period at the initiation of engagement in friction clutches is considered to be one of the main reasons for the failure of the friction material. One way to reduce the risk of this problem is to increase the rate of heat transfer by convection or, in other words, reduce the heat content of the friction material (internal energy) and thereby increase the lifecycle of the friction clutch. In this paper, the finite element technique has been used to study the effect of radial circumferential grooves on the temperature distribution and the amount of energy transferred by convection for a dry friction clutch disk during a single engagement, assuming a uniform distribution for the thermal load between the contact surfaces (i.e., uniform wear on clutch surfaces). Three-dimensional transient simulations are conducted to study the thermoelastic coupling of the problem. The effect of the groove area ratio (GR, defined as the groove area divided by the nominal contact area) is investigated. Furthermore, this paper presents the equations for energy considerations and energy balance at any time for the friction clutch system. The numerical results show that the amount of energy transferred by convection from the friction material can be controlled (within a limitation) by adjusting the value of the groove area ratio. Commercial ANSYS13 software has been used to perform the numerical computations in this paper.

Statistical model of nearly complete elastic rough surface contact

In the area of homogeneous, isotropic, linear elastic rough surface normal contact, many classic statistical models have been developed which are only valid in the early contact when real area of contact is infinitesimally small, e.g., the Greenwood–Williamson (GW) model. In this article, newly developed statistical models, built under the framework of the (i) GW, (ii) Nayak–Bush and (iii) Greenwood’s simplified elliptic models, extend the range of application of the classic statistical models to the case of nearly complete contact. Nearly complete contact is the stage when the ratio of the real area of contact to the nominal contact area approaches unity. At nearly complete contact, the non-contact area consists of a finite number of the non-contact regions (over a finite nominal contact area). Each non-contact region is treated as a mode-I “crack”. The area of each non-contact region and the corresponding trapped volume within each non-contact region are determined by the analytical solutions in the linear elastic fracture mechanics, respectively. For a certain average contact pressure, not only can the real area of contact be determined by the newly developed statistical models, but also the average interfacial gap. Rough surface is restricted to the geometrically-isotropic surface, i.e., the corresponding statistical parameters are independent of the direction of measurement. Relations between the average contact pressure, non-contact area and average interfacial gap for different combinations of statistical parameters are compared between newly developed statistical models. The relations between non-contact area and average contact pressure predicted by the current models are also compared with that by Persson’s theory of contact. The analogies between the classic statistical models and the newly developed models are also explored.

Tribological evaluation of porcine skin

This research studies the effects of external parameters on the friction of porcine skin. A tribometer was used to evaluate the frictional behavior of the same. The effects of DI water and body oil on porcine skin against steel and glass balls were evaluated in terms of coefficient of friction (COF). The COF dropped rapidly when DI water/body oil was introduced into the sliding system and remained stable when the volume of the liquid exceeded a certain value. The COF increased with increasing sliding speed under dry conditions and decreased in wet. Under an increasing normal force, the COF decreased regardless of the presence of liquid. The ratio of the real contact area to the nominal contact area of the skin with the steel/glass ball was found to increase with a power law as the applied force was increased. These results reveal basic tribological properties of the skin in contact with a hard slider. These properties could be used as reference for the design and development of artificial skin in prosthetic applications.

Thermal interface resistance: cross-over from nanoscale to macroscale

In a recent work Gotsmann and Lantz have observed that the thermal interface conductivity for a nano-sized nominal contact area is proportional to the normal contact pressure in a wide pressure range, with a prefactor which is typically ∼103 higher than observed for the same materials at the macroscale. Here I discuss the cross-over from the nanoscale to the macroscale. I show that for macroscopic solids the spreading resistance will dominate the interfacial resistance in most cases.

