title: Nonlinear force propagation during granular impact.
abstract: We experimentally study nonlinear force propagation into granular material during impact from an intruder, and we explain our observations in terms of the nonlinear grain-scale force relation. Using high-speed video and photoelastic particles, we determine the speed and spatial structure of the force response just after impact. We show that these quantities depend on a dimensionless parameter, M^{'}=t_{c}v_{0}/d, where v_{0} is the intruder speed at impact, d is the particle diameter, and t_{c} is the collision time for a pair of grains impacting at relative speed v_{0}. The experiments access a large range of M^{'} by using particles of three different materials. When M^{'}≪1, force propagation is chainlike with a speed, v_{f}, satisfying v_{f}∝d/t_{c}. For larger M^{'}, the force response becomes spatially dense and the force propagation speed departs from v_{f}∝d/t_{c}, corresponding to collective stiffening of a strongly compressed packing of grains.
authors.affiliation: Department of Physics & Center for Nonlinear and Complex Systems, Duke University, Durham, North Carolina 27708, USA.
authors.forename: Abram H
authors.initials: AH
authors.lastname: Clark
authors.affiliation: Department of Physics & Center for Nonlinear and Complex Systems, Duke University, Durham, North Carolina 27708, USA.
authors.forename: Alec J
authors.initials: AJ
authors.lastname: Petersen
authors.affiliation: Department of Mathematical Sciences, New Jersey Institute of Technology, Newark, New Jersey 07102, USA.
authors.forename: Lou
authors.initials: L
authors.lastname: Kondic
authors.affiliation: Department of Physics & Center for Nonlinear and Complex Systems, Duke University, Durham, North Carolina 27708, USA.
authors.forename: Robert P
authors.initials: RP
authors.lastname: Behringer