Modelling of rolling contact fatigue for rail materials being subject to severe plastic deformation

Modern railway systems are a beneficial way of transport. However, the operation requires maintenance to compensate for rail degradations. Key issues of degradation regard severe plastic deformation and rolling contact fatigue, which lead to the formation of cracks. A better understanding contributes to improve maintenance and operations. Numerical simulations offer ways to predict these degradations. State of the art for the simulation of solid mechanics is the Finite Element Method. An inherent limitation, though, is the basic assumption that the material is a continuum. Discontinuities like cracks violate this assumption. Thus, alternatives like Discrete Element Method models and Peridynamics were developed recently. These methods describe the material by a modular assembly of elements. This arrangement allows for a better modelling of discontinuities. Though, modelling of rolling contacts is a case where compressive loads dominate. This fact challenges discrete modelling approaches due to the common assumption that the failure of an element implies its removal. Another challenge models of this type face regards the validation of rolling contact fatigue. The aim of this work was to develop a discrete element model, which is optimised for the rolling contact application. For this reason, it was called the "Discrete Element Rolling Contact" model. Based on a linear-elastic model, a fatigue capability was introduced. This capability was transferred to the set-up of the rolling contact, which highlighted the first limitation stated above. In order to address this issue, a solution was developed and validated to calibrate the model to crack closure. Experimental results indicate that the fatigue crack growth behaviour of rails is governed by the severe plastic deformation the materials are subject to. For this reason, data of undeformed and deformed materials was parameterised and validated. A method to interpolate materials of varying degrees of deformation was introduced. Further, a method to transfer the material parameters to other materials of similar strength was developed. The result was a material library which is available for the model. In order to transfer the fatigue parameters to the rolling contact condition, a set-up to model the behaviour of highly shear deformed material was introduced. In this way, a drawback of the adopted fatigue law was highlighted, which emphasised the requirement to weight the influence of compressive strains. Finally, a procedure for the validation of the rolling contact model was proposed. In this work, a novel rolling contact fatigue model and a procedure for validation are presented, that consider the anisotropy of severe plastic deformation. With regard to the validation of discrete rolling contact models, the proposed procedure delivers a contribution that addresses key aspects that are not matured yet. These aspects, however, are critical for the development of dependable discrete models to predict rolling contact fatigue

Modernisierung eines Gasturbinenprüfstandes

Der Inhalt dieser Arbeit umfasst einen von mehreren Modernisierungsschritten eines Kleingasturbinen-Prüfstandes. Bei der Kleingasturbine handelt es sich um eine ROVER 1S60 des Instituts Energietechnik und Thermodynamik der TU Wien. Bei der Modernisierung wurde die Messtechnik erneuert, und eine neue Leistungsbremse angeschlossen, was jedoch nur dokumentiert wurde, und nicht Teil der Arbeit ist. Weiters wurde ein Betriebsmanual erstellt, mit dem die Maschine sicher betrieben werden kann.The thesis is about the upgrade of a stationary gas turbine testbed used by the Vienna Technical University / Institute for Energy Technology and Thermodynamics. The testbed uses a ROVER 1S60 which was developed for low-power applications. In the diploma thesis the renewal of measurement instruments, and the influence on operation of the new eddy current brake are discussed. Further an operation manual is added to the thesis in order to operate the testbed in a correct and safe way.von Markus JanuschewskyAbweichender Titel laut Übersetzung der Verfasserin/des VerfassersWien, Techn. Univ., Dipl.-Arb., 2009(VLID)160955

