iMechanica - Comments for "Frictional energy dissipation in contact of nominally flat rough surfaces under harmonically varying loads"
https://imechanica.org/node/11246
Comments for "Frictional energy dissipation in contact of nominally flat rough surfaces under harmonically varying loads"enbut the enigma remains! Any suggestions?
https://imechanica.org/comment/17537#comment-17537
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<p><em>In reply to <a href="https://imechanica.org/node/11246">Frictional energy dissipation in contact of nominally flat rough surfaces under harmonically varying loads</a></em></p>
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Although the analytical solution for halfspace contact problem under harmonic loading (normal and tangential), is simplifying considerably the Mindlin solution, the result is also negative.
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Indeed, we also found that any halfspace geometry including roughness or any other geometrical effect, and not just Hertzian geometry studied by Johnson in 1961, would show cubic dependence of the energy dissipation on tangential load amplitude (even when normal load is changing), so the "challenge" to justify the non-cubic dependence seen in all experiments (power law is usually between 2 and 3) cannot be justified within this model.
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Hence, the enigma originally noted by Johnson in 1961, and later observed by others and never explained, remains.... Room for brigth and energetic young people to think about it!
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<p>Johnson,K.L.,1961. Energy dissipation at spherical surfaces in contact transmitting oscillating forces. J. Mech. Eng.Sci.3,362–368. </p>
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</ul>Fri, 14 Oct 2011 14:22:13 +0000Mike Ciavarellacomment 17537 at https://imechanica.orgalso in a seminar at Caltech by Jim Barber October 18
https://imechanica.org/comment/17520#comment-17520
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<p><em>In reply to <a href="https://imechanica.org/node/11246">Frictional energy dissipation in contact of nominally flat rough surfaces under harmonically varying loads</a></em></p>
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<span class="h2">October 18, 2011</span>, <span class="h2">Tuesday at 11:00 a.m.</span> <br /><strong>James R. Barber, University of Michigan, Ann Arbor, Michigan <br /></strong><span class="h3">Roughness, Fractality, Contact and Friction<br /></span>The multiscale features of rough surfaces lead to approximately linear relations between macroscale physical quantities, such as normal force, electrical contact conductance, friction force, etc., even when the microscale relations between these quantities are highly non-linear. Surfaces are often found to be quasi-fractal at fine scales, in which case the resulting contact problem is best approached by considering the incremental change in the statistical distribution of (for example) contact pressure when the high frequency cutoff is slightly extended, and then using inductive arguments. The parameters of macroscale phenomena are generally determined by deviations from fractality, or by the occurrence of a length scale in the governing physical laws such as the constitutive equation. Some important quantities, including incremental stiffness and electrical contact resistance, are determined by the coarse scale ‘roll-off’ of the power spectral density, and rigorous bounds can then be established using relatively unsophisticated numerical models. By contrast, although multiscale arguments provide a plausible ‘explanation’ of Coulomb’s law of friction, the friction coefficient is determined at the fine scale<br />
and is therefore difficult to predict.</p>
<p>If two conforming bodies are in nominally static contact but subjected to vibration, ‘microslip’ will occur between some of the contacting asperities, leading to fretting fatigue and hysteretic damping. The energy dissipation per unit nominal area can be related to the tangential contact stiffness, using an extension of a general result for elastic contact problems due to Ciavarella and J¨ager. Since this stiffness is determined at the coarse scale, it proves unecessary to investigate the morphology of the fine scale microslip process. A general sinusoidal loading cycle shows that energy dissipation (and hence potential fretting damage) is significantly higher when the oscillations in normal and tangential forces are out of phase with each other.
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</ul>Wed, 12 Oct 2011 12:52:21 +0000Mike Ciavarellacomment 17520 at https://imechanica.org