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 concentr ated fiber suspensions in molten polypropylene has been combined with the R utgers-Delaware model for Herschel-Bulkley materials. At high fiber volume fractions, Coulombic friction forces and hydrodynamic lubrication forces ge nerated at the contact points between fibers are the dominant fiber-fiber i nteraction mechanisms. This feature has been shown here in both steady-stat e and oscillatory shear. The complex viscosity and the steady-state viscosi ty of the suspensions were measured as a function of an effective strain ra te, which was defined as the strain rate in steady-state shear and the prod uct of the strain and the frequency in oscillatory shear. Four distinct str ain regions were identified: viscoelastic behavior below an apparent yield stress, pseudosolid plasticity at low effective sheer rates above the yield stress, a viscous Newtonian plateau at medium effective shear rates, and a shear thinning region at high rates. The two latter regions were related t o the viscosity-shear rate curves of the neat resins. The testing method, n ew for such fiber suspensions, allowed simple and rapid access to the shear parameters of the materials. Effects of mat structure, resin viscosity, an d differences between different materials may be obtained. (C) 1999 The Soc iety of Rheology. [S0148-6055(99)00404-6].