La durabilité des miroirs solaires est un point clé pour le développement des centrales solaires à concentration, du fait de l’investissement important, des 30 ans d’opération visés et des zones d’implantations aux climats agressifs. Ces travaux de recherche portent ainsi sur l’étude des modes de dégradation et de la durabilité de miroirs solaires en verre monolithiques ou laminés. Cette thèse a permis d’appliquer avec succès au CSP une méthodologie de prédiction de la durée de vie développée dans des domaines plus matures. Les corrélations entre les vieillissements accélérés et les expositions extérieures réalisées sur trois sites ont mis en avant le rôle prédominant de l’eau et de l’irradiance dans la dégradation des peintures de protection des miroirs monolithiques. Les essais en chaleur humide ont quant à eux été jugés trop agressifs pour les miroirs, considérant de plus les conditions climatiques des sites d’application potentiels. Les résultats expérimentaux des essais de vieillissements accélérés ont conduit à la détermination des paramètres cinétiques des miroirs inclus dans les relations mathématiques modélisant les principaux facteurs de stress identifiés précédemment. Les facteurs d’accélération ont ainsi été calculés pour des essais standards par rapport à des sites comportant des centrales CSP opérationnelles. Ce travail a permis au final d’estimer des durées de vie des miroirs sur les sites choisis, en considérant que seuls les facteurs de stress étudiés intervenaient dans la dégradation. / Durability of solar mirrors is a key point for the development of concentrating solar power plants, because of the large investment, the goal of 30 years lifetime and of the implantations areas with hostile climates. This research work concerns the study of degradation modes and of monolithic or laminated glass solar mirrors durability. This thesis allows to successfully apply to CSP a lifetime prediction method already developed in more mature fields. Correlations between accelerated ageing tests and natural outdoor exposures performed on three sites evidence the predominant role of water and irradiance in protection paints of monolithic mirrors degradation. Damp heat tests were evaluated too aggressive for mirrors, considering thus the climatic conditions of potential application sites. Experimental results of accelerated ageing tests lead to the determination of kinetic parameters included in mathematical relationships modelling the main stress factors previously identified. Acceleration factors were then calculated for standard tests compared to sites with operational CSP plants. This work finally allows to estimate lifetimes of mirrors on specific sites, assuming that only the studied stress factors take part in degradation.

Le PA 6,6 est maintenant envisagé pour des pièces de robinetterie sur le réseau domestique de distribution d'eau potable. Cependant, ce matériau ne pourra être utilisé que si sa durabilité à long terme, en conditions réelles de service, est clairement démontrée. L'analyse cinétique des principaux mécanismes de vieillissement chimique susceptibles de se produire et d'interagir sur le réseau domestique de distribution d'eau potable : l'hydrolyse, la thermo-oxydation et l'attaque chimique par le dioxyde de chlore, a permis au final d'élaborer un modèle cinétique général de dégradation du PA 6,6. Ce modèle donne accès aux modifications structurales du PA 6,6 aux différentes échelles pertinentes (moléculaire, macromoléculaire, morphologique et macroscopique) en conditions de service. Il prédit la fragilisation du PA 6,6 lorsque la masse molaire moyenne en nombre atteint une valeur critique de l'ordre de 17 kg/mole. De nombreuses perspectives de recherche sont présentées comme l'introduction de ce modèle cinétique dans les codes courants de calcul mécaniques. / PA 6,6 is now considered for the elaboration of valve parts in the domestic network of drinking water distribution. However, this material will be used only if its long-term durability, in real use conditions, is clearly demonstrated. The kinetic analysis of the main chemical ageing mechanisms which may occur and interact in the domestic network of drinking water distribution: hydrolysis, thermal oxidation and chemical attack by chlorine dioxide, has allowed to finally build a general kinetic model for PA 6,6 degradation. This model gives access to PA 6,6 structural modifications at the different pertinent scales (molecular, macromolecular, morphological and macroscopic scales) in use conditions. It predicts the embrittlement of PA 6,6 when the number average molar mass reaches a critical value of the order of 17 kg/mole. Many research prospects are presented such as the introduction of this kinetic model in common mechanical calculation codes.