Influence of Vibration Time and Frequency on Surface Finishing Using Vibration-Assisted Micro-Forging

Vibration-assisted micro-forging was proposed for metal foil surface finishing. The mechanism was investigated by analysis of strain, surface roughness, microhardness, real / nominal contact area ratio and forming work at different vibration time and frequencies. Results show that vibration time and frequency influence the surface deformation by means of real contact area and forming work accordingly.

Static or breakloose friction for lubricated contacts: the role of surface roughness and dewetting

We present experimental data for the static or breakloose friction for lubricated elastomer contacts, as a function of the time of stationary contact. Due to fluid squeeze-out from the asperity contact regions, the breakloose friction force increases continuously with the time of stationary contact. We consider three different cases: (a) PDMS rubber balls against flat smooth glass surfaces, (b) PDMS cylinder ribs against different substrates (glass, smooth and rough PMMA and an inert polymer) and (c) application to syringes. Due to differences in the surface roughness and contact pressures the three systems exhibit very different time dependences of the breakloose friction. In case (a) for rough surfaces the dry contact area A is a small fraction of the nominal contact area A0, and the fluid squeeze-out is fast. In case (b) the dry contact area is close to the nominal contact area, A/A0 ≈ 1, and fluid squeeze-out is very slow due to percolation of the contact area. In this case, remarkably, different fluids with very different viscosities, ranging from 0.005 Pa s (water–glycerol mixture) to 1.48 Pa s (glycerol), give very similar breakloose friction forces as a function of the time of stationary contact. In case (c) the contact pressure and the surface roughness are larger than in case (b), and the squeeze-out is very slow so that even after a very long time the area of real contact is below the percolation threshold. For all cases (a)–(c), the increase in the breakloose friction is mainly due to the increase in the area of real contact with increasing time, because of the fluid squeeze-out and dewetting.

Sliding friction of wiper blade: Measurement, FE modeling and mixed friction simulation

In this paper, the dry and wet friction of a commercial wiper blade is investigated. The effect of load and velocity on sliding friction is studied at specimen-level experimentally, the material response is characterized by DMA tests, while the contact behavior is analyzed by plain strain FE models. Holding the normal force constant the numerical analysis makes it possible to follow the variation of interference between the wiper blade and glass surface, the change of friction force and contact pressure distribution, as well as the variation of nominal contact area during the transition from wet to dry friction. As a last step, the steady-state elastohydrodynamic lubrication problem of wiper blade/glass contact is solved by using a recent mixed friction model.

Evolution of ultrasonic velocity and dynamic elastic moduli with shear strain in granular layers

Ultrasonic wave transmission has been used to investigate processes that influence frictional strength, strain localization, fabric development, porosity evolution, and friction constitutive properties in granular materials under a wide range of conditions. We present results from a novel technique using ultrasonic wave propagation to observe the evolution of elastic properties during shear in laboratory experiments conducted at stresses applicable to tectonic faults in Earth’s crust. Elastic properties were measured continuously during loading, compaction, and subsequent shear using piezoelectric transducers fixed within shear forcing blocks in the double-direct-shear configuration. We report high-fidelity measurements of elastic wave properties for normal stresses up to 20 MPa and shear strains up to 500 % in layers of granular quartz, smectite clay, and a quartz-clay mixture. Layers were 0.1–1 cm thick and had nominal contact area of \(5 \mathrm{cm} \!\times \! 5 \mathrm{cm}\). We investigate relationships among frictional strength, granular layer thickness, and ultrasonic wave velocity and amplitude as a function of shear strain and normal stress. For layers of granular quartz, P-wave velocity and amplitude decrease by 20–70 % after a shear strain of 0.5. We find that P-wave velocity increases upon application of shear load for layers of pure clay and for the quartz-clay mixture. The P-wave amplitude of pure clay and quart-clay mixtures first decreases by \(\sim \)50 and 30 %, respectively, and then increases with additional shear strain. Changes in P-wave speed and wave amplitude result from changes in grain contact stiffness, crack density and disruption of granular force chains. Our data indicate that sample dilation and shear localization influence acoustic velocity and amplitude during granular shear.

