Abstract
This paper is devoted to an analytical, numerical, and experimental analysis of adhesive contacts subjected to tangential motion. In particular, it addresses the phenomenon of instable, jerky movement of the boundary of the adhesive contact zone and its dependence on the surface roughness. We argue that the “adhesion instabilities” with instable movements of the contact boundary cause energy dissipation similarly to the elastic instabilities mechanism. This leads to different effective works of adhesion when the contact area expands and contracts. This effect is interpreted in terms of “friction” to the movement of the contact boundary. We consider two main contributions to friction: (a) boundary line contribution and (b) area contribution. In normal and rolling contacts, the only contribution is due to the boundary friction, while in sliding both contributions may be present. The boundary contribution prevails in very small, smooth, and hard contacts (as e.g., diamond-like-carbon (DLC) coatings), while the area contribution is prevailing in large soft contacts. Simulations suggest that the friction due to adhesion instabilities is governed by “Johnson parameter”. Experiments suggest that for soft bodies like rubber, the stresses in the contact area can be characterized by a constant critical value. Experiments were carried out using a setup allowing for observing the contact area with a camera placed under a soft transparent rubber layer. Soft contacts show a great variety of instabilities when sliding with low velocity — depending on the indentation depth and the shape of the contacting bodies. These instabilities can be classified as “microscopic” caused by the roughness or chemical inhomogeneity of the surfaces and “macroscopic” which appear also in smooth contacts. The latter may be related to interface waves which are observed in large contacts or at small indentation depths. Numerical simulations were performed using the Boundary Element Method (BEM).
Highlights
Since the famous work by Johnson, Kendall, and Roberts (JKR) from 1971 [1], adhesive contacts have remained in focus of research in contact mechanics and tribology
It is important to note that the absence of relative tangential movement of surfaces in the case of a rolling contact suppresses the contribution from shearing of the contact area, while in a sliding contact this contribution exists but presumably represents the main contribution to friction in most cases
Simulations were performed using the FFT-assisted Boundary Element Method (BEM) [29] under displacementcontrolled conditions and with the same assumptions as in JKR theory: The materials behave as linear elastic half-spaces with surface slopes being low and adhesion only acts in the regions of intimate contact
Summary
Since the famous work by Johnson, Kendall, and Roberts (JKR) from 1971 [1], adhesive contacts have remained in focus of research in contact mechanics and tribology. The present paper is devoted to an analytical, numerical, and experimental investigation of adhesive contacts under tangential loading and rolling. Both mechanisms are investigated analytically and numerically For the former one, the numerical study is conducted by use of the BEM for the JKR-type adhesive contact. The analysis is restricted to the elastic contact under very slow normal and tangential movement (quasi-static contact), so that viscoelastic and inertia properties can be neglected We will consider both basic types of friction due to relative movement of bodies: rolling and sliding friction. It is important to note that the absence of relative tangential movement of surfaces in the case of a rolling contact suppresses the contribution from shearing of the contact area, while in a sliding contact this contribution exists but presumably represents the main contribution to friction in most cases.
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