Le développement d'outils numériques de prédiction de la durée de vie des polymères constitue un levier prometteur pour réduire les durées des processus de certification de ces matériaux dans le domaine automobile sans sacrifier leur fiabilité. Cette thèse s'applique à la modélisation de la photo-thermo-oxydation du polypropylène isotactique (iPP), laquelle est responsable de l'altération de ses propriétés mécaniques et d'aspect. L'approche adoptée consiste à coupler la cinétique des réactions de photo- et thermo-oxydation avec des phénomènes physiques, comme le transport du dioxygène et l'atténuation de la lumière UV dans l'épaisseur du matériau, pour décrire l'ensemble des évolutions physico-chimiques. Les propriétés aux échelles supérieures, sur lesquelles seront définis les critères de fin de vie, seront calculées a posteriori en appliquant les relations structure-propriété adéquates. Le principal enjeu était d'étendre le modèle cinétique de vieillissement thermique préexistant au vieillissement photo-thermique en prenant en compte les réactions d'amorçage photolytique. De lourdes campagnes d'essais de vieillissement et de caractérisation menées sur un iPP de référence, ainsi qu'une capitalisation exhaustive des données de la littérature d'autres iPPs, ont permis de mettre au point un modèle cinétique de photo-thermo-oxydation et de le généraliser à l'ensemble de la famille des iPPs dans de larges domaines de pression partielle d'oxygène (de 0.2 à 50 bars), de température (de 40 à 230°C) et d'exposition à la lumière UV (intensités et sources lumineuses variables) décrivant des conditions de vieillissements naturels et accélérés. La validation expérimentale du modèle a permis d'étayer l'approche cinétique et de montrer ses limites, mais aussi de révéler un certain nombre d'enjeux numériques. Le modèle a été conçu pour être un outil numérique évolutif qui permettra, à terme, d'optimiser la représentativité des méthodes d'essais de vieillissement et la performance des formulations commerciales d'iPP. L'ensemble de ces développements théoriques et numériques peut être appliqué à la photo-thermo-dégradation d'autres types de polymères, mais aussi dans d'autres champs d'application de la photochimie macromoléculaire telle que la photo-polymérisation UV.Mots-Clés : Polypropylène, photo-thermo-oxydation, contrôle par la diffusion d'oxygène, effet d'écran, modélisation cinétique, prédiction de durée de vie. / Developing numerical tools for polymer lifetime prediction constitutes a promising opportunity for shortening the duration of material certification procedures in the automotive industry without decreasing their reliability. This PhD thesis aims at modeling the photothermal oxidation of isotactic polypropylene (iPP), which is responsible for the alteration of both its mechanical and aspect properties. The adopted approach consists in coupling the kinetics of photo- and thermo-oxidation reactions with physical phenomena, such as oxygen transport and UV-light attenuation in the material thickness, in order to describe all the physico-chemical changes. Upper-scale properties, from which will be defined the end-of-life criteria, will be calculated afterwards by applying the suitable structure-property relationships. The main challenge was to extend the pre-existing kinetic model of thermal ageing to photothermal ageing by taking into account initiation reactions of photolysis. Heavy campaigns of ageing and characterization tests made on a reference iPP, as well as an exhaustive capitalization of literature data of other iPPs, have allowed elaborating a kinetic model of photothermal oxidation and to generalize it to the whole iPP family in large domains of oxygen partial pressure (from 0.2 to 50 bars), temperature (from 40 to 230°C) and UV-light exposure (variable intensities and light sources) describing both natural and accelerated ageing conditions. The experimental validation of the model has allowed substantiating the kinetic approach and showing its limitations, as well as highlighting some numerical issues. The model has been designed in order to be an upgradable numerical tool which will allow, at term, optimizing the representativeness of the ageing testing devices and the performance of commercial iPP formulations. All these theoretical and numerical developments are prone to be applied to the photothermal degradation of other types of polymer substrates, but also in other application fields of the macromolecular photochemistry such as UV-photopolymerization.Keywords: Polypropylene, photothermal oxidation, oxygen diffusion control, screen effect, kinetic modeling, lifetime prediction.

