The main focus of this research is the transformation induced fatigue behavior of shape memory alloy (SMA) actuators undergoing thermally induced martensitic phase transformation. The recent development of aerospace applications employing shape memory alloys (SMAs) has expanded the need for fatigue life characterization and modeling. Lightweight, compact and with a great work output, SMAs are ideal materials for actuated structural components. However, fatigue life becomes a key factor in applications such as commercial airplanes. Therefore, it is necessary to not only perform fatigue testing but also to investigate the causes of fatigue failure. As a new class of materials, SMAs have unique characteristics and require novel test methodologies to conduct repeatable and reliable fatigue testing. For this research, two materials are being investigated: TiNiCu and Ni-rich NiTi. The experiments performed on the first selected alloy, i.e. TiNiCu SMA, explore three major parameters: the applied stress level, the amount of actuation, and the corrosive nature of the environment. Experimental results show that SMAs undergoing transformation induced fatigue exhibit a low-cycle fatigue behavior and the measurement of the accumulated plastic strain at failure is associated to a Manson-Coffin type failure criterion. Investigations conducted on the post-mortem microstructure showed evidence of a multiphysical coupling between corrosion and cyclic phase transformation, from which a novel cyclic damage mechanism is proposed and explained using the micromechanical shear lag model accounting for actuation and accumulated plastic strains. Thereafter, based upon the identified failure mechanism and considering damage accumulation through crack formation, a stress renormalization procedure is proposed in combination with the Miner’s rule to predict the reduction of number of cycles to failure due to cyclic phase transformation and corrosion. A direct method is first presented and the predictions show good agreement with experimental results. However, both corrosion and corrosion-free fatigue data are required. Therefore, a new approach is proposed: the inverse Miner’s rule, which requires corrosion fatigue data only to predict corrosion-free life. The new and attractive properties of the selected second alloy, i.e. Ni-rich NiTi SMA, have revived the motivation of the aerospace industry to design SMA actuators. One particular property is cyclic stability generated by precipitation hardening mechanism using precipitates. However, are also precipitates due to high Nickel content (60 wt.% or 55 at.%). Parameters such as processing, heat treatments, size effects, surface quality and environment are investigated. Thermomechanical response and fatigue life are discussed and the greatest impact is found to come from specimen surface quality. Finally, a detailed fractography presents the different microstructural aspects of the fatigue damage and concludes to a precipitation driven fatigue failure mechanism cause by precipitates.

Cette thèse traite du problème de l'écoulement d'un fluide dans des interfaces étroites entre des solides en contact sous un chargement normal, ce qui est important pour de nombreuses applications en tribologie, ingénierie et géophysique. Le traitement de ce problème nécessite de prévoir un couplage entre la mécanique des fluides et celle des solides. Les contraintes liées à la présence du contact, ainsi que les caractéristiques complexes de la géométrie de surface rajoutent un niveau de complexité significatif. Dans cette thèse, un solveur monolithique par éléments finis permettant la gestion du contact frottant, des écoulements visqueux incompressibles et du transfert des efforts induits par le fluide sur le solide est développé. De plus, la possibilité que le fluide se retrouve piégé dans des cavités délimitées par des zones de contact est prise en compte par l'élaboration d'un nouvel élément dit "de fluide piégé", qui utilise une loi de comportement compressible non linéaire. Le code résultant de cette méthode comprend des algorithmes d’analyse d’image permettant de distinguer les zones de contact, d’écoulement de fluide et de fluide piégé. En outre, le code convient aux approches de couplage uni- et bidirectionnel. Le cadre développé a été appliqué dans un premier temps à l'étude d'un fluide piégé entre un solide déformable présentant une surface de contact ondulée et un plan rigide. Pour un système soumis à une charge externe croissante, nous avons examiné l'évolution de la surface de contact et du coefficient de frottement global en fonction des propriétés du fluide et du solide, ainsi que de la pente du profil de surface. Nous avons ensuite étudié l’écoulement d’un fluide entre un plan rigide et un solide déformable avec une géométrie modèle ou une surface rugueuse. Nous avons obtenu une solution analytique approchée qui gouverne le flux de fluide à travers une interface de contact ondulée, et cette dernière a été comparée à nos résultats numériques. Enfin, nous avons montré pour un intervalle de paramètres physiquement pertinents, que le couplage unidirectionnel sous-estime, par rapport à une approche bidirectionnelle, la perméabilité de l’interface ainsi que la charge externe critique nécessaire à la fermeture de l’interface. Une loi phénoménologique raffinée de perméabilité macroscopique des interfaces de contact rugueuses a été proposée. Enfin, le cadre développé a été utilisé pour calculer l'évolution de la fuite de fluide à travers une interface de contact métal sur saphir en utilisant un comportement matériau élasto-plastique et des mesures réelles de la rugosité de surface. / This thesis deals with the problem of a thin fluid flow in narrow interfaces between contacting solids subject to a normal loading, which is relevant for a range of tribological and engineering applications, as well as for geophysical sciences. The treatment of this problem requires coupling between fluid and solid mechanics, further complicated by contact constraints and potentially complex geometrical features of contacting surfaces. In this thesis a monolithic finite-element framework for handling frictional contact, thin incompressible viscous flow and transfer of fluid-induced tractions to the solid is developed. Additionally, we considered fluid entrapment in "pools" delimited by contact patches and formulated a novel trapped-fluid element using a non-linear compressible constitutive law. This computational framework makes use of image analysis algorithms to distinguish between contact, fluid flow and trapped fluid zones. The constructed framework is suitable for both one- and two-way coupling approaches. First, the developed framework was applied to a study of a fluid trapped between a deformable solid with a wavy surface and a rigid flat. We showed how the contact area and the global coefficient of friction evolve under increasing external load, depending on fluid and solid properties and on the slope of the surface profile. Next, we studied a thin fluid flow between a rigid flat and a deformable solid with a model geometry or random surface roughness. An approximate analytical solution for the fluid flow across a wavy contact interface was derived and compared with numerical results. We showed that for a range of physically relevant parameters, one-way coupling underestimates the interface permeability and the critical external load needed to seal the interface, compared to the two-way approach. A refined non-local phenomenological law for macroscopic permeability of rough contact interfaces was proposed. Finally, the developed framework was used to calculate the evolution of the fluid leakage through a metal-to-sapphire contact interface using an elasto-plastic material behaviour and real measurements of surface roughness.