Oxidative aging and fracture behavior of polymers and composites: theory, modeling and experiments

Polymers and their composites (PMC) have emerged as effective alternative materials in structural, aerospace, and automotive industries due to their lightweight and tunable properties compared to metals. However, these materials tend to degrade during their operations in extreme environments. In this work, two extreme conditions are considered: - i) high-temperature oxidative degradation of polymers and polymer-based composites ii) Fracture and damage of polymer-based composites under thermo-mechanical loading. Polymer oxidation starts when oxygen from the ambient diffuses into the bulk material and initiates chemical reactions to develop a coarse, brittle oxide layer on the exposed surface. The oxidative degradation process is inherently complex in nature, as it involves a coupling between diffusion, reaction, and mechanics. As oxygen diffuses into the polymer, a series of chain reactions occur, resulting in residual shrinkage strain on the oxidized layer of the material due to escaping of the volatiles. Consequently, residual stress develops within the material, causing spontaneous cracking even without the application of external loading. Thus, the oxidative aging can cause premature cracking in the material and requires a better understanding of the interaction between the chemistry and mechanics at different length scales and timescales to comprehend the effect of thermo-oxidative aging of polymeric materials. In this work, a fully coupled thermodynamically consistent chemo-mechanical phase-field fracture model is developed that attempts to bridge the gap between the experimental observations and a constitutive theory for thermo-oxidative aging in polymeric materials. To accomplish this, a novel approach has been adopted considering the chemical reactions at the polymer macromolecular level, a reaction-driven transient network evolution theory at the microscale, and a constitutive model at the macroscale. Finally, a phase-field fracture theory is added to the chemo-mechanical model to predict the oxidation-induced fracture in the polymer under mechanical loading. The model has been further extended to a homogenized continuum theory to capture the anisotropic oxidation characteristic of the fiber-reinforced polymer matrix composites. Specialized forms of the constitutive equations and the governing partial differential equations have also been developed for the polymers and the composite systems and numerically implemented in finite elements by writing ABAQUS user-defined element (UEL) subroutine. Lastly, a unified phase-field fracture model is developed to create an experimentally validated, physically motivated, and computationally tractable model to predict the fracture response of the unidirectional fiber reinforced polymer matrix composites. A homogenized, coupled thermo-mechanical model is developed considering a thermo-viscoelastic polymer matrix. The model is numerically implemented by writing a ABAQUS user-element subroutine (UEL). The model can predict the constitutive response and direction-dependent damage propagation and final fracture in commercially acquired unidirectional glass-fiber-reinforced epoxy composite both at different fiber orientations and at different temperatures in substantial agreement with the experiments

Elastic and viscoelastic deformation of glass plates by excimer laser induced stresses and patterned silicon suboxide films