Using experimentally measured temperature-process-dependent model parameters, the failure analysis and life prediction were conducted for Atmospheric Plasma Sprayed Thermal Barrier Coatings (APS-TBCs) and electron beam physical vapor deposition thermal barrier coatings (EB-PVD TBCs) with Pt-modified -NiAl bond coats deposited on Ni-base single crystal superalloys. For APS-TBC system, a residual stress model for the top coat of APS-TBC was proposed and then applied to life prediction. The capability of the life model was demonstrated using temperature-dependent model parameters. Using existing life data, a comparison of fitting approaches of life model parameters was performed. The role of the residual stresses distributed at each individual coating layer was explored and their interplay on the coating’s delamination was analyzed. For EB-PVD TBCs, based on failure mechanism analysis, two newly analytical stress models from the valley position of top coat and ridge of bond coat were proposed describing stress levels generated as consequence of the coefficient of thermal expansion (CTE) mismatch between each layers. The thermal stress within TGO was evaluated based on composite material theory, where effective parameters were calculated. The lifetime prediction of EB-PVD TBCs was conducted given that the failure analysis and life model were applied to two failure modes A and B identified experimentally for thermal cyclic process. The global wavelength related to interface rumpling and its radius curvature were identified as essential parameters in life evaluation, and the life results for failure mode A were verified by existing burner rig test data. For failure mode B, the crack growth rate along the topcoat/TGO interface was calculated using the experimentally measured average interfacial fracture toughness.

Power electronics, which fully covers the generation, conversion, transmission and usage of electrical energy, is a key technology for human welfare. With the development of technologies, the requirements on the reliability of power electronic systems are keep increasing. Long term operation under harsh environments is often accompanied by higher switching frequency and higher power density. To allow a reliable and sustainable performance of the power electronic systems, precise lifetime estimation of the power semiconductor devices is of significant importance. This work covers some aspects in the lifetime prediction of power semiconductor devices, especially IGBT and diode, in power module and transfer-molded discrete package. Difference in device temperature determination was illustrated using analytical calculation, simulation and measurement. In addition, temperature calculation in the frequency domain was demonstrated which gives benefits in the application with several hundred devices. Furthermore, different control strategies in the power cycling test were compared. The linear cumulative damage theory was validated by using the power cycling test. For the high power IGBT module used in the MMC HVDC application, power cycling lifetime with 50 Hz heating processes was investigated. For the transfer-molded discrete package, the first lifetime model with comparable scope like the lifetime model of power modules was proposed. / Leistungselektronik, welche direkt relevant für die Erzeugung, Umwandlung, Übertragung und Nutzung elektrischer Energie ist, ist eine Schlüsseltechnologie für das Wohl der Menschen. Mit der Entwicklung von Technologien steigen die Anforderungen an die Zuverlässigkeit leistungselektronischer Systeme. Der Langzeitbetrieb unter rauen Umgebungsbedingungen geht häufig mit einer höheren Schaltfrequenz und einer höheren Leistungsdichte einher. Um eine zuverlässige und nachhaltige Operation der leistungselektronischen Systeme zu ermöglichen, ist die genaue Lebensdauerabschätzung der Halbleiter-Leistungsbauelemente von großer Bedeutung. Diese Arbeit befasst sich mit einigen Aspekten der Lebensdauerabschätzung von den Halbleiter-Leistungsbauelementen. Unterschied in der Temperaturabstimmung der Halbleiter-Leistungsbauelemente wird anhand von Berechnung, Simulation und Messung veranschaulicht. Darüber hinaus bietet die Temperaturberechnung im Frequenzbereich Vorteile bei der Anwendung mit mehreren hundert Bauelementen. Darüber hinaus wurden verschiedene Regelstrategien im Lastwechseltest verglichen. Die lineare kumulative Alterungstheorie wurde unter Verwendung des Lastwechseltests validiert. Für das in der MMC-HGÜ-Anwendung verwendete Hochleistungs-IGBT-Modul wurden Alterungsprozesse bei 50 Hz Erwärmung untersucht. Für das Diskrete-Gehäuse wird das erste Lebensdauermodell vorgestellt, welches ein vergleichbares Anwendungsbereich wie das Lebensdauermodell von Leistungsmodulen hat.

