Power law scaling of early-stage forces during granular impact

We experimentally and computationally study the early-stage forces during intruder impacts with granular beds in the regime where the impact velocity approaches the granular force propagation speed. Experiments use 2D assemblies of photoelastic disks of varying stiffness, and complimentary discrete-element simulations are performed in 2D and 3D. The peak force during the initial stages of impact and the time at which it occurs depend only on the impact speed, the intruder diameter, the mass density of the grains, and the elastic modulus of the grains according to power-law scaling forms that are not consistent with Poncelet models, granular shock theory, or added-mass models. The insensitivity of our results to many system details suggest that they may also apply to impacts into similar materials like foams and emulsions.Comment: 5 pages, 4 figure

Power-Law Scaling of Early-Stage Forces during Granular Impact

17 USC 105 interim-entered record; under review.We experimentally and computationally study the early-stage forces during intruder impacts with granular beds in the regime where the impact velocity approaches the granular force propagation speed. Experiments use 2D assemblies of photoelastic disks of varying stiffness, and complimentary discreteelement simulations are performed in 2D and 3D. The peak force during the initial stages of impact and the time at which it occurs depend only on the impact speed, the intruder diameter, the stiffness of the grains, and the mass density of the grains according to power-law scaling forms that are not consistent with Poncelet models, granular shock theory, or added-mass models. The insensitivity of our results to many system details suggests that they may also apply to impacts into similar materials like foams and emulsions.Funding from the Office of Naval Research under Grant No. N0001419WX01519.

Viscous-like forces control the impact response of shear-thickening dense suspensions

We experimentally and theoretically study impacts into dense cornstarch and water suspensions. We vary impact speed as well as intruder size, shape, and mass, and we characterize the resulting dynamics using high-speed video and an onboard accelerometer. We numerically solve previously proposed models, most notably the added-mass model as well as a class of viscous-like models. In the viscous-like models, the intruder dynamics are dominated by large, viscous-like forces at the boundary of the jammed front where large shear rates and accompanying large viscosities are present. We find that our experimental data are consistent with this class of models and inconsistent with the added mass model. Our results strongly suggest that the added-mass model, which is the dominant model for understanding the dynamics of impact into shear-thickening dense suspensions, should be updated to include these viscous-like forces.Office of Naval ResearchOffice of Naval Research Global Visiting Scientist ProgramNo. N0001419WX01519VSP 19-7-00

NONTRIVIAL POWER-LAW SCALING OF PEAK FORCES DURING GRANULAR IMPACT

Ballistic impact into a soil target has broad military relevance. Understanding the forces during impact is crucial to predicting damage and survivability. This process involves several nonlinear physical mechanisms, making it difficult to describe. While some existing models of ballistic impact characterize the average response during penetration well, these models fail during the initial stages of impact when forces are the largest. There currently is no theoretical framework for understanding the forces and dynamics during these crucial early stages. In this thesis, we use numerical simulations of intruders impacting granular media, coupled with existing experimental data, to understand the forces during the initial stages of impact. For slow impacts, forces are independent of speed and set by the weight of the intruder. For fast impacts, the impact forces grow as a non-linear power law in the impact velocity with exponent 4/3. This scaling depends on the size of the intruder and stiffness of the grains, and it is insensitive to gravity, friction, the nonlinear force law between grains, and the density of the intruder. We use dimensional analysis to collapse all data onto a single curve, providing a first step toward a comprehensive theoretical description of this process.http://archive.org/details/nontrivialpowerl1094562766Outstanding ThesisMajor, Canadian ArmyApproved for public release; distribution is unlimited

NONTRIVIAL POWER-LAW SCALING OF PEAK FORCES DURING GRANULAR IMPACT [video]

NPS Physic

Viscous-like forces control the impact response of shear-thickening dense suspensions

We experimentally and theoretically study impacts into dense cornstarch and water suspensions. We vary impact speed as well as intruder size, shape and mass, and we characterize the resulting dynamics using high-speed video and an onboard accelerometer. We numerically solve previously proposed models, most notably the added-mass model as well as a class of viscous-like models. In the viscous-like models, the intruder dynamics is dominated by large, viscous-like forces at the boundary of the jammed front where large shear rates and accompanying large viscosities are present. We find that our experimental data are consistent with this class of models and inconsistent with the added-mass model. Our results strongly suggest that the added-mass model, which is the dominant model for understanding the dynamics of impacts into shear-thickening dense suspensions, should be updated to include these viscous-like forces