On the initiation of boudinage and folding instabilities in layered elasto-visco-plastic solids - Insights from natural microstructures and numerical simulations

The present understanding of the initiation of boudinage and folding structures is based on viscosity contrasts and stress exponents, considering an intrinsically unstable state of the layer. The criterion of localization is believed to be prescribed by geometry-material interactions, which are often encountered in natural structures. An alternative localization phenomenon has been established for ductile materials, in which instability emerges for critical material parameters and loading rates from homogeneous conditions. In this thesis, conditions are sought under which this type of instability prevails and whether localization in geological materials necessarily requires a trigger by geometric imperfections. The relevance of critical deformation conditions, material parameters and the spatial configuration of instabilities are discussed in a geological context. In order to analyze boudinage geometries, a numerical eigenmode analysis is introduced. This method allows determining natural frequencies and wavelengths of a structure and inducing perturbations on these frequencies. In the subsequent coupled thermo-mechanical simulations, using a grain size evolution and end-member flow laws, localization emerges when material softening through grain size sensitive viscous creep sets in. Pinch-and-swell structures evolve along slip lines through a positive feedback between the matrix response and material bifurcations inside the layer, independent from the mesh-discretization length scale. Since boudinage and folding are considered to express the same general instability, both structures should arise independently of the sign of the loading conditions and for identical material parameters. To this end, the link between material to energy instabilities is approached by means of bifurcation analyses of the field equations and finite element simulations of the coupled system of equations. Boudinage and folding structures develop at the same critical energy threshold, where dissipative work by temperature-sensitive creep overcomes the diffusive capacity of the layer. This finding provides basis for a unified theory for strain localization in layered ductile materials. The numerical simulations are compared to natural pinch-and-swell microstructures, tracing the adaption of grain sizes, textures and creep mechanisms in calcite veins. The switch from dislocation to diffusion creep relates to strain-rate weakening, which is induced by dissipated heat from grain size reduction, and marks the onset of continuous necking. The time-dependent sequence uncovers multiple steady states at different time intervals. Microstructurally and mechanically stable conditions are finally expressed in the pinch-and-swell end members. The major outcome of this study is that boudinage and folding can be described as the same coupled energy-mechanical bifurcation, or as one critical energy attractor. This finding allows the derivation of critical deformation conditions and fundamental material parameters directly from localized structures in the field.

What makes next generation members found their own firm: knowledge, contacts, or money?

To increase our understanding of the formation of students' intentions to found an own firm, research needs to systematically integrate theory of planned behavior, resource-based view, and family business literature. To date, however, an explicit and systematic integration of these perspectives cannot be found. We attempt to close this gap by explicitly investigating founding intentions of students with family business background. More specifically, we examine how the provision of human, social, and financial resources by the family affects students' desirability and feasibility perceptions, and ultimately founding intentions. Our analysis based on a sample of 14'290 students from 26 countries reveals that both desirability and feasibility perceptions mediate the relationships between all three types of resources and founding intentions. Interestingly, the provision of financial resources is negatively related to both desirability and feasibility perceptions. These findings illustrate the research potential of a combination of theory of planned behavior with the resource-based view, especially in the family business context. Our study thus offers valuable contributions to literature on career choices, theory of planned behavior, and family business, as well as to practice.

History of monetary systems: record of actual experiments in money made by various states of the ancient and modern world,

Mode of access: Internet.

The meaning of money /

Mode of access: Internet.