In this paper, a new methodology is presented to estimate the lifetime impact of design changes, called Failure Likelihood Assessment (FLA). The discussion in this paper is on the fatigue lifetime prediction of axial piston units, especially after a design change. The demonstration object is an axial piston pump due to extreme environmental conditions and high specification demands, where the FLA is applied to a manufacturing change in an existing product and delivers an effect on the unit reliability. The resulted reliability imp rovement, if combined with typical calculation methods like Weibull analysis, delivers an increase in predicted lifetime considering the intended modification. As demonstration subje ct, a change of the manufacturing process of the cylinder block hub in an axial piston pump is used. The effect to the lifetime is predicted via the FLA-method and the results are calculated with test data and compared to theoretical results. The paper shows that the methodology delivers highly accurate results providing that the FLA is a powerful tool to analyze design changes as weil as new designs in regard to reliability and lifetime. The benefit for the user of this methodology will hence be more reliable products with optimized designs tobest fulfil customer's performance requirements.

The thermal-oxidative degradation of a crosslinked hydroxy-terminated polybutadiene (HTPB)/cycloaliphatic diisocyanate (H12MDI) based polymer, which is commonly used as a polymeric binder in propellants, is investigated at temperatures from 95°C to 125°C with the aim of estimating the lifetime of the material in storage conditions (20°C) using the Arrhenius approach. Furthermore, the effect of antioxidants and to a lesser extent plasticizer on the degradation process was also studied. Diffusion-limited oxidation (DLO) was theoretically modelled and DLO conditions were estimated by gathering oxygen permeability and consumption data from similar studies. It was concluded that DLO-effects might be present at the highest experiment temperature (125°C) depending on the actual properties of the material investigated. The mechanical degradation was monitored by conducting tensile tests in a DMA apparatus and photographs using a microscope was taken to examine potential DLO effects. The degradation process of the stabilized polymer (with antioxidant) did not showcase Arrhenius behaviour, which was confirmed by the failure to construct a satisfactory mastercurve. This was most likely due to loss of antioxidants, resulting in autocatalytic oxidation(acceleration of the oxidation process). However, the induction period of the stabilized polymer showcased Arrhenius behaviour in the temperature region 95-125°C with an ~E_a = 90 kJ/mol. If the activation energy E_a is assumed to remain constant, the lifetime at ambient temperature (20°C) is predicted to be approximately 176 Years for a 2mm thick sample. However, this is probably an overestimation since curvature in the Arrhenius plot has been observed for many rubber materials in the lower temperature region. Assuming the E_a drops from ~90 kJ/mol to~71 kJ/mol, a more conservative lifetime prediction of 58 Years was estimated.

