Evaluating Rigid and Semiflexible Fiber Orientation Evolution Models in Simple Flows

Abstract

As American vehicle fuel efficiency requirements have become more stringent due to the CAFE standards, the auto industry has turned to fiber reinforced polymer composites as replacements for metal parts to reduce weight while simultaneously maintaining established safety standards. Furthermore, these composites may be easily processed using established techniques such as injection molding and compression molding. The mechanical properties of these composites are dependent on, among other variables, the orientation of the fibers within the part. Several models have been proposed to correlate fiber orientation with the kinematics of the polymer matrix during processing, each using various strategies to account for fiber interactions and fiber flexing. However, these all require the use of empirical fitting parameters. Previous work has obtained these parameters by fitting to orientation data at a specific location in an injection-molded part. This ties the parameters to the specific mold design used. Obtaining empirical parameters is not a trivial undertaking and adds significant time to the entire mold design process. Considering that new parameters must be obtained any time some aspect of the part or mold is changed, an alternative technique that obtains model parameters independent of the mold design could be advantageous. This paper continues work looking to obtain empirical parameters from rheological tests. During processing, the fiber–polymer suspension is subjected to a complex flow with both shear and extensional behavior. Rather than use a complex flow, this study seeks to isolate and compare the effects of shear and extension on two orientation models. To this end, simple shear and planar extension are employed, and the evolution of orientation from a planar random initial condition is tracked as a function of strain. Simple shear was imparted using a sliding plate rheometer designed and fabricated in-house. A novel rheometer tool was developed and fabricated in-house to impart planar extension using a lubricated squeeze flow technique, where a low-viscosity Newtonian lubricant is applied to the solid boundaries to minimize the effect of shearing due to the no-slip boundary condition. The Folgar–Tucker model with a strain reduction factor is used as a rigid fiber model and compared against a bead–rod model (a semiflexible model) proposed by Ortman. Both models are capable of predicting the data, with the bead–rod model performing slightly better. Orientation occurs at a much faster rate under startup of planar extension and also attains a much higher degree of flow alignment when compared with startup of steady shear.

A Flow-Dependent Fiber Orientation Model

“…There have been no experimental works so far conducted for elongational flow using short fiber reinforced thermoplastics. However, there were two experiments conducted by Lambert et al [17] and Lambert and Baird [18] using long fibers, under the influence of elongational flow. In the following sections, we use the mechanistic model for the simulation of elongational flow with long fibers to compare the results with the experimental work.…”

“…Lambert and Baird [18] used a 10% wt. glass fiber reinforced PP for the constant planar extension experiment.…”

“…All stated approaches determine fiber orientation parameters in shear flow. Lambert et al [17,18] determined fiber orientation parameters for the first time in elongational flow.…”

“…Current fiber orientation models, on both scales, are mostly focused on the effects of shear flow. However, experiments in elongation flow [17,18] showed that fiber orientation develops differently under elongational flows. On the other hand, during injection molding, complex flow fields combining shear and elongational flow usually take place.…”

The Role of Reinforcing Particle Size in Tailoring Interfacial Microstructure and Wear Performance of Selective Laser Melting WC/Inconel 718 Composites

“…The preparation of particle-reinforced metal matrix composites (PRMMCs) shows the great potential to address this concern through adding certain harder and stiffer ceramic reinforcements into the metal matrix. The PRMMCs have illustrated their superiority compared with the conventional metals for significantly increased hardness, wear resistance, and hightemperature mechanical performance [8][9][10][11]. The ceramics widely used in PRMMCs include TiC [12], WC [13], CrC [14], etc.…”