Wear Simulation of Metal-on-Metal Hip Replacements With Frictional Contact

Preclinical wear evaluation is extremely important in hip replacements, wear being one of the main causes of failure. Experimental tests are attractive but highly cost demanding; thus predictive models have been proposed in the literature, mainly based on finite element simulations. In such simulations, the effect of friction is usually disregarded, as it is considered not to affect the contact pressure distribution. However, a frictional contact could also result in a shift of the location of the nominal contact area, which can thus modify the wear maps. The aim of this study is to investigate this effect in wear prediction for metal-on-metal implants. Wear assessment was based on a purpose-developed mathematical model, extension of a previous one proposed by the same authors for metal-on-plastic implants. The innovative aspect of the present study consists in the implementation of a modified location of the nominal contact point due to friction, which takes advantage of the analytical formulation of the wear model. Simulations were carried out aimed at comparing total and resurfacing hip replacements under several gait conditions. The results highlighted that the adoption of a frictional contact yields lower linear wear rates and wider worn areas, while for the adopted friction coefficient (f=0.2), the total wear volume remains almost unchanged. The comparison between total and resurfacing replacements showed higher scaled wear volumes (wear volume divided by wear factor) for the latter, in agreement with the literature. The effect of the boundary conditions (in vivo versus in vitro) was also investigated remarking their influence on implant wear and the need to apply more physiological-like conditions in hip simulators. In conclusion although friction is usually neglected in numerical wear predictions, as it does not affect markedly the contact pressure distribution, its effect in the location of the theoretical contact point was observed to influence wear maps. This achievement could be useful for increasing the correlation between numerical and experimental simulations, usually based on the total wear volume. In order to improve the model reliability, future studies will be devoted to implement the geometry update by combining the present model to finite element analyses. On the other hand, further experimental investigations are required to get out from the wide dispersion of wear factors reported in the literature.

Modeling and verification of ultra-high molecular weight polyethylene wear in multi-directional sliding

In a separate paper in this volume, the authors have discovered that the total wear of ultra-high molecular weight (UHMWPE) polyethylene moving along a circular path is directly proportional to the number of motion cycles and independent of the sliding distance per cycle. This is in conflict with common tribological principles particularly the Archard Equation that states that wear is proportional to load and sliding distance. The authors have also discovered that when the wear rate is defined as the volume loss per unit sliding distance (mm3/mm) rather than volume loss per cycle (mm3/cycle) it is directly proportional to the curvature (k) or the inverse of the sliding circle radius (1/r). Curvature is the mathematical equivalence to “cross-shear”. The present study attempts to quantify the effects of the “cross-shear” motion, sliding distance, load and contact area on the wear of UHMWPE both theoretically and experimentally. The theoretical approach starts with a single postulate that wear is proportional to the frictional work input multiplied by a “cross-shear” factor—sliding path curvature. For an UHMWPE with a nominal contact area A moving along a circular path l (l=2πr, r is the radius or 1/r is the curvature) under a normal load W, the volumetric wear rate per cycle of motion (ΔV/N) is then deduced, which follows a simple equation: ΔV/N=KW2/3A1/3or ΔV=KW2/3A1/3N. A comprehensive set of experiments were carried out on a hip joint simulator with UHMWPE having different nominal contact areas (A) and under different peak loads (W). The relationship between wear rate and contact area and load was verified with near-perfect accuracy.

