  We study the impact of a projectile onto a bed of 3 mm grains immersed in an
index-matched fluid. Specifically, we vary the amount of prestrain on the
sample, strengthening the force chains within the system. We find this affects
only the prefactor of linear depth-dependent term in the stopping force. We
therefore attribute this term to pressure within the material, and not the
grain-intruder friction as is sometimes suggested. Using a laser sheet scanning
technique to visualize internal grain motion, a high-speed camera, and particle
tracking, we can measure the trajectory of each grain throughout an impact
event. Microscopically, our results indicate that weaker initial force chains
result in more irreversible, plastic rearrangements during impact, suggesting
static friction between grains does play a substantial role in the energy
dissipation within the granular material.
