Local Mechanical Properties of Granular Media

Granular materials consist of athermal conglomerates of macroscopic particles which interact via dissipative short-range potentials. The global behaviour of large granular systems away from a transition can be described quite well by conventional continuum mechanics. For smaller system sizes, unusual properties appear which originate from the macroscopic scales of the constituent particles. In this thesis, we will investigate the micromechanical response of granular media in different physical conditions. We therefore use techniques to track the structural and mechanical properties of the packings on the grain scale. The dynamics of granular media strongly depends on the external driving conditions. We built setups to expose granular particles to different states of aggregation. We then probe the micromechanical response of the systems to external stresses. In a first experiment, a macroscopic stress field is applied to a granular solid through isotropic compression. We evaluate the micromechanic response by refining current techniques of stress-birefringence measurements. This provides access to the distribution of contact forces in the system. We also analyse the linked distribution of local moduli in a coarse-graining approach which relates local structural and mechanical properties of the material. For crystalline structures we find a narrow distribution of moduli. In contrast, amorphous media shows heterogeneous mechanical properties on microscopic scale. The distribution becomes more homogeneous for higher compression states. As we vary mixture compositions we can attribute the level of heterogeneity to the amount of disorder in the packing. In a second experiment, we expose granular particles to strong external driving which induces a fluidlike dynamics. We probe the micromechanic response by a local pertubation with an intruder. We measure the viscosity of the system by tracking the intruders motion. We demonstrate access to three viscosity regimes: linear response, shear thinning and shear thickening; depending on the shear rate. We attribute the effect of shear thinning to a decrease of entropic forces from the bath when the intruder is fast enough to break cages. Shear thickening appears due to the formation of particle clusters in front of the intruder for very high velocities. In a third experiment, granular particles show gaseous dynamics when exposing them to a zero gravity environment. We find that the long time cooling behaviour is in accordance with Haff's law. By tracking the particles' motion we furthermore see indications for clustering

Local Mechanical Properties of Granular Media

Granular materials consist of athermal conglomerates of macroscopic particles which interact via dissipative short-range potentials. The global behaviour of large granular systems away from a transition can be described quite well by conventional continuum mechanics. For smaller system sizes, unusual properties appear which originate from the macroscopic scales of the constituent particles. In this thesis, we will investigate the micromechanical response of granular media in different physical conditions. We therefore use techniques to track the structural and mechanical properties of the packings on the grain scale. The dynamics of granular media strongly depends on the external driving conditions. We built setups to expose granular particles to different states of aggregation. We then probe the micromechanical response of the systems to external stresses. In a first experiment, a macroscopic stress field is applied to a granular solid through isotropic compression. We evaluate the micromechanic response by refining current techniques of stress-birefringence measurements. This provides access to the distribution of contact forces in the system. We also analyse the linked distribution of local moduli in a coarse-graining approach which relates local structural and mechanical properties of the material. For crystalline structures we find a narrow distribution of moduli. In contrast, amorphous media shows heterogeneous mechanical properties on microscopic scale. The distribution becomes more homogeneous for higher compression states. As we vary mixture compositions we can attribute the level of heterogeneity to the amount of disorder in the packing. In a second experiment, we expose granular particles to strong external driving which induces a fluidlike dynamics. We probe the micromechanic response by a local pertubation with an intruder. We measure the viscosity of the system by tracking the intruders motion. We demonstrate access to three viscosity regimes: linear response, shear thinning and shear thickening; depending on the shear rate. We attribute the effect of shear thinning to a decrease of entropic forces from the bath when the intruder is fast enough to break cages. Shear thickening appears due to the formation of particle clusters in front of the intruder for very high velocities. In a third experiment, granular particles show gaseous dynamics when exposing them to a zero gravity environment. We find that the long time cooling behaviour is in accordance with Haff's law. By tracking the particles' motion we furthermore see indications for clustering

Enhanced granular medium-based tube press hardening

Active and passive control strategies of internal pressure for hot forming of tubes and profiles with granular media are described. Force transmission and plastic deformation of granular medium is experimentally investigated. Friction between tube, granular medium and die as also the external stress field are shown to be essential for the process understanding. Wrinkling, thinning and insufficient forming of the tube establishes the process window for the active pressure process. By improving the punch geometry and controlling tribological conditions, the process limits are extended. Examples for the passive pressure process reveal new opportunities for hot forming of tubes and profiles.Comment: 4 pages, 11 figure

Granular cooling of ellipsoidal particles in microgravity

A three-dimensional granular gas of ellipsoids is established by exposing the system to the microgravity environment of the International Space Station. We use two methods to measure the dynamics of the constituent particles and report the long-time development of the granular temperature until no further particle movement is detectable. The resulting cooling behavior can be well described by Haff’s cooling law with time scale τ. Different analysis methods show evidence of particle clustering towards the end of the experiment. By using the kinetic theory for ellipsoids we compare the translational energy dissipation of individual collision events with the overall cooling time scale τ. The difference from this comparison indicates how energy is distributed in different degrees of freedom including both translation and rotation during the cooling