No description available.

Thermal barrier coatings (TBCs) are advanced material systems used in the hot sections of gas turbines. The TBCs are designed to provide insulation against hot gases by a ceramic top coat and to provide oxidation and corrosion resistance by a metallic bond coat. As the operating environment is harsh and complex, the TBC often requires stricter material properties. Failure of TBCs can limit the longevity of the turbine severely. In this study, failure caused by erosion has been the main focus. The erosion behaviour of TBCs processed by atmospheric plasma spay (APS), electron beam physical vapour deposition (EB-PVD), and plasma spray physical vapour deposition (PS-PVD) has been studied by an experimental investigation and a literature study. The erosion performance of different TBCs was studied by conducting erosion tests under 90° and 15° alumina particle impact (50 μm) and measuring the weight loss and thickness loss of the ceramic top coat. Variables affecting the erosion behaviour were studied by means of scanning electron microscopy (SEM), investigating the microstructure, the erosion damage, porosity content, and column density. Hardness tests were also conducted to investigate a potential correlation between hardness and erosion performance. It was evident that the 8YSZ top coat processed by EB-PVD had higher erosion resistance than APS, which in turn had higher erosion resistance than PS-PVD. Their microstructures are significantly different, resulting in different erosion failure mechanisms. APS TBCs have a splat-on-splat lamellar microstructure, and the failure mechanismsare ploughing of furrows, splat boundary failure, and tunneling via pores. In contrast, EB-PVD TBCs have columnar microstructure and fail by near-surface cracking. The investigated PS-PVD TBC had a feathery columnar microstructure, containing many large grain boundaries and flaws, making grain boundary failure the governing mechanism. The APS and EB-PVD TBCs impacted at a 90° angle had significantly higher erosion rates than those eroded at 15°, which also was reported in literature. However, the opposite was observed for the PS-PVD TBCs. The level of porosity and hardness of the TBC top coat was found to affect the erosion rate, even though no evident correlations could be observed in this study. No factor alone was found to dictate the erosion behaviour of the investigated TBCs. Based on the literature study and findings in the experimental study, a TBC with good erosion performance has, in general, low porosity, few defects, high hardness and high fracture toughness. Specifically for APS TBCs, good splat bonding is favourable and for EB-PVD and PS-PVD it recommended to have high column density, columns orthogonal to the substrate, and low gap width between the columns.

<p>Power system maintenance optimisation involves obtaining the minimum total costs, including preventive and corrective maintenance costs and the cost of failures for both supplier and customer. To calculate the cost of failure, information is needed about the equipment reliability characteristics. It is also necessary to know how maintenance affects component reliability. The aim of the work leading up to this thesis has been to develop reliability models that include the effect of maintenance.</p><p>Three case studies have been carried out for different types of power system components using three distinct methods. In the first study the reliability of the first generation XLPE cables was modelled with respect to failures caused by water treeing using load-strength modelling. The model was based on assumptions of the ageing process and the distribution system characteristics. This study showed that it is possible to and overvoltage and insulation characteristics that can be fitted to agree with failure statistics for water tree ageing in XLPE cables. The second case study included a study of all circuit breaker failures in the Swedish transmission grid during the period from 1 January 1999 to 30 June 2003. In a subsequent investigation a method to combine information from the design process with maintenance records and failure statistics was employed using Bayesian methods. The resulting reliability model is continuously updated as more failure and maintenance data</p><p>becomes available. This case study showed that it is possible to develop reliability models for components that have not yet failed by utilising information from the design process and right-censored observations from inspections. Finally, in the third case study a quantitative method for establishing the condition of disconnector contacts by the use of thermography was developed. Two sets of measurements on disconnector contacts in the Swedish transmission</p><p>grid were carried out to establish the accuracy of the method. By utilising the results from the measurements estimates of the statistical distributions of the error sources were produced.</p><p>The results from the case studies show that the lack of detailed, high-quality data remains a critical problem when modelling reliability of power system equipment, even when using methods that require a minimum of data.</p>

No description available.

