Etude et modelisation du comportement rheologique de suspensions de fibres rigides en regime non-dilu

Several studies have investigated the rheological behavior of fiber suspensions in simple shear (transient and steady) and oscillatory shear (small deformations) flow. However in many transformation processes, the elongational component of the flow cannot be neglected, while in a process such as push-pull injection, knowledge of the rheological properties in large amplitude oscillatory shear (LAOS) may be of great interest. Thus, a thorough study of the rheological behavior of fiber-filled fluids subjected to elongational and LAOS flows is presented in this work. The second objective is to evaluate the performances of rheological models available in the literature and highlight their drawbacks in order to propose a more adequate approach.;The last point tackled in this work concerns the mathematical development of a rheological model for fiber suspensions with a more precise modeling of fiber-fiber interactions. This constitutive equation consists in an extension of the Dinh and Armstrong (1984) model, in addition to the hydrodynamic contribution of the fibers, the total stress results from an additional component due to fiber-fiber interactions. Accordingly, novel second and fourth order interaction tensors are defined. Moreover, a new time evolution equation is proposed, where the diffusion term is proportional to the average number of contacts between fibers. The model is validated using experimental data in transient and steady shear flows for concentrated fiber suspensions in a polybutene. Measured reduced viscosity and primary normal stress differences exhibit quadratic functions, as predicted by the model. The transient behaviour is also fairly well estimated, considering that the model only requires three parameters to fit the behavior of suspensions over a wide range of fiber concentrations. (Abstract shortened by UMI.).;Results from large amplitude oscillatory shear flow showed that for short glass fiber reinforced polypropylene (PP) the shear stress amplitude decreased gradually over the cycles investigated. However, an opposite behavior was observed for a reinforced polybutene matrix (PB) containing the same fibers: the stress amplitude increased continuously until reaching a constant value after several cycles. The shear stress increase (PB) and decrease (PP) are especially pronounced as the fiber loading increases. Fiber orientation is at the origin of these two phenomena. It was observed that the primary normal stress differences of the two matrices are more sensitive to fiber orientation than the shear stresses: the normal stress-strain loops showed a symmetrical shape if the fiber orientation was also symmetric with respect to the flow direction. Fast Fourier Transforms were also performed for the PB- and PP-based systems: harmonic responses were observed for shear stress, whereas non-harmonic signals were obtained for the primary normal stress differences. In addition the viscous dissipation energy increased with increasing fiber content, and composites based on polypropylene showed an increased Newtonian character with increasing deformation.