Structural and Property Changes in Granular Materials Subjected to Thermal Cycling

  • Rotta Loria, Alessandro Francesco (Northwestern University)
  • Pan, Yize (Northwestern University)
  • Coulibaly, Jibril Birante (Northwestern University)

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Over the past 50 years, numerous investigations have shown that the application of cycles of heating and cooling to granular materials causes a permanent densification through irreversible deformations associated with particle rearrangements [1]. Due to such microstructural changes, the application of thermal cycles has the potential to significantly change both the structure and properties of granular materials as they are inherently linked. Despite many advances in the understanding of the impacts of thermal cycles on the physics of granular materials, evidence about the changes in their properties due to thermal cycles remains scarce. The objective of this work is to assess the processing-structures-properties relationship in granular materials subjected to thermal cycling. This subject is addressed by gathering the results of recent experiments [2,3] and computations [4]. Thermal cycling of rounded, subangular, and angular sands subjected to constant vertical stress under loose and dense states is performed in oedometric conditions. Coupled thermo-mechanical discrete element simulations are employed under similar conditions to provide complementary information on this problem. The analyses expand on the microstructural changes caused by thermal cycling and the interconnected impacts on the macroscopic physical properties of the studied materials. Thermally induced densification is observed experimentally to be more significant for more rounded particles. The microstructural changes probed numerically reveal an increase in coordination number, an increase in fabric anisotropy and important horizontal stress relaxation. Granular topology evolves non-monotonically and presents a maximum for a critical temperature amplitude, indicative of an optimal microscopic reorganization. Consequently, mechanical stiffness changes depending on fabric anisotropy and substantially increases in the vertical direction. Thermal conductivity correlates with the coordination number and shows a peak 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.