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yes / In order to study the mechanical behaviors of engineered cementitious composites (ECC) reinforced with various types of polyvinyl alcohol (PVA) fibers and different fiber volume fractions after the freeze-thaw cycles, the rapid freeze-thaw method was used to test the mass loss ratios, longitudinal relative dynamic elastic modulus, compressive strength and flexural strength. The results showed that specimens incurred more serious damage with the increasing of freeze-thaw cycles; however their performance would be improved by fiber type and dosage. Mass loss rate of JPA (specimen with 2% volume content of JP fiber) was lower than JPB (specimen with 1% volume content of JP fiber). Furthermore, the mass loss rate of JPB was lower than that of CPB (specimen with 1% volume content of CP fiber). The longitudinal relative dynamic elastic modulus of JPA was higher than that of JPB while the longitudinal relative dynamic elastic modulus of JPB was higher than that of CPB. In addition, the compressive strength and flexural strength decreased with the increasing of freeze-thaw cycles. Mechanical behaviors of specimens with fiber exhibited better strength than specimens without fiber. Based on the SL 211-2006 code for the design of hydraulic structures against ice and freezing action, JPA and JPB specimens are adequate for use in severe cold regions, while specimen CPA and CPB can be used in cold regions, specimen JPC only can be used in warm regions.

Les Alliages à Mémoire de Forme Magnétiques (AMFM) sont des matériaux actifs qui présentent des comportements inhabituels par rapport aux matériaux " classiques ". Ils peuvent par exemple présenter de larges déformations réversibles sous l'action d'un champ magnétique ou sous une action mécanique. Ce sont des candidats potentiels pour des applications dans des domaines de pointe (automobile, aéronautique, spatial, etc.). Les AMFM présentent par ailleurs un avantage indéniable par rapport aux matériaux à mémoire de forme " thermique " en raison de leur réponse dynamique à haute fréquence. Il est bien connu que ces comportements sont dus à un couplage magnéto-mécanique et à un phénomène physique lié à l'orientation des variantes de martensite. L'objectif de cette thèse est d'analyser les comportements magnéto-mécaniques des AMFM. Pour ce faire, nous étudions expérimentalement et théoriquement, la réorientation martensitique dans les AMFM. Tout d'abord, une analyse énergétique en 2D/3D est proposée et intégrée dans des diagrammes d'état pour une étude systématique de la réorientation martensitique dans les AMFM sous chargements tridimensionnels quelconques. Ainsi, des critères de large déformation réversible sous des chargements cycliques sont obtenus. L'analyse énergétique montre que les AMFM, sollicités sous chargement multiaxiaux présentent plus d'avantages que ceux sollicités en 1D ; en particulier, on montre que l'état multiaxial permet d'augmenter (d'améliorer) la contrainte fonctionnelle, ce qui augmente le champ d'application des ces matériaux. Ensuite, afin de valider les prédictions de l'analyse énergétique, des expériences bi-axiales ont été effectuées sur des éprouvettes en AMFM. Les résultats révèlent que la dissipation intrinsèque et la déformation de transformation dues à la réorientation martensitique sont constantes dans tous les états de contraintes. De plus, les résultats ont permis de valider nos prédictions théoriques quant à l'augmentation de la contrainte fonctionnelle. Enfin, afin de prédire les comportements magnéto-mécaniques des AMFM sous des chargements multiaxiaux, un modèle tridimensionnel est développé dans le cadre de la thermodynamique des processus irréversibles avec liaison interne. Toutes les variantes de martensite ont été considérées et l'effet de température a également été pris en compte. Les simulations numériques montrent un très bon accord (rejoignent/confirment les résultats) avec les résultats expérimentaux existant dans la littérature. Le modèle a ensuite été programmé dans un code de calcul par éléments finis afin d'étudier les comportements non linéaires de flexion des poutres en AMFM. L'effet géométrique et l'effet d'anisotropie du matériau ont été systématiquement pris en compte.

