Fiber Orientation In Flow Research Articles

Effects of geometry constraints and fiber orientation in field assisted extrusion-based processing

Composite manufacturing processes adapted for assisted-additive manufacturing (AM) have recently been proposed. Extrusion-based AM utilizes shear-driven alignment in producing printed structures where polymers and fibers naturally align parallel to the material flow. Convergent flow geometries become the dominant processing route for thermoplastic-melts and thermoset polymer extrusions. For rotational fibers, the phenomenon known as Jeffrey orbits poses issues during extrusion through a convergent channel, resulting in a randomized fiber architecture. Methods of minimizing Jeffrey orbits include the application of an additional external force such as a magnetic field to arrest or counteract the rotation. This work explores a combination of magnetic forces in conjunction with adjusted channel geometries using theory and experimental observations. The findings suggest the ability to alter fiber orientation in flow in a 300 cP viscosity matrix by modifying the extrusion channel geometry.

Modified hybrid closure approximation for prediction of flow-induced fiber orientation

Distribution of fiber orientation in flow molding processes with short-fiber reinforcements is of great importance because it affects the mechanical properties of molded parts. Due to trade-off between the computational efficiency and accuracy, a second-order orientation tensor has been widely used to describe the fiber orientation distribution. For calculation of this fiber orientation tensor, a closure approximation has been introduced to reduce a higher fourth-order orientation tensor to a lower second order. In the present investigation, a hybrid closure approximation has been modified. Two parametric forms of the distribution function, which accurately describe the random-in-space, random-in-plane, and uniaxial distributions of the fiber orientation were linearly interpolated. The interpolating factor was obtained as a function of the fiber interaction coefficient by fitting the distribution function calculations. Test simulation in homogeneous flow and nonhomogeneous flow fields, respectively, showed that the proposed closure approximation gives good performance for a wide range of CI values without showing nonphysical behavior.

Experimental and theoretical study of short fiber orientation in diverging flows

Two types of experiments have been carried out to study the fiber orientation in flow through a divergent channel. First, a reinforced polyamid mold sprue containing two types of orientation was investigated: near the center, the fibers are mostly oriented perpendicular to the flow lines, whereas on the periphery, they are oriented parallel to them. Second, direct observation of copper fibers moving in a corn syrup was performed in a transparent diverging device: the fibers rapidly become oriented transverse to the flow lines. The solution of Stokes equations for the undisturbed fluid motion gives the shear rate and elongation rate, which are then substituted in Jeffery's orientation equations. The resolution shows two types of behavior: in a large area in the center, the fiber tends to a stable equilibrium position which depends strongly on the flow line on which it moves. On the periphery, the fiber follows a shear-like behavior. The strong influence of the elongational component relative to the shear component is demonstrated and the time necessary for orientation is calculated. The theoretical results are found to be in agreement with the observations.