The deformation mechanisms and contact response of TiN-based thin films deposited onto a soft substrate using a physical vapour deposition (PVD) technique is still an area of both technological importance and considerable discussion. These coatings are commonly applied to various kinds of steel cutting tools, creating surfaces with enhanced tribological properties. However, no extensive systematic study of the deformation mechanisms in these thin film systems has been performed to date. In the present study, the effect of the coating microstructure, indenter geometry, coating thickness and substrate hardness on the deformation mechanisms in both TiN and TiAlN coatings of varying thickness deposited onto ductile steel substrates has been investigated using a combination of nanoindentation and microstructural analysis, including focused ion beam (FIB) milling and transmission electron microscopy (TEM). Different modes of cracking, such as columnar and transverse cracking, as well as shear steps at the coating/substrate interface, were observed. The microstructure of the TiN coatings was found to be very important in controlling their modes of deformation. Thicker coatings were seen to contain more equiaxed grains, so less columnar shearing occurred and inclined cracks were found to be a more dominant fracture type in the thicker coating. Also, it was found that soft substrates absorbed most of the energy from indentation by plastic deformation. It was found that both the TiN and TiAlN/TiN dual-layer coatings exhibited broadly similar mechanisms of deformation. The epitaxial interface between the TiAlN and TiN in the dual-layer coating did not appear to affect the deformation behaviour. As a further investigation of the overall deformation behaviour for the coating/substrate systems studied, a DualBeam FIB was used to generate three dimensional images of the indented regions which provided additional information on the crack morphology. For the first time, a systematic study of the deformation behaviour of TiN and TiAlN coatings upon indentation has been carried out. FIB milling was demonstrated to be a highly appropriate technique for characterization of the deformation behaviour of these coatings, allowing detailed, high resolution microstructural investigations to be performed in both two and three dimensions.

La partie 1 releve de l'Analyse Numerique. Partant de l'interpretation Element Finis Mixtes des schemas volumes finis classiques, l'estimation a posteriori de l'erreur est analysee dans la hierarchie des elements de Raviart-Thomas. Un estimateur calculable est explicite pour ces schemas volumes finis.<br />La partie 2 introduit, d'abord un maillage rectangulaire, puis un maillage structure, une famille de schemas volumes finis de type differences finies. Des essais numeriques sur des problemes modeles montrent que l'ordre prevu par l'analyse peut etre atteint.<br />La partie 3 presente l'application de ces schemas volumes finis a la simulation numerique du comportement d'un bloc de gomme en presence d'une fissure finie. Il s'agit d'un materiau hyperelastique compressible en grandes deformations et differents tenseurs de contraintes, avec tests en quasi-incompressible et des simulations d'endommagement.

DANS MON TRAVAIL DE THESE JE M'INTERESSE A LA MODELISATION GEOMETRIQUE, MECANIQUE ET NUMERIQUE DU MYOCARDE. LA PARTIE GEOMETRIQUE CONSISTE A VERIFIER UNE CONJECTURE SELON LAQUELLE LES FIBRES MYOCARDIQUES COURENT COMME DES GEODESIQUES SUR DES SURFACES EMBOITEES. POUR CELA, LES TRAJECTOIRES ET LES SURFACES DE FIBRES ONT ETE IDENTIFIEES. DANS LA PARTIE MECANIQUE, NOUS AVONS ETABLI UNE NOUVELLE LOI DE COMPORTEMENT MACROSCOPIQUE DU MYOCARDE PAR UNE TECHNIQUE D'HOMOGENEISATION DISCRETE A PARTIR DE LA DESCRIPTION MICROSCOPIQUE DE L'ARRANGEMENT DES CELLULES CARDIAQUES ET DE LEUR COMPORTEMENT MECANIQUE INDIVIDUEL. CETTE LOI DE COMPORTEMENT PREND EN COMPTE LA STRUCTURE FIBREUSE. DE PLUS, NOUS AVONS APPLIQUE NOTRE METHODE D'HOMOGENEISATION AUX NANOTUBES DE CARBONE.

