Le déphasage entre augmentation de température et augmentation de gaz à effet de serre (GES) dans l'atmosphère lors des grandes transitions climatiques passées est estimé grâce aux mesures effectuées dans les carottes de glace polaires dans la phase glace pour la température et dans la phase gaz (bulles d’air piégées) pour la concentration en GES. Ce déphasage est encore mal contraint et, pour résoudre ce problème, il est nécessaire de mieux comprendre le processus mécanique de transformation de neige en glace près de la surface de la calotte (centaine de mètres supérieurs, le névé). En l'absence de fusion, la transformation de la neige (matériau à porosité ouverte en contact avec l'atmosphère) en glace (matériau contenant des bulles d'air isolées) s'effectue progressivement sous l'action des gradients de température près de la surface, puis sous l'action du poids des couches de neige situées au-dessus. Selon les conditions de température et précipitation, ce processus peut prendre quelques décennies à plusieurs millénaires et s'étend sur une centaine de mètres de profondeur. Il contrôle la différence d'âge entre la glace et les gaz qu'elle renferme. La prédiction de la profondeur de piégeage des gaz représente un enjeu majeur pour la paléoclimatologie en particulier sur la séquence des changements relatifs de température et de concentration en gaz à effet de serre.Un modèle thermo-mécanique de densification de la neige a été conçu et développé au LGGE en intégrant la formulation des processus mécaniques, des propriétés thermiques, et des critères de piégeage des gaz. Les performances de ce modèle peuvent être testées et améliorées grâce à des études de structure de névés actuels (densité, rapport porosité ouverte/fermée, …). Pour les périodes plus anciennes, des mesures d’isotopes des gaz inertes d15N et d40Ar) dans l’air piégé dans les carottes de glace polaire permettent d’obtenir des informations directes sur les variations passées de la structure du névé (e.g. épaisseur de la zone diffusive). Les larges divergences observées en Antarctique entre les sorties de modèle de densification et les mesures isotopiques de gaz piégé dans la glace génèrent une grande incertitude sur les reconstructions climatiques passées et comprendre ce désaccord est un défi majeur de la paleoclimatologie actuelle.Dans le cadre de cette thèse, nous avons pris en compte les influences de la dépendance à la température des énergies d'activation et des impuretés (poussières) sur la vitesse de densification. Cela a permis de concilier les données et le modèle. Les résultats du modèle modifié sont globalement cohérents avec les profils de densité mesurés pour des névés actuels et les données d'isotopes de gaz inertes pour les déglaciations (aussi appelées terminaisons). Nous avons également présenté de nouvelles mesures à haute résolution de d15N et d40Ar pour les terminaisons 2 (129-138ka) et 3 (243-251ka) des carottes antarctiques de Dôme C et Vostok. Nous avons montré que les différentes évolutions de d15N entre les différents sites et différentes déglaciations s’expliquaient largement par les différences de taux d’accumulation qui contrôlent la profondeur de transition neige-glace. Nous avons aussi montré que l’utilisation des isotopes de l’air était un complément important à l’utilisation des isotopes de l’eau pour contraindre la dynamique climatique locale en Antarctique de l’Est lors des déglaciations. / The phasing between increases in temperature and greenhouse gas concentrations during large climatic variations in the past is classically estimated using analyses in polar ice cores, in the ice phase for the temperature and in the gas phase (trapped air bubbles) for the concentration of greenhouse gases. This phasing is still insufficiently constrained and solving this problem requires a better understanding of the mechanical process of snow to ice metamorphism near to the top of the ice sheet (i.e. the firn, about 100 m deep). In the absence of melting, the transformation of snow (a material with open porosity in contact with the atmosphere) into ice (a material containing isolated bubbles) occurs progressively as a response to temperature gradients near the surface, and the weight of overlying snow in deeper layers. Depending on temperature and precipitation conditions, this process occurs in a few decades to several millennia and a ~100 meters depth range. It controls the age difference between the ice and the entrapped gases. Predicting the gas trapping depth is a major issue in paleoclimatology, especially in order to understand the phasing between temperature changes and changes in greenhouse gas concentrations.A thermo-mechanical model of snow densification has been developed at LGGE, it includes the main mechanical processes, the thermal properties of ice, and gas trapping criteria. The model performances can be tested and improved using experimental studies of modern firns (density, open/closed porosity ratio, etc). For firnification under ancient climates, measurements of isotopes of inert gases (d15N et d40Ar) in the air trapped in ice cores provide direct informations about past variations of firn structure (e.g. diffusive zone thickness). Large differences between firn densification model outputs and gas isotopic data are obtained in Antarctica, and imply a large uncertainty on past climatic reconstructions. Understanding this discrepancy is a major issue in paleoclimatology.As part of this thesis work, we took into account the effects of the temperature dependence of activation energies and impurities (dust) on the firn densification speed. It allowed to reconcile the model results with available data. The modified model results show an overall consistency with measured density profiles of present-day polar firns and isotopes of inert gases over deglaciations (also called terminations). We also analyze new high resolution measurements of d15N and d40Ar over Terminations 2 (129-138ka) and 3 (243-251ka) on the Dome C and Vostok ice cores. We have shown that the different evolutions of d15N between different sites and different deglaciations are largely explained by differences in accumulation rates that control the snow/ice transition depth. We also showed that the use of air isotopes was an important complement to the use of water isotopes to constrain local climatic dynamics in eastern Antarctica during deglaciations.