Measurement and Scaling of Hydrodynamic Interactions in the Presence of Draining Channels

Central to the adhesion and locomotion of tree frogs are their structured toe pads, which consist of an array of 10 μm hexagonal epithelial cells separated by interconnected channels that are 1 μm wide and 10 μm deep. It has been proposed that the channels facilitate the drainage of excess fluid trapped between the toe pads and the contacting surface, and thus reduce the hydrodynamic repulsion during approach. We performed direct force measurement of the normal hydrodynamic interactions during the drainage of fluid from the gap between a structured and a smooth surface using surface force apparatus. The structured surface consisted of a hexagonal array of cylindrical posts to represent the network of interconnected channels. The measured hydrodynamic drainage forces agree with the predictions from Reynolds' theory for smooth surfaces at large separations. Deviations from theory, characterized by a reduction in the hydrodynamic repulsion, are observed below some critical separation (h(c)), which is independent of drive velocity. We employ a scaling analysis to establish the relationship between structural features (channel depth, width, and post diameter) and the critical separation for the onset of deviations. We find agreement between our experiments and the scaling analysis, which allows us to estimate a characteristic length scale that corresponds to the transition from the fluid being radially squeezed out of the nominal contact area to being squeezed out through the network of interconnected channels.

The Fluid Mechanics of Slag-Metal Interactions in Ladle Metallurgy

In Ladle Metallurgy operations following steelmaking, the control of the interactions between slag and metal is crucial for the production of high-quality steel. While the chemical aspects of the refining have been well studied, the fluid dynamic aspects have received less attention. Ladles are often gas-stirred by porous plugs pushing the slag to the periphery of the ladle and opening “eyes” of steel exposed to the atmosphere. The nominal slag-metal contact area is reduced, but there may be entrainment of slag in metal and vice versa which increases the effective interfacial area. Experiments were conducted with water models and the work of others in metallic systems has been included in the study. The eye size has been found to be quite different for thin and thick slag layers, and mathematical models of the controlling factors for the eye size has been determined for these two regimes. The models show that the important factor is the balance between the inertia from the gas-liquid plume and height of the slag, so the governing equations are based on Froude number control. The conditions for closure of the eye have been identified and mathematically modeled. At higher gas flow rate, droplets of slag are entrained into metal which is quantitatively linked to Kelvin-Helmholtz instability. The role that physical properties of the slag play in these phenomena is discussed.

The contact problem for a periodic cluster of microcontacts

Abstract The three-dimensional contact problem of the linear theory of elasticity is investigated for a system consisting of a large number of small punches, periodically arranged within a limited area on the boundary of an elastic semi-infinite body. The problem is investigated by a method which combines the averaging method and the method of matched asymptotic expansions. It is assumed that the ratios of the diameters of the actual contact areas to the distance between them are small, and each such ratio for neighbouring contact areas is in proportion to the ratio of the dimension of the periodic cell to the diameter of the nominal contact area. The asymptotic form of the doubly periodic contact problem for an elastic half-space is constructed. An approximate solution, in explicit form, is constructed for circular and elliptic contact areas. In the first case, the results agree with the well-known solution in the literature, obtained by another method.

Determination of flow factors for the semi-analytical prediction of friction coefficients

Friction conditions in sheet metal forming are mainly influenced by the surface topography of blank and tool, its mechanical properties and the properties of the intermediate medium, the lubricant. Aiming towards an analytical determination of friction coefficients for sheet metal forming, such factors should be included in suitable models and be weighted accordingly. In addition to the elastic–plastic deformation of the surface topography of the blank as a result of increasing nominal surface pressure, the influence of the lubricant can be considered using the Reynolds equation. In the present investigation, the flow factors of the elasto-hydrodynamic lubrication and mixed friction, which take into account the effects of surface topography and orientation, were calculated as a function of nominal surface pressure and nominal contact area in terms of plastically deformed surfaces. Asperity deformation is evaluated with a numerical contact model using the flow curve of base metal to calculate local contact forces.

