Browsing by Author "Hueckel, Tomasz A"

Intense studies on evaporation in fluid systems have been conducted over the past decades, mainly on sessile droplets which are the simplest bodies to simulate. In this thesis, the evaporation process of a water bridge between two spherical grains is studied. A multi-physics approach is adopted focusing on the evaporation dynamics, the flow motion in liquid, the pressure distribution inside the water bridge, and the vapor behavior combined with the movement of free interface.

This dissertation studies the chemo-hygro-mechanical coupling involved in the process of crack propagation encountered both in natural and engineered context. Chemical processes are likely to affect the mechanical properties of geo-materials, resulting in possible weakening effect. The deformation and micro-cracking induced by material weakening in turn enhances the overall mass removal. In this study, several models within both elasticity and plasticity domain are developed for a better understanding of the enhanced crack propagation. A deformational plasticity model based on experimental observations is addressed. Rigid-plasticity models are applied to various boundary conditions. In the chemo-elasticity model, chemical dissolution is assumed to be a function of a comprehensive strain invariant. One-way coupling and two-way coupling models are discussed. In the two-way coupling model, volumetric strain coupling and deviatoric strain coupling are compared. A variety of loading modes are adopted to investigate the chemical enhancement of propagation of a single crack. The behavior of the material is either rigid-plastic, or elastic with the variable of mass removal enters the constitutive equation as a chemical strain. Comparison between the results from two models is presented and discussed.

Propagation of subcritical cracks is studied in a geomaterial subject to weakening by the presence of water, which dissolves a mineral component of it. Such weakening is common when tensile micro-cracks develop, constituting sites of an enhanced mineral dissolution. Meanwhile, the dissolution process at each active site of the inter-surface is affected by the chemical properties of the environment, e.g. the PH value. In this research, a previous concept of reactive chemo-plasticity is adopted with the yield limit depending on the mineral mass dissolved and causing a chemical softening. The dissolution is described by a rate equation and is a function of a variable internal specific surface area, which in turn is assumed to be a function of the dilative plastic deformation. Two loading modes are adopted to investigate the chemical enhancement of propagation of a single crack. The behavior of the material is rigid-plastic with a chemical softening. The extended Johnson approximation is adopted, meaning that all the fields involved are axisymmetric around the crack tip with a small, unstressed cavity around it. An initial dissolution proportional to the initial porosity activates the plastic yielding. The total dissolved mass diffuses out from the process zone, and the exiting mineral mass flux can be correlated with the displacement of the crack tip. A calibration against available data will be performed in the future, followed by a series of experiments to simulate the real case.

Laboratory tests on microscale are reported in which amorphous silica grains were compressed in a liquid environment, namely in solutions with different silica ion concentrations for up to four weeks. Such an arrangement represents an idealized representation of two sand grains. The grain surfaces and asperities were examined in Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) for fractures, silica polymer growth, and polymer strength. Single chains of silica polymers are found to have a failure pulling force of 330 - 450 nN.

A chain of observations are reported for the first time, using Pneumatic Grain Indenter and Grain Indenter-Puller apparatuses, confirming a long-existing hypothesis that a stressed contact with microcracks generates dissolved silica in the contact (asperity) vicinity, which eventually polymerizes, forming a structure between the grains on a timescale in the order of weeks. Such structure exhibits intergranular tensile force of 1 - 1.5 mN when aged in solutions containing silica ion concentrations of 200- to 500 ppm. Stress appears to accelerate the generation of silica polymers around stressed contact regions, so does mica-silica contacts. The magnitude of intergranular tensile force is 2 to 3 times greater than that of water capillary effect between grains.

Laboratory tests on microscale are reported in which two amorphous silica cubes were compressed in a liquid environment, namely in solutions with different silica ion concentrations for up to four weeks. Such an arrangement represents an idealized representation of two sand grains. The grain surfaces and asperities were examined in Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) for fractures, silica gel growth, and polymer strength. In 500ppm solution, silica gel structures a few hundred microns long appeared between stressed silica cubes. In 200ppm solution, silica deposits were found around damaged grain surfaces, while at 90ppm (below silica solubility in neutral pH), fibers a few microns in length were found growing in cube cracks. AFM pulling tests found polymers with strength in the order of 100nN and length between 50 and 100 nm. After aging, size of silica gel is in the order of 10-100 µm with intergranular strength in the order of 0.01-1 mN. We concluded that prolonged compression produced damage in grains, raising local Si ion concentration, and accelerating precipitation, polymerization and gelation of silica on grain surfaces enhancing soil strength at the microscale, hence most likely contributing to the aging phenomenon observed at the macroscale. Mica surfaces near stressed silica contacts were also found to enhance silica gel growth.