Fractal contact spot and its application in the contact model of isotropic surfaces

Abstract

All engineering surfaces are rough and composed of asperities with different scales. In conventional fractal contact models, asperities are used to build contact models between two rough surfaces. However, it is found that the deformation of asperities in the contact process is rather complex because only a small part of the asperities deform elastically or plastically. Moreover, the asperities used in conventional fractal contact models are not constructed exactly from contact mechanism and, of course, they are not fractal or self-affine. So, these fractal contact models seem like statistical models using fractal parameters rather than pure-fractal contact models. In the present paper, instead of asperities, fractal contact spots are utilized to describe the contact process of isotropic rough surfaces. The differences between fractal contact spots and asperities are analyzed to prove that fractal contact spots are more suitable to reveal the nature of contact process. Numerical method is used to simulate the contact process and study the characteristics of fractal contact spots, and the load-separation relationship of fractal contact spots is obtained using a fractal method. Then, a pure-fractal contact model is proposed and extended to calculate the contact stiffness of rough surfaces. To verify the contact model proposed, an experiment of stiffness identification of isotropic metal surfaces is carried out. The identified stiffness is in good agreement with that calculated by the contact model proposed, which indicates that the pure-fractal contact model applying the fractal contact spots is more suitable to reveal the contact mechanism of isotropic surfaces.