Rheological properties of short fiber model suspensions
Résumé
The 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.