Sjöfarten står för 3 % av världens totala växthusgaser och är idag högst beroende av fossila bränslen. Ett alternativ för att gå över till en fossilfri flotta är användning av bränsleceller och vätgas som drivmedel. Om vätgasen produceras från elektrolys med förnyelsebara energikällor så är driften utsläppsfri och koldioxidneutral. Bränsleceller kan användas i många olika sammanhang, men har ännu inte slagit igenom med full kraft. En anledning till detta är livslängden som är för kort. För att sjöfarten ska kunna implementera bränslecellsdrift behöver nedbrytningen av bränslecellen bli vidare utforskad och motverkad. Syftet med detta examensarbete var att hitta de mest signifikanta nedbrytningsmekanismerna för sjöfarten samt att bygga en modell för att förutspå livslängden utifrån lastprofiler från fartyg.  Rapportens avgränsningar var att enbart studera PEM-bränsleceller tack vare dess höga energitäthet och att teknologin är närmast marknaden för mobila applikationer. En litteraturstudie genomfördes för att fastställa de viktigaste nedbrytningsmekanismerna samt de faktorer som begränsar livslängden. Dessa bestämdes till start/stop cykler samt lastcykler vilka försämrar konduktiviteten i membranet samt minskar den elektrokemiska ytarean. En empirisk modell från experiment från litteraturen etablerades för att hitta relationen mellan parametrarna konduktivitet och elektrokemisk ytarea som funktion av start/stop cykler respektive lastcykler. En Comsol-modell användes för att utvärdera bränslecellens prestanda med dessa försämrade parametrar. Två lastprofiler analyserades och tolkades som antal start/stop cykler samt lastcykler för att utvärdera prestandan som funktion av antal år i drift. Båda fallen var i drift till dess att prestandan minskat med 20 % utvärderat vid strömtätheten 0.6 A/cm2. Båda lastprofilerna var utvärdera med olika körstrategier för att jämföra den modellerade livslängden beroende på ingångsvärden. Den första lastprofilen delades in i Case 1a och Case 1b där antalet start/stop cykler och lastcykler varierade. Resultatet visade att antalet timmar i drift mer än tredubblades i Case 1b där båda ingående indata hade minskats.   Case 2 delades upp i tre olika körstrategier där Case 2a var en referens som jämfördes mot Case 2b respektive Case 2c. Skillnaden mot Case 2b var att antalet start/stop cykler per dag multiplicerades med faktor 4. Resultatet från modellen var att livstiden minskade från 6 till 4 år. Vidare utvärderades Case 2c där istället antalet lastcykler dividerades med faktor 4, allt annat identiskt med Case 2a. Utfallet var en ökad livslängd från 6 år till 11 år, med totalt 32 032 timmar i drift. Denna livslängd kan jämföras med kommersialiserade marina produkter från Ballard och Powercell, där livslängden är 30 000 respektive 20 000 timmar i drift.  Sammanfattningsvis kan det konstateras att både start/stop cykler och lastcykler bryter ner bränslecellen och därmed minskar dess prestanda. Lastcykler var den faktor som var mest förödande gällande livslängden. Den modellerade livslängden på 32 032 timmar indikerar att den empirisk modellen tillsammans med Comsol-modellen genererade realistiska resultat. Slutligen kan sägas att ett område för framtida forskning inom ämnet innefattar lastcykler eftersom denna hade störst påverkan på modellen. Att särskilja olika typer av lastcykler och koppla till olika degradering skulle skapa än mer förståelse för livslängden. Då denna studie genomfördes på bränslecellsnivå skulle framtida studier kunna inkludera att analysera degradering på systemnivå för att få mer insikt i dessa processer. / The shipping industry emits 3 % of the global GHG-emissions and is highly dependent on fossil fuels. One alternative to decarbonise the sector is by utilising hydrogen in fuel cells. The propulsion is free from emissions if hydrogen is produced from renewables. The fuel cell technology can be applied in various applications but have not been fully commercialised. One of the biggest bottlenecks for fuel cell technology is the durability that needs to be improved. In order for marine vessels to implement hydrogen propulsion, the degradation need to be further understood and mitigated. The purpose of this thesis was to assess the most significant degradation mechanisms for marine applications and to build a model to evaluate the lifetime depending on load profiles. The scope of the thesis was to include PEMFCs since they have the highest power density and are closest to commercialisation for transport applications. A literature review was conducted to assess the most important degradation mechanisms and operating conditions that limits the lifetime, which concluded in start/stop cycling and load cycling. These conditions deteriorate the membrane conductivity and the electrochemical surface area. An empirical model based on experimental data from the literature was conducted to find relationships for conductivity and ECSA as function of start/stop cycling and load cycling, respectively. A Comsol model was established to evaluate performance reduction with degraded parameters. Two different load cycles were interpreted as number of start/stop cycles and load cycles in order to simulate the degradation. The output was polarization curves as function of operating years. Each case was operated until a performance reduction of 20 % was obtained at the current density of 0.6 A/cm2.  