Quantifying 3D time-resolved kinematics and kinetics during rapid granular compaction, Part II: Dynamics of heterogeneous pore collapse

Pores in granular materials may occupy significant material volume. Pore-scale dynamics, therefore, strongly influence the macroscopic response of these materials when they are subjected to rapid compaction. In Part I of this series, Ghosh et al. (2024) employed in-situ X-ray imaging coupled with mesoscale finite element modeling to reconstruct the 3D time- resolved kinematics and kinetics of aluminum and soda lime glass powders subjected to rapid compaction. In Part II of this series, presented here, we use the same approach to examine the dynamics of the pores and the phenomenon of pore collapse during rapid compaction while also expanding our materials of interest to include Ottawa sand. We find that pore collapse is a highly spatially and temporally heterogeneous process in which pores reach their maximum compacted states across a broad range of timescales dictated by local microstructure, boundary conditions, and grain interactions. Using our data, we assess the validity of common pressure- based (P-alpha) and strain-based (epsilon-alpha) porosity evolution models at different length scales, and as a function of grain size, strain rate, and material ductility. We emphasize the importance of boundary conditions when interpreting theoretical porosity evolution models. Overall, our study provides deep new insight into pore collapse and porosity evolution during rapid granular compaction and highlights the importance of accounting for heterogeneous porosity evolution when modeling this process.