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International audienceThe rheological behavior of two series of model suspensions containing the same glass fibers in a Newtonian polybutene and in a Boger fluid has also been investigated. The steady-state shear viscosity of both supensions increased with fiber content, but the suspensions in the Boger fluid became shear thinning. Both types of suspension exhibited non-negligible normal stresses. The steady-state viscosity and normal stress difference of the supensions in the polybutene are well predicted by the Lipscomb ~1987! equation coupled with the Folgar–Tucker ~1984! model. Both types of fiber suspensions were shown to exhibit shear and normal stress overshoots in stress growth experiments. Under flow reversal, a shear stress overshoot was observed at a larger deformation compared to the primary overshoot. The reverse overshoot has been attributed to tumbling of fibers that are not totally aligned in the flow direction even after a very long time. When the flow was reversed, the normal stress difference took initially mimimum values ~negative values in the polybutene case! and then depicted a smaller positive overshoot before reaching a steady-state value. The normal stress undershoot has been attributed to a transient fiber-oriented structure. The shape and the magnitude of these overshoots depend on the fiber content, nature of the matrix, and time delay between consecutive experiments

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International audienceA set of rheological equations is developed for semiconcentrated suspensions of rigid fibers in a Newtonian fluid taking into account hydrodynamic and fiber-fiber interactions. The force generated by the fiber interactions is modeled using a linear hydrodynamic friction coefficient proportional to the relative velocity at the contact point, and weighted by the probability for contacts to occur. The equation of evolution of the second-order orientation tensor, containing advection and diffusion terms due to fiber interactions, is derived to predict fiber orientation under flow. The well known fourth-order orientation tensor, related to the hydrodynamic contribution, and a newly proposed fourth-order interaction tensor are used to evaluate the total stress in the composite. A linear and a quadratic closure approximation are proposed to describe the fourth-order interaction tensor. Results are presented using the quadratic form, which is found to be more accurate than the linear one. The model is shown to describe well simple shear data of suspensions of glass fibers in a Newtonian polybutene. Moreover, fiber orientation and the average number of contacts per fiber are predicted. The newly proposed interaction coefficient varies with fiber orientation, which appears to be realistic