Modelling internal erosion of cohesionless soils using a microstructural parameter

Internal erosion processes are of vital importance for the risk management of geotechnical structures as well as for the understanding of the macroscopic mechanical and hydraulic properties of the subsoil in various man-made constructions. Here, a 4-phase continuum model is presented and numerically applied to illustrative applications. The role of interfacial area and related microstructural parameters is addressed. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical analysis of wave propagation in fluid-filled deformable tubes

The theory of Biot describing wave propagation in fluid saturated porous media is a good effective approximation of a wave induced in a fluid-filled deformable tube. Nonetheless, it has been found that Biot's theory has shortcomings in predicting the fast P-wave velocities and the amount of intrinsic attenuation. These problems arises when complex mechanical interactions of the solid phase and the fluid phase in the micro-scale are not taken into account. In contrast, the approach proposed by Bernabe does take into account micro-scopic interaction between phases and therefore poses an interesting alternative to Biot's theory. A Wave propagating in a deformable tube saturated with a viscous fluid is a simplified model of a porous material, and therefore the study of this geometry is of great interest. By using this geometry, the results of analytical and numerical results have an easier interpretation and therefore can be compared straightforward. Using a Finite Difference viscoelastic wave propagation code, the transient response was simulated. The wave source was modified with different characteristic frequencies in order to gain information of the dispersion relation. It was found that the P-wave velocities of the simulations at sub-critical frequencies closely match those of Bernabe's solution, but at over-critical frequencies they come closer to Biot's solution. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

About the transition frequency in Biot's theory

Biot's theory of wave propagation in porous media includes a characteristic frequency which is used to distinguish the low-frequency from the high-frequency range. Its determination is based on an investigation of fluid flow through different pore geometries on a smaller scale and a subsequent upscaling process. This idea is limited due to the assumptions made on the smaller scale. It can be enhanced for a general two-phase system by three properties: Inertia of the solid, elasticity of the solid, and frequency dependent corrections of the momentum exchange. They become important for highly porous media with liquids.     

Dynamic mechanical properties of epoxy/novolac system modified with reactive liquid rubber and carbon filler

This paper reports on the work carried out to evaluate the frequency dependent viscoelastic properties of epoxy/novolac compositons modified with a liquid reactive rubber and carbon filler. For epoxy systems modified with elastomer, three typical transitions were observed: the α-relaxation deeply related to the glass transition of epoxy, the β-transition of epoxy, and the glass transition of rubber appeared near to the β-relaxation of epoxy resin. Considering an Arrhenius equation, the activation energies of β-relaxation were estimated. In the region of glass transition and rubbery state the temperature dependence of the shift factor (α_T) was determined through Williams-Landel-Ferry (WLF) equation.     

Residual saturated porous media – from oscillating water blobs to waves in ground earth

A model for wave propagation in residual saturated porous media is presented distinguishing enclosed fluid clusters with respect to their eigenfrequency and damping properties. The additional micro-structure information of cluster specific damping is preserved during the formal upscaling process and allows a stronger coupling between micro- and macro-scale than characterisation via eigenfrequencies alone. A numerical example of sandstone filled with air and liquid clusters of different eigenfrequency and damping distributions is given. If energy dissipation due to viscous damping dominates energy storage due to cluster oscillations, the damping distribution is more influential than the eigenfrequency distribution and vice versa. Spreading the damping distribution around a constant mean value supported the effect of capillary forces and spreading the eigenfrequency distribution around a constant mean value supported the effect of viscous damping in the investigated samples. For a wide distribution of the liquid clusters' damping properties, two damping mechanisms of propagating waves occur at the same time: damping due to viscous effects (for highly damped clusters) and energy storage by cluster oscillations (for underdamped clusters). (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental evaluation of phase velocities and tortuosity in fluid saturated highly porous media

In the current contribution, we present a novel method for the determination of the high frequency tortuosity parameter, α_∞ in high porous media. Therefore, time-domain measurements of ultrasonic signals are performed with a transmission technique. Aluminium foams with different pore fluids will be under the scope of experimental investigation. Finally, the experimental results are compared with analytical wave propagation tests. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Frequency-dependent properties of multiphase materials: origins,needs, and effective use in geophysics and modern synthetic materials.

Dynamic processes in multiphase materials, e.g., fluid-filled sandstones or bones, are described by models that include frequency-dependent properties. The origin of such properties is introduced as an averaged representation of frequency-dependent microscale motions. In addition to classical frequency-dependence of fluid flow, the influence of weak, high-porosity materials and fluid-fluid interfaces is discussed. The relevant characteristic numbers are contrasted and specific situations are demonstrated, in which frequency dependence has to be considered or not. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of coal on the rheological and mechanical properties of epoxy matrix

An experimental study was conducted to investigate the effect of coal used as a new type of filler in epoxy networks. For this purpose two series of epoxy composites with different coal weight fraction were studied and their thermo-mechanical properties were characterized. The work demonstrated that coal-filled epoxy networks show a typical behavior of a particulate filled material and follows the theoretical models already used.