From 2D to 3D: Resolving Flow Fields Around Sand Penetrators From Flash X-ray Imaging*

The dynamic penetration of projectiles into brittle granular materials is important in a broad range of fields including planetary science and defense. As a projectile impacts a granular material, various material mechanisms are activated, including grain fracture, plasticity, fragmentation, granular flow, and pore collapse.   Experimental visualization and quantification of these mechanisms is extremely difficult without disrupting the mechanisms at work; however, quantifying of the role of each mechanism in projectile behavior is essential for validation of constitutive laws.  In this presentation we discuss a novel approach to visualizing and quantifying the 3D flow fields inside a sand sample during penetration by using lead tracer particles that are embedded in matrix of sand, flash x-ray imaging, and a radiographic image generation algorithm.  The 3D positions of the lead particles are known prior to impact from X-ray computed tomography.  During impact, two orthogonally-placed flash x-ray sources capture the displacement of the lead particles in a single instance of time.  By using the known position of particles both before impact, the observed position of particles during penetration from flash x-ray imaging, and an optimization scheme employing the image generation algorithm, the full 3D flow field can be obtained.  The 3D flow fields are investigated for both dry and saturated sand samples at different times during penetration and for different impact chamber pressures surrounding the samples.