Adaptive FEM-DEM bridging coupling for third-body evolution in sheared granular media

The frictional behavior of dry solids in sliding contact is significantly affected by the formation and accumulation of wear particles at the interface. These particles create an amorphous layer known as the third body, which undergoes intense deformation. We show that accurately capturing the evolution of the third body requires a precise modeling of the physical dimensions of the surrounding elastic bodies and boundary conditions. This implies that a computational tribology simulation engine must resolve both the fine-scale dynamics of wear particles and the large-scale structural dimensions of the sliding bodies. To address this challenge, we employ a coupled Finite Element Method (FEM) and Discrete Element Method (DEM) approach. The DEM is used to model the third body, where grain-scale discretization is crucial, while the FEM represents the first bodies as a continuum, reducing computational cost. The coupling between FEM and DEM is achieved using the bridging method. To allow unrestricted growth of the third body beyond the initial discrete region, we implement an adaptive coupling mechanism that enables FEM regions to transition into DEM regions. This transition occurs when a predefined criterion is met, based on the average change in neighboring particles. Our approach is validated for both amorphous materials and ordered crystalline lattices. Finally, as a proof of concept, we apply the adaptive coupling framework to simulate the evolution of a third body initially composed of stiff elliptical particles.