Upgrade of a gas turbine testbed

Abweichender Titel laut Übersetzung der Verfasserin/des VerfassersDer Inhalt dieser Arbeit umfasst einen von mehreren Modernisierungsschritten eines Kleingasturbinen-Prüfstandes. Bei der Kleingasturbine handelt es sich um eine ROVER 1S60 des Instituts Energietechnik und Thermodynamik der TU Wien. Bei der Modernisierung wurde die Messtechnik erneuert, und eine neue Leistungsbremse angeschlossen, was jedoch nur dokumentiert wurde, und nicht Teil der Arbeit ist. Weiters wurde ein Betriebsmanual erstellt, mit dem die Maschine sicher betrieben werden kann.The thesis is about the upgrade of a stationary gas turbine testbed used by the Vienna Technical University / Institute for Energy Technology and Thermodynamics. The testbed uses a ROVER 1S60 which was developed for low-power applications. In the diploma thesis the renewal of measurement instruments, and the influence on operation of the new eddy current brake are discussed. Further an operation manual is added to the thesis in order to operate the testbed in a correct and safe way.8

Investigations on structure-property relations of (La,Sr)CoO3-[delta] thin film SOFC cathodes

In this thesis (La,Sr)CoO3-d thin films were prepared by PLD and a simple sol-gel route. The electrochemical performance towards oxygen reduction was investigated by impedance spectroscopy. Structural and morphological investigations were performed using XRD, SEM, AFM and TEM.The surface composition was investigated by LEIS and XPS. From that data several structure-property relations were found.The structure and electrochemical performance of (La,Sr)CoO3-d thin-film electrodes prepared by PLD on single crystalline YSZ strongly depends on the deposition temperature of the films. Surprisingly thin films deposited at rather low temperatures (340-510°C) and exhibiting hardly any signal in Bragg-Brentano XRD patterns showed the best electrochemical performance. It was possible to prepare LSC film electrodes with polarization resistances as low as 0.1 Ohmcm² at 600°C.Many electrodes showed a pronounced degradation when operating between 500 and 700°C. Interestingly, the films exhibiting the lowest resistances showed the lowest degradation within a few days of operation. It was further possible to employ a sol-gel route to prepare electrodes with the same performance as the electrodes prepared by PLD. Again it seems that there is a correlation between the degree of crystallinity and the resistance. As the sol-gel thin films are smooth and dense and have no open porosity this extremely fast oxygen reduction cannot be attributed to a strongly increased surface area. It seems to be related to the lower crystallinity or a consequence of the lower crystallinity. The surface composition was investigated by LEIS and XPS. Different results concerning the surface composition of as prepared and heat treated samples deposited at 450°C and 650°C are achieved.10

Discrete element modelling of rolling contact fatigue and crack closure with different bond laws

The prediction of rolling contact fatigue requires modelling methods that efficiently simulate the propagation, branching and coalescence of fatigue cracks. The developed Discrete Element Method (DEM) model is a novel approach to provide this capability. The model is based on discrete elements (bonds) that are subject to a fatigue law. With it, the failure of elements due to cyclic loads can be simulated. The propagating cracks are not confined to predefined paths, but develop according to the local loading conditions. These conditions depend on an appropriate set-up of the element interaction, i.e. the bond law, which is calibrated, taking into account crack closure effects. The method delivers a relevant improvement for DEM- and peridynamic models, where compressive loads dominate. The current development of the model is summarised in this paper

La0.6Sr0.4CoO3-δ (LSC) thin film electrodes with very fast oxygen reduction kinetics prepared by a solgel route

La0.6Sr0.4CoO3-δ (LSC) thin film electrodes of about 80 nm thickness were prepared via a sol-gel route on yttria-stabilized zirconia (YSZ) and gadolinium doped ceria (GDC) solid electrolytes. Impedance measurements on microelectrodes fabricated from these films revealed very low polarization resistances for electrochemical oxygen exchange at intermediate temperatures (500–600 ºC). Prerequisite for the very fast oxygen reduction kinetics was a limitation of the preparation (annealing) temperature to 600 ºC. Already after ca. 15 minutes annealing of the sol-gel prepared layer at this temperature, electrodes were electrochemically highly active despite low crystallinity (almost XRD amorphous). LSC electrodes on GDC showed much higher polarization resistances. However, this originated from an additional interfacial resistance rather than from kinetically slow LSC surfaces