Practical Green's function approach to the simulation of elastic semi-infinite solids

This paper is concerned with the principles of Green's function-based molecular dynamics (GFMD) simulations of semi-infinite elastic solids and their application to various contact mechanical problems. A methodology to compute the (renormalized) elastic interactions of surface atoms is presented first. It is based on the fluctuation-dissipation theorem, with the help of which thermal fluctuations of atomic displacements can be related to the elastic Green's functions and thus to the effective coupling between surface atoms. We suggest a sparse representation of these renormalized spring constants and present numerical results for some simple two- and three-dimensional lattices. The renormalized elastic interactions can be obtained for relatively small systems and then be extrapolated to large systems. They incorporate the full elastic response of semi-infinite solids in a way that only surface atoms have to be considered in molecular dynamics simulations. The usefulness of GFMD is demonstrated by applying it to various idealized contact models, such as nonadhesive Hertzian contacts as well as nonadhesive contacts between flat, semi-infinite elastic solids and a self-affine, rigid substrate. In all cases, a zero probability density P(p) for infinitesimally small contact pressures p is found, as predicted theoretically. If the self-affine, nonadhesive surfaces are under such high loads that the contact is complete, the pressure histogram can be represented by a Gaussian also in accordance with theoretical predictions. However, if the topography of the substrate resembles that of industrial steel surfaces and the loads are moderate, P(p) decays exponentially for medium and large p in contradiction to theoretical predictions for randomly rough surfaces.