Die vorliegende Dissertation beschäftigt sich mit der Untersuchung des Ermüdungsverhaltens einer neu entwickelten Ti-40(wt%)Nb Legierung für die Anwendung als Implantatwerkstoff aus der Materialklasse der β-Titanlegierungen. Darüber hinaus werden auch vergleichende Ermüdungsuntersuchungen zu den Werkstoffen Ti-45(wt%)Nb als weitere binäre β-Titanlegierung sowie an α-Titan als klassischem Referenzwerkstoff im industriellen Einsatz durchgeführt. Ziel ist es, grundlegende Erkenntnisse zu den zyklischen Verformungsmechanismen binärer Ti-Nb Legierungen in Abhängigkeit ihres Werkstoffzustandes und damit Ihrer Mikrostruktur zu erhalten. Im Vergleich mit anderen Implantatwerkstoffen soll daraus eine mögliche Eignung für den Einsatz als nicht resorbierbares Osteosynthesematerial abgeleitet werden. Als übergeordnetes Ziel wird hierbei eine Minimierung des Elastizitätsmodules bei gleichbleibend hohen statischen und zyklischen Festigkeiten angestrebt, um einerseits schnellere Heilungserfolge zu erzielen als auch eine möglichst dauerhafte Implantatzuverlässigkeit zu gewährleisten. Für diesen Zweck wurden umfangreiche Ermüdungsprüfungen an Werkstoffproben und Osteosyntheseplatten in Abhängigkeit von Werkstoff, Mikrostruktur, Oberfläche, Umgebung und Geometrie durchgeführt. Mit Hilfe von strukturanalytischen Verfahren vor und nach der Werkstoffermüdung wurden diese begleitend detailliert charakterisiert. Insbesondere die Ergebnisse zum Einfluss des Werkstoffzustandes von Ti-40Nb erbrachten hierbei direkte Unterschiede und eine um ca. 30 % höhere Schwingfestigkeit im ausgelagerten Zustand aufgrund von vorliegenden isothermen Ausscheidungen der ω-Phase. Zurückgeführt wurde diese Erhöhung zum einen auf einen dominanten Effekt der Ausscheidungshärtung sowie die Unterdrückung von einer in binären Ti-Nb Legierungen möglichen Phasenumwandlung während der zyklischen Belastung. Stattdessen wurden nur im Ermüdungsrissbereich den Gleitbändern ähnliche Deformationsbänder (DB’s) beobachtet, die als ω-verarmte Versetzungskanäle die plastische Verformung tragen, während im rekristallisierten Zustand eine nahezu komplette Transformation in die martensitische α‘‘-Phase infolge der Ermüdung generiert wird. Der Einfluss der Oberfläche wird hierbei als signifikant herausgestellt, da vor allem Kerbeffekte die erreichbaren Schwingfestigkeiten der untersuchten Ti-Nb Legierungen massiv beeinflussen und zum vorzeitigen Versagen führen. Ein überlagerter Geometrieeinfluss, wie im Fall untersuchter Osteosyntheseplatten, hat einen zusätzlichen dominanten negativen Effekt auf das Ermüdungsverhalten. Ein negativer Einfluss eines korrosiven Umgebungsmediums am Beispiel einer Lösung aus simulierter Körperflüssigkeit (SBF) kann hingegen nicht festgestellt werden, was auf die exzellenten Selbstpassivierungseigenschaften von β-Titanlegierungen auf Ti-Nb Basis zurückgeführt wird. Insgesamt ergibt sich auf Grundlage der Ergebnisse dieser Arbeit eine nur bedingte Einsatzfähigkeit als Werkstoff für die Osteosynthese. Vorteile, wie der sehr geringe Elastizitätsmodul und die exzellente Widerstandfähigkeit gegen Korrosionsermüdung stehen einer allgemein nur ausreichenden Schwingfestigkeit sowie hoher Empfindlichkeit für Kerbeffekte jeglicher Art gegenüber. Eine effiziente Ausnutzung der Auslagerungseffekte sowie eine Oberflächennachbehandlung auf Basis von Druckeigenspannungen bieten jedoch weiteres hohes Potential für das Ermüdungsverhalten binärer Ti-Nb Legierungen. / Present thesis investigates the fatigue behavior of a newly developed Ti-40(wt%)Nb alloy from the material class of β-titanium alloys for application as implant material. Furthermore it investigates the fatigue behavior of the binary β-titanium alloy Ti-45(wt%)Nb and α-titanium as standard material already in industrial application for reference. Main focus are the detection of cyclic deformation mechanisms in dependence of material condition and the underlying microstructure. In comparison with other implant alloys, suitability of the new Ti-40Nb alloy as osteosynthesis material shall be evaluated. The overall aim is thereby the minimization of the Young’s modulus with simultaneously high values of static and cyclic strength for better healing chances and long term implant reliability. For this objective, comprehensive fatigue testing on material samples as well as osteosyn-thesis plates was carried out in dependence of material, microstructure, surface, environ-ment and sample geometry. Detailed structural analyses were additionally carried out be-fore and after fatigue testing. Compared with the recrystallized condition of the Ti-40Nb al-loy, fatigue results of the aged condition with present isothermal ω-precipitates in the β-matrix show a 30% higher fatigue strength. Uncovered reasons for this behavior are a domi-nant effect of precipitation hardening as well as the complete suppression of for β-titanium alloy commonly observed phase transformations during fatigue. Whereas the recrystallized condition of the Ti-40Nb alloy is characterized by significant martensitic α’’-phase formation and occasional mechanical twin development during fatigue, the aged condition shows none of these characteristics. Instead deformation bands (DB’s), similar to commonly overserved slip bands during fatigue, are detected in the immediate area of the fatigue crack. These localized DB’s carry all of the plastic deformation in the aged condition of the Ti-40Nb alloy as ω-depleted dislocation channels. Furthermore, the influence of the surface condition regarding the fatigue properties is sig-nificant. The reason are notch effects leading to localized stress concentration and early fa-tigue failure. Tests on the osteosynthesis plates show in addition a superimposed effect of the geometry, which leads to an even more decreased fatigue strength of Ti-40Nb and the reference α-titanium. A negative influence of corrosion fatigue in simulated body fluid (SBF) is on the contrary not detected, which is caused by the excellent self-passivating properties of β-titanium alloys on the basis of Ti-Nb. An overall evaluation of the material leads to an only limited suggestion as osteosynthesis material. Advantages are the very low Young’s modulus and the excellent corrosion fatigue resistance. Disadvantages are the only insufficient fatigue strength compared to other standard implant materials and the high susceptibility for notch effects leading to a rapid decrease of fatigue strength. Nevertheless, an efficient use of the positive effects of the aged condition as well as a surface post-treatment to induce positive residual compression stresses provide high potential for Ti-40Nb or the binary Ti-Nb alloys in general.

