One of the important factors determining the lifetime of polymer electrolyte membrane fuel cells (PEMFCs) is membrane degradation and failure. The lack of effective mitigation methods is largely due to the currently very limited understanding of the underlying mechanisms for mechanical and chemical degradations of fuel cell membranes. In order to understand degradation of membranes in fuel cells, two different experimental approaches were developed; one is fuel cell testing under open circuit voltage (OCV) with bi-layer configuration of the membrane electrode assemblies (MEAs) and the other is a modified gas phase Fenton's test. Accelerated degradation tests for polymer electrolyte membrane (PEM) fuel cells are frequently conducted under open circuit voltage (OCV) conditions at low relative humidity (RH) and high temperature. With the bi-layer MEA technique, it was found that membrane degradation is highly localized across thickness direction of the membrane and qualitatively correlated with location of platinum (Pt) band through mechanical testing, Infrared (IR) spectroscopy, fluoride emission, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) measurement. One of the critical experimental observations is that mechanical behavior of membranes subjected to degradation via Fenton's reaction exhibit completely different behavior with that of membranes from the OCV testing. This result led us to believe that other critical factors such as mechanical stress may affect on membrane degradation and therefore, a modified gas phase Fenton's test setup was developed to test the hypothesis. Interestingly, the results showed that mechanical stress directly accelerates the degradation rate of ionomer membranes, implying that the rate constant for the degradation reaction is a function of mechanical stress in addition to commonly known factors such as temperature and humidity. Membrane degradation induced by mechanical stress necessitates the prediction of the stress distribution in the membrane under various conditions. One of research focuses was on the developing micromechanism-inspired continuum model for ionomer membranes. The model is the basis for stress analysis, and is based on a hyperelastic model with reptation-inspired viscous flow rule and multiplicative decomposition of viscoelastic and plastic deformation gradient. Finally, evaluation of the membrane degradation requires a fuel cell model since the degradation occurs under fuel cell operating conditions. The fuel cell model included structural mechanics models and multiphysics models which represents other phenomena such as gas and water transport, charge conservation, electrochemical reactions, and energy conservation. The combined model was developed to investigate the compression effect on fuel cell performance and membrane stress distribution.

This thesis presents the simulation of chip formation in orthogonal metal cutting to evaluate the predictive capabilities of finite element code DYNA 3D. The Johnson and Cook constitutive model for materials, OFHC Copper, Aluminum 2024 T351, and Aluminum 6061 T6 alloy were incorporated into the simulation to account for the effects of strain hardening, strain rate hardening, and thermal softening effects during machining. Calculated values for the Johnson and Cook constitutive constants for Aluminum 6061 T6 alloy were determined from the literature. The model was compared to experimentally measured shear angles, chip thickness, chip velocity, and forces from the literature to evaluate the accuracy of the finite element code for a range machining strain rates. In an attempt to determine the predictive capabilities of DYNA 3D a strain rate regime of 10+3 s-1 to 10+4 s-1 was defined as the optimal strain rate regime for the orthogonal metal cutting application.

No description available.

No description available.

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No description available.

