High-resolution X-ray and neutron computed tomography of partially saturated granular materials subjected to projectile penetration

• We examine microstructures of partially saturated sand after projectile penetration.
• Damaged microstructures are evaluated from Neutron and X-ray computed tomography.
• We present cavity structures developed after projectile penetration in wet sand.
• High concentrations of water around cavity and penetration path are revealed.

To improve fundamental understandings of projectile penetration through partially saturated sand at a meso-scale and provide controlled experimental data to validate future numerical simulations, high-resolution computed tomography of granular materials after projectile penetration using two different imaging modalities (X-rays and neutrons) was implemented. This paper presents the novel imaging techniques and results of studying the variation of geometric structure (arrangement of solids, voids, liquid films) at a meso-scale after the penetration of projectiles (ex-situ) under controlled conditions. Attenuation contrast obtained from X-rays and neutrons provided complementary information that would not have been possible to obtain by using X-rays only. X-rays identified silica particles and their boundaries at 14.8 µm/pixel resolution, while neutrons spatially resolved small amounts of moisture at 15.6 µm/pixel resolution due to a large neutron cross-section of hydrogen. Lead (Pb) contained in most projectile tips is opaque to X-rays but transparent to neutrons at the energy range generally used for imaging. Indeed, the need for using multi-modal imaging is further emphasized for non-invasive damage diagnostics. Partially saturated crystalline sand specimens with bulk gravimetric water contents of 5~6% were studied at two different initial packing states (dense and loose) using two different types of commercially available projectiles. A novel image registration technique combined 3-D attenuation data obtained at different spatial resolution with two imaging modalities (X-ray and neutron). The role of moisture in sand on penetration resistance and related effects on the meso-structure has not been studied in detail in the past, and the results from the current study show important preliminary insights on this topic. The effects of projectile impact and penetration on particle rearrangements, fractures, and water re-distributions are summarized as a function of the initial specimen state of compaction and projectile parameters (mass, impact velocity, twist during penetration, and with use of full metal jacket ammunition). Observations of damage patterns including penetration depth and cavity formation are summarized in this paper, and the findings form an important basis for an improved understanding of penetration mechanics in porous materials and related multiscale modeling framework.