Light weighting has become an integral part of vehicle design. This strategy makes use of fiber-reinforced thermoplastics, which can be injection molded into complex shapes. Composite design work requires knowledge of the orientation state throughout a part to predict properties like stiffness and strength. Although evolution models for the orientation state have been developed, each requires empirical parameters, and no standard method for obtaining these exists. This work continues efforts to find such a test, particularly for long (average length>1 mm) glass fiber composites. Here, a polypropylene loaded with 30, 40, and, 50 wt% long glass fiber is subjected to startup of shear and nonlubricated squeeze flow (NLSF). The orientation evolution was reported for both flows and an attempt was made at fitting several orientation models to the data. The measured orientation evolution from startup of shear was slower than expected, and this slower evolution appears to result from the combination of the initial orientation state and fiber concentration. On the other hand, the measured orientation profile from NLSF was found to be essentially independent of fiber concentration. However, the NLSF orientation profile could not be replicated with a single set of parameters. Rather, shear-like parameters made reasonable predictions in shear-dominated regions, and extension-like parameters made reasonable predictions in extension-dominated areas, in concordance with results from short glass fiber composites.

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. (C) 2019 The Society of Rheology.

To predict fiber orientation for injection molded parts, it is important to use a slow-kinetics orientation model and pay careful attention to the role of flow kinematics on orientation evolution speed. A model incorporating the impact of flow type on the fiber reorientation rate was tested by comparing with experimental data measured in an injection molded center-gated disk for both short and long glass fiber thermoplastics. Unlike the existing orientation models using a constant factor, a variable strain reduction factor (SRF) is expressed as a function of an objective flow-type parameter reflecting local flow kinematics. The use of this model improved the fiber orientation predictions at locations where the constant SRF, obtained from simple shear flow, deviated significantly from that based on the local flow kinematics.

Accurate models of fiber orientation dynamics during the processing of polymer-fiber composites are needed for the design work behind important automobile parts. All of the existing models utilize empirical parameters, but a standard method for obtaining them independent of processing does not exist. This study considers non-lubricated squeeze flow through a rectangular channel as a solution. A two-dimensional finite element method simulation of the kinematics and fiber orientation evolution along the centerline of a sample is developed as a first step toward a fully three-dimensional simulation. The model is used to fit to orientation data in a short-fiber-reinforced polymer composite after squeezing. Fiber orientation model parameters obtained in this study do not agree well with those obtained for the same material during startup of simple shear. This is attributed to the vastly different rates at which fibers orient during shearing and extensional flows. A stress model is also used to try to fit to experimental closure force data. Although the model can be tuned to the correct magnitude of the closure force, it does not fully recreate the transient behavior, which is attributed to the lack of any consideration for fiber-fiber interactions.

Experimental fiber evolution data from a rheometer can be used to determine material parameters in fiber orientation models independent from processing flows. The Folgar-Tucker orientation model was demonstrated in simple shear flow to predict distinctly different trends in the orientation evolution depending on the initial fiber orientation. Repeatable fiber orientation evolution data in a simple shear flow was used to determine material parameters in the strain reduction factor (SRF) and reduced strain closure (RSC) orientation models. The SRF and RSC orientation models were tested with different initial fiber orientations created from an injection molded plaque in order to evaluate the orientation model parameters. A stress equation incorporating fiber-fiber interactions was evaluated independently of the fiber orientation models using experimental orientation evolution data with different initial orientations. (C) 2016 Elsevier B.V. All rights reserved.

Fiber orientation simulation is conducted for the Center-Gated-Disk (CGD) geometry and compared with experimental data. Long-fiber thermoplastic composites (LFTs) possess competitive advantages over short glass fiber composites in terms of their mechanical properties while retain the ability to be injection molded. Mechanical properties of LFTs are highly dependent on the microstructural variables imparted by the injection molding process including fiber orientation and fiber length distribution. As the fiber length increased, the mechanical properties of the composites containing discontinuous fibers can approach those of continuous fiber materials. Several researchers have reported that flexural, creep and charpy impact properties increase as fiber length increases, while tensile modulus will plateau for glass fibers above 1 mm in length. Fibers less than the 1 mm threshold have been considered to be short while fibers with lengths greater than 1 mm are considered long. For long fibers, they will have the ability to deform, bend and even break during any stage of polymer processing. There is a lack of knowledge about the effects of fiber length and fiber length variation on fiber orientation kinetics. This lack of information provides an opportunity to understand the length effect inherent to long fibers systems. The Bead-Rod fiber orientation model takes into account the flexibility of semi-flexible fibers that show small bending angles. In this model, a flexibility parameter representing the resistive bending potential is fiber length dependent (detailed explanation can be found in the reference)1. This work is concerned with the effect of fiber length on the performance of the Bead-Rod fiber orientation model which takes into account the flexibility of semi-flexible fibers. Different averaging techniques are used to represent the average fiber length for the population of fibers, which give different fiber length parameters for the Bead-Rod model. The sensitivity of the Bead-Rod model is evaluated with regard to the fiber flexibility parameter, k, and length parameter, lb. The other phenomenal parameters within the orientation model are obtained via basic rheological measurements using simple shear flow. As the value of average fiber length Lav increases and the corresponding flexibility parameter value decreases, the core regions become wider and the flow direction orientation gradually decreases especially near the walls for the Bead-Rod model predictions. In addition, as the parameters favor longer fiber lengths, the predicted extent of fiber bending increases. The simulation results are also compared with the experimental obtained fiber orientation at different flow length along the thickness direction. The Bead-Rod model shows improvement over the rigid rod model.

