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Elastohydrodynamic Lubrication

Measured and Predicted Static Friction for Real Rough Surfaces in Point Contact

[+] Author and Article Information
Jose M. García

Armour College of Engineering,  Illinois Institute of Technology, Chicago, IL 60616

Ashlie Martini

School of Engineering,  University of California, Merced, CA 95343

J. Tribol 134(3), 031501 (Jun 12, 2012) (8 pages) doi:10.1115/1.4006917 History: Received August 25, 2011; Revised May 22, 2012; Published June 12, 2012; Online June 12, 2012

A numerical model to predict static friction for metallic point contacts was developed and validated by comparison to experimental measurements using a specially designed test rig. Key aspects of the numerical model were the incorporation of a digitized real rough surface profile, application of discrete convolution fast Fourier transform (DC-FFT) to predict local asperity interference, and modification of the yield strength to capture the effect of cold hardening. It was found that these model features are critically important to quantitative prediction of static friction. The model significantly underestimated the static friction coefficient if randomly generated surfaces having statistical parameters the same as the measured rough surface were used; digitized real rough surfaces enabled accurate predictions. Further, the model was able to describe the static friction of worn surfaces after cold hardening was introduced through modification of material yield strength. This work illustrates the importance of incorporating the surface features and the change of those features with wear to accurately and reliably predict static friction.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Graphical representation of an asperity and its relation to a discrete rectangular element

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Figure 2

Graphical representation of an idealized rough surface (asperities represented as spheres of the same radius but different heights) against a smooth flat

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Figure 3

Simulated point contact of two rough surfaces

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Figure 4

Asperity deformation regimes

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Figure 5

Estimation of the surface shear stress for a 52100 stainless steel sphere on a 304 stainless steel flat

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Figure 6

Test set up schematic; inset is a photograph of the contacting spheres

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Figure 7

Experimental measurement of the static friction coefficient at a 50 N normal force

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Figure 8

Initial (left) and final (right) surface maps of the test surfaces

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Figure 9

Experimental and simulation results of the static friction coefficient for a metallic point contact

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Figure 10

Simulated progression of the static friction coefficient with cold hardening (Y.S. 3x indicates three times the yield strength and Y.S. 5x indicates five times the yield strength)

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Figure 11

Model-predicted static friction coefficient using the digitized real surface profile and three sets of randomly generated profiles with the same statistical parameters

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Figure 12

Comparison of the static friction coefficient predictions using the CEB, KE, and present model and the experimental values

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Figure 13

Elastic-plastic and plastic limits for a sphere contact simulation

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