The topic of chemical oxidative degradation in rotational molded polyethylene (high-density cross-linked) chemical (sodium hypochlorite) storage tanks is an industry problem that ranks at the top of current business issues for manufacturers of chemical storage tanks. The degradation of these tanks not only compromises the physical and mechanical properties of the tank material, but reduces the life expectancy of the tank, eventually resulting in catastrophic tank failure. Premature tank failure comes at a hefty cost. The reputation of the manufacturer is questioned often resulting in immediate loss of customer satisfaction and future business. The leaking of the chemical from the failed tank serves as a liable environmental hazard that jeopardizes the safety and welfare of its surroundings – people and environment. And the associated manufacturer of the failed tank is almost certainly responsible for the repair or replacement of the tank. All these associated problems and many more related to chemical tank failure cost this relatively small industry millions of dollars annually. The need to determine the failure mechanisms of these tanks is critically important. Such an understanding will provide industry with useful knowledge that will open the door for improvements in tank performance. There is no question that a deeper understanding of failure mechanisms will improve a tank manufacturer's reputation, increase business sales, and assure environmental safety. The addition of this knowledge will also instill consumer confidence in an industry that is considered to lack refined manufacturing processes and proven quality controls. Such advancements are keys to making rotational molding a cutting-edge, technology-driven process that prepares industry for future growth and development. The purpose of this research is to provide tested empirical data and proven expert analysis that can be utilized by companies in understanding the failure mechanisms of these tanks. The information regarding this topic was collected from various tank samples taken from Poly Processing, a leading manufacturer of rotationally molded polyethylene chemical storage tanks and producer of the examined samples, and Odyssey Manufacturing, a manufacturer of bulk sodium hypochlorite and the end user of the examined samples. In the final chapter of this research, a summary is presented of the important findings regarding the purpose of the thesis study.

