Prof. Michele Ciavarella (Politecnico di Bari, Italy

Is there a contact area between elastic bodies in contact? & Some possible new ideas for fretting fatigue
When May 15, 2017
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283446
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Part I - Is there a contact area between elastic bodies in contact?

I try to comment on the big problem of tribology: what is the real contact area? There are direct measurement methods (in situ electron microscopy), as well as indirect methods (which are based on contact resistance, contact stiffness, lateral forces, and topography). There are simulation techniques and theories from single-contact continuum mechanics; to multi-contact continuum mechanics; and atomistic accounting. I remark that the “real contact area” is often an ill-defined quantity (for the elastic model, it is a fractal!), which perhaps is not possible to define at all. But even if we knew the “contact area”, how would we use this information? For what is called today “multiscale roughness”, the contact area is considered “magnification” dependent and the problem remains to introduce a truncation to the fractal process, what was called in the 1970’s “functional filtering”, a forgotten expression. Even then, most of the times the truncation is not real “atomic roughness”, but an arbitrary convenient fitting parameter for a numerical solution: the “multiscale” approach may look fancier, but is not any different from many engineering empirical approaches (we make the example of rubber friction). Even force-separation law (on which contact stiffness and conductance depend) which Persson recently claimed is not “resolution” dependent, in fact is. Hence, measuring an ill-defined quantity like contact area via another ill-defined one by indirect methods could be misleading. We remark that plasticity and adhesion could lead to some regularization of the problem, but are far from being well understood in general.

 

Part II – Some possible new ideas for fretting fatigue

MIT introduced in the late 90’s an interesting “crack analogue” model, which has been generalized by the present author to include bulk stresses in the contact and also the case of finite stress concentration, building on the so called concepts of “crack-like notches” and “blunt notches” with the simplified Atzori-Lazzarin criterion. This essentially suggests that many details of the fretting fatigue induced stress field, are often unimportant, and despite the contact mechanical problems can lead to a never ending number of new variant, the understanding of the mechanics of fretting fatigue has not advanced much. The role of finite friction, wear, and surface damage in initiation of crack has never been quantified, and experiments have been attempted mostly on well defined conditions, without being really useful for the real conditions in turbines. We suggest two possible new ideas: first, use in phase loading so that no surface damage is produced, so see the effect of fretting “in the pure”; second, to investigate variable amplitude fatigue as a more realistic condition, and also varying contact area.

 

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