Numerical Modeling of Dynamic Friction Phenomena

New Micro- and Macroscopic Models of Contact and Friction

Sponsored by: The Air Force Office of Scientific Research
Project duration: 1986-1993

These projects, which were sponsored over many years by the Air Force Office of Scientific Research, were dedicated to understanding, modeling and prediction of phenomena of dynamic friction, in particular of friction-induced oscillations, squeaks, stick-slip motion, and the difference between the static and kinetic coefficient of friction. Importantly, these complex phenomena have been a subject of numerous experimental, theoretical and numerical studies. Until recently, most of these studies had a very limited success, in that no general understanding and modeling methodology were developed for the stick-slip, squeak, chatter and related phenomena. Some previous approaches based on velocity-dependence of friction were partially successful in explaining selected cases and in modeling selected apparatus, but were not general enough to represent a wide class of friction-induced oscillations. In particular, it was observed that the results obtained on one apparatus usually did not transfer to a different setup, even with the same frictional samples. For a review of these and other efforts, see an exhaustive paper by Ibrahim [1]. During the course of the project, new understanding of these phenomena was developed and a new models of contact and friction were formulated. Some of the most important findings of this research [2,3,4] are listed as follows:
  1. Contrary to intuition of many investigators in the past, it is the normal compliance rather than tangential properties of the interface that must be modeled adequately to capture the appropriate friction phenomena. The authors of this summary have formulated new phenomenological models of contact and friction (Oden-Martins model [2]) which take into account both normal and tangential properties of the interface. These models are capable of delivering results in good correlation with experimental observations.
  2. A basic mechanism of self-excited frictional oscillations for mechanical systems has been identified as unstable, friction-induced vibrations triggered by coupling between normal and rotational motion of the components of the sliding system, with additional presence of velocity-dependence of the coefficient of friction [3]. This conclusion is in accord with recent experimental observations of Dweib and D'souza, and has been used successfully in modeling of friction-induced oscillations of representative mechanical systems.
  3. A new class of asperity-based models of frictional interfaces has been developed [4]. These models combine accurate finite element analysis of micro-scale asperities with statistical homogenization methods, to produce macro-scale models of contact and friction. Such an approach provides a new insight into the phenomena occurring on frictional interfaces and defines a new class of constitutive interface models, consistent with both micro-scale behavior and macro-scale phenomenological observations.
As a result, COMCO has presently a leading edge expertise in numerical simulation and prediction of friction-induced vibrations. This expertise is presently being used in practical industrial applications.

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