Many researchers reporting the transient rheology of fiber suspensions have not experimentally verified the initial fiber orientation. An assumption for the initial orientation is then required to simulate the fiber orientation evolution and stress response. Measurements of fiber orientation obtained prior to testing in a rheometer can confirm the homogeneity of fiber orientation throughout a sample and repeatability between multiple samples. In this work, the transient rheology of glass fiber suspensions above 0.5 mm in length was measured in a sliding plate rheometer with the initial fiber orientation generated through compression molding, flow reversal, and injection molding. Measurements of shear stress and fiber orientation were obtained to evaluate each sample preparation method and to gain insight into the stress -microstructure relationship. Preshearing and applying flow reversal was used in an effort to control the initial fiber orientation for transient shear stress measurements, but fiber orientation did not change significantly, and the stress response was variable with little stress overshoot. Samples created through injection molding provided a repeatable transient stress response with measurable changes in fiber orientation. Components of the fourth order orientation tensor were also reported to compare with current theories for stress. An overshoot profile in fiber orientation was not observed during the startup of flow, and little change in orientation occurred during the flow reversal tests. Based on experimental values of fiber orientation, a stress theory dependent on the fourth order orientation tensor was not able to reflect an overshoot in shear stress and could, at best, capture the steady state. (C) 2016 The Society of Rheology.

In order to use rheological measurements as a tool to investigate fiber orientation in simple flows, the relationship between stress and fiber orientation must be understood. In this work, a sliding plate rheometer was used to measure the shear stress growth during the startup of simple shear flow of a polymer melt containing long glass fibers. The concentrations of the suspensions were varied from 10 to 40wt% and tested over three shear rates spanning an order of magnitude. Significant shear thinning was observed in the suspension as concentration increased. Additionally, the magnitude of stress and breadth of the stress growth overshoot increased with concentration. A larger distinction between the different concentrations is observed in the shear stress growth than the measured evolution of fiber orientation. Measured values of fiber orientation were used with a semi-dilute stress equation to show that the fiber motion in these experiments was not responsible for the stress overshoot and that additional stress contributions must be considered.

Fiber orientation in the frontal region of a center-gated disk was measured for the first time and compared with orientation predictions using standard Folgar-Tucker and the reduced strain closure (RSC) model in coupled flow simulations. Fiber orientation was experimentally measured along three different heights representative of shell, transition and core layers, in order to understand the evolution of orientation along the radial direction in the frontal region. Orientation predictions of the Folgar-Tucker model and its two modified versions, the delayed Folgar-Tucker model and the RSC model were assessed against the measured experimental data. Orientation predictions with all three models showed a drop in orientation near the front, which was in qualitative agreement with the experimental data. Modified versions of the Folgar-Tucker model showed a relatively larger drop in orientation in the shell layer with predictions being relatively closer to experimental values. However, no significant slowdown was observed with the modified versions in the transition and core layers. With coupling of flow and orientation, the frontal flow region was slightly larger and orientation predictions showed only slight improvement. A significant improvement in the frontal region was obtained when a lower value of the interaction coefficient was used. Published by Elsevier B.V.

The evolution of fibre orientation in transient simulations with the Folgar–Tucker model and its slip and reduced strain closure versions was assessed against experimental data measured in the shell, transition, and core layers. Decoupled and coupled transient simulations were performed using a measured orientation profile at the inlet of the mould. All three orientation models match the experimental data only in the steady region of the shell layer but are unable to predict the evolution of orientation in all three layers. Effects of coupling are very small and unable to account for the discrepancies between the experiments and simulations.

