Thesis (Ph.D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Rami Haj-Ali; Committee Member: Arash Yavari; Committee Member: Bruce R. Ellingwood; Committee Member: Kenneth M. Will; Committee Member: Richard W. Neu.

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

A series of composite beam models are constructed for efficient high-fidelity beam analysis based on the variational-asymptotic method (VAM). Without invoking any a priori kinematic assumptions, the original three-dimensional, geometrically nonlinear beam problem is rigorously split into a two-dimensional cross-sectional analysis and a one-dimensional global beam analysis, taking advantage of the geometric small parameter that is an inherent property of the structure. The thermal problem of composite beams is studied first. According to the quasisteady theory of thermoelasticity, two beam models are proposed: one for heat conduction analysis and the other for thermoelastic analysis. For heat conduction analysis, two different types of thermal loads are modeled: with and without prescribed temperatures over the crosssections. Then a thermoelastic beam model is constructed under the previously solved thermal field. This model is also extended for composite materials, which removed the restriction on temperature variations and added the dependence of material properties with respect to temperature based on Kovalenoko’s small-strain thermoelasticity theory. Next the VAM is applied to model the multiphysics behavior of beam structure. A multiphysics beam model is proposed to capture the piezoelectric, piezomagnetic, pyroelectric, pyromagnetic, and hygrothermal effects. For the zeroth-order approximation, the classical models are in the form of Euler-Bernoulli beam theory. In the refined theory, generalized Timoshenko models have been developed, including two transverse shear strain measures. In order to avoid ill-conditioned matrices, a scaling method for multiphysics modeling is also presented. Three-dimensional field quantities are recovered from the one-dimensional variables obtained from the global beam analysis. A number of numerical examples of different beams are given to demonstrate the application and accuracy of the present theory. Excellent agreements between the results obtained by the current models and those obtained by three-dimensional finite element analysis, analytical solutions, and those available in the literature can be observed for all the cross-sectional variables. The present beam theory has been implemented into the computer program VABS (Variational Asymptotic Beam Sectional Analysis).

Yes / Hot spotting is a thermal localisation phenomenon in which multiple hot regions form on a brake disc surface during high energy and/or high speed braking events. As an undesired problem, hot spots can result in high order brake judder, audible drone and thermal cracking. This paper presents a finite element model for hot spot modelling which introduces the classical axisymmetric assumptions to the brake pad in 3D by scaling the material properties combined with a subroutine to simulate the heat generation instead of modelling the rotation of the brake pad. The results from the initial feasibility models showed significant improvement in computing efficiency with acceptable accuracy when compared to a traditional FE model without such simplifications. This method was then applied to the 3D simulation of hot spotting on a realistic ventilated brake disc/pad pair and the results showed good correlation with experiments. In order to improve the understanding of the hot spotting mechanism, parametric studies were performed including the effects of solid and ventilated disc geometry, rotational speed and energy, pins, disc run-out, and brake pad length. Based on the analysis of the results, it was identified that the vents and pins affected the hot spot distribution. Speed was shown to be more important on the hot spot generation time and distribution than either the pressure or total energy input. Brake disc run-out was shown to affect the magnitude of both hot spot temperature and height due to the non-linear relationship between local deformation, contact pressure and heat generation. Finally, increasing the brake pad length generated fewer hot spots but the temperature of each hot spot increased.