A soil-bentonite cutoff wall is a type of subsurface vertical barrier constructed by back-filling a trench with a mixture of soil, bentonite, and water. Although soil-bentonite cutoff walls are common, their mechanical behavior is not well understood. Current design procedures do not consider the final stress state of the consolidated soil-bentonite backfill or deformations in adjacent ground. The final stress state in the completed wall is important because it influences the hydraulic conductivity of the cutoff (Barrier 1995), the cutoff's susceptibility to hydraulic fracture, and the magnitude of deformations adjacent to the cutoff wall. Deformations adjacent to the cutoff wall can be significant and can cause damage to adjacent structures. The objectives of this research are to 1) add to the current body of knowledge of the properties of soil-bentonite mixtures, 2) evaluate constitutive models and select a model to represent soil-bentonite, 3) model a soil-bentonite cutoff wall using finite elements, and 4) investigate the influence of several factors on the deformations in adjacent ground. These objectives were met by first summarizing information from the literature on soil-bentonite properties and then performing a laboratory testing program on different soil-bentonite mixtures. Five constitutive models were evaluated to determine how well they match the data from the laboratory testing program. A model referred to as the RS model was chosen to best represent soil-bentonite, and provided a good match of the soil-bentonite behavior. The RS model, which is a special case of a more complicated existing model, is a non-associative Modified Cam Clay type model that has parameters to change the yield surface and plastic potential surface into ellipses of varying shapes. The RS model was implemented into the finite element program SAGE. A finite element model was developed using SAGE to simulate all stages of construction of a soil-bentonite cutoff wall including excavation of a trench under bentonite-water slurry, replacement of the bentonite-water slurry with soil-bentonite backfill, and consolidation of the soil-bentonite backfill. The model was calibrated with a well-documented case history, and predicted deformations in adjacent ground were close to measured deformations. Evaluation of the model indicates that there is good confidence in the prediction of deformations in adjacent ground, but there is lower confidence in the predicted stresses in the consolidated soil-bentonite and settlement of the backfill in the trench. A parametric study was then performed using the finite element model assuming sand sites of varying density and OCR. Deformations in adjacent ground were calculated for various soil conditions, soil-bentonite properties, and trench configurations. A correlation was found between maximum calculated settlement in adjacent ground and factor of safety against trench / Ph. D.

The coastal population is growing, putting extra stress on coastal sediments and protection features, such as beach dunes. Moreover, global warming will increase the frequency of storms, and coastal dunes and other defense infrastructure will be subjected to increased erosion and scouring, endangering the people they are meant to protect. Understanding soil dynamics and fluid interaction is crucial to predict the effects of sand erosion. In particular, the study of wind erosion of sands in coastal dunes is essential due to the protective role these earthen structures have during storm events. One of the challenges about predicting wind erosion in coastal dunes is its extended spatial scale and the associated economic and logistics costs of sampling and characterizing the sediments. Because of this, in-situ testing for sediment characterization is essential. In particular, the usage of free-fall penetrometers (FFP) is appealing due to their portability and robustness. The sediment properties obtained with this type of testing can later be used to assess wind erosion susceptibility by determining, for example, the wind velocity to initiate the erosion process. FFP testing involves dropping an instrumented probe that impacts the soil and measures the kinematics or kinetics during the penetration process. For example, deceleration measurements are used to compute an equivalent quasi-static failure, which is not in line with the dynamic process characteristic of FFP testing. This preassumed failure mechanism is used to back-calculate the sand's geomechanical properties. However, soil behavior is highly complex under rapid loading, and incorporating this behavior into FFP sediment characterization models is challenging. Advanced numerical modeling can improve the understanding of the physics behind FFP testing. This thesis presents various advancements in numerical modeling and erosion models to bridge FFP in-situ testing with predicting the initiation of wind erosion of sands. First, improvements oriented to the Material Point Method (MPM) for modeling in-situ FFP testing are proposed. The numerical results show that the simulation of FFP deployment in sands is affected by strain localization and highlight the importance of considering constitutive models sensitive to different loading rates. Because of the importance of rate effects in soil behavior, the second aspect of this thesis proposes a novel consistency framework. Two constitutive models are adapted to study strain-rate sensitive non-cohesive materials: i) a strain-softening Mohr-Coulomb, and ii) a NorSand model. In addition to increased strength, the proposed framework captures increased dilatation, an early peak deviatoric stress, and relaxation. Finally, a novel sand erosion model is derived using a continuum approximation and limit equilibrium analysis. The erosion law considers geotechnical parameters, the effects of slope, and moisture suction, in a combined manner. The proposed model is theoretically consistent with existing expressions in the literature. It covers a wide range of environmental and geometrical conditions and helps to reconcile the results from FFP testing with the prediction of the initiation of wind erosion. The model was validated in a wind tunnel and is demonstrated to be a viable alternative for predicting sand erosion initiation. This thesis opens up new research prospects, such as improving the soil characterization models or the direct prediction of sand erosion using rapid, reliable, and efficient in-situ testing methods. / Doctor of Philosophy / With global warming and climate change, it is expected that the frequency and intensity of storms will increase. This increment will put extra stress on coastal sediments such as beach sand and coastal dunes, making them prone to erosion. Coastal dunes lose their ability to withstand storms as they erode, potentially making coastal flooding more frequent. In light of this, all stakeholders involved in the protection against coastal disasters must have the tools to predict, prepare for, and mitigate for situations like the ones stated above. An essential aspect of the prediction component is dependent on a successful sediment characterization, for example, determining how much wind the sand can withstand before it erodes. Free-fall penetrometers (FFP) are devices designed to conduct the characterization mentioned above. However, the procedures used to perform this characterization are mainly based on empirical or semi-empirical expressions. Computer models, capable of simulating the physics behind FFP testing, can bring more insight into the process of interaction between FFP devices, sands, and water and can be the basis to improve the characterization methods. The latter results can be utilized for instance to predict wind erosion, including several properties of the sand, such as its mineralogy and shape. This study contributes to developing the computer simulations of FFP deployment and the wind erosion prediction models. Eventually, these developments can help engineers and coastal managers to anticipate and prepare for more frequent coastal hazards.

