Pore collapse during impacts in granular materials

¹ Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218
² Hopkins Extreme Materials Institute, Johns Hopkins University, Baltimore, MD 21218
³ Earth and Environmental Sciences Division, Los Alamos National Laboratory, Albuquerque, NM 87545

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Published online: 1 December 2025

Abstract

The rapid compaction of grains and pores during the initial stages of impact into granular materials controls crater formation and ejecta release. In hydrocode modeling of these impacts, pore collapse is assumed to occur homogeneously throughout the material, with no consideration of local variations in porosity, microstructure, pressure, and volumetric strain. In this work, we employ the methodology of [1–3] to infer the kinematics and kinetics of the grains subjected to impact by combining in-situ X-ray imaging of impact with numerical modeling of a “digital twin” of the same material. We then use the validated numerical model to quantify pore space evolution in 3D with excellent temporal and spatial resolution. Our results demonstrate that porosity evolution is indeed heterogeneous, and local porosity evolution can deviate significantly from the average porosity evolution due to the intricacies associated with the granular microstructure. Pore pressures and volumetric strains computed in simulations allow us to verify the efficacy of these two popular pore collapse models at lower length scales – the P−α and ϵ−α models. The P-α model was determined to be able to capture the statistical distributions of evolving pore pressures. In contrast, the ϵ-α model was shown to have limitations due to its functional form. Our work is the first to investigate pore collapse using validated digital twin models. This study advances our understanding of the complexities involved in rapid granular compaction.