L'objectif de ce travail est une contribution à la compréhension des mécanismes de déformation plastique monotone et cyclique d'un acier duplex (50% [Alpha]-50%[Gamma]) en utilisant la microscopie à force atomique comme technique d'investigation. Les analyses après sollicitation monotone valident le potentiel de la technique AFM et imposent une identification minutieuse des lignes de glissement en fonction de leurs morphologies et dimensions, dans les grains de ferrite et d'austénite. En fatigue oligocyclique à faible variation de déformation, en faisant varier la dureté de la ferrite, nous montrons que la réponse macroscopique à la sollicitation cyclique de l'acier duplex hypertrempé résulte de la contribution effective des deux phases, Alpha et Gamma, contrairement à ce qui est proposé dans la littérature. En réalisant des essais de fatigue à variation de déformation élevée et interrompus à différents pourcentages de la durée de vie, les mécanismes d'amorçage de fissure ont pu être proposés. En fonction de la morphologie des îlots d'austénite, de leur répartition et de leur cohérence cristallographique avec la matrice ferritique, certaines extrusions dans l'austénite peuvent générer des zones de haute rugosité dans la ferrite, à la frontière Alpha/Gamma, qui constituent les sites d'amorçage de fissure dans la phase ferritique. De plus, l'interactivité des deux phases est de nouveau démontrée dans le mécanisme de plasticité cyclique par le transfert progressif de la plasticité de la phase austénitique vers la phase ferritique. L'ensemble des résultats propose que la plasticité cyclique des aciers duplex est accommodée progressivement et de manière interactive par les deux phases Alpha et Gamma. De ce fait, la séparation du comportement en fatigue en régimes différents, concept mettant en avant une activité ou passivité de chaque phase selon la déformation imposée, semble être une approche simplifiée ne reflétant pas nécessairement le comportement réel de ce matériau biphasé. / In order to increase the understanding of monotonic and cyclic plastic deformation mechanisms in multiphase alloys, an intensive research task has been carried out on a Duplex Stainless Steels (DSS-50%α-50%γ) using Atomic Force Microscopy (AFM). After monotonic deformation, AFM investigations of the surface reveal that this technique is especially promising for the plasticity studies of DSS. It allows high detailed characterisation of different types of slip lines, depending of their morphology, dimensions and the analysed phase, austenite or ferrite. Concerning the Low Cycle Fatigue behaviour at low strain range, contrary to earlier works, the comparison of the surface topography between two alloys differing by their ferrite hardness (annealed and aged), suggests that the macroscopic cyclic behaviour of the annealed DSS is a consequence of the mutual contribution of the two phases. Performing interrupted fatigue tests at high strain range, High-Rugged (HR) areas were identified in ferritic grains and near the α/γ interfaces. The formation of HR areas is a consequence of the high surface activity (extrusions) in an austenitic neighbour grain, the relative crystallographic “compatibility” between α/γ grains and the phase distribution. In addition, the close interaction between the two phases was still evidenced, this time as a transfer of the plastic activity from austenite to ferrite. It turns out that, taking into account the whole results set obtained in this work, the cyclic plasticity of the duplex stainless steels could be explained like a progressive contribution of the activity in the two phases and their interactions. Therefore, the utilisation of different regimes depending of the deformation levels, supported by the individually activity or passivity of each phase, appears to be a simplified explanation that does not illustrate the real cyclic behaviour of this material.

Thesis (M.S.)--University of Michigan, 1973.

Thesis (M.S.)--University of Michigan, 1972.

Thesis (M.S.)--University of Michigan, 2003. / Includes bibliographical references.

Thesis (M.S.)--University of Michigan, 1972.

Thesis (M.S.)--University of Michigan, 1973.