Both load profiles were analysed with different operation strategies to compare the resulting lifetime. The first load curve was divided into Case 1a and Case 1b where start/stop cycles and load cycles were altered. The results showed that the lifetime in operation hours was more than three-folded in Case 1b when the number of start/stop cycles and load cycles was reduced.  Case 2 was divided into three operating strategies. For comparison with Case 2a, the number of start/stop cycles per day in Case 2b was increased by a factor of 4. The result was that the lifetime declined from 6 to 4 years. Furthermore, Case 2c evaluated the lifetime if the number of load cycles per day decreased by a factor of 4, all else being equal to Case 2a. The outcome was an increment in lifetime from 6 to 11 years, with a total of 32 032 hours of operation. This lifetime can be compared to commercialised products from Ballard and Powercell with lifetimes of 30 000 and 20 000 operating hours, respectively. Conclusively, the simulations showed that both start/stop cycling and load cycling deteriorates the fuel cell performance. Load cycling is the operating condition that cause the most severe degradation. Moreover, the modelled lifetime of 32 032 hours indicates that the empirical model in combination with the Comsol model generate realistic lifetimes. Finally, since load cycling had the most vital impact on the lifetime, one of the topics for future research would be to distinguish different types of load cycles and connect to separate degradation outcomes. Since the model was on fuel cell level, future work could also include systems effects such as ripple current or distributions within fuel cell stacks.

Polymer electrolyte membrane (PEM) fuel cell (FC) stacks require elastomeric gaskets for each cell to keep the reactant gases within their respective regions[1]. If any gasket degrades or fails, the reactant gases can leak or mix with each other directly during operation or standby, affecting the overall operation and performance of the FC. The elastomeric gaskets used as FC seals are exposed to a range of environmental conditions, and concurrently, subjected to mechanical compression between the bipolar plates forming the cell. The combination of mechanical stress and environmental exposure may result in degradation of the seal material[2] over a period of time. In order to address the durability and make reliability predictions, the long-term stability of the gaskets in FC assemblies is critical. The aim of this study is to investigate the performance of elastomeric seals in a simulated FC environment in the presence of mechanical stresses. The overall scope of the study includes mechanical and viscoelastic properties characterization, and lifetime durability predictions based on an accelerated characterization approach. With the help of finite element analysis software, ABAQUS, a fixture was designed to perform strain-based accelerated characterization of seal material in air, deionized (DI) water, 50v/50v ethylene glycol/water solution, and 0.1M sulfuric acid solution. Dogbone samples were strained to different levels in the custom fixture and submerged in liquid solutions at 90°C and in air at 90°C and 120°C. It was observed that mechanical properties such as tensile strength, strain to break, 100% modulus, crosslink density, and tensile set degrade due to aging and the extent of change (increase or decrease) depends significantly on the strain level on the specimen. Trouser tear tests were conducted on reinforced specimens in air and deionized water (DI) to evaluate the tear resistance of an elastomeric seal material intended for proton exchange membrane fuel cells. Plots relating the crack growth rate with tearing energy were obtained at various temperatures and provided significant insight into the rate and temperature dependence of the tearing strength of the seal material. Stick-slip crack propagation was observed at all temperatures and loading rates, although the behavior was suppressed significantly at low loading rates and high temperatures. Crack growth rate versus tearing energy data at different temperatures was shifted to construct a master curve and an estimate on the threshold value of tear energy was obtained which may be helpful in designing components where material tear is of concern. Strain energy release rate (SERR) value, calculated using the J-integral approach for a pre-existing crack in ABAQUS, was used to estimate the crack growth rate in a given seal cross-section to predict lifetime. In order to assess