Eine mechanisch vorgespannte Schicht kann eine signifikante Verformung des darunterliegenden Substrats verursachen. Üblicherweise handelt es sich hierbei um eine elastische Verformung, aber unter bestimmten Umständen kann auch eine plastische oder viskose Verformung des Substrats beobachtet werden. In den meisten Fällen ist diese Verformung unerwünscht. Allerdings wurden auch Methoden zur gezielten Verformung von Substraten entwickelt, die auf dem Einbringen von ebenen Spannungskomponenten in eine oberflächennahe Schicht basieren. Hierdurch können Formfehler von optischen Komponenten korrigiert werden. Es wurde auch vorgeschlagen, die viskose Verformung eines Glassubstrats aufgrund einer strukturierten Spannungsschicht zu nutzen, um eine angestrebte Oberflächentopographie zu erhalten. Excimerlaser werden zur präzisen Strukturierung von Glasoberflächen und dünnen dielektrischen Schichten auf Glassubstraten eingesetzt. In dieser Arbeit berichte ich über die Verformungen und Spannungen in zwei Systemen mit Bezug zur Anwendung eines ArF-Excimerlasers (193 nm, 20 ns). Beim ersten System handelt es sich um dünne Platten aus dem Borosilikatglas Schott D263M. Durch Bestrahlung wird eine starke Zugspannung in einen oberflächennahen Bereich der Proben eingebracht. Über die Verformung der Proben habe ich die integrierte Spannung in Abhängigkeit von den Bestrahlungsparametern, das Langzeitverhalten der integrierten Spannung und die Spannungsverteilung in Richtung senkrecht zur Oberfläche gemessen. Ich zeige, dass antibiaxiale Spannungskomponenten mittels Bestrahlung mit einem Linienmuster eingebracht werden können. Und ich zeige die Möglichkeit auf, die eingebrachten Spannungen zur Formkorrektur von Glassubstraten zu nutzen. Die Ergebnisse betonen auch die Bedeutung von dauerhaften thermischen Spannungen in der Bearbeitung von Gläsern mit Excimerlasern. Beim zweiten System handelt es sich um kontinuierliche oder laserstrukturierte dünne Schichten aus einem substöchiometrischen Siliziumoxid auf plattenartigen Quarzglassubstraten. Tempern der Proben bei Temperaturen nahe der Glasübergangstemperatur führt zu einer starken viskosen Verformung der Substrate, deren Ausprägung von der Schichtstruktur abhängt. Ich habe die Verformung für unterschiedliche Schichtstrukturen und Atmosphären gemessen. Durch Vergleich mit Finite Elemente Simulationen und analytischen Berechnungen zeige ich, dass die Verformung in Analogie zum Verhalten eines elastischen Substrats und durch Spannungen, die durch die Oxidation der Schicht entstehen, verstanden werden kann. Die Ergebnisse zeigen die Möglichkeit eines neuen Verfahrens zur Formung von Glasplatten über eine strukturierte Spannungsschicht auf.A mechanically stressed film can cause a bending of the underlying substrate. Usually, the substrate deforms elastically, but in special cases a plastic or viscous deformation can be observed. Such deformations are mostly unwanted. However, methods have been developed by several people to specifically deform plate-like substrates via the generation of plane stress components inside a near-surface region, and by this to correct errors in the surface topography of thin optical components (figure correction). It was also proposed to make use of the viscous deformation of a glass substrate due to a stressed film for forming of optical components. Excimer lasers are applied in precise structuring of glass surfaces and patterning of dielectric films on glass substrates. In this thesis, I report on the deformations and stresses in two systems related to the application of an ArF excimer laser (193 nm, 20 ns). The first system consists of sheets of the borosilicate glass Schott D263M. When irradiated, a large tensile stress is generated in a surface-near region. From the deformation of the samples, I measured the integrated stress in dependence on the irradiation parameters, its long-term temporal evolution and the stress distribution in direction normal to the surface. I show that antibiaxial plane-stress components can be generated by irradiation with a line pattern. I performed a proof of principle for an application of the laser induced stresses in figure correction. The results also demonstrate the relevance of long-term thermal stresses in excimer laser ablation of glasses. The second system consists of continuous or laser patterned thin films of a substoichiometric silicon oxide on plate-like substrates of vitreous silica. Annealing of the samples at a temperature close to the glass transition causes a large viscous deformation of the substrates, which is determined by the film pattern. I measured the deformation for different film patterns and for different atmospheres during annealing. By comparison to the results of a finite element simulation and analytic calculations, I show that the deformation can be understood by analogy to the case of an elastic substrate and stresses due to the oxidation of the film. The results pave the way for an application for forming of plates by a patterned film.2021-11-0

Fundamental Study Of Mechanical And Chemical Degradation Mechanisms Of Pem Fuel Cell Membranes

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\u27s 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\u27s 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\u27s 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

Use of AFM Indentation to Quantify Mechanical Properties of the Interphase Region in Fiber-Reinforced Composites

Interphase is the region in the vicinity of the reinforcing fiber in the polymer composites with properties distinct from the bulk matrix. Investigation of these nano- and micro interphase regions have been done using conventional indentation techniques, followed by closed-form solutions applicable to the indentation in a semi-infinite space. However, due to the presence of the fiber the interphase region can be considered only as a semi-infinite space with rigid constraint. An integrated approach using AFM (Atomic Force Microscopy) based indentation and FEA (Finite Element Analysis) is used to investigate the effect of the fiber constraint on the mechanical properties of the interphase region. Obtained results indicate that the thickness of the interphase region is approximately 250 nm, based on the gradient in the elastic modulus as a function of radial distance from the fiber. 3D FEA using an elasto-plastic material model indicates that the fiber constraint effect is considerable in the region less than 40 nm away from the fiber. The time-dependent behavior of the interphase region is studied using a constant displacement approach, and the distinct viscoelastic response of the interphase region is observed as a function of the radial distance to the fiber. FEA using a linear viscoelastic material model shows that the influence of fiber constraint on the evaluation of the viscoelastic properties is distinguishable only within 20 nm away from the fiber. Due to the limited extent of influence of fiber constraint effect, the distinct behavior of the interphase region in terms of elastic and viscoelastic properties is confirmed. Additionally, it is shown that consideration of an axisymmetry assumption for modeling the interphase region leads to an overestimation of the properties of the region. This technique is further implemented to demonstrate the effect of UV irradiation on the interphase region. Methodical analysis of the data indicates that the response of epoxy to UV irradiation is influenced by the proximity to the reinforcement and carbon fiber reinforcement hinders the photo-degradation of epoxy. Furthermore, the influence of the thermal mismatch between the fiber and the matrix on the formation of the interphase and the effect of post-curing are examined using the approach considered in this study. Results indicate that the presence of thermal stresses greatly impact the width of the interphase region and its behavior

