This work is concerned with determining empirical parameters in stress and fiber orientation models required to accurately simulate the fiber orientation in injection molded composites. An independent approach aims to obtain the material parameters using a sliding plate rheometer to measure the rheology of fiber suspensions at increased fiber lengths subjected to transient shear flow. Fiber orientation was measured in conjunction with shear stress to determine the relationship between stress and fiber orientation. Using a compression molding sample preparation procedure, the transient shear stress response was measured for glass and carbon fiber suspensions up to a number average fiber aspect ratio (length/diameter) of 100. Increases in concentration or fiber aspect ratio caused the magnitude of the stress response to increase by as much as an order of magnitude when compared to the suspending matrix. The degree of shear thinning at low shear rates also increased with increases in aspect ratio and concentration. The compression molding sample preparation procedure provided poor control of the initial fiber orientation which led to the investigation of samples subjected to flow reversal and samples generated through injection molding. The samples prepared through injection molding provided improved repeatability in the measured shear stress response and fiber orientation evolution during the startup of flow compared to compression molded samples and samples subjected to flow reversal. From repeatable stress and orientation evolution data, models for stress and fiber orientation were assessed independently. Current theories for stress were unable to reflect the overshoot in the measured stress response and could at best capture the steady state. The transient behavior of the fiber orientation models were found to be highly dependent on the initial fiber orientation. The repeatable orientation data obtained from the injection molding sample preparation procedure provided material parameters in the strain reduction factor and reduced strain closure models. The injection molded samples provided evolution data from different initial fiber orientations to provide further scrutiny or validation of the material parameters. Orientation model parameters that provided reasonable agreement to multiple sets of fiber evolution data in simple shear flow should allow for a better assessment of the orientation models in complex flow simulations. / Ph. D.

Dans ce travail, nous étudions le comportement rhéologique de suspensions concentrées de fibres non-browniennes. Dans un premier temps, nous avons élaboré de nouvelles méthodes expérimentales en géométrie torsionnelle plan-plan, pour mesurer la viscosité, les deux différences de contraintes normales, ainsi que les contraintes normales d'origine particulaire. Nous avons été en mesure d'apporter des résultats originaux qui ont été interprétés en termes d’évolution de la microstructure des suspensions de fibres. Les mesures de la viscosité en régimes stationnaire et transitoire ont permis de mettre en évidence l'importance de l'orientation et de l'effet du confinement sur la viscosité. Par ailleurs, nous avons obtenu pour la première fois des mesures indépendantes des deux différences de contraintes normales en géométrie torsionnelle plan-plan. Les mesures de la pression dans le fluide interstitiel nous ont donné accès aux contraintes normales particulaires et apporté une première mise en évidence expérimentale du phénomène de migration des fibres dans les suspensions non-browniennes. Dans un deuxième temps, nous avons établi deux modèles théoriques qui tentent d'expliquer la rhéofluidification des suspensions de fibres concentrées par une compétition entre des forces adhésives entre fibres et les forces hydrodynamiques et qui proposent un scénario de formation et de destruction d'agrégats. Les résultats de ces modèles sont alors confrontés aux mesures expérimentales et rendent bien compte du comportement de la viscosité sur une large gamme de taux de cisaillement. / In this manuscript we investigate, both experimentally and theoretically, the rheological behavior of concentrated non-Brownian fiber suspensions. The experiments consist in developing new methods for measuring the viscosity, the two normal stress differences as well as the particle normal stresses, in torsional plate-plate geometry. We were able to bring original results that have been interpreted by the evolution of the microstructure of the fiber suspensions during the flow. The experiments in stationary and transient regime highlight the importance of fiber orientation and the effect of the confinement on the viscosity measurement. For the first time, the first and the second normal stress differences have been measured separately in a torsional flow. Furthermore, thanks to the measurement of the pore pressure in the suspensions, an estimation of the particle normal stresses has been carried out and, for the first time the phenomenon of fiber migration in non-Brownian suspensions has been evidenced. From a theoretical point of view, we developed two complementary models that tend to explain shear-thinning behavior in concentrated fiber suspensions by a balance between adhesive and hydrodynamic forces and propose a scenario for the formation and the destruction of aggregates. These models are then tested against experimental measurements in a wide range of shear rates.