the viscoelastic behavior and to investigate the long term stress relaxation behavior of the seal material, compression stress relaxation (CSR) tests were performed on molded seals, called as SMORS, over a range of environmental conditions using a custom-designed fixture. The effect of temperature and environment was evident on material property changes and presented in terms of momentary properties and stress relaxation behavior. Various mechanisms involved in material degradation, chain scission and crosslinking, were suggested and insights were gained into how cure state and level of antidegradants in a material dictate the material behavior during the first phase of environmental exposure leading to change in material properties. Ring samples made of silicone were also tested using the fixture to obtain insight additional into material degradation due to aging. Results presented from testing on SMORS showed a lot more variation in data as compared to neat silicone rings due to the complexity involved in making SMORS. For understanding the deformation behavior of an elastomeric seal and its sealing performance, finite element characterization of seal cross-section was carried out on O-ring and SMORS cross-section. The effect of a seal's layout on distribution and magnitude of contact stresses and contact width was investigated for the O-ring and the information obtained thereby helped to analyze a complex assembly such as SMORS, where several interfaces and boundary conditions are involved. Stress/strain profiles were generated to visualize their concentration and distribution in the seal cross-section. Frictionless and rough interfacial conditions between seal material and platens were assumed and it was found that its effect on contact width and peak contact pressure was insignificant. Results obtained from FEA on SMORS were validated through comparison with contact mechanics approach and experimental data and it was found that Lindley's equation correlates well with experimental data whereas ABAQUS overestimates the load values at a given compression. Lindley's approach may be used to develop contact pressure profiles that may help estimate peak contact pressure at a given time so leaking can be avoided. / Ph. D.

This dissertation aims to develop methods for statistical predictions based on various types of data from different areas. We focus on applications from reliability and spatial epidemiology. Chapter 1 gives a general introduction of statistical predictions. Chapters 2 and 3 investigate the photodegradation of an organic coating, which is mainly caused by ultraviolet (UV) radiation but also affected by environmental factors, including temperature and humidity. In Chapter 2, we identify a physically motivated nonlinear mixed-effects model, including the effects of environmental variables, to describe the degradation path. Unit-to-unit variabilities are modeled as random effects. The maximum likelihood approach is used to estimate parameters based on the accelerated test data from laboratory. The developed model is then extended to allow for time-varying covariates and is used to predict outdoor degradation where the explanatory variables are time-varying. Chapter 3 introduces a class of models for analyzing degradation data with dynamic covariate information. We use a general path model with random effects to describe the degradation paths and a vector time series model to describe the covariate process. Shape restricted splines are used to estimate the effects of dynamic covariates on the degradation process. The unknown parameters of these models are estimated by using the maximum likelihood method. Algorithms for computing the estimated lifetime distribution are also described. The proposed methods are applied to predict the photodegradation path of an organic coating in a complicated dynamic environment. Chapter 4 investigates the Lyme disease emergency in Virginia at census tract level. Based on areal (census tract level) count data of Lyme disease cases in Virginia from 1998 to 2011, we analyze the spatial patterns of the disease using statistical smoothing techniques. We also use the space and space-time scan statistics to reveal the presence of clusters in the spatial and spatial/temporal distribution of Lyme disease. Chapter 5 builds a predictive model for Lyme disease based on historical data and environmental/demographical information of each census tract. We propose a Divide-Recombine method to take advantage of parallel computing. We compare prediction results through simulation studies, which show our method can provide comparable fitting and predicting accuracy but can achieve much more computational efficiency. We also apply the proposed method to analyze Virginia Lyme disease spatio-temporal data. Our method makes large-scale spatio-temporal predictions possible. Chapter 6 gives a general review on the contributions of this dissertation, and discusses directions for future research. / Ph. D.