TUNING THE ELECTRONIC AND SPINTRONIC PROPERTIES OF GRAPHENE BY DOPING AND ATOM ADSORPTION

Ph.DDOCTOR OF PHILOSOPH

Influence of geometrical imperfections and flaws at welds of steel liners on fatigue behavior of pressure tunnels and shafts in anisotropic rock

The recent development of high-strength (HSS) weldable steels has enlarged the range of design alternatives for the optimization of high-head steel-lined pressure tunnels and shafts (SLPT&S) in the hydropower industry. With the liberalization of the European energy market and increasing contribution of new renewable volatile energies in the electricity grid due to high subsidies, storage hydropower and pumped-storage plants are subject to more and more severe operation conditions resulting in more frequent transients. The use of HSS allows the design of thinner and thus more economic steel liners. However, welded HSS do not provide higher fatigue resistance than lower steel grades, and may be particularly subject to the risk of cold cracking in the weld material as dramatically illustrated by the failure of the Cleuson-Dixence pressure shaft in 2000. Fatigue behavior may become the leading limit state criterion. This research project aims at improving the comprehension of the mechanical behavior of SLPT&S and at developing a framework for probabilistic fatigue crack growth and fracture assessment of crack-like flaws in the weld material of longitudinal butt welded joints, considering all possible steel grades for high-head hydropower schemes. The influence of anisotropic rock behavior and geometrical imperfections at the longitudinal joints on the structural stresses have been studied by means of the finite element method accounting for the interaction with the backfill concrete-rock multilayer system. Parametric correction factors have been derived to estimate stress concentrations and structural stresses in steel liners with ease in practice, allowing the use of $S$-$N$ based engineering fatigue assessment approaches. Stress intensity factors (SIF) for axial cracks in the weld material of the longitudinal joints have also been obtained by means of computational linear elastic fracture mechanics (LEFM). The use of the previously developed parametric equations in the classical formulas for SIF in cracked plated structures has been validated, and new parametric equations for the weld shape correction have been proposed. A probabilistic model for fatigue crack growth assessment has been developed in the framework of LEFM in combination with the Paris-Erdogan law. The probability of failure is estimated by means of the Monte Carlo simulation procedure, in which the crack growth rate parameters and the crack shape ratio are defined as stochastic variables. A week-long normalized loading spectrum derived from prototype measurements on an alpine pumped-storage hydropower plant in Switzerland is used. This approach provides relative and quantitative results through parametric studies, giving new insights on the fatigue behavior of steel liners containing cracks in the weld material of the longitudinal joints. Finally, a fatigue assessment case study is presented, detailing the entire calculation procedures developed in this research. It aims at ensuring the transfer of knowledge toward practitioners

Parametric Study of Steel-Lined pressure Shafts in Anisotropic Rock

Steel-lined pressure tunnels and shafts are used to convey water from the reservoir to the turbines in high-head hydroelectric power plants. An axisymmetrical multilayer model is of-ten considered by engineers for the design of the steel liner. Stresses and deformations can then be computed with a closed-form analytical solution in isotropic rock. When the rock is anisotropic, the lowest elastic modulus of the rock measured in situ is often considered in the analytical solu-tion, which is regarded as conservative. In this work, the behaviour of steel liners in anisotropic rock was studied systematically by means of the Finite Element Method (FEM). The materials, namely steel, backfill concrete, near-field rock and far-field rock were modelled as linear and elas-tic, and a tied contact was assumed between the layers. The influence of geometrical and material parameters under a quasi-static internal water pressure was studied through an extensive para-metric study. It was observed that, compared to the corresponding results in isotropic rock, maxi-mum stresses in the steel-liner can be reduced up to 25% when anisotropy is considered. On the contrary, the maximum stresses in the far-field rock can be largely underestimated, namely up to 65%

Flow field UVP measurements of a Y-shape outlet structure

Y-shape outlet structures are used to release the turbine water into a reservoir. Classical design oftenuses a Y-shape over a certain length and with a specific opening angle. Depending on the geometry,especially in turbine mode, significant undesired flow separations may occur at the bifurcation.Investigations were performed with physical model tests using ultrasonic profilers at 1 and 2 MHzemission frequencies in order to assess an approach to control the design of an outlet structure bymeasuring the flow field and its fluctuations. The distribution and fluctuation of the flow velocities, bothat the exit section as well as along the tunnel axes for both outlet openings of the Y-shape geometryallow detecting potential flow detachment at the bifurcation

Contraintes aux soudures longitudinales des blindages sous l'effet de l'anisotropie du rocher et des imperfections géométriques

L’utilisation des aciers à haute-résistance permet de concevoir des blindages plus fins pour les puits et galeries blindés en charge des aménagements hydroélectriques. Cependant, ces aciers, bien que présentant une meilleure résistance élastique sous tension, ne sont en général pas plus résistants à la fatigue que les nuances d’acier plus basses lorsqu’ils sont soudés. Dans le contexte changeant du marché électrique européen, les conditions opératoires deviennent de plus en plus extrêmes pour les aménagements hydroélectriques, et plus particulièrement pour les aménagements de pompage-turbinage, qui opèrent pour balancer le réseau. Pour l’estimation de la durée de vie à la fatigue des blindages, les contraintes structurelles et locales aux soudures doivent être connues. En vue de cette analyse, cet article présente une méthode de calcul efficace de ces contraintes aux soudures longitudinales des viroles en acier, notamment en tenant compte des imperfections géométriques et de l’interaction avec une roche anisotrope

Advanced models for stress evaluation and safety assessment in steel-lined pressure tunnels

An accurate estimation of the stress in a steel liner is required for the design of steel-lined pressure shafts, both with regard to safety assessment of ageing steel-lined waterways of hydropower plants built in the 20th century, and for new projects which may be subject to harsher operational conditions. This paper proposes some new considerations for enhancing the standard calculation model, developed by the authors in the scope of practical engineering needs, by introducing more complexity

Scattering theory of spin-orbit active adatoms on graphene

The scattering of two-dimensional massless Dirac fermions from local spin-orbit interactions with an origin in dilute concentrations of physisorbed atomic species on graphene is theoretically investigated. The hybridization between graphene and the adatoms' orbitals lifts spin and valley degeneracies of the pristine host material, giving rise to rich spin-orbit coupling mechanisms with features determined by the exact adsorption position on the honeycomb lattice - bridge, hollow, or top position - and the adatoms' outer-shell orbital type. Effective graphene-only Hamiltonians are derived from symmetry considerations, while a microscopic tight-binding approach connects effective low-energy couplings and graphene-adatom hybridization parameters. Within the T-matrix formalism, a theory for (spin-dependent) scattering events involving graphene's charge carriers, and the spin-orbit active adatoms is developed. Spin currents associated with intravalley and intervalley scattering are found to tend to oppose each other. We establish that under certain conditions, hollow-position adatoms give rise to the spin Hall effect, through skew scattering, while top-position adatoms induce transverse charge currents via trigonal potential scattering. We also identify the critical Fermi energy range where the spin Hall effect is dramatically enhanced, and the associated transverse spin currents can be reversed

Anomalous spectral features of a neutral bilayer graphene

10.1038/srep10025Scientific Reports