The effects of roughness on the area of contact and on the elastostatic friction - FEM simulation of micro-scale rough contact and real world applications

You are cordially invited to attend the PhD Dissertation Defense of Alessandro Rigazzi on Thursday, November 27th 2014 at 10h30 in room A32 (Red Building)

Abstract:

Every object, every surface we touch or look at, even if it looks smooth, is actually rough. Most of the rough structures, which can be observed at a given length scale, repeat themselves at smaller length scales, as in a fractal. The first implication of the rough nature of surfaces is that what we perceive as a full, solid, and smooth contact area, is in reality a collection of fragmented microscopical contact patches, composed of single contact points. As it is easy to imagine, the real area of contact is crucial for many real-world applications, such as the prediction of wear and fretting, charge and heat conduction, and frictional effects.

The importance of roughness, together with our knowledge in parallel computing and fast solution methods, are the premises of the current work. In this study, we analyze rough contact between realistic surfaces, and we resolve it numerically at micro-scale, to understand its meso- and macro-scale effects. We do this by means of the Finite Element Method, in combination with an optimal multi-grid strategy and a spatial decomposition to perform the computations on highly parallel super-computers.

In this work, we simulate the contact between an elastic cube and diverse rigid rough surfaces, under different loading conditions, and we derive empirical laws which describe the influence of well-known roughness parameters on important features such as contact evolution and elastostatic friction production. We also define bounds on the uncertainty of our measurements, to make clear the level up to which our predictions have to be considered reliable and applicable.

We then compare our results on the real area of contact to the predictions of two widely accepted theories (one by B. N. J. Persson, the other by A. W. Bush, R. D. Gibson, and T. R. Thomas), which have often proved to be interpretable as asymptotical bounds, for systems at low pressures. For large pressures, and consequent large areas of contact, we also compare our results to the newly developed and semi-empirical theory by V. A. Yastrebov, G. Anciaux, and J.-F. Molinari. Finally, we test our method on the real world problem of tyre-asphalt interactions on wet roads, comparing the results obtained by our method to data from other studies, collected on real highways and runways, and to a theoretical model, which is close, in the assumptions, to our numerical experiments.

Dissertation Committee:

• Prof. Rolf Krause, Università della Svizzera italiana, Switzerland (Research Advisor)
• Prof. Igor Pivkin, Università della Svizzera italiana, Switzerland (Internal Member)
• Prof. Illia Horenko, Università della Svizzera italiana, Switzerland (Internal Member)
• Prof. Marco Paggi, Politecnico di Torino, Italy (External Member)
• Prof. Friedermann Schuricht, Technische Universität Dresden, Germany (External Member)