This Bachelor thesis at KTH Royal Institute of Technology in Stockholm covers a model-based analysis of thrust washer bearings in heavy trucks. This work treats Scania CV AB’s range-splitter gearbox. In this gearbox there are thrust washer bearings which are analyzed in this study. The purpose of the study is to examine the stresses the bearings are experiencing during load, determine which bearing (front or rear) that is most highly stressed, and discuss the cause for that. At large axial forces, which in turn lead to greater friction and thermal stress, there is a large risk for severe wear. Theoretically, there should not be any axial forces in a planetary gearbox with straight cut-gears, but because of the shape of the planet wheel carrier and the torque from the engine, the carrier and the needle bearings are distorted, which will cause an axial force. The axial force is not easily measured. Furthermore, it is also constantly changing during an actual driving sequence. Based on engineering judgement, the axial force was estimated to be 647 N. The axial force leads to all parts of the gearbox being pushed together axially, and cause friction forces to appear in surfaces that rotate relative each other. This process is analyzed with the FEM-program Ansys. Three possible scenarios were identified: (1) when the planetary wheel rotates relative the thrust washer bearing; (2) when the planetary wheel and needle bearing rotate relative the thrust washer bearing and (3) when the planet wheel carrier rotates relative the thrust washer bearing. The frictional heat power was simplified and analyzed as a heat power source and estimated to be 90 W. Cooling was modeled in Ansys as convection, with a film coefficient of 25 W/m2C. The initial temperatures used in the analysis were 22 and 90 °C. According to this, an equivalent stress of 12.3 MPa, and a maximum principal stress of 7.63 MPa, was located at the front thrust washer bearing. This occurs when the planet wheel and the needle bearing rotate relative to the thrust washer bearing, with an initial temperature of 90 °C. To validate the model, temperature sensors could be placed near the thrust washer bearings. However, since everything is rotating, this can be a difficult task. This report gives Scania a potential explanation to why the front and rear bearings in the gearbox are worn differently. / I det här kandidatexamensarbetet som genomfördes vid Kungliga Tekniska Högskolan i Stockholm gjordes en modellbaserad analys av axialglidlager som används i tunga lastbilar. Arbetet baserades på Scania CV ABs växellådor med range-splitter. I dessa sitter axialglidlager (även kallade brickor). Syftet var att undersöka hur stor påfrestning dessa utsätts för och försöka utröna vilken av brickorna (fram eller bak) som utsätts för störst belastning, samt diskutera vad detta skulle kunna bero på. Vid oönskade axiella krafter, som i sin tur leder till högre friktion och termoelastiska spänningar, är risken för nötning på brickorna stor. Teoretiskt sett ska det inte uppstå några axiella krafter i en planetväxel med rakkugg, men på grund av planethjulsbärarens utformning och det ingående momentet från motorn kan det uppstå en förvridning av hela bäraren och nållagren. Detta leder i sin tur till att en axiell kraft uppkommer. Denna kraft beräknades till 647 N med konservativa ingenjörsmässiga antaganden, eftersom axiella kraften inte kan mätas och ständigt varieras beroende på körprofil. Den axiella kraften medför att alla delar i planetväxeln pressas samman axiellt. En friktionskraft uppstår då mellan ytor som roterar i förhållande till varandra. Detta förlopp har analyserats med hjälp av FEM-programmet Ansys. Tre möjliga belastningsfall identifierades: (1) när planethjulet roterar mot axialglidlagret; (2) när planethjulet och nållagret roterar mot axialglidlagret, samt (3) när planethjulsbäraren roterar mot axialglidlagret. Den friktionsvärme som uppstår förenklades i modellen till en värmeeffekt på 90 W. Den värme som leds bort representerades som konvektion, med ett konvektionstal på 25 W/m2C. Som initialtemperaturer för FEM-analyserna användes 22 °C och 90 °C. En effektivspänning på 12,3 MPa och en största huvudspänning på 7,63 MPa uppstod i den främre brickan vid fallet när planethjul och nållager roterade mot axialglidlagret vid en intialtemperatur på 90 °C. För att kontrollera modellens rimlighet skulle temperaturgivare kunna installeras vid brickorna. Detta kan i praktiken dock bli svårt att genomföra då hela systemet roterar. Detta arbete ger Scania en möjlig förklaring till vad som skulle kunna orsaka en ojämn nötning fram och bak.