Rubber-like materials are submitted to cyclic loading conditions in various applications. Fillers are always incorporated within rubber compounds. They improve the mechanical properties but induce a significant stress-softening under cyclic loadings. The physical source of the softening is not yet established and its modeling remains a challenge. For a better understanding of the softening, filled rubbers are submitted to cyclic loadings. In order to quantify the effects of the loading intensity and the number of cycles, original methods are proposed to characterize the softening. To study the influence of the material microstructure on the softening, compounds with various compositions are considered.Non proportional tensile tests including uniaxial and biaxial loading paths are applied in order to highlight the softening induced anisotropy. Such unconventional experimental data are used to provide a general criterion for the softening activation. A constitutive modeling grounded on a thorough analysis of experimental data is proposed. The model is based on a directional approach. The Mullins softening is accounted for by the strain amplification concept and is activated by a directional criterion. The model ability to predict non proportional softened material responses is demonstrated

L’étude présentée porte sur le développement d’un modèle numérique destiné à évaluer les performances balistiques d’une structure multicouche polymère/métal frittée par procédé SPS. Les matériaux sont un alliage d’aluminium 7020 et un polyimide thermoplastique amorphe qui sont ensuite assemblés avec une résine epoxy. Le comportement mécanique de ces trois matériaux a été étudié sur de larges gammes de vitesses de déformations (de 0.0001 /s à 50.000 /s) et de températures (de -70°C à 500°C) correspondant aux conditions extrêmes rencontrées lors d’impacts à hautes vitesses. Afin d’améliorer la précision des résultats, des approches analytiques ont été développées autant pour la modélisation du métal que pour celle les polymères. Après la calibration des modèles, ces derniers ont été implémenté dans ABAQUS®/Explicit (éléments finis) via des subroutines VUMAT en code FORTRAN. Des essais d’impacts de billes à hautes vitesses ont été réalisés sur des cibles monocouches pour valider les modèles numériques. De nombreuses configurations de composites multicouches ont ensuite été étudiées numériquement et leurs performances balistiques ont été comparées. / The present study deals with the development of a numerical model to evaluate the ballistic performance of a polymer/metal multilayer structure sintered by SPS. The materials are an aluminum alloy 7020 and an amorphous thermoplastic polyimide which are then assembled using an epoxy resin. The mechanical behavior of these three materials has been studied over wide ranges of strain rates (from 0.0001 / s to 50,000 / s) and temperatures (from -70 °C to 500 °C) corresponding to the extreme conditions encountered during impacts at high velocities. In order to improve the accuracy of the results, analytical approaches have been developed both for the modeling of the metal and for the polymers. After the calibration of the models, these models were implemented in ABAQUS® / Explicit (finite elements) via VUMAT subroutines in FORTRAN code. Ball impact tests at high velocities were performed on monolayer targets to validate numerical models. Numerous configurations of multilayer composites were then studied numerically and their ballistic performances have been compared.