Thermal distress in asphalt concrete pavements is a widespread problem around the world. Thermal cracking can be divided into two modes of distress: low temperature cracking and thermal fatigue cracking. Low temperature cracking results from extremely cold temperatures; thermal fatigue cracking results from daily temperature cycles. Low temperature cracking is attributed to tensile stresses induced in the asphalt concrete pavement as the temperature drops to an extremely low temperature. If the pavement is cooled, tensile stresses develop as a result of the pavement's tendency to contract. The friction between the pavement and the base layer resists the contraction. If the tensile stress equals the strength of the mixture at that temperature, a micro-crack develops at the surface of the pavement. Under repeated temperature cycles, the crack penetrates the full depth and across the asphalt concrete layer. The thermal stress restrained specimen test (TSRST) was identified as an accelerated laboratory test to evaluate the thermal cracking resistance of asphalt concrete mixtures. The TSRST system developed at OSU includes a load system, data control/acquisition system and software, temperature control system, and specimen alignment stand. The overall system is controlled by a personal computer. A TSRST is conducted by cooling an asphalt concrete specimen at a specified rate while monitoring the specimen at constant length. A typical thermally-induced stress curve is divided into two parts: relaxation and non-relaxation. The temperature at which the curve is divided into two parts is termed the transition temperature. The temperature at fracture is termed the fracture temperature and the maximum stress is the fracture strength. An extensive number of TSRSTs over a wide range of conditions were performed to investigate the thermal cracking resistance of asphalt concrete mixtures. The TSRST results provided a very strong indication of low temperature cracking resistance for all mixtures considered. A ranking of mixtures for low temperature cracking resistance based on the TSRST fracture temperature was in excellent agreement with a ranking based on the physical properties of the asphalt cements. It is highly recommended that the TSRST be used in mix evaluation to identify low temperature cracking resistance of asphalt concrete mixtures. The TSRST showed very promising results regarding the effect of all variables which are currently considered to affect the low temperature cracking of mixtures. The variables considered to have significant affect on the low temperature cracking resistance of mixtures in this study include asphalt type, aggregate type, degree of aging, cooling rate, and stress relaxation. / Graduation date: 1994

Thermally cycling a thin plate of nickel-based superalloy with an intense in-plane thermal gradient, or hot spot, produces thermally induced crack growth not represented by classic thermo-mechanical fatigue (TMF). With the max hot spot temperature at 1093 C (2000 F) of a 1.5 mm thick, 82.55 mm diameter circular plate of B-1900+Hf, annular buckling and bending stresses result during each thermal cycle which drive the crack initiation and propagation. A finite element analysis (FEA) model, using ANSYS 7.1, has been developed which models the buckling and as well as represents the stress intensity at simulated crack lengths upon cool down of each thermal cycle. The model approximates the out-of-plane response at heat-up within 5% error and a difference in the final displacement of 0.185 mm after twelve thermal cycles. Using published da/dN vs. Keff data, the number of cycles needed to grow the crack to the experimental arrest distance is modeled within 1 mm. The number of cycles to this point is within 5 out of 462 in comparison to the experimental test.

Duplex stainless steels have a microstructure that consists of almost equal shares of austenite and ferrite, which leads to excellent material properties. During production and processing, the steel can be exposed to high temperatures which leads to the development of a third (sigma) phase, and thus to a change in material properties. The objective of this research is to assess the material damage in thermally degraded 2205 duplex stainless steel using nonlinear ultrasonics (NLU). Seven 2205 duplex stainless steel specimens are thermally degraded at 700 C for a series of different time durations. Nonlinear Ultrasonic measurements are conducted in a pitch-catch setup to avoid any adverse nonlinear influences of reflections and wave interference. The material nonlinearity parameter, beta, is then obtained by directly applying the fast Fourier Transform (FFT) to the measured time-domain signal. The results show that the nonlinearity parameter beta increases to a peak at 30 minutes aging time, then droppes to a low at 360 minutes and then increases again for increasing thermal damage. This demonstrates that the nonlinearity parameter has the potential to be used as a quantitative tool to estimate thermal damage in a specimen.