In this paper, a numerical method is used to predict the response of an elastic body during a collision in which both normal and tangential impulses are important. Results are compared with those from simplified prediction procedures, which stem from the assumptions that the energy-returning capacity of the normal deformation mechanism is constant and the tangential compliance is neglected. The finite-element predictions indicate the importance of the tangential compliance for elastic collisions wherein friction forces are significant. The results of both methods of prediction point up the roles of friction and inertia coupling in determining the normal velocity ratio (the "coefficient of restitution"). / Graduation date: 1992

An elastohydrodynamic analysis of a rotary lip seal is performed numerically, incorporating both the fluid mechanics of the lubricating film and the elastic deformation of the lip, by solving the Reynolds equation with flow factors. Asperities on the lip surface dominate the behavior of the flow field in the lubricating film and the elastic deformation of the lip. Since previous analyses treated those asperities deterministically, they required very large computation times. The present approach is much less computationally intensive because the asperities are treated statistically. Since cavitation and asperity orientation play important roles, these are taken into account in the computation of the flow factors. An asperity distortion analysis is introduced to obtain a more realistic model of the complex variations in the asperity distribution on the surface of the seal. Results of the analysis show how the operating parameters of the seal and the characteristics of the asperities affect such seal characteristics as the thickness of the lubricating film, reverse pumping rate, power dissipation and load carrying capacity.

In recent impact and penetration mechanical tests, steel projectiles (AISI4340) were impacted into targets like concrete with striking velocities (1200 m/s to 1500 m/s). Results indicated a material removal from the nose of the projectile, phase changes of the projectile materials, a reduction in the length of the projectile, and a blunting of the nose shape. These observations cannot be explained by current theories and numerical integration code that are used to study impact and penetration mechanics. Thus, the objectives of the thesis research are to (a) formulate and characterize the mechanisms responsible for the material erosion of the impacting projectile and the mass loss from the nose region; and (b) to determine the physical properties of alloy steels that are important to penetration mechanics from ab initio methods. The results can be used to design new projectile materials that can provide the desired penetration characteristics. These objectives are accomplished by investigating two related problems. The first problem is to formulate simplified models that can explain the penetration mechanics. The new models include the varying cross-section nose, changes of yield stress behind the shock wave and high strain rate phase transitions. Nose erosion effects, and time-dependent penetration path can be determined by integrating ODEs. A cavity expansion theory model is used to obtain the target resistance that is responsible slowing and deforming the penetrating projectile. The second problem concerns the determination of the constitutive relations from ab initio methods. The equation of state (EOS) and magnetic moments for alloy steels are investigated by using a special quasirandom structure technique and ab initio methods. Specifically, EOS for an interstitial disordered alloy Fe1-x-yNixCy is developed. First, the EOS of iron and phase transition of iron are studied and validated. Second, Nickel is considered to investigate the substitutional disordered alloy Fe1-x-yNixCy. Third, Carbon is placed at an interstitial position in the substitutional disordered alloy. These investigations will form foundation for future work involving new projectile with steel nose and shank made of multifunctional structural energetic materials.

Nanowires are an exciting class of novel materials that have potential applications in areas including biological sensing, photonics, and electronics. The promise of these future applications relies on the production of nanowires of controlled size, shape, and crystal structure, in reasonable quantities, and further, ultimately requires that the nanowires be mechanically stable in the application environment. This research is aimed at understanding the mechanical behavior of metallic nanowires, through the use of atomistic simulations. At the nanometer scale, where the surface-area-to-volume ratio is substantial, the effects of free surfaces have a primary influence on the physical properties of a material. Surface energy arises from unsatisfied bond coordination at the surface of a solid and results in a surface stress as the surface atoms contract into the bulk of the material to increase their local electron density. The magnitude of surface energy and surface stress is dependent on the orientation of the surface and the compliance of the structure. In bulk materials, the effects of surfaces are negligible; however, at the nanometer scale, surface effects become quite significant. In metallic nanowires, these surface effects strongly influence mechanical properties, and the characteristics of plastic deformation. The mechanical testing of nanowires is precluded by the difficulties of accurately applying and measuring forces on the nanometer scale. For this reason, computational simulations are a primary tool for investigating the mechanical behavior of nanowires. In this work, we have performed atomistic simulations to examine the mechanical response of silver nanowires. We have conducted studies to determine the deformation characteristics of experimentally observed nanowire geometries subjected to tensile and bending loads. We have also developed a technique to probe the energetics of mechanical deformation, in order to elucidate the energetically favored deformation pathways in nanowires. Our results show that nanowires may be tailored for specific mechanical requirements based on geometry and free surface orientation and provide insight to the effect of free surfaces in the mechanical deformation of nanometer scale structures.

