Stagnant zone formation in a 2D granular bed under penetration

Penetration into a granular material is a process that is both rich in physics and important in engineering applications. Here, we present experiments with a rectangular indenter penetrating horizontally into 2D granular beds. The penetration force experienced by the indenter shows a linear increase with the penetration depth, and this force is larger for penetration into elongated particles than that into circular-shaped particles. Particle positions are tracked, and local particle-based strains are calculated, showing a triangular-shaped stagnant zone in front of the indenter, and two shear bands along the edge of the stagnant zone. A structural function, Q_k, is used to characterize the local packing anisotropy, which reveals unique structural signatures as well as different structure-dynamic relations for the stagnant zone and the shear bands. The linear force law and the distinct regions observed here agree with a recent continuum model for granular penetration [1], while the structural information extracted here further demonstrates the microscopic difference between shear jammed and shear localized regions.

[1] W. Kang, Y. Feng, C. Liu, R. Blumenfeld, Nat. Commun. 9, 1101 (2018).