Short glass fiber composites represent an interesting class of materials where the properties can be highly anisotropic. This is a direct consequence of the anisotropic fiber orientation generated during processing and makes simulation of the fiber orientation and stresses imperative to part stability and functionality. Since most parts manufactured from short glass fiber composites are processed in the melt state the rheological behavior and associated fiber microstructure of the composite fluid is of technical relevance to constitutive relation model development and flow simulation.

The purpose of this work is to give a quantitative experimental description of the transient shear rheological behavior as it relates to the microstructure of a concentrated short glass fiber commercially available composite fluid. A series of stress growth measurements in start-up of flow and flow reversal tests are performed at various shear rates. Rheological measurements were performed with a Rheometrics Mechanical Spectrometer (RMS-800) using a novel sample geometry which minimizes excessive fiber boundary interaction while maintaining a homogeneous shear field. The fiber orientation is characterized using confocal laser microscopy at points of interest on a stress vs. strain plot. Experimental results are compared to predictions based on the generalized Jeffery equation with the addition of a non-affine motion function that slows the rate of fiber reorientation. Model predictions show good agreement with experimental results.