A complete sample preparation procedure used to determine three-dimensional fiber orientation from optical micrographs of glass fiber-reinforced thermoplastic composites is presented. Considerations for elimination of irregularities in the elliptical footprints, contrast enhancement between fibers and surrounding polymer matrix, controlled-etching that allows the identification of small shadows where fiber recedes into the matrix, and topographical reconstruction of the elliptical footprint are described in the procedure. This procedure has produced high-quality optical micrographs employed to obtain accurate fiber orientation data for thermoplastic composites using the method of ellipses. The optimal definition of the nonelliptical footprints borders allows an accurate measurement of orientation in small sampling areas.

A modified version of the method of ellipses was successfully developed, validated, and implemented to characterize short fiber orientation. Unambiguous orientation and a small sampling area were obtained by distinguishing between in-plane angles of ϕ and ϕ+π for every fiber and characterizing non-elliptical footprints, respectively. Measurements in two thin center-gated disks showed the existence of an asymmetric profile of orientation at the gate, which differs from the 3D random orientation commonly assumed in simulations. This profile washed out gradually at the entry region until disappearing at about 40% of flow length. A detailed set of orientation data for comparing fiber orientation to calculated values was obtained.

The properties of long glass fiber reinforced parts are highly dependent on the fiber orientation generated during processing. In this research, the orientation of concentrated long glass fibers generated during the filling stage of a center-gated disk (CGD) mold was simulated. The orientation of the fibers was calculated using both the Folgar-Tucker model and a recently developed semiflexible Bead-Rod model. Rheologically consistent model parameters were used in these simulations, as determined from a previously proposed method, using a sliding plate rheometer and newly modified stress theory. The predicted CGD orientations were compared with experimentally measured values obtained from the parts. Both models performed very well when using model parameters consistent with the independent rheological study, and the results provide encouragement for the proposed method. Comparatively, the Folgar-Tucker model provided slightly better orientation predictions up to 20% of the fill radius, but above 20% the Bead-Rod model predicted better values of the orientation in both the radial and circumferential directions. The Folgar-Tucker model, however, provided better orientation values perpendicular to the flow direction. Lastly, both models only qualitatively represented the orientation above 70% of the fill radius where frontal flow effects were suspected to be non-negligible. The uniqueness of this research rests on a method for obtaining model parameters needed to predict fiber orientation which are independent of the experiments being simulated and a method for handling long semiflexible fiber suspensions. POLYM. COMPOS., 2012. (c) 2012 Society of Plastics Engineers

The properties of long glass fiber reinforced parts, such as those manufactured by means of injection molding and compression molding, are highly dependent on the fiber orientation generated during processing. A sliding plate rheometer was used to understand the transient stress and orientation development of concentrated long glass fibers during the startup of steady shear flow. An orientation model and stress tensor combination, based on semiflexible fibers, was assessed in its ability to predict fiber orientation when using model parameters obtained from the fits of the stress responses. Specifically, samples of different initial fiber orientations was subjected to the startup of steady shear flow, and an orientation model based on bead and rod theory was coupled with a derived stress tensor that accounts for the semiflexibility of the fibers to obtain the corresponding model parameters. The results showed the semiflexible orientation model and stress tensor combination, overall, provided improved rheological results as compared to the Folgar-Tucker model when coupled with the stress tensor of Lipscomb et al. [J. Non-Newtonian Fluid Mech. 26, 297-325 (1988)]. Furthermore, it was found that both stress tensors required empirical modification to accurately fit the measured data. Finally, orientation models provided encouraging results when predicting the transient fiber orientation for all initial fiber orientations explored. (C) 2012 The Society of Rheology. [http://dx.doi.org/10.1122/1.4717496]

In order to eventually predict the behavior of long fiber suspensions in complex flows commonly found in processing operations, it is necessary to understand their rheology and its connection to the evolution of fiber orientation and configuration in well defined flows. In this paper we report the transient behavior at the startup of shear flow of a polymer melt containing long glass fibers with a length (L) > 1 mm, using a sliding plate rheometer (SPR). The operation of the SPR was confirmed by comparing the transient shear viscosity (eta) for a polymer melt and a melt containing short glass fibers (L < 1 mm) with measurements obtained from a cone-and-plate device, using a modified sample geometry that was designed to avoid wall effects. For the long fiber systems, measurements could only be obtained in the SPR because these systems would not stay within the gap of the rotational rheometer. Transient stress growth behavior of the long fiber systems was obtained as a function of shear rate and fiber concentration for samples prepared with three different initial orientations. Results showed that, unlike short fiber systems (with a random planar initial orientation) that usually exhibit a single overshoot peak followed by a steady state, eta* of the long fiber suspensions often passed through multiple transient regions, depending on the fiber concentration and applied shear rate. Additionally, eta*. of the long fiber suspensions was found to be highly dependent on the initial orientation of the sheared samples. Finally, the initial and final fiber orientations of the long glass fiber samples were measured and used to initiate an explanation of the viscosity behavior. The results obtained in this research will be useful for future assessment of a quantitative correlation between transient rheology and the evolution of fiber orientation. (C) 2011 Elsevier B.V. All rights reserved.

