Colin Servais

Fiber–fiber interaction in concentrated suspensions: Disperse fibers

A hypothesis for fiber–fiber interaction in planar randomly oriented concentrated fiber suspensions is proposed and tested. The idea is that at sufficiently high fiber concentrations, friction and lubrication at fiber–fiber contact points are the dominant interaction mechanisms. A fiber pull-out technique is introduced to measure the force per unit fiber length on a single longitudinally moving fiber embedded in a volume of bulk suspension. By varying both the fiber velocity and the fiber volume fraction, the lubrication and frictional components of the force are identified. Furthermore, the corresponding bulk shear viscosity resulting from the same mechanisms is derived and compared with experimental data. The results support the hypothesis.

Fiber–fiber interaction in concentrated suspensions: Dispersed fiber bundles

A model is proposed to describe the rheology of planar randomly oriented concentrated fiber bundle suspensions in a shear-thinning matrix. The approach is that, at high fiber volume fractions, the dominant interaction mechanisms are friction and lubrication at the fiber–fiber contact points. A fiber pull-out technique is used to measure the force per unit length exerted on a single fiber tow of elliptical cross section embedded in a bulk suspension. By varying the tow velocity, fiber volume fraction, resin viscosity, and suspension structure, the factors affecting the lubrication and frictional components of the interaction forces were analyzed. The lubrication force is related to the flow behavior of the neat resin. The theoretical equations derived in this work allow for the computation of a shear viscosity of the suspension, which is in good agreement with experimental evidence. It is shown that dispersed fiber and dispersed fiber bundle suspensions are yield stress fluids.

The relationship between steady-state and oscillatory shear viscosity in planar randomly oriented concentrated fiber suspensions

A micromechanical model for the flow of in-plane randomly oriented concentrated fiber suspensions in molten polypropylene has been combined with the Rutgers–Delaware model for Herschel–Bulkley materials. At high fiber volume fractions, Coulombic friction forces and hydrodynamic lubrication forces generated at the contact points between fibers are the dominant fiber–fiber interaction mechanisms. This feature has been shown here in both steady-state and oscillatory shear. The complex viscosity and the steady-state viscosity of the suspensions were measured as a function of an effective strain rate, which was defined as the strain rate in steady-state shear and the product of the strain and the frequency in oscillatory shear. Four distinct strain regions were identified: viscoelastic behavior below an apparent yield stress, pseudosolid plasticity at low effective shear rates above the yield stress, a viscous Newtonian plateau at medium effective shear rates, and a shear thinning region at high rates. The two...