Investigation of friction hysteresis using a laboratory-scale tribometer

The current paper addresses the characterization of dynamic friction by using a laboratory-scaletribometer. A special post-processing script in MatLab has been developed in order to analyse the datafrom the experiments. A sine wave signal for the velocity is imposed, with three different frequencies and,consequently, acceleration and deceleration rates. A friction material from brakes, with nominal contactarea of 254 mm², was subjected to sliding against a commercially available brake disc (gray cast iron, diameter of 256 mm). Some technical details and adjustments from the designed tribometer are showed and the results from the experiments are discussed. A friction hysteresis has been observed for all experimental curves, which exhibit loops in elliptical shape. A negative slope has been encoThe current paper addresses the characterization of dynamic friction by using a laboratory-scaletribometer. A special post-processing script in MatLab has been developed in order to analyse the datafrom the experiments. A sine wave signal for the velocity is imposed, with three different frequencies and,consequently, acceleration and deceleration rates. A friction material from brakes, with nominal contactarea of 254 mm², was subjected to sliding against a commercially available brake disc (gray cast iron,diameter of 256 mm). Some technical details and adjustments from the designed tribometer are showedand the results from the experiments are discussed. A friction hysteresis has been observed for allexperimental curves, which exhibit loops in elliptical shape. A negative slope has been encountered for thecurves when the imposed frequency is 1 Hz and 2 Hz, while for the highest frequency (4 Hz) the slope ispositive. The laboratory-scale tribometer, associated to the post-processing stage, is capable tosuccessfully be used to characterize friction hysteresis effect. untered for the curves when the imposed frequency is 1 Hz and 2 Hz, while for the highest frequency (4 Hz) the slope is positive. The laboratory-scale tribometer, associated to the post-processing stage, is capable to successfully be used to characterize friction hysteresis effect.

The influence of manufacturing method on the running-in of gears

Striving for higher gear transmission efficiency by using a low-viscosity transmission lubricant affects the lubricant film thickness. To keep the Λ-ratio (the ratio between the film thickness and the surface roughness) intact, more effort must be put on the surface topography. This paper presents a study of how running-in affects the dry elastic contact area ratio in spur gears using real surface topographies from three common manufacturing methods (green-shaving, honing, and grinding). The test gears were manufactured in case-hardened steel, similar to 20MnCrS5. Surface topography measurements were used as input to a contact analysis boundary element software program. An important hypothesis used in this work is that the dry elastic contact area ratio, i.e. the ratio between real and nominal contact area, can be used as a measure of the contact conditions in gears. It is concluded that running-in differs significantly for the studied manufacturing methods and that the dry elastic contact area ratio gives a consistent description of these changes. The shaved gears have the highest dry elastic contact area ratio after running in, where the ground gears have the lowest dry elastic contact area ratio. The increase in dry elastic contact area ratio is about 30 per cent for the shaved gears, 12 per cent for the honed gears and less than 5 per cent for the ground gears. Extreme value parameters, such as Rp and Rz, showed relatively good correlation to dry elastic contact area ratio. However no genuine correlation could be found between dry elastic contact area ratio and two-dimensional (2D) surface roughness parameters. Furthermore, traditional gear metrology form parameters do not serve as a good measure on the changes occurring during the running-in.

Numerical analysis on the clamping reliability of fixture–workpiece facility considering partial slip

Fixture is a mechanical component to clamp a workpiece and to move it to a specific position. This paper analyses partial slip at the fixture–workpiece interface and addresses the effects of design parameters and working conditions of a fixture–workpiece facility on clamping reliability. A stick ratio, defined as the ratio of the stick area to the entire nominal contact area, is introduced in this study to characterize the clamping reliability. If the value of stick ratio equals to one, the clamping is most reliable, but when it drops to zero, a gross sliding will occur, which means clamping failure. This paper investigates the partial slip phenomenon by solving three-dimensional contacts between elastically dissimilar materials, using the conjugate gradient method (CGM) and discrete convolution-fast Fourier transform (DC-FFT) techniques, but focuses on its industrial applications. Results have demonstrated how the inferential factors, such as material property, normal and tangential loads, operational parameters, and geometric features of the fixture, would affect the clamping reliability, which can be used for guiding the design of the fixture, for example, the choice of material, friction coefficient, fixture’s opening angle, acceleration in manipulation, and so on.

