Nonisothermal transient filling simulation of fiber suspended viscoelastic liquid in a center‐gated disk

The present study numerically investigates a fiber orientation in injection‐molded short fiber reinforced thermoplastic composite by using a rheological model, which includes the nonlinear viscoelasticity of polymer and the anisotropic effect of fiber in the total stress. A nonisothermal transient‐filling process for a center‐gated disk geometry is analyzed by a finite element method using a discrete‐elastic‐viscous split stress formulation with a matrix logarithm for the viscoelastic fluid flow and a streamline upwind Petrov–Galerkin method for convection‐dominated problems. The numerical analysis result is compared to the experimental data available in the literature in terms of the fiber orientation in center‐gated disk. The effects of the fiber coupling and the slow‐orientation kinetics of the fiber are discussed. Also, the effect of the injection‐molding processing condition is discussed by varying the filling time and the mold temperature. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Dynamic viscoelasticity of hybrid kevlar and glass fiber reinforced LLDPE in the molten state

Kevlar and glass fibers were used to reinforce linear low density polyethylene (LLDPE), and composite sheets of 0.8, 1.5 and 2.5 mm thicknesses were obtained by using a compression molding technique. Dynamic viscoelastic properties of non‐hybrid and hybrid composites of various compositions at 200°C are evaluated. Storage modulus (G′) and loss modulus (G″) increase with angular frequency (ω) and reinforcement. Replacement of glass fiber by Kevlar at constant loading of fibers in LLDPE increases the value of G′, G″ and η′. The fractured surface of composite shows the gradient orientation of fibers particularly in 2.5 mm thick sheet. Top and bottom layers show relatively two‐dimensional orientation as compared to the middle layer, which shows random orientation. The orientation of fibers decreases G′ and η′ of Kevlar fiber and hybrid fiber hybrid fiber reinforced LLDPE composites. The effect of change in distance between parallel plate of rheometer (change in strain amplitude) on dynamic rheological properties is studied and reported here.     

Influence of MMT nanoclay on impedance spectroscopy analysis of naturally woven coconut sheath/polyester hybrid composite

In this work, the impedance spectroscopy study has been carried out for the naturally woven coconut sheath fiber reinforced composite with the effect of adding different weight percentages of montmorillonite nanoclay and chemical activation of fiber. The dispersion mechanism of nanoclay with polyester, and the modification of the fiber surface have been studied by using X–ray diffractogram, transmission electron microscope and scanning electron microscope respectively. Infra‐red spectra have also been taken to study the reactive compounds of treated fibers. The impedance spectrum shows significant improvement in A.C. conductivity, dielectric constant and loss tangent values by the fiber modification, using alkali and silane treatments, due to the structural topography changes at the fiber surface. The highest dielectric strength was found in alkali‐treated coconut sheath/polyester composite due to increase in orientation polarization by the absorption of moisture content at fiber surface. The addition of different weight percentages of nanoclay with coconut sheath in all the treated conditions shows significant changes in the dielectric properties. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Fiber Suspensions in Complex Geometries: Flow/Orientation Coupling

A Galerkin finite element solution is developed for the flow of fiber suspensions. Primary variables are velocity, pressure, and a second‐order tensor describing the fiber orientation. The model treats the orientation as three‐dimensional, includes fiber—fiber interaction effects, and uses an orthotropic closure approximation. The flow and orientation are strongly coupled through an orientation‐dependent constitutive equation. We explore the effect of this coupling on the fluid mechanics of fiber suspensions by studying three flows: an axisymmetric contraction, an axisymmetric expansion, and a center‐gated disk. Coupling enhances the corner vortex in the contraction, in quantitative agreement with published experiments and calculations. The expansion results demonstrate that the aligned‐fiber approximation is not valid for this flow. In the center‐gated disk the effects of coupling are modest and are only noticeable near the center of the disk. This supports the use of decoupled models for injection molding in thin cavities.     