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The convergence of the digital and physical world encourages advances in high-speed telecommunication and fifth generation technology. Two-phase heat transfer systems are common engineering solutions. However, due to the large frequency spectra in 5G, the systematic heat generation increases requiring more efficient thermal management. The surface characteristics of solid materials in these systems is vital making micro-architectured materials a novel pathway to improve heat transfer. The coefficient of thermal expansion and thermal conductivity of the Schoen-Gyroid, a triply periodic minimal surface structure is studied along with a classical cylindrical porous structure. Graphite and graphene are considered as materials with excellent thermal and mechanical properties and are thus the base materials considered in this project. A comprehensive manufacturability study was conducted in order to gain knowledge regarding different graphite/graphene options and it was concluded that commercially available isotropic graphite was the best suited material for the purpose of this project. A decoupled thermo-mechanical analysis of the coefficient of thermal expansion and thermal conductivity of said structures as a function of volume fraction was conducted using computational homogenization with finite element analysis. A linearly elastic constitutive material model in COMSOL Multiphysics was used. As expected, the homogenized effective material is governed by linear constitutive model. Moreover, the results displayed a linear dependency on the porosity for both the CTE and thermal conductivity. The mechanical FEM model was validated using an analytical model derived by Gibson and Ashby and the thermal conductivity FEM model was validated using experimental data.

This research aims to investigate the influence of 3D through-thickness stitching on the gas permeability and transverse microcracking of cryogenically cycled carbon/epoxy composites. 3D through-thickness stitching can be used to improve the interlaminar properties of polymer matrix composites (PMCs) and produce lightweight, unitized structures for cryogenic storage tanks. To fully utilize stitched composite structures for these applications, their inherent gas permeability challenges must be understood. Therefore, in this study, the stitched composites' damage evolution and gas permeability was experimentally characterized under a) pure thermal stress, b) thermal and uniaxial mechanical stress, and c) thermal and biaxial mechanical stress. Helium gas permeability was measured for each specimen at room or cryogenic temperatures under a mechanically strained state following the thermo-mechanical cycles. Optical microscopy was used to measure microcrack densities and monitor their evolution through the thickness of the composite specimens. Thin plies, graphene nanoplatelets (GNP) modified resin, and a hybrid barrier layer comprising of both were incorporated in the stitched specimens as barrier layers to reduce their gas permeability. The dependence of gas permeability of stitched composites on the mechanical strain, test temperature, and load history was evaluated and correlated to microcrack density. A significant reduction in permeability and damage evolution (transverse microcracks and delaminations) was obtained for all thermo-mechanical cases using the hybrid barrier layer laminate. Additionally, the permeability was several orders of magnitude lower than the allowable. Overall, the hybrid barrier layer shows tremendous promise as a viable barrier layer for stitched/unstitched composites undergoing thermo-mechanical fatigue involving a cryogenic environment.

