Modelling the mechanical behaviour of porous cohesive granular material using discrete elements: Application to snow failure
Jonas Ritter
Department of Mechanical Engineering, KIT
12. Dezember 2018, 17.00
WW8, Raum 2.018, Dr.-Mack-Str. 77, Fürth
The mechanical behaviour of loose, cohesive, granular assemblies was investigated through load-controlled discrete element simulations. As initial particle configurations, existing three dimensional assemblies of 2048 particles were used based on the Baxter sticky hard sphere (SHS) model. The assemblies are characterizedby the average coordination number $z_c$ and volume fraction $φ$. The SHS model allows to generate highly porous assemblies of particles with a given volume fraction and average coordination number. Both quantities can be modified independently and obtained directly based on micro-computed tomography. Cohesion was added through cohesive bonds between adjacent particles which can break under tension and shear. Mixed-mode loading simulations of cubic samples were performed. The load was applied from the top of the sample while the bottom remained fixed. Failure of the specimens was detected systematically by a two step criterion involving kinetic energy and stress. Simulations allowed to evaluate the yield surface of the samples which was fitted with an ellipse function, similar to the cohesive cam clay model. The parameters of the fitted yield surface were determined by a least-squareestimation. The compressive strength as well as the ratio between the tensile and compressive strengths show a strong relation to the initial cohesive contact density $ν_{c,0} = z_{c,0} φ_0$ as a power law. Furthermore, the slope of the critical state line, shows a large scattering and seems independent of $ν_{c,0}$. Additionally the post-peak behaviour under uniaxial compression was investigated. After the failure point an increase of the strain rate can be observed with a simultaneously decrease of the normal stress. Hence the samples show a strain-softening behaviour followed by the volumetric collapse of the porous structure and ultimately jamming.