Superalloys are widely applied as materials for components in the hot section of gas turbines. As superalloys have a limited oxidation life, the application of a coating is vital. The most commonly applied coatings in stationary gas turbines are MCrAlY coatings. Since the turbine components are exposed to high cyclic thermal stresses, MCrAlY coatings must also show a high thermal fatigue resistance. In this thesis, the effect of Cobalt and Rhenium on microstructure, oxidation and thermal fatigue of NiCoCrAlY coatings is presented. Additionally the condition of the coatings after testing in an industrial gas turbine is shown. The influence of Cobalt and Rhenium on coating microstructure was investigated by thermodynamic modelling and by metallography. It could be shown that both elements reduce the γ`-phase fraction and increase the β-phase fraction owing to an expansion of the γ+β field in the phase diagram. Modelling showed that Rhenium promotes the formation of α-Cr, which could be explained by a shift of the α-Cr solvus to higher temperatures and lower Cr concentrations. In the real coatings Re causes the precipitation of TCP-phase. The oxide scale growth rate is increased by Cobalt and Rhenium and it appears that Yttrium plays a significant role for that effect. Coating consumption due to simultaneous oxidation and interdiffusion could be decreased by the application of Cobalt and Rhenium. In thermal fatigue testing Rhenium reduces the time to crack initiation and increases crack propagation rate, although it could be shown that Rhenium increases the creep resistance of the coating. The effect could be explained by the influence of Rhenium on the microstructure, which increases creep resistance, but also reduces the ductility of the coating.

abstract: Solar photovoltaic (PV) industry is tipped to be one of the front-runners in the renewable industry. Typically, PV module manufacturers provide a linear or step warranty of 80% of original power over 25 years. This power loss during the field exposure is primarily attributed to the development of performance affecting defects in the PV modules. As many as 86 different defects can occur in a PV module. One of the major defects that can cause significant power loss is the interconnect metallization system (IMS) degradation which is the focus of this thesis. The IMS is composed of cell-interconnect (cell-ribbon interconnect) and string-interconnect (ribbon-ribbon interconnect). The cell interconnect is in turn composed of silver metallization (fingers and busbars) and solder bonds between silver busbar and copper ribbon. Weak solder bonding between copper ribbon and busbar of a cell results in increase of series resistance that in turn affects the fill factor causing a power drop. In this thesis work, the results obtained from various non-destructive and destructive experiments performed on modules exposed in three different climates (Arizona - Hot and Dry, Mexico - Warm and Humid, and California - Temperate) are presented. These experiments include light I-V measurements, dark I-V measurements, infrared imaging, extraction of test samples from the modules, peel strength measurements and four-point resistance measurements. The extraction of test samples was performed using a mechanical method and a chemical method. The merits and demerits of these two methods are presented. A drop of 10.33% in fill factor was observed for a 0.05Ω increase in the series resistance of the modules investigated in this work. Different combinations in a cell that can cause series resistance increase were considered and their effect on fill factor were observed using four-point probe experiments. Peel test experiments were conducted to correlate the effect of series resistance on the ribbon peel strength. Finally, climate specific thermal modelling was performed for 4 different sites over 20 years in order to calculate the accumulated thermal fatigue and also to evaluate its correlation, if any, with the increase of series resistance. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2016

