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Research Papers: Contact Mechanics

Asperity Creep Measured by the Reflection of Ultrasound at Rough Surface Contact

[+] Author and Article Information
M. Gonzalez-Valadez

 CIATEQ, A.C., Avenida Manantiales 23-A, Parque Industrial Bernardo Quintana, Querétaro, México, C.P. 76246; Department of Mechanical Engineering, The University of Sheffield, Mapping Street, Sheffield, S1 3JD UKmiguel.gonzalez@ciateq.mx

R. S. Dwyer-Joyce

Department of Mechanical Engineering, The University of Sheffield, Mapping Street, Sheffield, S1 3JD UK

J. Tribol 131(2), 021410 (Mar 09, 2009) (8 pages) doi:10.1115/1.3089217 History: Received November 21, 2008; Revised January 27, 2009; Published March 09, 2009

The asperity contact regions in static contact are subjected to very high stress. Deformation is plastic and the material can suffer localized creep that is not usually observed at conventional stress levels. Creep of the asperity contacts causes an increase in contact area and hence an increase in the adhesive component of friction. The strains are so small compared with the bulk deflections that they are hard to measure by displacement of strain transducers. However, one measurement approach is to use the reflection of an ultrasonic pulse since this depends only on the interface behavior, specifically its stiffness. In this study, ultrasound was used to investigate the increase in interfacial stiffness with time. A power law relationship between stiffness and hold time was observed for both steel and aluminum surfaces pressed together. An analytical model that assumes a simple geometry for the contact has been developed. A single asperity was considered to determine the geometry of the whole interface by superposition. The stiffness predicted by the model was compared with experimental data and was used to determine the creep rate exponent for the material.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematically showing a representation of the ultrasonic response of rough surface contact: (a) reflection, (b) loading and deflection, and (c) the spring model representation

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Figure 2

Schematic loading-unloading cycle for a rough interface in elastic-plastic contact, (a) a loading-unloading cycle, (b) a similar cycle for a rougher contact, (c) a loading cycle carried to a higher end point, and (d) a cycle held at the maximum load point

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Figure 3

Schematic showing loading rig, specimens, and ultrasonic measuring apparatus

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Figure 4

Different aluminum grit-blasted upper specimens used in the experiments

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Figure 5

Different steel grit-blasted upper specimens used in the experiments

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Figure 6

Interface stiffness variation under constant nominal contact pressure for a steel-steel interface

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Figure 7

Interface stiffness variation under constant nominal contact pressure for an aluminum-aluminum interface

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Figure 8

Schematic of an idealized interface composed of an array of plastic hemispherical asperities

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