Investigation of the Friction Variation with Sliding which is Commonly Observed in Individual Fretting Test Cycles

Many researchers that have carried out fretting wear tests have presented results which show a variation (usually an increase) in the friction force during the sliding phase of individual cycles. This phenomenon is contrary to the Amontons/Coulomb model of friction which predicts a constant friction force as sliding proceeds. An in-line fretting test involving an abrupt increase in amplitude was used to show that the effect is a result of wear scar interaction effects. A pair of rotational fretting tests were then carried out to determine whether these interaction effects originate from interaction of the wear scar ends, or whether such interaction occurs throughout the nominal contact area. A new experimental rig designed to adapt conventional in-line test machines for rotational fretting was designed and built for this purpose. Results show that the friction variation occurs whether or not wear scar ends are present. After testing, the worn surface topography of each fretting pair was scanned using a focus variation microscope, and these surface images revealed the existence (and size) of many distributed local peaks and troughs distributed throughout the contact area. In conclusion, the friction variation commonly observed in the literature arises predominantly from the interaction of local wear scar features distributed over the contact region.

Effects of selected nanoadditives on the friction and wear performance of carbon–carbon aircraft brake composites

Abstract Nanomaterials are being produced in large volumes for the development of advanced new materials. Composites can easily make use of the nanomaterials because of the small size of the particle. With the large surface area of nanomaterials very small quantities are needed to observe significant friction altering effects as observed by numerous researchers. Specifically, when the nanomaterials become part of the friction layer they will increase the nominal area of contact, the mechanical properties and typically increase or stabilize the coefficient of friction and reduce wear. Nanomaterials were added (nanoadditives) to commercial C/C composites using ultrasonics attempting to fill in some of the open porosity. The nanoadditives used were Si, SiC, and SWCNT's. Two different concentrations of each nanoadditive in isopropyl alcohol were attempted. A total of 6 pairs of nanoadditive disks and 1 baseline pair were made. The finished 2.75 in. (69.85 mm) diameter disks were tested on a LINK ring-on-ring subscale aircraft brake dynamometer. A series of braking engagements simulating an aircraft taxiing to the runway, landing, and taxiing back to the terminal were performed totalling 50 landings. Wear was measured as mass and thickness changes of the original disks. It was found that Si and SiC had significant effects on the friction performance in the “hot” taxi after a landing with very small weight percentages. The presence of Si led to chemical changes of the friction layer and the formation of carbides. These changes would occur when the interface reaches a temperature high enough to activate the chemical process. As a consequence, the dominating oxidative wear mechanism changed to mild abrasion. Friction and wear performance is shown and discussed. Analysis of the friction layer was performed using SEM and polarized light microscopy and is presented.

Investigation of the dynamic response in a dry friction process using a rotating stick–slip tester

In this paper, the friction stability of brake materials is investigated by means of a rotating stick–slip tester. Experimental results are compared with a computational model implemented in Simulink. The main goal of this work is to find out how vibrations induced by friction are related to mechanical system parameters such as inertia (mass), stiffness, damping and sliding speed. At the same time, a computational model is developed in order to analyse the dynamic response of a dry friction process at different conditions. A friction sample with nominal contact area of 254 mm2 was subjected to sliding against a gray cast iron disk. The sliding speed was varied between 0.28 and 10 mm/s, while the normal load was kept constant (400 N, equivalent nominal pressure 1.57 MPa). Moreover, tests were carried out in three different damping values and two different stiffnesses. Experimental results show typical stick–slip behaviour for the lowest speeds while for the highest speeds stick–slip disappears and smooth sliding prevails. The computational model can be used to evaluate the dynamic response in dry friction process, as far as static/dynamic coefficient of friction of the mating materials are known as well as the mechanical characteristics of the system.