O objetivo desse trabalho é apresentar o desenvolvimento de modelos numéricos baseados no método dos elementos finitos, para investigar o comportamento termoelástico, de um freio a tambor utilizado em veículos pesados. O acoplamento de fenômenos físicos e mecânicos durante o processo de frenagem é uma etapa fundamental para o projeto com foco em confiabilidade. A identificação de falhas potenciais decorrentes dos diferentes processos de frenagens, ainda na fase do projeto preliminar, permite o desenvolvimento de projetos mais robustos, incluindo reduções de tempo e nos custos de ensaios experimentais. Este trabalho apresenta o desenvolvimento e implementação de um modelo computacional baseado no comportamento termoelástico dos principais elementos mecânicos de um freio a tambor envolvidos no processo de frenagem. Foram consideradas três condições principais de análise: estática de corpos rígidos, estática de corpos flexíveis e dinâmica em regime permanente com corpos flexíveis. O objetivo é investigar e estimar valores de pressão de contato, cargas resultantes nos apoios, bem como obter o gradiente de temperatura e tensão devido à carga térmica em regiões de interesse. Essa análise pode melhorar o entendimento dos fenômenos envolvidos nos diferentes processos de frenagens, auxiliando as tomadas de decisões técnicas, principalmente na antecipação de falhas prematuras relacionadas com fadiga térmica e desgaste adesivo. / This work presents the development of a numerical model based on thermoelastic behavior of a drum brake applied to heavy vehicles. The coupling of physics and mechanical phenomena during the braking process represents an important phase in the design vehicle systems. The identification of potential failures during the braking processes, considering the preliminary design phase gives robustness to the design. This characteristic allows reducing the time and cost of experimental set-up. This work presents the development and preliminary results of a numerical model based on thermal-elastic behavior of an automotive drum brake. It was considered three main conditions for the analysis: rigid bodies statics, flexible bodies statics and flexible bodies steady state dynamic analysis. The main objective of this study is investigate and estimate the relationship between the design parameters, as strain, stress as well thermal loads and temperature gradient, in conditions of static and quasi-static steady state regime. The estimative of these parameters to support the technical decisions related to reliability design, mainly to anticipate failures arising to thermal fatigue and adhesive wear.

O objetivo desse trabalho é apresentar o desenvolvimento de modelos numéricos baseados no método dos elementos finitos, para investigar o comportamento termoelástico, de um freio a tambor utilizado em veículos pesados. O acoplamento de fenômenos físicos e mecânicos durante o processo de frenagem é uma etapa fundamental para o projeto com foco em confiabilidade. A identificação de falhas potenciais decorrentes dos diferentes processos de frenagens, ainda na fase do projeto preliminar, permite o desenvolvimento de projetos mais robustos, incluindo reduções de tempo e nos custos de ensaios experimentais. Este trabalho apresenta o desenvolvimento e implementação de um modelo computacional baseado no comportamento termoelástico dos principais elementos mecânicos de um freio a tambor envolvidos no processo de frenagem. Foram consideradas três condições principais de análise: estática de corpos rígidos, estática de corpos flexíveis e dinâmica em regime permanente com corpos flexíveis. O objetivo é investigar e estimar valores de pressão de contato, cargas resultantes nos apoios, bem como obter o gradiente de temperatura e tensão devido à carga térmica em regiões de interesse. Essa análise pode melhorar o entendimento dos fenômenos envolvidos nos diferentes processos de frenagens, auxiliando as tomadas de decisões técnicas, principalmente na antecipação de falhas prematuras relacionadas com fadiga térmica e desgaste adesivo. / This work presents the development of a numerical model based on thermoelastic behavior of a drum brake applied to heavy vehicles. The coupling of physics and mechanical phenomena during the braking process represents an important phase in the design vehicle systems. The identification of potential failures during the braking processes, considering the preliminary design phase gives robustness to the design. This characteristic allows reducing the time and cost of experimental set-up. This work presents the development and preliminary results of a numerical model based on thermal-elastic behavior of an automotive drum brake. It was considered three main conditions for the analysis: rigid bodies statics, flexible bodies statics and flexible bodies steady state dynamic analysis. The main objective of this study is investigate and estimate the relationship between the design parameters, as strain, stress as well thermal loads and temperature gradient, in conditions of static and quasi-static steady state regime. The estimative of these parameters to support the technical decisions related to reliability design, mainly to anticipate failures arising to thermal fatigue and adhesive wear.

