Erich Bauer

Initial response of a micro-polar hypoplastic material under plane shearing

The behavior of an infinite strip of a micro-polar hypoplastic material located between two parallel plates under plane shearing is investigated. The evolution equation of the stress tensor and the couple-stress tensor is described using tensor-valued functions, which are nonlinear and positively homogeneous of first order in the rate of deformation and the rate of curvature. For the initial response of the sheared layer an analytical solution is derived and discussed for different micro-polar boundary conditions at the bottom and top surfaces of the layer. It is shown that polar quantities appear within the shear layer from the beginning of shearing with the exception of zero couple stresses prescribed at the boundaries.

Simulation of the Influence of Grain Damage on the Evolution of Shear Strain Localization

The influence of grain damage on shear strain localization in a lateral infinite granular layer under monotonic plane shearing is simulated using a micro-polar continuum description. The change of the grading of the grain sizes caused by grain damage is taken into account in a simplified manner by a reduction of the mean grain diameter, which is embedded in the constitutive model as an internal length. It is assumed that a reduction of the mean grain diameter caused by grain breakage and grain abrasion is related to an increase in the pressure, the micro-rotation and the micro-curvature. A reduction of the grain sizes is accompanied by a reduction of the limit void ratios and a reduction of the material against compaction. In the constitutive model the decrease of the mean grain diameter is linked to the so-called solid hardness, which is defined within a continuum description. The proposed concept of reduction of the mean grain diameter and the solid hardness is embedded in a micro-polar hypoplastic model. The results of the numerical simulations show that the reduction of the mean grain diameter has a significant effect on the evolution of the void ratio within the zone of shear strain localization.

Numerical investigations of the interface behaviour between granular soil and a geogrid reinforcement

In this paper the interface behaviour between a cohesionless granular soil and a rigid geogrid reinforcement under shearing is numerically investigated using a continuum approach. Particular attention is paid to the influence of grain size, initial density, vertical pressure and fluctuations of the rotation resistance of soil particles interlocked within the geogrid cells. To model the essential mechanical properties of granular soil a micro-polar hypoplastic description is used which takes into account stress and couple stress, pressure dependent limit void ratios and the mean grain size as the characteristic length. The results obtained from the 2D-finite element calculations show that the interaction of the reinforcement with the soil material has a strong influence on the deformation of the soil around the reinforcement.

Modelling Limit States Within the Framework of Hypoplasticity

The focus of this paper is on modelling limit stress states or so‐called critical states in which a cohesionless granular body can be deformed continuously at a constant stress and a constant volume. The constitutive equations are based on the framework of hypoplasticity, which is an alternative concept to elasto‐plasticity. The requirements for modelling limit stress states are studied for both a standard and a micro‐polar hypoplastic model. As in a micro‐polar description the stress tensor is usually non‐symmetric, the question arises as to whether the result for the limit stress ratio based on the symmetric Cauchy stress tensor in the standard continuum is the same as the one obtained from the micro‐polar continuum description. To this end the limit stress state is analysed under plane shearing of a lateral infinite granular strip located between parallel rough platens under a constant normal stress. It is shown that the stress ratio in the limit state obtained from the micro‐polar hypoplastic model ca...

Hypoplastic Modeling of the Disintegration of Dry and Saturated Weathered Broken Rock

For weathered broken rock materials the resistance of particles against abrasion and breakage is strongly influenced by their moisture content. The focus of the present paper is on the constitutive modeling of the mechanical behaviour of weathered granular materials under dry and wet conditions using a moisture dependent granular hardness. It will be shown that the results obtained from numerical simulations of element tests are in a good agreement with laboratory experiments carried out with weathered granite.

Hypoplastic Constitutive Modelling of Grain Damage Under Plane Shearing

In this paper a new concept is proposed for the constitutive modeling of grain damage of a cohesionless and unweathered granular material within the framework of micro-polar hypoplasticity. The effect of the change of the grain size distribution as a result of grain abrasion and grain rupture is taken into account in a simplified manner by reducing the mean grain diameter and modifying the constitutive relation for the incremental stiffness. For constitutive modeling it is convenient to distinguish different causes of particle damage such as the increase of the isotropic and deviatoric stress, the increase of the rotation resistance of particles and the abrasion caused by large particle rotation. The focus of this paper is mainly on modeling particle damage as a result of an increase of the curvature and particle rotation, which is investigated for monotonic plane shearing of an infinite granular layer under a constant normal stress.

Modeling of Weathered and Moisture Sensitive Granular Materials

The focus of this paper is on modeling the essential mechanical properties of weathered and moisture sensitive granular materials under dry and water saturated conditions using a state dependent solid hardness. Herein the solid hardness is related to the grain assembly in the sense of a continuum description and does not mean the hardness of an individual grain. Particular attention is paid to creep deformation initiated by water saturation of an initially dry material. First the constitutive model is outlined for isotropic loading paths. For general stress paths the influence of the stress deviator on creep deformation and stress relaxation is investigated with an extended hypoplastic model. The performance of the refined model is demonstrated for triaxial compression tests where the saturation of the material is investigated for different deviator stresses. The predictions of the model are compared with experiments.

Modeling the Influence of Pressure and Moisture Content on the Disintegration of Weathered Rockfill Materials

In this paper the essential mechanical behavior of rockfill materials is modeled using a hypoplastic continuum approach. Critical states are included in the model for large shearing. With respect to a pressure dependent relative density the model can capture the essential mechanical properties of initially loose and dense granular materials with a single set of constants. While the calibration and application of hypoplastic models has already been extensively investigated for fine-grained materials like sand and powders, the present application to weathered rockfill materials is a first attempt to describe coarse-grained materials with a low and decreesing grain hardness. Particular attention is paid to modeling the influence of the initial density, the pressure and the moisture content of weathered broken rock on the incremental stiffness. An increase of the compressibility and a decrease of the limit void ratios with an increase of the moisture content of the grains is modeled in a simplified manner using only a moisture dependent granular hardness. The comparison of the numerical simulations of isotropic compression tests and triaxial compression test with experiments shows that the model captures the essential properties of weathered rockfill materials for both dry and water saturated grains. The possibility of spontaneous shear band bifurcation under plane strain compression is analyzed and discussed for different initial densities.

The Influence of the Fluctuation of Micro-Polar Boundary Conditions on the Evolution of Shear Strain Localization in Granular Materials

In this paper the interface behavior between an infinite strip of a granular mate- rial and a rough boundary under plane shearing is numerically investigated using a micro-polar continuum approach. Particular attention is paid to the influence of a fluctuation of micro-polar boundary conditions along the interface on the evolution of shear strain localization within the granular material. The mechanical behavior of the cohesionless granular material is described with a micro-polar hypoplastic model. The evolution equations for the stress and the couple stress are non-linear tensor valued functions which model inelastic behavior. The investigations show that the micro-polar boundary conditions have a strong influence on the location and thickness of the zone of strain localization when relative displacements within the interface are excluded.

DEM Simulation of Oedometer Tests with Grain Crushing Effects

The grain crushing effects in oedometer tests are studied in this paper with discrete element method (DEM) in both 2D and 3D. The crushing of grain is modeled with a novel method which combines the replacement method and agglomerate method together. The results of oedometer tests with and without grain crushing are compared.