Fiber migration in shear flow: Model predictions and experimental validation

Understanding transient flow behavior of polymer composites is crucial for improving the predictions of final composite properties in polymer processing operations. This work is aimed at measuring concentration migration of fiber reinforcing particles in addition to fiber orientation evolution during simple shear flow. Center-gate injection molded samples using Nylon-6 containing 33 wt% short glass fibers (PA6-33GF) were prepared. Samples were imaged using X-ray computed tomography (X-CT) to obtain fiber orientation tensors and fiber volume fraction distributions after different strain units were imposed. With higher applied strains, the orientation of fibers evolved from a shell-core structure typical of injection molded composites to a preferential alignment in the shearing direction. As for fiber concentration, an initial shell-core concentration profile established during injection molding was found to diminish towards a uniform concentration distribution upon shearing. This experimentally observed concentration migration was modeled using the suspension balance model (SBM) which predicts concentration changes based on changes in particle stress contributions. The SBM uses a particle stress contribution model that is designed for spherical particles. Predictions from this model gave reasonably accurate steady state results and the evolution of the concentration profile. A modified fiber stress model that incorporates fiber orientation effects was also tested for its applicability for fiber-filled systems. Predictions from this model yielded reasonable agreement with measured concentration evolution data but with a slower attainment of a uniform concentration profile. Further investigation on the empirical constants used in the fiber stress model warrants a rheological assessment to improve concentration evolution predictions for use in constitutive rheological modeling.