Lors du chargement cyclique d’une pièce métallique fissurée, un champ de température hétérogène est créé à la pointe de la fissure. Ce champ de température est dû à trois types de sources de chaleur : (i) la première source est une source de couplage thermoélastique liée à la partie hydrostatique du tenseur des contraintes résultant de la sollicitation mécanique cyclique. Elle fluctue périodiquement dans le temps et l’énergie qui lui est associée est nulle à la fin de chaque cycle de chargement ; (ii) la deuxième source de chaleur est une source dissipative et intrinsèque au comportement du matériau. Elle est reliée au phénomène de l’auto-échauffement dû à la microplasticité dissipée en chaleur dans le matériau à l’échelle microscopique. Elle est positive et s’accumule dans le temps ; (iii) enfin, la troisième source de chaleur a les mêmes origines et propriétés que la deuxième source, mais elle est associée à la plasticité, à l’échelle macroscopique, qui se dissipe en chaleur dans la zone de plasticité cyclique à la pointe de la fissure. En présence de ces trois sources de chaleur, le champ de température résultant génère un champ de contrainte dû au phénomène de la dilatation thermique. Ce nouveau champ des contraintes s’ajoute au champ des contraintes dû au chargement mécanique cyclique, et donc l’état des contraintes sur la fissure est modifié. En conséquence, le facteur d’intensité des contraintes, qui est un paramètre clé dans la modélisation de la propagation des fissures, est modifié. D’où l’objectif de cette thèse qui vise à quantifier les conséquences de ces trois sources de chaleur sur le facteur d’intensité des contraintes, et ce dans le cas d’une fissure longue de fatigue. / By subjecting a cracked specimen to a cyclic loading, thermal effects take place and create a heterogeneous temperature field around the crack tip. Those thermal effects are associated with coupling and dissipative heat sources, namely: (i) the heat source due to thermoelastic coupling generated by the hydrostatic part of the stress tensor related to cyclic mechanical loading; (ii) the heat source due to intrinsic dissipation associated with the self-heating phenomena originating from plasticity at the microscopic scale; (iii) and the heat source due to cyclic plasticity, at the macroscopic scale, which occurs in the reverse cyclic plastic zone ahead of the crack tip, and dissipates into heat. The overall heterogeneous temperature field resulting from the heat sources induces a heterogeneous stress field due to thermal expansion phenomena. As a consequence, the stress state over the crack is modified and leads to modify the stress intensity factor, which is a key parameter in modeling fatigue crack growth. Therefore, the aim of this PhD thesis is to quantify the consequences of the heat sources on the stress intensity factor, in the case of a long propagating fatigue crack.