Estudaram-se os efeitos da fração volumétrica de carboneto eutético e da dureza da matriz sobre a resistência à fadiga térmica de ferros fundidos brancos multicomponentes. Utilizaram-se ligas do sistema Fe-4Cr-V-2Mo-2W-2C, V variando entre 5 e 8% e Fe-4Cr-8V-Mo-2W-2C, Mo variando entre 2 e 5%. Corpos-de-prova com a geometria de discos com seção variável foram temperados e revenidos para obtenção de dois valores de microdureza da matriz: 450 HV e 650 HV. Conduziram-se ensaios de fadiga térmica por 100 e 500 ciclos. Cada ciclo envolveu aquecimento por indução da superfície até a temperatura de 600°C em 10 segundos e subseqüente resfriamento em água por 45 segundos (equalização das temperaturas do núcleo e da superfície). Os corpos-de-prova foram caracterizados antes e após os ensaios de fadiga térmica. Antes dos ensaios, caracterizaram-se os carbonetos eutéticos (tipos, morfologia, fração volumétrica, tamanho, forma e distribuição dos carbonetos) e a microdureza da matriz. Após os ensaios, caracterizaram-se as trincas de fadiga térmica macroscópicas e microscópicas (número e profundidade) e a microdureza da matriz. A nucleação de trincas de fadiga térmica ocorre predominantemente na superfície do corpo-de-prova, induzidas por concentradores de tensão mecânicos e metalúrgicos. As trincas nucleiam na matriz (a rugosidade age como concentrador de tensão mecânico) e em carbonetos (interface carboneto/matriz ou no próprio carboneto). A taxa de nucleação sofre influência da fração volumétrica de carboneto eutético (seu aumento promove aumento da taxa de nucleação) e da dureza da matriz (seu aumento promove diminuição da taxa de nucleação). A propagação de trincas ocorre predominantemente pela interface carboneto/matriz ou através do carboneto. A taxa de propagação sofre influência da distribuição de carboneto eutético. Quanto maior a relação “continuidade de carbonetos/distância livre média entre carbonetos”, maior a taxa de propagação de trincas. A taxa de propagação de trinca diminui com o aumento do tempo de ensaio, independentemente da fração volumétrica de carboneto eutético e da dureza da matriz. O regime de propagação durante os primeiros 100 ciclos é caracterizado pela propagação instável da trinca controlada pela tenacidade à fratura do material; de 100 a 500 ciclos, a propagação é controlada pela magnitude da tensão. O tamanho do corpo-de-prova também influenciou os resultados dos ensaios: o aumento do tamanho promove aumento das taxas de nucleação e propagação. Este resultado é atribuído ao aumento do gradiente térmico ao longo do corpo-de-prova com o aumento do seu diâmetro. / The effects of the volume fraction of eutectic carbides and of the matrix hardness on the thermal fatigue resistance of multicomponent white cast iron were investigated. Alloys Fe-4Cr-V-2Mo-2W-2C, V ranging from 5 to 8 wt% and Fe-4Cr-8V-Mo-2W-2C, Mo ranging from 2 to 5 wt % were used. Disc shaped samples were quench and tempered for obtaining two matrix microhardness levels: 450 HV and 650 HV. Thermal fatigue tests were carried out for 100 and 500 cycles. Each cycle involved high frequency induction heating of the surface to 600°C and subsequent cooling in water during 45 seconds (equalization of the bulk and surface temperature). The test specimens were characterized before and after the thermal fatigue tests. Before the tests, eutectic carbide (type, morphology, volume fraction, syze, shape and distribution of carbides) and matrix microhardness were characterized. After the tests, the macroscopic and microscopic thermal fatigue cracks (number and depth) and matrix microhardness were characterized. The nucleation of the thermal fatigue cracks takes place mostly at the specimen surface, induced by mechanical and metallurgical stress risers. The crack nucleates at the matrix (roughness as mechanical stress risers as well as at carbides (at the carbide/matrix interface or at the carbide itself). The nucleation rate is influenced by the volume fraction of eutectic carbide (the higher the volume fraction, the higher the nucleation rate) and by the matrix microhardness (the higher the microhardness, the lower the nucleation rate). The crack propagation mostly takes place at the carbide/matrix interface or through the carbide. The propagation rate is affected by the carbide distribution. The higher the “carbide continuity/carbide free path” ratio, the higher the propagation rate. The propagation rate decreases with increasing test time, regardless the eutectic carbide volume fraction and the matrix microhardness. The propagation behaviour during the first 100 cycles is characterized by instable crack propagation controlled by the fracture toughness of the material; from 100 to 500 cycles, the propagation is controlled by the stress magnitude. The syze of the test specimen also influenced the tests results: the larger the specimen syze, the higher the nucleation and propagation rates. This is attributed to the effect of increasing thermal gradient across the specimen with increasing specimen diameter.