<div>The mechanical behaviors of different kinds of biological tissues, including muscle tissues, cortical bones, cancellous bones and skulls, were studied under various loading conditions to investigate their strain-rate sensitivities and loading-direction dependencies. Specifically, the compressive mechanical behaviors of porcine muscle were studied at quasi-static (<1/s) and intermediate (1/s─10^2/s) strain rates. Both the compressive and tensile mechanical behaviors of human muscle were investigated at quasi-static and intermediate strain rates. The effect of strain-rate and loading-direction on the compressive mechanical behaviors of human frontal skulls, with its entire sandwich structure intact, were also studied at quasi-static, intermediate and high (10^2/s─10^3/s) strain rates. The fracture behaviors of porcine cortical bone and cancellous bone were investigated at both quasi-static (0.01mm/s) and dynamic (~6.1 m/s) loading rates, with the entire failure process visualized, in real-time, using the phase contrast imaging technique. Research effort was also focused on studying the dynamic fracture behaviors, in terms of fracture initiation toughness and crack-growth resistance curve (R-curve), of porcine cortical bone in three loading directions: in-plane transverse, out-of-plane transverse and in-plane longitudinal. A hydraulic material testing system (MTS) was used to load all the biological tissues at quasi-static and intermediate loading rates. Experiments at high loading rates were performed on regular or modified Kolsky bars. Tomography of bone specimens was also performed to help understand their microstructures and obtain the basic material properties before mechanical characterizations. Experimental results found that both porcine muscle and human muscle exhibited non-linear and strain-rate dependent mechanical behaviors in the range from quasi-static (10^(-2)/s─1/s) to intermediate (1/s─10^2/s) loading rates. The porcine muscle showed no significant difference in the stress-strain curve between the along-fiber and transverse-to-fiber orientation, while it was found the human muscle was stiffer and stronger along fiber direction in tension than transverse-to fiber direction in compression. The human frontal skulls exhibited a highly loading-direction dependent mechanical behavior: higher ultimate strength, with an increasing ratio of 2, and higher elastic modulus, with an increasing ratio of 3, were found in tangential loading direction when compared with those in the radial direction. A transition from quasi-ductile to brittle compressive mechanical behaviors of human frontal skulls was also observed as loading rate increased from quasi-static to dynamic, as the elastic modulus was increased by factors of 4 and 2.5 in the radial and tangential loading directions, respectively. Experimental results also suggested that the strength in the radial direction was mainly depended on the diploë porosity while the diploë layer ratio played the predominant role in the tangential direction. For the fracture behaviors of bones, straight-through crack paths were observed in both the in-plane longitudinal cortical bone specimens and cancellous bone specimens, while the cracks were highly tortuous in the in-plane transverse cortical bone specimens. Although the extent of toughening mechanisms at dynamic loading rate was comparatively diminished, crack deflections and twists at osteon cement lines were still observed in the transversely oriented cortical bone specimens at not only quasi-static loading rate but also dynamic loading rate. The locations of fracture initiations were found statistical independent on the bone type, while the propagation direction of incipient crack was significantly dependent on the loading direction in cortical bone and largely varied among different types of bones (cortical bone and cancellous bone). In addition, the crack propagation velocities were dependent on crack extension over the entire crack path for all the three loading directions while the initial velocity for in-plane direction was lower than the other two directions. Both the cortical bone and cancellous bone exhibited higher fracture initiation toughness and steeper R-curves at the quasi-static loading rate than the dynamic loading rate. For cortical bone at a dynamic loading rate (5.4 m/s), the R-curves were steepest, and the crack surfaces were most tortuous in the in-plane transverse direction while highly smooth crack paths and slowly growing R-curves were found in the in-plane longitudinal direction, suggesting an overall transition from brittle to ductile-like fracture behaviors as the osteon orientation varies from in-plane longitudinal to out-of-plane transverse, and to in-plane transverse eventually.</div>