The effect of thermal cycling on the state of stress in polymeric materials bonded to stiff elastic substrates was investigated using numerical techniques, including finite element methods. The work explored the relationship between a cyclic temperature environment, temperature-dependent viscoelastic behavior of polymers, and thermal stresses induced in a constrained system. Due to the complexity of developing a closed-form solution for a system with time, temperature, material properties, and boundary conditions all coupled, numerical techniques were used to acquire approximate solutions. Descriptions of attempted experimental verification are also included. The results of the numerical work indicate that residual stresses in an elastic-viscoelastic bimaterial system incrementally shift over time when subjected to thermal cycling. Tensile axial and peel stresses develop over a long period of time as a result of viscoelastic response to thermal stresses induced in the polymeric layer. The applied strain energy release rate at the crack tip of layered specimens is shown to similarly increase. The rate of change of the stress state is dependent upon the thermal cycling profile and the adhesive’s thermo-mechanical response. Discussion of the results focuses on the probability that the incrementing tensile residual stresses induced in an adhesive bond subjected by thermal cycling may lead to damage and debonding, thus reducing durability. / Master of Science

Si un métal est soumis à un chargement cyclique autour d'une contrainte moyenne non nulle, de la déformation plastique peut s'accumuler au cours des cycles, généralement dans la direction de la contrainte moyenne : c'est le phénomène de déformation progressive ou phénomène dit de rochet.<br />Le contexte de cette thèse était l'amélioration de la description de ce phénomène dans les structures métalliques. Pour cela, nous avons choisi d'axer nos efforts sur les modèles macroscopiques phénoménologiques qui décrivent le comportement de matériaux dans des calculs de structure.<br />Comme cela était suggéré dans plusieurs articles récents, une voie d'amélioration de ces modèles était de prendre en compte la distorsion du domaine d'élasticité, phénomène maintes fois observé expérimentalement, mais jusqu'alors négligé par soucis de simplicité. Pour introduire convenablement ce nouvel ingrédient dans la modélisation, nous nous sommes appuyés sur un dialogue entre un modèle micro-macro d'une part (plus pertinent mais aussi plus coûteux en temps de calcul) et le modèle macroscopique à construire d'autre part. Nous avons sû tirer profit des informations données par le modèle micro-macro pour constuire un modèle macroscopique capable de décrire à la fois le phénomène de rochet multiaxial et la distorsion du domaine d'élasticité prévus par le modèle micro-macro, et ce sur une large base d'"essais virtuels". Ensuite, le modèle développé a été identifié et validé avec succès sur plusieurs résultats d'essais complexes obtenus sur un élément de volume d'un matériau réel. Une validation finale du modèle sur un essai non-homogène (de structure) est en cours d'étude.

In the present thesis, fatigue behavior of nodular cast iron (NCI) is investigated using micromechanical simulations. An elastic-plastic porous material experiences an increase in a void volume fraction with each cycle of loading. This is called void ratchetting. The hypothesis of this thesis is to explain the fatigue failure of NCI using void ratchetting mechanism. The strain-life, stress-life, notch support effect, and fatigue crack growth are studied using the micromechanical simulations. In all these studies, matrix material is defined as an elastic-plastic with isotropic/kinematic hardening. No damage law is used to define material degradation. The axisymmetric cell model is developed to study strain-life and stress-life approaches for fatigue. The cell model is subjected to cyclic loading and cycle by cycle simulations are carried out until failure. The failure of the cell model is defined based on the drop in the macroscopic response of the cell model. The notch support effect is investigated using a 2D plane strain model within stress-life concept. From the simulation results, strain-life and stress-life curves are extracted, and they are in qualitative and quantitative agreement with experimental data collected from literature. The fatigue crack growth is studied using a micromechanical cell model under small scale yielding conditions. The graphite particles are considered as voids, and they are resolved discretely in fracture process zone. The region outside of the fracture process zone is considered as a homogenized medium. When positive alternating loads are applied, ligaments in the fracture process zone show ratchetting behavior, which is responsible for an effective fatigue crack growth. This mechanism is relevant for the fatigue crack growth in NCI. The 2D plane strain boundary layer model is able to predict the effect of load ratio on threshold for the fatigue crack growth and the fatigue crack growth rate. The fatigue crack growth rate curves obtained from the simulations are compared with experimental data. It is essential to note that the void ratchetting (plastic collapse of the intervoid ligaments) is a crucial mechanism in NCI and more focus should be given to this mechanism as it is simple to implement and gives satisfying simulation results.