Embora a literatura disponível sobre conformação mecânica disponha de grande número de trabalhos relacionados à fadiga térmica em cilindros de laminação a quente, poucos autores abordam como esse tipo de dano é influenciado por algumas características do processo - em especial a oxidação na superfície de trabalho dos cilindros, tanto enquanto formação de uma camada de óxido uniforme sobre essa superfície quanto na forma de corrosão. Essa lacuna ocorre a despeito do volume de referências que relacionam a oxidação à fadiga térmica em escala laboratorial. O presente trabalho busca relacionar ambas as linhas de investigação, utilizando métodos analíticos e numéricos comparados com experimentos em diferentes escalas, para compreender sob quais condições são formados os óxidos presentes nos cilindros, qual seu comportamento mecânico no decorrer dos ciclos de laminação e como os óxidos influenciam na ocorrência de trincas de laminação a quente. A análise da superfície de trabalho de um cilindro em escala piloto revela a presença de trincas térmicas na superfície de trabalho. Os padrões de defeitos obtidos se mostram de acordo com o exposto em diversas referências publicadas sobre ensaios de fadiga térmica e termomecânica, em relação à presença e distribuição de óxidos no interior das trincas e na superfície externa das amostras. Em paralelo, foram desenvolvidos modelos numéricos do processo de laminação a quente, inicialmente em escala macroscópica, para obter detalhes sobre distribuições de temperatura, tensões e deformações nos cilindros, de difícil obtenção experimental. Estes modelos foram simulados e seus resultados comparados com valores medidos no laminador piloto, validando seu uso para avaliação do comportamento dos cilindros durante uma série de campanhas de laminação. Os resultados obtidos mostram que o mecanismo de fadiga de baixo ciclo normalmente associado ao dano térmico não é suficiente para explicar a nucleação das trincas de fadiga térmica observadas no experimento. O erro na predição pela teoria de fadiga de baixo ciclo tradicional, em conjunto com a caracterização dos cilindros em escala piloto e com observações de cilindros em escala industrial presentes na literatura, permite levantar a hipótese de que os fenômenos de oxidação e corrosão em curso durante o processo de laminação contribuem para uma redução significativa da resistência dos cilindros à fadiga termomecânica. Cálculos analíticos e simulações adicionais em escala microscópica foram realizados, incluindo os efeitos da oxidação e da corrosão, confirmando a hipótese e oferecendo uma linha de investigação mais assertiva para a nucleação e evolução do dano nos cilindros. / Although the literature on metal forming presents a considerable amount of references related to thermal fatigue in hot rolling, only few authors discuss how this type of damage is influenced by some characteristics of the rolling process - specially the oxidation that develops at the surface of the rolls. This gap occurs despite the amount of references relating oxidation to thermal fatigue in laboratory scale. The current work aims to relate both investigation paths, employing analytical and numerical methods in comparison with experiments in different scales, to provide understanding about the conditions at which the oxides observed upon the rolls are formed, their mechanical behaviour during the rolling process and how they influence thermal fatigue cracking in the rolls. The analysis of the roll surface in a pilot scale mill reveals the presence of thermal fatigue cracks on the work surface. The patterns of defects observed are in accordance with several references on thermal and thermomechanical fatigue, regarding the presence and distribution of oxides inside the cracks and at the external surface of the samples. In parallel, numerical models of hot rolling were developed, initially at macroscopic scale, to obtain details on temperature, stress and strain distribution in the rolls, which are difficult to obtain experimentally. These models were simulated and their results were compared with values measured in the pilot scale mill, validating their use to evaluate the behaviour of the rolls throughout a series of rolling campaigns. The obtained results show that the low-cycle fatigue mechanism commonly associated to thermal damage is not enough to explain the nucleation of thermal cracks seen in the experiments. The error in the prediction using the traditional low-cycle fatigue theory, in conjunction with the characterization of the pilot-scale rolls and with observations of industrial scale rolls, yields the hypothesis that oxidation and corrosion phenomena occurring throughout the rolling process contribute to a significative reduction in the resistance of the rolls to thermomechanical fatigue. Analytical calculations and additional simulations in microscopic scale were conducted, including effects of oxidation and corrosion, confirming the hypothesis and providing a more assertive line of research for damage nucleation and evolution in the rolls.