Na última década, muitas minas a céu aberto têm alcançado alturas de 600 metros ou mais, algumas com perspectiva em projeto de alcançar mais de 1100 m. A literatura especializada revela que os mecanismos de ruptura para taludes altos ainda não são bem entendidos. Existem dúvidas tanto em relação aos mecanismos de ruptura, como quanto à estimativa da resistência do maciço rochoso em tal escala. Recentemente, há uma tendência crescente da aplicação de análises numéricas para estudar a estabilidade de taludes altos, mas ainda não se consegue reproduzir todos os fenômenos envolvidos. Análises reportadas na literatura consideram apenas a configuração final da cava, sem levar em conta o processo evolutivo da escavação, e o dano induzido ao maciço decorrente deste processo. Este trabalho analisa este efeito e suas conseqüências na avaliação da segurança. Realizaram-se análises bidimensionais de tensão-deformação em taludes de rocha. Tais análises foram realizadas com modelos elástico linear e elasto plástico de amolecimento da coesão e de endurecimento do atrito, considerando a mobilização não simultânea das componentes de resistência no critério de Mohr-Coulomb, e a danificação do maciço rochoso. Avaliação preliminar da segurança de um talude hipotético mostrou que estas considerações são muito importantes. Foram considerados a altura do talude, o ângulo do talude e as tensões in situ. 0 histórico de tensões modifica os parâmetros de resistência do maciço ao longo do talude por danificação. A região do pé do talude, em cada estágio de escavação, está sujeita a concentração de tensões induzidas que geram danificação ao maciço nestas áreas. A danificação em regiões do pé do talude pode explicar o inicio do processo de rupturas do tipo progressivo. / Along the last decade, many open pit mines have reached up to 600 meters or more in height, and some of them are planned to reach more than 1100 meters. The specialized literature shows that the failure mechanisms for high rock slopes are not well understood as yet. Doubts exist in relation to failure mechanisms, as well as to rock mass strength estimation in such scale. In recent years, there is a growing trend for the use of numerical analyses in order to study high rock slope stability, but they are not capable to reproduce all the phenomena involved. Analyses reported in the literature consider only the final configuration of the open pit, without taking into consideration the excavation evolution process, and damage induced to the rock mass resulting from this process. This work analyzes this effect and its consequences on the slope safety evaluation. Two dimensional stress strain analyses in rock slopes are described. Such analyses were conducted with linear elastic model and elasto plastic strain cohesion softening - friction hardening model considering the non-simultaneous mobilization of the strength components in the Mohr-Coulomb criterion, by including the rock mass damage. An approximate safety evaluation of a hypothetical slope shows that these considerations are very important. The stress path modifies the rock mass strength parameters close to the slope face by damage. The regions of the slope toe at each excavation stage are subjected to induced stress concentration causing damage to rock. This damage can explain the beginning of the progressive failure mechanism.

Prognostics and health management (PHM) is a major tool enabling systems to evaluate their reliability in real-time operation. Despite ground-breaking advances in most engineering and scientific disciplines during the past decades, reliability engineering has not seen significant breakthroughs or noticeable advances. Therefore, self-awareness of the embedded system is also often required in the sense that the system should be able to assess its own health state and failure records, and those of its main components, and take action appropriately. This thesis presents a radically new prognostics approach to reliable system design that will revolutionise complex power electronic systems with robust prognostics capability enhanced Insulated Gate Bipolar Transistors (IGBT) in applications where reliability is significantly challenging and critical. The IGBT is considered as one of the components that is mainly damaged in converters and experiences a number of failure mechanisms, such as bond wire lift off, die attached solder crack, loose gate control voltage, etc. The resulting effects mentioned are complex. For instance, solder crack growth results in increasing the IGBT’s thermal junction which becomes a source of heat turns to wire bond lift off. As a result, the indication of this failure can be seen often in increasing on-state resistance relating to the voltage drop between on-state collector-emitter. On the other hand, hot carrier injection is increased due to electrical stress. Additionally, IGBTs are components that mainly work under high stress, temperature and power consumptions due to the higher range of load that these devices need to switch. This accelerates the degradation mechanism in the power switches in discrete fashion till reaches failure state which fail after several hundred cycles. To this end, exploiting failure mechanism knowledge of IGBTs and identifying failure parameter indication are background information of developing failure model and prognostics algorithm to calculate remaining useful life (RUL) along with ±10% confidence bounds. A number of various prognostics models have been developed for forecasting time to failure of IGBTs and the performance of the presented estimation models has been evaluated based on two different evaluation metrics. The results show significant improvement in health monitoring capability for power switches. Furthermore, the reliability of the power switch was calculated and conducted to fully describe health state of the converter and reconfigure the control parameter using adaptive algorithm under degradation and load mission limitation. As a result, the life expectancy of devices has been increased. These all allow condition-monitoring facilities to minimise stress levels and predict future failure which greatly reduces the likelihood of power switch failures in the first place.

