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Adhesion in viscoelastic contacts

MichelleLOyen's picture

Yesterday I had the distinct pleasure of seeing a mechanics seminar delivered "tag-team" by Ken Johnson and Jim Greenwood. (I know several people have thought I was a bit mad for jumping "across the pond" but there are really some amazing benefits of being part of the Cambridge Engineering faculty!)

The talk was on "adhesion with just a bit of viscoelasticity" and largely concerned recent works on oscillatory loading, with experiments done in collaboration with the group of Kathy Wahl at the Naval Research Lab in Washington, DC. The work of Wahl et al. was mentioned elsewhere on this site in the more general discussion of viscoelastic contacts. The work is really interesting, but the experienced viscoelasticity researcher in me sees warning lights flashing at the use of a single time-constant standard linear solid model to describe anything in the real word. In this case, the single time constant is extremely small, comparable to the strain rates associated with the adhesion process (i.e. at the edge of contact) but not comparable to the average overall rates seen in the general indentation testing. Perhaps this work will be extended to a multiple time-constant model in future work.

References:

Greenwood JA and Johnson KL, Oscillatory loading of a viscoelastic adhesive contact. Journal of Colloid and Interface Science 296 (2006) 284–291

Wahl KJ et al. Oscillating adhesive contacts between micron-scale tips and compliant polymers. Journal of Colloid and Interface Science 296 (2006) 178–188

(I was amused to find out that the letters "JKR" are in that order because they are alphabetical!)

 

 

Comments

Dear Michelle,

It was a great talk. I attended it as well and really enjoyed. I agree with you that a simple dash-pot model with a single relaxation time is unrealistic for most real materials. However the demonstrated agreement between the model and experimental data was impressive, wasn't it? Is there a chance you have the original JKR paper from 1971(is it?)?

Best regards

Vladimir

MichelleLOyen's picture

Whenever there is good agreement in viscoelastic modeling where the model appears "too simple" it usually indicates that the experiment did not probe a sufficient range of behavior (i.e. sufficiently different time-frames).  I'd love to see the experimental data expanded in range to more time-scales and at the same time perform the experiment on a material with substantially more viscosity in the response.  PDMS really is nearly elastic in most reasonable experimental time-frames.  I did not see what the authors did in this work to guarantee that the dissipation observed was even in the material and not in the instrument... overall I believe this is a brilliant first study in the subject but as is often the case it seemed to me that it raised more questions than answers!

Indeed I probably have a copy of it but my files are still a mess after the trans-Atlantic relocation so I don't know that I can find anything... we should probably just ask Prof. Johnson for a copy!

 

For a non - control film we can't fit a straight line in usual manner for finding work of adhesion by using JKR equation(especially for  unloading). Can u send to me any mathematical procedure to find the work of adhesion for non-control films?

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