Modeling of combustion processes of stick propellants via combined Eulerian-Lagrangian approach

This research is motivated by the improved ballistic performance of large-caliber guns using stick propellant charges. A comprehensive theoretical model for predicting the flame spreading, combustion, and grain deformation phenomena of long, unslotted stick propellants is presented. The formulation is based upon a combined Eulerian-Lagrangian approach to simulate special characteristics of the two phase combustion process in a cartridge loaded with a bundle of sticks. The model considers five separate regions consisting of the internal perforation, the solid phase, the external interstitial gas phase, and two lumped parameter regions at either end of the stick bundle. For the external gas phase region, a set of transient one-dimensional fluid-dynamic equations using the Eulerian approach is obtained; governing equations for the stick propellants are formulated using the Lagrangian approach. The motion of a representative stick is derived by considering the forces acting on the entire propellant stick. The instantaneous temperature and stress fields in the stick propellant are modeled by considering the transient axisymmetric heat conduction equation and dynamic structural analysis

Study on the deformation behavior of the cathode collector bar at high temperature and low levels

L'étude de la déformation de la barre collectrice dans les conditions subies au sein de la cellule de réduction d'aluminium est d'une grande importance pour l'optimisation de l'efficacité et l'augmentation de la durée de vie de la cellule. Ce mémoire nous informe des résultats d'un programme expérimental réalisé sur une barre de collectrice en acier. Le but, est d' étudier son comportement en tenant compte de ses propriétés thermiques, mécaniques et de fluage. Des essais ont été effectués en compression à de basses tensions, de 0,5 à 2MPa et à une température élevée, de 900°C. Différents comportements ont été observés à de faibles contraintes, jusqu'à 2MPa, cela peut être justifié par le temps et le niveau de pression appliqué. L'inspection métallographique a montré l'apparition d'oxydation et de la corrosion sur des échantillons testés, ceci est dû à l'environnement agressif des conditions du test. D'importants efforts et modifications ont été fournis pour éradiquer cet effet et pour améliorer l'exactitude des données de test de fluage obtenus.The study of the deformation behaviour of the collector bar at conditions experienced within the aluminium reduction cell is of great importance to optimizing the efficiency and increasing the life span of the cell. This mémoire communicates the results of an experimental program carried out on the steel collector bar material (AISI 1006) to investigate its behaviour in relation to its thermal, mechanical and the creep properties. Tests were carried out in compression at low stresses, 0.5 to 2 MPa and high temperature, 900 °C. Different behaviour was observed at low stresses up to 2 MPa, which can be characterised by time and applied stress level. Metallographic inspection showed effect of oxidation and corrosion on tested samples due to the aggressive environment of the test condition, major efforts and modification were made to eradicate this effect and to improve the accuracy of obtained creep test data

FRICTION MEASUREMENT IN PRECISION GLASS MOLDING

Extensive growth of state-of-the-art technologies has created a demand for high quality lenses and has driven the industry toward an inexpensive process for manufacturing of aspheric glass lenses called Precision Glass Molding (PGM). Finite Element Analysis (FEA) has been used to predict the right mold geometry. Having a realistic simulation to predict mold geometry depends on the correct model of material behavior and friction coefficient at elevated temperature. Finding the static and dynamic coefficient of friction experimentally between two flat surfaces at elevated temperature is the subject of this research. The equipment used in this study was originally designed for the Precision Glass Molding (PGM) process and was modified for friction measurement by using molds designed specifically for the friction test. The performance of this apparatus was validated using a steel-steel friction pair at room temperature and a steel-BK7 pair at elevated temperature. The frictional behavior of two different types of oxide glasses; BK7 and Soda-Lime-Silica glass have been studied. During trials at which the temperature is above the glass transition temperature, the results show the effect of glass viscoelasticity in the friction data. This effect is in the form of exponential increase in friction force data prior to the onset of sliding. Moreover, the effect of stick-slip phenomenon can be seen as a jump in the position data (in the order of microns in tangential direction). Coulomb\u27s Law has been used to calculate the friction coefficient. An average friction coefficient has been defined and calculated for some trials, providing a quantitative value for dynamic friction coefficient at different process parameters. The final part of the investigation involved using the Design of Experiment approach to include a broader range of processing parameters and do a sensitivity analysis to find the effect of temperature, normal force, feed rate, and surface finish on dynamic friction coefficient. The finding from the current investigation demonstrates reasonable changes in dynamic friction of glass due to its viscoelastic properties close to its transition temperature. These friction data can be used to improve the accuracy of simulations of the PGM process