Phase transition processes of pore fluids in porous materials

Taking a closer look on, e. g., storage processes of greenhouse gases in deep geological aquifers or pressure decreases in dilatant shear bands, the observation can be made that pressure and temperature changes in porous materials can induce phase transition processes of the respective pore fluids. For a numerical simulation of this behaviour, a continuum mechanical model based on a multiphasic formulation embedded in the well-founded framework of the Theory of Porous Media (TPM) is presented in this contribution. The single phases are an elasto-viscoplastic solid skeleton, a materially compressible pore gas consisting of the components air and gaseous pore water (water vapour) and a materially incompressible pore liquid, i. e., liquid pore water. The numerical treatment is based on the weak formulations of the governing equations, whereas the primary variables are the temperature of the mixture, the displacement of the solid skeleton and the effective pressures of the pore fluids. An initial boundary-value problem is discussed in detail, where the resulting system of strongly coupled differential-algebraic equations is solved by the FE tool PANDAS. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Discussion on appropriate material parameters for a porous media model of the IVD

The determination of the theoretically introduced material parameters is always a big issue, especially in computational biomechanics. In this context it is difficult to obtain the appropriate intervertebral disc (IVD) specimens within 24 h post decease and to perform reliable experiments on them, respectively. A complex test setup is needed for most of the cases and the measured results are very ‘patient specific’. On the other hand, the parameters obtained from literature are often also not reliable, because they are widespread, the experimental setup is not clearly described or was deficient. Moreover, as soft tissues generally exhibit a coupled dissipative behaviour, it is often not possible to measure these effects independently. Hence, the parameters have to be identified by inverse computations, i. e., axial compression, bending or torsion of a whole motion segment. The goal of this contribution is to proceed from an extended biphasic model and a set of suitable material parameters obtained from literature, in order to find the influence of certain parameters on a numerical compression-bending experiment of a L4-L5 motion segment. Finally, a comment on how the respective parameters should be determined is given. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Granular materials: Parameter identification and modelling of localisation problems

The prediction of landsliding requires an exact knowledge of the mechanical behaviour of granular materials. This kind of materials, e. g., sand, have a very complex deformation behaviour, which depend on the stress state and on the loading history. In this work, the deformation behaviour of the solid skeleton is characterised via homogeneous triaxial tests on dry sand specimens. Additionally, an appropriate elasto-plastic material law to describe the solid skeleton in the frame of Theory of Porous Media (TPM) is used, which is implemented in the FE tool PANDAS. Furthermore, a single-surface yield criterion with isotropic hardening, which limits the elastic domain, and a non-associated plastic flow are employed. The determination of the material parameters of the linear elasticity law as well as the single-surface yield criterion are based on test data of triaxial experiments. The material parameters are identified using a derivative-based optimisation method (donlp2), which is coupled with PANDAS. Finally, a simulation of a benchmark test is presented to show shear band localisation effects, where the material behaviour is described by a triphasic porous media model based on the TPM, where the constituents are a deformable solid skeleton and two pore fluids, water and air. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

In-plane magnetic pattern separation in NiFe/NiO and Co/NiO exchange biased bilayers investigated by magnetic force microscopy

Ion bombardment induced magnetic patterning (IBMP) was used to write in-plane magnetized micro and submicron patterns in exchange biased magnetic bilayers, where the magnetization directions of the adjacent patterns are antiparallel to each other in remanence. These magnetic patterns were investigated by non-contact magnetic force microscopy (MFM). It is shown that the recorded MFM images of the IBMP patterns in two exemplarily chosen standard layer systems (NiFe (4.8 nm)/NiO (68 nm) and Co (4.8 nm)/NiO (68 nm)) can be well described by a model within the point-dipole approximation for the tip magnetization. For 5 and 0.9 μm wide bar patterns the domain wall widths between adjacent magnetically patterned areas were determined to a≈1 μm. The minimum magnetically stable pattern width was estimated to be 0.7 μm in the standard system Co (4.8 nm)/NiO (68 nm).     

Synthesis and Reactivity of the Phosphorus Analogues of Cyclopentadienone,Tricyclopentanone, and Housene

The phosphorus analogues of cyclopentadienone, tricyclopentanone, and housene were accessed from bis(cyclopropenyl)diphosphetanedione 3 , which was prepared by mixing 1,2,3‐tris‐tert‐butylcyclopropenium tetrafluoroborate ( 1 ) and sodium phosphaethynolate [Na(OCP)(dioxane)_n]. While photolysis of 3 results in decarbonylation, yielding bis(cyclopropenyl)diphosphene 4 and after rearrangement diphosphahousene 5 , thermolysis of 3 leads to phosphatricyclo[2.1.0.0]pentanone 7 . Metal‐mediated valence isomerization of 7 and subsequent demetalation provides access to phosphacyclopentadienone 12 .     

Newtonian Cosmology in Lagrangian Formulation: Foundations and Perturbation Theory

The Newtonian theory of spatially unbounded, self-gravitating, pressureless continua in Lagrangian form is reconsidered. Following a review of the pertinent kinematics, we present alternative formulations of the Lagrangian evolution equations and establish conditions for the equivalence of the Lagrangian and Eulerian representations. We then distinguish open models based on Euclidean space R³ from closed models based (without loss of generality) on a flat torus T³. Using a simple averaging method we show that the spatially averaged variables of an inhomogeneous toroidal model form a spatially homogeneous background model and that the averages of open models, if they exist at all, in general do not obey the dynamical laws of homogeneous models. We then specialize to those inhomogeneous toroidal models whose (unique) backgrounds have a Hubble flow, and derive Lagrangian evolution equations which govern the (conformally rescaled) displacement of the inhomogeneous flow with respect to its homogeneous background. Finally, we set up an iteration scheme and prove that the resulting equations have unique solutions at any order for given initial data, while for open models there exist infinitely many different solutions for given data.