Delay in fiber orientation evolution based on the Folgar-Tucker model with a slip parameter correction has been proposed as a simple alternative to improve predictions of fiber orientation in injection molded parts. Predictions based on this model and model parameters fitted to data from simple shear experiments were compared with experimentally determined fiber orientation in a center-gated disk. Three methods of fitting to simple shear data were assessed to obtain the isotropic diffusivity and the slip parameter. The model parameters and orientation data evaluated at the entry, lubrication and near-end-of fill regions in a center-gated disk for 30 wt.% short glass fiber-filled polybutylene terephtalate (PBT) were obtained from earlier efforts in our laboratory. Simulation results based on the Folgar-Tucker model with the slip correction using customarily assumed inlet orientation being random and experimentally measured at the gate disagreed with measured orientation values at certain positions along the disk. However, improvement in the prediction of orientation due the slip correction was found at the core and transition layers in the lubrication region. The use of inlet conditions washes out quickly in the absence of the slip correction and induced a general reduction of orientation towards the center of the sample causing underestimation of orientation at the entry and lubrication region. Model predictions combining the slip correction and experimentally determined orientation at the gate are in agreement with the experimental data for the core layers near the end-of fill region.

The common approach for simulating the evolution of fiber orientation during flow in concentrated suspensions is to use an empirically modified form of Jeffery's equation referred to as the Folgar–Tucker (F-T) model. Direct measurements of fiber orientation were performed in the startup of shear flow for a 30 wt% short glass fiber-filled polybutylene terephthalate (PBT-30); a matrix that behaves similar to a Newtonian fluid. Comparison between predictions based on the F-T model and the experimental fiber orientation show that the model over predicts the rate of fiber reorientation. Rheological measurements of the stress growth functions show that the stress overshoot phenomenon approaches a steady state at a similar strain as the fiber microstructure, at roughly 50 units. However, fiber orientation measurements suggest that a steady state is not reached as the fiber orientation continues to slowly evolve, even up to 200 strain units. The addition of a "slip" parameter to the F-T model improved the model predictions of the fiber orientation and rheological stress growth functions.

The mechanical properties of injection molded short-fiber reinforced thermoplastic composite parts are highly dependent on the orientation distribution of the fibers. A simulation tool capable of predicting fiber orientation accurately as a function of mold design and processing conditions is required as the predicted fiber orientation capabilities in commercial software show large discrepancies when compared with experimentally measured orientation. In this work a two dimensional coupled Hele-Shaw approximation for predicting the flow-induced orientation of glass fibers in injection molded composite parts is presented. In addition to coupling the stresses to fiber orientation for a highly concentrated short glass fiber PBT suspension, the model considers the slowdown of the evolution of orientation due to fiber interaction. Material parameters in the model are determined from basic rheometry rather than using data from injection molding experiments. The equation of motion coupled with stress equations are discretized using the discontinuous Galerkin Finite Element Method. Flow simulations are performed using a measured orientation profile at the gate instead of random orientation assumed in previous studies. Finally, the evolution of fiber orientation in the cavity is determined experimentally using a modified version of the method of ellipses and results are compared against the predicted values of orientation. The fiber orientation predicted in the entry region and the core layer structure at the end of fill region are now in closer agreement with the experimental values, but there are still some discrepancies.

Short and long fiber reinforced thermoplastics are a feasible alternative to develop lightweight materials for semi-structural and structural applications, respectively. These composites present a layered structure showing a complex tridimensional structure along the molding, created during the forming stage. The details of short fiber orientation in a center-gated disk with diameter of 1.38 mm were obtained in several regions including the gate and advancing front. Several modifications were introduced in the method of ellipses to obtain unambiguous orientation of 30 % short glass-fiber PBT. The unambiguous Arθ component was successfully obtained with the modified method. The results also showed an asymmetric distribution of fiber orientation that gradually washout as the flow progress. In addition, the initial orientation measured at the gate presented a fiber distribution different from the random orientation that is assumed in literature for a center-gated disk. Additionally, the initial results of this modified method to assess the complex structure of a 30 % long glass-fiber PP end-gated plaque are shown.