Viscoelasticity induces anisotropy in contacts of rough solids

One of most peculiar issues related to soft viscoelastic contacts is the anisotropy induced on the contact solution: when sliding a perfectly isotropic surface over a viscoelastic half-space, the solution in terms of contact areas and displacement distribution is anisotropic. This is due to the different behaviour, in each contact patch, between the leading edge and the trailing one, where the viscoelastic material needs time to relax. Things become even more complicated when the surfaces into contact are already affected by an anisotropic degree, as occurring in many cases due to the manufacturing process. In this paper, by means of a proper Boundary Element Formulation, based on the definition of a viscoelastic periodic Green's function, we carry out, on these topics, a systematic investigation aimed to study the relation between the rigid surface and the deformed half-space anisotropy tensors. In detail, we show that the two tensors are related by means of a linear transformation, based on a rotation and a ”stretching” of the roughness anisotropy tensor. The role of anisotropy is, then, studied in relation with the friction force and with the lateral anisotropy induced force originally defined in this study.