This work focuses on the thermomechanical characterization and constitutive model calibration of shape memory polymers (SMPs). These polymers have the ability to recover seemingly permanent large deformations under the appropriate thermomechanical load path. In this work, a contribution is made to both existing experimental and modeling efforts. First, an experimental investigation is conducted which subjects SMPs to a thermomechanical load path that includes varying the value of applied deformations and temperature rates. Specifically, SMPs are deformed to tensile extensions of 10% to 100% at temperature rates varying from 1 degree C /min to 5 degree C/min, and the complete shape recovery profile is captured. The results from this experimental investigation show that the SMP in question can recover approximately 95% of the value of the applied deformation, independent of the temperature rate during the test. The data obtained in the experimental investigation are then used to calibrate, in one-dimension, two constitutive models which have been developed to describe and predict the material response of SMPs. The models include a model in terms of general deformation gradients, thus making it capable of handling large deformations. In addition, the data are used to calibrate a linearized version of the constitutive model for small deformations. The material properties required for calibrating the constitutive models are derived from portions of the experimental results, and the model is then used to predict the shape memory effect for an SMP undergoing various levels of deformation. The model predictions are shown to match well with the experimental data.

In order to study suspensions of deformable particles, a hybrid numerical technique was developed that combined a lattice-Boltzmann (LB) fluid solver with a finite element (FE) solid-phase solver. The LB method accurately recovered Navier-Stokes hydrodynamics, while the linear FE method accurately modeled deformation of fluid-filled elastic capsules for moderate levels of deformation. The LB/FE technique was extended using the Message Passing Interface (MPI) to allow scalable simulations on leading-class distributed memory supercomputers. An extensive series of validations were conducted using model problems, and the LB/FE method was found to accurately capture proper capsule dynamics and fluid hydrodynamics. The dilute-limit rheology was studied, and the individual normal stresses were accurately measured. An extension to the analytical theory for viscoelastic spheres [R. Roscoe. J. Fluid Mech., 28(02):273-93, 1967] was proposed that included the isotropic pressure disturbance. Single-body deformation was found to have a small negative (tensile) effect on the particle pressure. Next, the rheology and microstructure of dense suspensions of elastic capsules were probed in detail. As elastic deformation was introduced to the capsules, the rheology exhibited rapid changes. Moderate amounts of shear thinning were observed, and the first normal stress difference showed a rapid increase from a negative value for the rigid case, to a positive value for moderate levels of deformation. The particle pressure also demonstrated a decrease in compressive stresses as deformation increased. The corresponding changes in microstructure were quantified. Changes in particle self-diffusivity were also noted.

To better understand consequences of classical assumptions regarding deformation mechanisms at the mesoscale, experimental observations of mesoscale deformation are presented. In light of actual micrographics of deformed polycrystals, the Von Mises criterion which states that 5 independent plastic deformation sources are needed at each material point to satisfy compatibility is studied, and the consequences of violating this assumption are presented through comprehensive parametric studies. From these studies, it can be concluded that not only are 5 independent plastic deformation sources not needed or observed at each point, but if less than 5 sources are allowed to be active a new physical understanding of a mechanism for kinematic hardening emerges. Furthermore, for enhanced subgrain rotation and evolution the Von Mises criterion must be violated. The second focus of this work is looking at studies, experiments, and models of mesoscale deformation in order to better understand controlling deformation length scales, so that they can be fed into a combined top-down, bottom-up, non-uniform crystal plasticity model that captures the variability provided by the mesoscale during deformation. This can in turn be used to more accurately model the heterogeneity provided by the response of each grain. The length scale intuited from insight into mesoscale deformation mechanisms through observation of experiments and analytical models is the free slip line length of each slip system, which informs non-uniform material parameters in a crystal plasticity model that control the yielding, hardening, and subsequent softening of each individual slip system. The usefulness of this non-uniform multiscale crystal plasticity model is then explored with respect to its ability to reproduce experimentally measured responses at different strain levels for different size grains. Furthermore, a "Mantle-Core" type model which combines both the non-uniform material parameter model and the limited slip model is created, in which the majority of plastic deformation is accommodated near the grain boundary under multi-slip, and uniform plastic deformation occurs in the bulk dominated by double or triple slip. These models are compared for similar levels of hardening, and the pole figures that result from their deformation are compared to experimental pole figures. While there are other models that can capture the heterogeneity introduced by mesoscale deformation at the grain scale, this combined top-down, bottom-up multiscale crystal plasticity model is by far one of the most computationally efficient as the heterogeneity of the mesoscale is does not emerge by introducing higher order terms, but rather by incorporating the heterogeneity into a simple crystal plasticity formulation. Therefore, as computational power increases, this approach will be among the first that will be able to perform accurate polycrystal level modeling while retaining the heterogeneity introduced by non-local mesoscale deformation mechanisms at the sub-grain scale.