Diss.--Universität Basel. / Bibliography: l. [1]-[3]

Thesis (Ph. D.)--Drexel University, 2005. / Includes abstract and vita. Includes bibliographical references (leaves 71-78).

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 197-212). Also available in electronic version.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

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We present a computational framework for modeling geomaterials undergoing failure in the brittle and ductile regimes. This computational framework introduces anisotropic gradient regularization to replicate a wide spectrum of size-dependent anisotropic constitutive responses exhibited in layered and sedimentary rock. Relevant subsurface applications include oil/gas wellbore completions, caprock evaluation for carbon sequestration in saline aquifers, and geothermal energy recovery. Considered failure modes are mixed-mode fracture, shear band formation due to plastic strain localization, and rate-dependent frictional slip along the propagated fracture's rock surface, subsequent to fracture closure. Our nonlocal modeling framework extends the state-of-the-art gradient-enhanced plasticity and damage mechanics for frictional materials with a special treatment that injects bias for the regularization for different orientations. A novel contribution is that the formulations not only contains a regularization, but that the regularization also provides a method to introduce size-dependent anisotropies. Consequently, this treatment provides a new means to create non-associative flow via a variational framework while introducing different anisotropic responses for specimens of different sizes (introduced in Chapter 1). These anisotropic regularization modeling techniques are then applied to three classes of common geomechanics problems: critical state plasticity of clay and shale rock (Chapter 2), brittle fracture of rock (Chapter 3), and the plastic slip of interfaces and cracks (Chapter 4). This combination, of established rock physics, local anisotropy, and size-dependent anisotropy enfranchised with diffusive regularization, is investigated. For instance, experimentation on uniaxially compressed specimens failing in the brittle regime reveals a repeatable typology of wing- and coalescent-crack patterns, broadly taken to indicate a mixed-mode fracture phenomenon particular to rock-like materials. In the ductile regime, biaxially compressed shale rock displays orientation-dependence of the plastic deformation difficult to capture merely by attributing anisotropy to the elastic response, with localization at or near the critical state. We numerically capture both these phenomena. Verification and/or validation is provided for proposed constitutive relations.

No description available.

This thesis presents a study of magnetic anisotropies of transition metal based thin films investigated by magnetooptic Kerr effect (MOKE) and x-ray radiation techniques. Thin Co films of different thickness deposited on glass are investigated to study the uniaxial magnetic anisotropy (UMA) of these films. The direction of the UMA is determined from the azimuthal dependence of the magnetic remanence and differs with increasing thickness of the Co film investigated by x-ray reflectivity (XRR). Strain in the substrate and possible textures in the film structure can be excluded as the origin of the magnetic behavior as studied by grazing incidence wide angle x-ray scattering (GIWAXS). Furthermore, a new combination of different vectorial magnetometry techniques using MOKE is developed (vector MOKE). The processing of the experimental data contains the separation of linear and quadratic parts of the magnetization curves and determination of all three components of the magnetization vector in units of Kerr rotation. The complete reversal process can be reconstructed and interpreted by monodomain states and domain splitting. The measurement procedure and the processing of the data is demonstrated for ultra-thin epitaxial Fe films on MgO(001), which are structurally characerized by XRR and (grazing incidence) x-ray diffraction (GI(XRD)). Magnetic properties of thin Fe films (of different thickness) and A2-type ordered Co50Fe50 films (prepared with different annealing temperatures) on MgO(001) are studied by this new combined vector MOKE technique. Here, a two-domain switching process can be distinguished from a four-domain switching process generated by the incoherent rotation between magnetization saturation and magnetic remanence. Furthermore, UMA constants and domain wall pinning energies are determined using the Stoner-Wohlfarth model. For the Fe films the domain wall pinning energies obtained from these analyses decrease with increasing lateral crystallite sizes determined by GIXRD. For the Co50Fe50 films the domain wall pinning energies decrease with the annealing temperature due to fewer defects in the film. The magnetic structure of a thin B2 ordered Co50Fe50 film reveal two in-plane magnetic easy axes of different strengths which are not orthogonal. This atypical magnetic structure can be explained by a cubic magnetic anisotropy (CMA) induced by the crystalline film structure superimposed by an additional UMA which is not parallel to one of the magnetic easy axes of the CMA.

No description available.