Abstract Low-temperature co-fired ceramic (LTCC) based system-in-package (SiP) is an emerging multilayer module technology for wireless communication applications, mainly due to its excellent high-frequency material properties. LTCC-SiP modules are typically soldered onto an organic motherboard, but the lifetime of the 2nd-level solder joints is often poor due to the high stress level of the joints in test/field conditions. Moreover, using lead-free solders in the interconnections of LTCC modules raised new questions about the feasibility and reliability of the solder joints in LTCC applications. Therefore, the characteristic features of the 2nd-level solder joint configuration were determined in this thesis work. It was proved that collapsible Sn4Ag0.5Cu spheres are not a feasible option in LTCC/PWB assemblies with a large global thermal mismatch; a non-collapsible ball grid array (BGA) joint with a plastic core solder balls (PCSBs) was required to attain an adequate lifetime for such assemblies. To enhance the thermal fatigue endurance of the non-collapsible lead-free joints, a novel BGA joint consisting of Sn7In4.1Ag0.5Cu solder and PCSBs was developed. Moreover, this work proved that there is a relationship between the primary failure mechanisms of various Sn-based lead-free solders and thermomechanically induced stress level in the present non-collapsible BGA joint configuration. The effect of the plating material of the solder lands on the failure mechanism of the BGA joints in the LTCC/PWB assemblies was studied. The results showed that the adverse phenomena related to the sintered Ag-based metallization materials can be avoided using electroless nickel with immersion gold (ENIG) as a deposit material. On the other hand, this study also demonstrated that the inadequate adhesion strength of the commercial base metallization in the ENIG-plated modules resulted in the disadvantageous failure mechanism of the test assemblies. Therefore, the criteria for material selection and the design aspects of reliable 2nd-level interconnections are discussed thoroughly in this thesis.

Mathematical models of heat flow and elastic stress generation based on the finite-element method have been developed and utilized to analyze the Epic-3 Monofrax-S casting process (Monofrax-S is primarily composed of 47-57% A1₂O₃, 34-41% ZrO₂ and 10-15% SiO₂). The results of the mathematical analysis, in conjunction with information obtained from a comprehensive industrial study, has led to the development of mechanisms for the formation of the various crack types found in this casting process. Thermal stresses have been predicted to be generated early in the solidification process in association with rapid cooling of the refractory surface as it contacts the initially cool mould and again later in the solidification process in conjunction with the tetragonal-to-monoclinic phase transformation which occurs in the zirconia component of Monofrax-S. The mathematical analysis has also helped to identify indirectly a potential mechanism for the generation of mechanical stresses. Based on an understanding of the generation of tensile stresses, recommendations have been made for modifications to the moulding and casting procedures in order to reduce the propensity for the formation of cracks. The modifications have included changes to the mould construction and geometry to reduce the generation of mechanical stresses and changes to the moulding materials to impact on the flow of heat at key times during solidification and cooling. With the recommendations in place, the casting process has been re-examined with the mathematical models to verify the impact of the modifications. The predictions show that the modifications have acted to reduce tensile stresses associated with the formation of Type-A and -B cracks. Preliminary industrial trials with the modified mould have yielded blocks free of these defects. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Superalloys are widely applied as materials for components in the hot section of gas turbines. As superalloys have a limited oxidation life, the application of a coating is vital. The most commonly applied coatings in stationary gas turbines are MCrAlY coatings. Since the turbine components are exposed to high cyclic thermal stresses, MCrAlY coatings must also show a high thermal fatigue resistance. In this thesis, the effect of Cobalt and Rhenium on microstructure, oxidation and thermal fatigue of NiCoCrAlY coatings is presented. Additionally the condition of the coatings after testing in an industrial gas turbine is shown. The influence of Cobalt and Rhenium on coating microstructure was investigated by thermodynamic modelling and by metallography. It could be shown that both elements reduce the γ`-phase fraction and increase the β-phase fraction owing to an expansion of the γ+β field in the phase diagram. Modelling showed that Rhenium promotes the formation of α-Cr, which could be explained by a shift of the α-Cr solvus to higher temperatures and lower Cr concentrations. In the real coatings Re causes the precipitation of TCP-phase. The oxide scale growth rate is increased by Cobalt and Rhenium and it appears that Yttrium plays a significant role for that effect. Coating consumption due to simultaneous oxidation and interdiffusion could be decreased by the application of Cobalt and Rhenium. In thermal fatigue testing Rhenium reduces the time to crack initiation and increases crack propagation rate, although it could be shown that Rhenium increases the creep resistance of the coating. The effect could be explained by the influence of Rhenium on the microstructure, which increases creep resistance, but also reduces the ductility of the coating.