Dans une première partie de cette thèse, l’indentation du verre par des sphères rigides a été analysée numériquement. Un modèle d’endommagement continu anisotrope a été implanté dans un code de calculs par éléments finis pour étudier l’endommagement du verre. Trois zones ont été mises en évidence : la première est liée à la fissure conique, la deuxième à la fissure médiane et la troisième à la déformation permanente. Les directions des fissures prédites via le critère de densité d’énergie minimale ont été trouvées en très bon accord avec ceux déterminées expérimentalement dans la littérature. Le modèle utilisé dans l’analyse statique a été étendu aux cas dynamiques dans une deuxième partie de cette thèse. Une attention particulière a été portée sur la fissure conique. Un modèle d’endommagement simplifié (seulement gouverné par la contrainte principale maximale) couplé à la technique de désactivation des éléments a été utilisé pour suivre la propagation de la fissureconique sans présumer du site d’initiation. Dans une dernière partie de cette thèse, le phénomène d’érosion du verre a été étudié expérimentalement (par sablage) et numériquement. Le modèle implanté a été utilisé pour expliquer les observations expérimentales, en particulier la dépendance de l’enlèvement de matière vis-à-vis de la taille des projectiles, la distance inter-projectiles, la vitesse d’impact, l’angle d’impact et le nombre d’impacts. En modélisant plusieurs tailles et vitesse de projectiles en accord avec les paramètres expérimentaux connus, la simulation numérique d’un seul impact a prédit une quantité de matière enlevée en très bon accord avec celui mesuré expérimentalement par profilométrie. / In a first part of this thesis, the indentation of glass bulk was numerically analyzed using small-sized rigid spheres loaded normally. An anisotropic continuum damage mechanics (CDM) model was implemented into a finite element program to study the damage pattern in glass. The CDM-based model pointed out three explicit sites for damage initiation: the first for cone crack, the second for median crack and the third for permanent deformation. The directions of crack propagation predicted via the criterion of minimum strain energy density were found in very good agreement with those experimentally obtained in the literature. The CDM framework used in the static modeling was extended to the dynamic cases in a second part of this thesis. A particular attention was paid to the cone crack pattern. A simplified CDM-based model (only governed by the maximum principal stress) coupled with the vanishing element technique was employed to follow the cone crack propagation without presuming the initiation site. In the last part of this thesis, the phenomenon of glass erosion was studied from experimental (sandblasting) and numerical approaches. The implemented CDMbased model was used to explain the experimental observations, especially the dependence of material removal on projectile size, inter-projectile spacing, velocity, angle and number of impacts. By modeling various projectile sizes and velocities according to those used in the experiments, the numerical simulation of a single impact predicted an amount of material removal in very good agreement with that measured experimentally using a profilometer.

Magister Scientiae Dentium - MSc(Dent) / BACKGROUND: The application of dental sealants is a recommended procedure to prevent and control dental caries. However, despite strong evidence for the safety and effectiveness of dental sealants, their use still remains low, especially among children from lower socioeconomic communities. The World Health Organization (WHO), Centres for Disease Control and Prevention (CDC) and the Association of State and Territorial Dental Directors (ASTDD) strongly endorse the implementation of school based dental sealant programmes as a community-based preventive strategy to increase sealant use and reduce dental caries. However, in the WHO African Region, oral health is seen as a very low priority and this is compounded by limited technical and managerial resources. The availability of human resources and equipment are crucial for the successful placement of dental sealants. A gap in the research literature was identified for determining the effectiveness of fissure sealants placed under field conditions. AIM: To evaluate the caries preventive effect as well as retention status of a resin-based fissure sealant that was placed under field conditions as part of a school based sealant programme. METHODOLOGY: A cross-sectional comparative study was conducted at two primary schools in close proximity of each other in the same low socio-economic area in Beaufort West, South Africa. The study population consisted of grade two children between the ages of 7-9 years who had fully erupted first permanent molar teeth. The case group consisted of 100 learners who received dental sealants on caries-free first permanent molar teeth 12 months earlier. The control group consisted of a random selection of the same number of learners from the adjacent school. Dental caries on the occlusal surfaces of the first permanent molar teeth was detected by making use of the decayed (D) portion of the decayed, missing and filled tooth (DMFT) score, while a separate diagnosis distinguished between cavitated and non-cavitated lesions. Sealant retention was determined by a calibrated examiner who was not involved in the placement of the sealants. RESULTS: The response rate of the study was 80.0% (n=100) and 78.9% (n=356) of the fissure sealants that were originally placed were evaluated. When the sealants were placed in 2013, 52.0% of the children were female and at the 12 month follow-up, 51.3% were female. The average age of the female children at follow-up was 8 years and 4 months (99.9 months) and 8 years and 5 months (101.8 months) for the males. The standard deviation of the gender profiles differed by 1 month only and implies an equal distribution of age between female and male children throughout the study. Just less than ten per cent (7.8%) of the sealants were fully intact at the 12 month follow-up examination and 91% were totally lost, which is a higher sealant loss rate than what is generally reported on in the literature. Of the 7.8% fully retained sealants, a statistically significant proportion (p=0.044) were found on the mandibular molar teeth. The caries incidence rate in the sealed group was 7.1% versus 9.1% in the control group. Relative risk (RR) calculations was slightly lower for the sealed (RR=0.79) than the unsealed (RR=1.02) teeth. CONCLUSION: The study showed a 2% lower caries prevalence rate on the occlusal surfaces of the sealed versus the unsealed teeth. However, this does not represent a statistically significant finding (P=0.39). The study also showed a low retention rate for the resin-based sealants placed under field conditions (12 month retention rate of 7.8%). The results from this study has therefore shown that resin-based fissure sealants placed on grade 1 learners under field conditions appear to be not ideal in preventing the onset of dental caries on the occlusal surfaces of the first permanent molar teeth.

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Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal