A model incorporating the effects of flow type on fiber orientation for flows with mixed flow kinematics

Abstract

Reliable predictions of the fiber orientation profile generated during processing are important for the improvement of mold design and processing conditions to optimize mechanical properties of the as-formed composites. To better reflect slower orientation evolution observed experimentally in shear flow dominated regions, various models were developed with a scalar strain-reduction factor (SRF). Normally, this scalar factor is obtained by fitting to orientation data in simple shear flow. However, during polymer processes such as injection molding, the flow types, including extensional, simple shear, rotational flow, and mixed flows, have a significant impact on fiber orientation kinematics. Nonlubricated squeeze flow (NLSF) is replicated through a rectangular channel, in order to simulate industrial polymer processing. This is accomplished because both shear and extensional flows are present. However, unlike industrial processes such as injection molding, the flow is well controlled, resulting in less fiber breakage and concentration variation. Therefore, an NLSF is ideal for investigating the effects of flow types on fiber orientation. In this study, we developed a model incorporating the effects of flow types on fiber orientation by using a variable SRF. Unlike the existing constant factor, this variable parameter can reflect the different degrees of less strain experienced by concentrated fibers in shear and extension (no strain reduction in extensional flow), respectively. The variable SRF is expressed as a function of a flow-type parameter, locally describing the relative magnitudes of shear and extension, which allows the fiber orientation speed to be dependent on local kinematics. The predicted orientation shows improved agreement with experimental data for the NLSF, because the use of a variable SRF can reflect the different rates at which fibers orient during different flow types.