Short glass fiber-reinforced thermoplastics are a class of engineering materials of growing commercial importance. These composites posses improved stiffness, strength, and heat distorsion temperature over unfilled polymers. As they are normally processed in the molten state, their rheology is of technical relevance. The ability to design feasable manufacturing processes depends in part on the ability to account for the interrelation between local fiber orientation and macroscopically observable rheological properties. I-n contrast to geometrically isotropic fillers the maximum reinforcement is only obtained when the fibers are properly oriented. Therefore, it is important to get insight into the change of fiber orientation during flow and the influence of fiber orientation on the flow properties.The rheological behaviour of fiber suspensions in a Newtonian fluid is difficult to characterize. A short review on the literature is given by Dinh and Armstrong [1, 2] who developed a rheological equation of state for suspensions. Still more complex are suspen-*) Dedicated to Dr. D. Heinze on the occasion of his 60th birthday.Abstract: The rheology of polyolefines, polyamide 6, and polystyrene-acrylonitrile filled with glass fibers of different concentrations and aspect ratios have been investigated in simple shear flow, capillary rheometry, and uniaxial elongation. A comparison is made with unfilled and glass bead-filled melts. Fiber orientation was investigated by X-ray microradiography. Steady-state viscosities are obtained on fibers aligned parallel to the direction of flow. Entrance pressure losses, the shape of the viscosity function, and the appearance of a yield stress are depending on the fiber aspect ratio. The temperature dependence of the viscosity function is not significantly different from that of the unfilled melt. Transient shear stresses were measured on samples of different initial orientations of the fibers. The change of fiber orientation during shear flow gives rise to a pronounced overshoot of shear stress and normal stress difference. Elastic strains in shear are increased by the fibers but elastic elongational strains are~reduced. Mechanisms are proposed to explain the experimental observations.
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