Discrete Element Modeling of Coupled Processes

Discrete element models (DEM) have been widely used to model deformation and fracturing processes in geological materials. A particular strength of the modeling approach is that internal and external surfaces are treated identically, and that the model can handle material behavior ranging from a completely dissociate particle assembly to an elastic body. We have used discrete element models to study the behavior of coupled deformation, fluid-flow and reaction processes ranging from hydro-fracturing, fluidization and fragmentation in fluid-filled solids, to diffusion-reaction processes such as dessication cracking, material decomposition during devolatilization, and hierarchical fracturing and fragmentation in weathering processes. A particular interesting class of processes is coupled diffusion-reaction-fracturing processes where the reaction leads to a change in local volume in the solid. As a fluid diffuses into the solid and reacts it generates stresses which eventually may lead to fracturing of the solid. As a result the boundary conditions for the diffusion-reaction process are changed, hence the need for a coupled model.

We have developed methods to couple a DEM-based description of a deforming solid to a continuum description of a diffusion-reaction process. We have applied this model to study self-propagating fracture fronts observed in drying and cooling fronts, and we demonstrate that the front propagates at a constant velocity with a constant width, contrary to diffusion-dominated processes. Recently, we have also used this model to study weathering and serpentinization processes in which the reaction leads to a local expansion. In this case, we observe both spalling (spheroidal weathering) and hierarchical fragmentation, resulting in a significant acceleration in the propagation of the reaction front, and we argue that this process may provide a first order control on weathering and serpentinization rates.