The purpose of the thesis work given is to gain a better understanding of the degating process in high-pressure die casting components, as well as the effectiveness of the thermo-mechanical vibration technique for detaching cast components from the gating system. In addition, to evaluate and establish the most appropriate degating procedure in order to reduce labor, material costs, and processing time.In recent days, many investigators have been looking at an automated technique that can help optimize and streamline the casting process. VOLVO CE is also exploring on ways to assess and establish the most acceptable degating process in order to optimize cost towards labor, material expense, and processing time. To begin with, VOLVO CE wants to develop the full Finite Element Method (FEM) based approach prior to implementationThis thesis consists of a substantial amount of numerical work, demonstrating the effectiveness of thermo-mechanical vibration technique for degating the casting high-pressure die casting component. The dynamic behavior of the component was studied in ANSYS WB where the impact of thermal loading has been taken into consideration.The component studied in this thesis is a transmission component (oil distributor) from a Volvo construction equipment, VOLVO group. It is cast in Aluminum alloy with the material composition AlSi13Fe. A oil distributor is a component which is bolted on the back side (place where electric motor is connected to the casing through shaft) of the transmission casing.

With the potential to improve upon silicon (Si ) based power electronics exhausted, the push for improvement now lies with wide bandgap (WBG) materials like gallium nitride (GaN). With a larger bandgap, higher electron mobility, and higher electrical field strength than Si, GaN high electron mobility transistors (HEMTs) can have lower on-state losses and higher switching frequencies in a smaller package. This makes GaN HEMTs an attractive choice for compact, high efficiency power devices. However, the package designs used for Si cannot be used for GaN HEMTs, requiring novel, chip-scale designs that are optimized for low electrical parasitics and low thermal resistance. Recent Center for Power Electronics (CPES) research culminated in a printed circuit board-direct bonded copper (PCB-DBC) hybrid package to house a 650 V, 150 A GaN HEMT. Called the PCB-Interposer-on-DBC package, it utilizes a DBC for heat extraction while using vertical pin interconnects to minimize electrical parasitics. Previous work did not investigate the design's locations of expected failure or reliability. With thermally generated mechanical fatigue a consistent cause of electronics failure, it must be investigated for the design to move beyond the prototyping phase. Thermo-mechanical fatigue failure is the brittle fracture of bonds caused by thermally induced warpage. The thermal warpage is the consequence of the bonded package components having a coefficient of expansion (CTE) mismatch while being subjected to temperature changes during operation. Multiphysics simulation software have previously quantified the fatigue placed on bonds exposed to these cyclic conditions, with a common metric being the volume-averaged inelastic strain energy density gained per cycle (ΔWavg). ΔWavg can identify which bonds are subjected to the greatest amount of fatigue and will thus fail first, and then quantify the effect of design alterations on those vulnerable bonds. A common design alteration seen in solder ball packaging is adding a polymeric material that encapsulates the bonds. If the polymer has a CTE like that of the package substrates and an elastic modulus (E) exceeding 1 GPa, it constrains the thermal warpage and lowers bond fatigue. This thesis uses thermo-mechanical simulations to provide evidence on which bonds fail first in the package, and that material-based methods of fatigue reduction used in solder ball packing apply to this novel package. Chapter 1 explains how a desire to reduce the cost and increase the performance of electric vehicles led to the development of the PCB-Interposer-on-DBC design, and that the package's response to thermo-mechanical fatigue is unknown. The concepts of thermo-mechanical fatigue and using encapsulants to reduce it are established, along with how simulations are used to study said fatigue. Chapter 2 serves two purposes, the first being an explanation of the simulation settings and metrics used to establish the quality and assumptions used, and the second being a beginners guide on how to create these simulations. Chapter 3 identifies the most probable locations of initial package failure and identifies what encapsulants minimize ΔWavg on those locations. The sintered silver bond expected to fail first is the Internal Gate bond, and an encapsulant with the maximum possible E and 8 ppm/°C minimizes ΔWavg. The Sn60Pb40 bond expected to fail first is the External Source 4 bond and using an encapsulant with the maximum possible E and a CTE of 24 ppm/°C minimizes ΔWavg. While ΔWavg cannot determine which of the two bonds fails first as they are made of different materials, the Internal Gate is prioritized as it has a higher per-cycle fatigue and to prevent loss of the gate signal. Chapter 4 demonstrates how to perform a brief encapsulant study while ranking the expected cycles to failure when using four different encapsulant options. The first two options are to use no encapsulant or silicone gel. As the elastic modulus of silicone gels are too low to restrict or couple the thermally generated warpage, using silicone gel results in a ΔWavg comparable to using no encapsulant. The rigid encapsulant with the properties most like the optimal encapsulant identified for