CERN is presently constructing the LHC, which will produce collisions of 7 TeV protons in 4 interaction points at a design luminosity of 1034 cm-2 s-1. The radiation dose resulting from the operation at high luminosity will cause a serious deterioration of the silicon tracker performance. The state-of-art silicon microstrip detectors can tolerate a fluence of about 3·1014 cm-2 of hadrons or charged leptons. This is insufficient for long-term operation in the central parts of the LHC trackers, in particular after the possible luminosity upgrade. By operating the detectors at cryogenic temperatures the radiation hardness can be improved by a factor 10. This work proposes a cryogenic microstrip detector module concept which has the features required for the upgraded LHC experiments at CERN. The module can hold an edgeless sensor, being a good candidate for improved luminosity and total cross-section measurements. The design of such a module is constrained by the requirements on radiation hardness and minimal mass. The choice of the component materials is guided by the properties of the silicon sensors, and the main criteria include best possible matching of the thermal dilatation, high thermal conductivity and appropriate elastic properties, in addition to the radiation resistance. A module design is proposed where, apart from the silicon sensor, both the support plate and the pitch adapter are processed on silicon. Future design options may also feature hybrids processed in silicon using thick-film techniques and cooling microchannels directly micromachined into the support plate. The best performance and highest degree of integration of the cooling is achieved with two-phase flow (high heat transfer coefficient) argon running through capillary pipes embedded in a CFC spacer close to the heat sources. A series of thermal tests have shown that silicon is an excellent heat spreader and its use as a structural material leads to a uniform temperature distribution in the sensor and support plate. The thermal resistance due to the glue layers dominates the thermal behaviour. The thermoelastic properties of the epoxies are key factors in the design. A series of samples were prepared to measure these properties of Araldite® 2011, Stycast® 1266 and Type L epoxies filled with fused quartz powder, as a function of temperature. Filling these epoxies reduces their thermal dilatation, nearly matching that of metals. This reduces the stress in the joints when cooling down. However, filling increases the Young modulus (E) of the epoxy so much, that the thermal stress increases with the filling factor. Furthermore, filling increases the viscosity and leads to thicker glue layers, which also increases the thermal stress in silicon. The idea of using filled epoxy was therefore abandoned. The E of unfilled epoxies at 77 K is between 4 and 8 times higher than that measured at 300 K. Thin layers of epoxy should be used to minimize the stress on the silicon substrate.Precision gluing jigs were designed and produced to assemble the prototype modules. The alignment with respect to the beam is done using a warm support plate, placed between the module and the vacuum chamber. The module is attached to this support structure through three thermally isolating precision support posts with dowels. The position of the module and its readout strips is thus accurately referred to the vacuum chamber, which itself can be aligned in the test beam line using optical targets.A first electrical prototype module was assembled using a 50 ?m pitch silicon microstrip sensor (32.5 cm2). Pitch adapter and support plate were processed on silicon. The CMS hybrid with APV25 readout chips was characterized at low temperature. First results were obtained down to 210 K, showing a decrease of the noise and the rise time and an increase of the pulse peak height with respect to the room temperature behaviour. A pair of edgeless silicon diode pad sensors was exposed to the X5 high-energy pion beam, in order to determine the edge sensitivity. A high-resistivity silicon p+-i-n+ planar diode detector (0.25 cm2) was diced through its front p+ implant to produce two halves of edgeless diode pad sensors. A large surface current on such an edge prevents the normal reverse biasing of this device but it can be sufficiently reduced by the use of a suitable cutting method, followed by edge treatment and by operating the sensor at low temperature. The gap width between the edgeless sensors, determined using the tracks measured by a reference telescope, was compared with metrology measurements. It was concluded that the depth of the dead layer is compatible with zero within the statistical accuracy of 8 µm and systematic accuracy of 6 µm.

Many studies with Thermoelastic Stress Analysis (TSA) and Infrared Thermography, in Fiber Reinforced Polymeric materials (FRPs), are concerned with surface detection of "hot spots" in order to locate and infer damage. Such experimental analyses usually yield qualitative relations where correlations between stress state and damage severity cannot be obtained. This study introduces quantitative experimental methodologies for TSA and Digital Image Correlation to expand the use of remote sensing technologies for static behavior, static damage initiation detection, and fatigue damage in FRPs. Three major experimental studies are conducted and coupled with nonlinear anisotropic material modeling: static and TSA of hybrid bio-composite material systems, a new stochastic model for fatigue damage of FRPs, and fracture analysis for FRP single-lap joints. Experimental calibration techniques are developed to validate the proposed macromechanical and micromechanical nonlinear anisotropic modeling frameworks under multi-axial states of stress. The High Fidelity Generalized Method of Cells (HFGMC) is a sophisticated micromechanical model developed for analysis of multi-phase composites with nonlinear elastic and elastoplastic constituents is employed in this study to analyze hybrid bio-composites. Macro-mechanical nonlinear anisotropic models and a linear orthotropic model for fracture behavior using the Extended Finite Element method (XFEM) are also considered and compared with the HFGMC method. While micromechanical and FE results provide helpful results for correlating with quasi-static behavior, analyzing damage progression after damage initiation is not straightforward and involves severe energy dissipation, especially with increasing damage progression. This is especially true for fatigue damage evolution, such as that of composite joints as it is associated with uncertainty and randomness. Towards that goal, stochastic Markov Chain fatigue damage models are used to predict cumulative damage with the new damage indices proposed using full-field TSA image analysis algorithms developed for continuously acquired measurements during fatigue loading of S2-Glass/E733FR unidirectional single-lap joints. Static damage initiation is also investigated experimentally with TSA in single-lap joints with thick adherends providing for new design limitations. The computational modeling, stochastic and experimental methods developed in this study have a wide range of applications for static, fracture and fatigue damage of different FRP material and structural systems.