abstract: Polymer matrix composites (PMCs) are attractive structural materials due to their high stiffness to low weight ratio. However, unidirectional PMCs have low shear strength and failure can occur along kink bands that develop on compression due to plastic microbuckling that carry strains large enough to induce nonlinear matrix deformation. Reviewing the literature, a large fraction of the existing work is for uniaxial compression, and the effects of stress gradients, such as those present during bending, have not been as well explored, and these effects are bound to make difference in terms of kink band nucleation and growth. Furthermore, reports on experimental measurements of strain fields leading to and developing inside these bands in the presence of stress gradients are also scarce and need to be addressed to gain a full understanding of their behavior when UDCs are used under bending and other spatially complex stress states. In a light to bridge the aforementioned gaps, the primary focus of this work is to understand mechanisms for kink band evolution under an influence of stress-gradients induced during bending. Digital image correlation (DIC) is used to measure strains inside and around the kink bands during 3-point bending of samples with 0°/90° stacking made of Ultra-High Molecular Weight Polyethylene Fibers. Measurements indicate bands nucleate at the compression side and propagate into the sample carrying a mixture of large shear and normal strains (~33%), while also decreasing its bending stiffness. Failure was produced by a combination of plastic microbuckling and axial splitting. The microstructure of the kink bands was studied and used in a microstructurally explicit finite element model (FEM) to analyze stresses and strains at ply level in the samples during kink band evolution, using cohesive zone elements to represent the interfaces between plies. Cohesive element properties were deduced by a combination of delamination, fracture and three-point bending tests used to calibrate the FEMs. Modeling results show that the band morphology is sensitive to the shear and opening properties of the interfaces between the plies. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2016

Na última década, muitas minas a céu aberto têm alcançado alturas de 600 metros ou mais, algumas com perspectiva em projeto de alcançar mais de 1100 m. A literatura especializada revela que os mecanismos de ruptura para taludes altos ainda não são bem entendidos. Existem dúvidas tanto em relação aos mecanismos de ruptura, como quanto à estimativa da resistência do maciço rochoso em tal escala. Recentemente, há uma tendência crescente da aplicação de análises numéricas para estudar a estabilidade de taludes altos, mas ainda não se consegue reproduzir todos os fenômenos envolvidos. Análises reportadas na literatura consideram apenas a configuração final da cava, sem levar em conta o processo evolutivo da escavação, e o dano induzido ao maciço decorrente deste processo. Este trabalho analisa este efeito e suas conseqüências na avaliação da segurança. Realizaram-se análises bidimensionais de tensão-deformação em taludes de rocha. Tais análises foram realizadas com modelos elástico linear e elasto plástico de amolecimento da coesão e de endurecimento do atrito, considerando a mobilização não simultânea das componentes de resistência no critério de Mohr-Coulomb, e a danificação do maciço rochoso. Avaliação preliminar da segurança de um talude hipotético mostrou que estas considerações são muito importantes. Foram considerados a altura do talude, o ângulo do talude e as tensões in situ. 0 histórico de tensões modifica os parâmetros de resistência do maciço ao longo do talude por danificação. A região do pé do talude, em cada estágio de escavação, está sujeita a concentração de tensões induzidas que geram danificação ao maciço nestas áreas. A danificação em regiões do pé do talude pode explicar o inicio do processo de rupturas do tipo progressivo. / Along the last decade, many open pit mines have reached up to 600 meters or more in height, and some of them are planned to reach more than 1100 meters. The specialized literature shows that the failure mechanisms for high rock slopes are not well understood as yet. Doubts exist in relation to failure mechanisms, as well as to rock mass strength estimation in such scale. In recent years, there is a growing trend for the use of numerical analyses in order to study high rock slope stability, but they are not capable to reproduce all the phenomena involved. Analyses reported in the literature consider only the final configuration of the open pit, without taking into consideration the excavation evolution process, and damage induced to the rock mass resulting from this process. This work analyzes this effect and its consequences on the slope safety evaluation. Two dimensional stress strain analyses in rock slopes are described. Such analyses were conducted with linear elastic model and elasto plastic strain cohesion softening - friction hardening model considering the non-simultaneous mobilization of the strength components in the Mohr-Coulomb criterion, by including the rock mass damage. An approximate safety evaluation of a hypothetical slope shows that these considerations are very important. The stress path modifies the rock mass strength parameters close to the slope face by damage. The regions of the slope toe at each excavation stage are subjected to induced stress concentration causing damage to rock. This damage can explain the beginning of the progressive failure mechanism.