Internal Gate has the lowest ΔWavg¬ of the encapsulants tested. Guidelines are established for what properties an encapsulant must have to outperform said rigid encapsulant. This work uses simulations to provide evidence that encapsulant methods used in ball grid array (BGA) packaging to reduce fatigue apply to a novel GaN HEMT package. By identifying the first-failure locations of the package, establishing what existing encapsulant should be used, and what encapsulation it should eventually be replaced with, these results provide the groundwork for both experimental temperatures cycling and more complex simulations. Such work fills the gap in understanding the reliable lifetime and common failure mechanisms of the PCB-Interposer-on-DBC package. / Master of Science / In modern engineering, the cause of failure in a well-designed electronic device is typically not a single event. Rather, it is the culmination of many smaller events that each cause a minor amount of damage. This cycle of repeated, minor damage is called fatigue. When working with power or IC electronics, the most common type of fatigue occurs due to the device's changing temperature. Electronics undergo continuously changing temperatures due to the environment and their own energy losses, causing repeated cycles of heating and cooling. All materials expand upon heating and contract upon cooling , and the magnitude of this change is the coefficient of thermal expansion (CTE). Electronic devices are comprised of dissimilar materials, so disparate components will expand and contract at different rates. Holding these disparate materials together are bonds, which in the process of holding this warped structure together, also deform. This deformation causes permanent damage, which accumulates in the bonds until they break. As these bonds often serve as pathways for the electrical signal or heat extraction, their failure either degrades or breaks the electrical devices. While preventing bond fatigue is impractical, there are strategies to extend the operating lifetime. A common option used elsewhere is to encase the bonds with a polymer. If the polymer's properties are carefully selected, they can reduce the structural warpage, thereby reducing the fatigue on the bonds. Previous Center for Power Electronics (CPES) research has culminated in a new electronics device called the Printed Circuit Board-Interposer-on-Direct Bonded Copper package (PCB-Interposer-on-DBC package). While general trends suggest which bonds will fail first and what kind of polymers reduce fatigue, this information has not yet been confirmed. This thesis uses computer simulations to identify which bonds will likely fail first, and to provide evidence that existing methods for reducing fatigue are viable for this unique package. The simulations work by subjecting a 3D model to a cycle of heating and cooling, called a temperature cycle, and quantifying the damage sustained by the bonds for every cycle. Chapter 1 describes the relevant details leading to this package design, the importance of thermo-mechanical reliability in the design of electronics, and how to use simulation software to quantify reductions in bond fatigue. Chapter 2 explains how to set up these simulations and evaluate their quality. Chapter 3 identifies the initial locations of package failure and identifies what are the most optimal encapsulants to use. Chapter 4 identifies what existing encapsulant will maximize the package lifetime in experimental temperature cycling.

<p>  </p> <p>The fundamental theory of the glass transition and the glassy state does not exist. Instead, over the years of research there have accumulated a vast number of experimental observations and phenomenological models developed in order to rationalize these observations. A case in point is the stress-strain behavior of a polymer glass during large constant strain rate deformation; for every feature of the stress-strain curve, such as yield, post-yield softening and post-yield hardening, a new mechanism is postulated. But do these mechanisms have a physical basis or are they merely curve-fits? The experiments included in this dissertation are purposefully designed to challenge the prevailing model assumptions. Four specific areas have been chosen: (1) the linear viscoelastic behavior above the glass transition temperature, Tg, (2) the effect of physical aging on the relaxation response of the glassy polymers well below Tg, (3) the behavior of the glassy polymers during a multi-step non-linear deformation, and (4) the effect of the large deformation of the glassy polymer on the enthalpy relaxation as measured by the DSC.</p> <p>(1) Linear viscoelastic isotherms were analyzed by performing dynamic mechanical analysis on a thermoset epoxy, EPON1009F-MDA (Tg: 102.5°C). Storage and loss moduli for the material were investigated for isotherms ranging from 90°C (Tg-12°C) to 180°C (Tg+78°C) for frequencies ranging from 10-2 Hz to 101.7 Hz. This linear viscoelastic dataset was augmented by performing stress relaxation experiments on the same material for temperatures ranging from 90°C (Tg-12°C) to 112.5°C (Tg+10.5°C). The transient results from the stress relaxation (SR) were converted to frequency domain. The resulting augmented dataset spanned 6 order of magnitude in frequency. The wide frequency window showed that the material is thermo-rheologically complex, precluding the creation of a master curve via horizontal shifting of the isotherms. This renders impossible the use of time-temperature superposition, and thus highlights the need for reevaluating its prominent use in glass studies.