Thermal cycling effects on the structure and physical properties of granular materials

Over the past two decades, various investigations have shown that the application of thermal cycles of heating and cooling to granular materials causes permanent densification. However, prior to this study, significant associated effects of thermal cycles on the structural reorganization and physical properties of granular materials remained mostly unknown. To address these elusive aspects, this paper investigates the relationship between the microstructural changes induced by thermal cycling and their influence on the physical properties of granular materials using a multiphysical and multiscale computational approach. Microstructural changes are analyzed using coupled thermo-mechanical discrete element simulations and macroscopic physical properties are upscaled using homogenization. Multiple initial porosities and particle size distributions are investigated for a large number of thermal cycles of varying amplitude. Results demonstrate significant variations in the structure and properties of granular materials. Volumetric densification and fabric anisotropy increase monotonically with the amplitude and number of cycles. Coordination number exhibits a maximum for a critical temperature amplitude established theoretically to correspond to an optimal particle rearrangement. Mechanical stiffness and thermal conductivity increase in anisotropic fashion due to stress relaxation and fabric anisotropy and typically exhibit the most variation at the critical temperature amplitude. Intrinsic permeability to fluid flow decreases isotropically and monotonically with the amplitude and number of thermal cycles due to pore volume reduction. The critical temperature amplitude provides a limit to the thermal energy that can seemingly induce optimal and permanent structural reorganizations, as well as maximum variations of specific physical properties of granular materials.