</p> <p>(2) Existing beliefs on the diminishing effects of physical aging at low temperatures were studied. Linear viscoelastic isotherms of EPON825-MDA (Tg: 182°C) that have been annealed for 2 to 600 hours at temperature ranging from -100°C (Tg-282°C) to 185°C (Tg+3°C) were investigated. At temperatures near Tg, no tangible effects of annealing were identified. At the lowest temperature of -100°C, no differences could be identified between 2 hour and 6 hour annealed specimens; however, annealing effects could possibly be observed at longer aging times based on the results of other isotherms. For all other isotherms between -50°C to 170°C, clear differences could be observed between 2 hour and 6 hour annealed specimens, where the storage moduli increased while the loss moduli decreased. In addition, the effects of annealing were unidentifiable when the material went through a temperature up-jump, but persisted when the material went through a temperature down-jump. The results of this study show that contrary to popular belief, annealing effects are not frozen and persist deep in the glass state and is observable even within experimental limits. Additionally, deep glass aging is fundamentally different from physical aging in that no master curve can be achieved via horizontal shifting along the frequency axis unlike physical aging, due to the change in shape and magnitude of the isotherms.</p> <p>(3) Many constitutive models that target prediction of mechanical behaviors are drawn from the results of single step deformation experiments. Multi-step non-linear deformation experiments were performed on a copolymer of PBMA and PMMA, to challenge the existing models, where the last step is a constant-strain-rate loading step that shows the effects of previous deformation histories on the stress overshoot. Various multi-step deformation histories were investigated, one being a sequence of constrain-strain-rate-loading/unload/creep/constant-strain-rate-loading. Contrary to previous literature reports, the results showed a dependence on the creep stress level of the last overshoot, which initially increased in peak magnitude with creep stress, reached a maximum, and decreased for further increase in creep stress. These results are not qualitatively predicted by any of the existing constitutive models, illustrating the need to rethink how the mechanisms behind stress-strain behaviors are approached. A new toy model is also discussed that can qualitatively predict these results as well as the results of other multi-step deformation histories that are discussed.</p> <p>(4) A new methodology for analysis of the differential scanning calorimetry (DSC) traces was proposed. The DSC trace is known to be sensitive to the thermo-mechanical history a material is subjected to prior to the DSC test; but, the true effects are convoluted with the experimental scatter. The conventional method consists in shifting of the data obtained for different thermo-mechanical histories to the same glassy asymptote, but this has no physical basis. In fact, we argue that this misses the actual effects. We propose that the shifting must be to match the liquid, i.e., equilibrium, asymptote as it is in the equilibrium state which is independent of the history. The new methodology was used to confirm literature reports on the effects of aging. DSC scans of deformed specimens were also studied, where the results showed a systematic effect in the heat capacity traces of deformed specimens, where an endothermic peak followed by an exothermic peak is observed. The peaks are not present in the case of an undeformed material, where a larger degree of strain led to a larger endothermic peak. The results indicate the possibility of a systematic effect where the magnitudes of the additional endothermic and exothermic peaks are controlled by the amount of mechanical work performed during the deformation.</p>

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One of the most critical issues for Polymer Matrix Composites (PMCs) in naval applications is the structural performance of composites at high temperature such as that experienced in a fire. A three-dimensional model including the effect of orthotropic viscoelasticity and decomposition is developed to predict the thermo-mechanical behavior and compressive failure of polymer matrix composites (PMCs) subjected to heat and compressive load. An overlaid element technique is proposed for incorporating the model into commercial finite element software ABAQUS. The technique is employed with the user subroutines to provide practicing engineers a convenient tool to perform analysis and design studies on composite materials subjected to combined fire exposure and mechanical loading. The resulting code is verified and validated by comparing its results with other numerical results and experimentally measured data from the one-sided heating of composites at small (coupon) scale and intermediate scale. The good agreement obtained indicates the capability of the model to predict material behavior for different composite material systems with different fiber stacking sequences, different sample sizes, and different combined thermo-mechanical loadings. In addition, an experimental technique utilizing Vacuum Assisted Resin Transfer Molding (VARTM) is developed to manufacture PMCs with a hypodermic needle inserted for internal pressure measurement. One-sided heating tests are conducted on the glass/vinyl ester composites to measure the pressure at different locations through thickness during the decomposition process. The model is employed to simulate the heating process and predict the internal pressure due to the matrix decomposition. Both predicted and measured results indicate that the range of the internal pressure peak in the designed test is around 1.1-1.3 atmosphere pressure. / Ph. D.