Prediction of fiber orientation in the injection molding of long fiber suspensions

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. © 2012 Society of Plastics Engineers     

Evaluation of dynamic elastic properties of wood‐filled polypropylene composites

Dynamic modulus of elasticity (MoE) and shear modulus of wood‐filled polypropylene composite at various filler contents ranging from 10% to 50% was determined from the vibration frequencies of disc‐shaped specimens. Wood filler was used in both fiber form (pulp) and powder form (wood flour). A novel compatibilizer, m‐isopropenyl‐α,α‐dimethylbenzyl‐isocyanate(m‐TMI) grafted polypropylene with isocyanate functional group was used to prepare the composites. A linear increase in dynamic MoE, shear modulus, and density of the composite was observed with the increasing filler content. Between the two fillers, wood fiber filled composites exhibited slightly better properties. At 50% filler loading, dynamic MoE of the wood fiber filled composite was 97% higher than that of unfilled polypropylene. Halpin‐Tsai model equation was used to describe the changes in the composite modulus with the increasing filler content. The continuous improvement in elastic properties of the composites with the increasing wood filler is attributed to the effective reinforcement of low‐modulus polypropylene matrix with the high‐modulus wood filler. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1706–1711, 2006     

Effects of abrupt expansion geometries on flow‐induced fiber orientation and concentration distributions in slit channel flows of fiber suspensions

Flow‐induced fiber orientation and concentration distributions were measured in channel flows of fiber suspension. The test fluids used are a concentrated fiber suspension (CFS), a semidilute one (SDFS), and a dilute one (DFS). The channel has a thin slit geometry with a 1:16 expansion. In the present work, fiber orientation and concentration distributions are quantitatively evaluated by direct observation of fibers even in the CFS flow. It is found that the weak fiber–fiber interaction of the SDFS largely affects the fiber orientation in the flow with a sudden change such as in the expansion flow, while it is ineffective upon the fiber orientation in the flow without a sudden change such as in the far downstream region. Fiber concentration in the CFS has a flat distribution over a channel width in both the entrance region of the expansion and the downstream region. However, fiber concentration distributions in the SDFS and the DFS have a small and a large peak near the sidewall in the entrance region, respectively, due to the fiber‐wall interaction at the channel wall. These peaks, however, disappeared in the far downstream region after the fibers passed through the expansion. POLYM. COMPOS., 26:660–670, 2005. © 2005 Society of Plastics Engineers.     

纤维增强复合材料的力学性能预测的数值模拟

纤维增强复合材料的力学性能和热物理性能依赖于纤维的取向状态.在注射成型过程中纤维最终的取向状态依赖于充填过程的速度场,因此最终的产品性质依赖于成型的详细过程.研究发现,注塑成型制品的结构呈层状分布,层数依赖于模具几何和成型条件,不过大多数的结构在成型表面为沿流动方向取向,而在中心层为横向排列,有时在制件表面还有一层薄的介于二者之间排列的取向层.本文主要给出两个简单模型中纤维取向预测的理论和数值方法,这两个模型分别为:中心浇口圆盘和边浇口长条.     

Mechanical characterization and application of Weibull statistics to the strength of softwood lignin‐based carbon fibers

Mechanical characterization of the first generation of softwood kraft lignin‐based carbon fibers (CF) was carried out. The single‐fiber tensile tests of filaments with different diameters and length were performed to evaluate stiffness and strength of carbon fibers. The average mechanical properties were measured as follows: tensile strength of approximately 300 MPa, the elastic modulus of 30 GPa and a strain at failure within interval of 0.7–1.2%. The fiber strength data was evaluated by the two‐parameter Weibull statistics and parameters of this distribution were obtained. Although strength of the produced fibers is still significantly lower than that of commercially available, the experimental results and predictions based on Weibull statistics show a fairly good fit. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3689–3697, 2013     

Analytical model to predict multiaxial laminate fracture toughness from 0° ply fracture toughness

The prediction of nominal strength is very important in the design and evaluation of materials especially polymer matrix composites. Various cohesive laws forms are successfully used in predicting the nominal strength of laminated composite structures. For composite structures, fracture toughness is dominated parameter when using cohesive laws to predict their nominal strength. In spite of complex reported models, this study propose an easy simple model to predict the fracture toughness of multidirectional composite laminates using the fracture toughness of the 0° ply ones. This model is mainly based on the geometry of fiber orientation and linear elastic fracture mechanics and uses the fracture toughness of the 0° ply obtained from compact tension test specimens. A good prediction is obtained by comparing the model results with experimental data which are obtained from center‐cracked specimens manufactured using different lay‐ups orientations and materials. POLYM. ENG. SCI., 54:234–238, 2014. © 2013 Society of Plastics Engineers     

Two‐dimensional long‐flexible fiber simulation in simple shear flow

Long‐flexible fiber orientation under the influence of a flow field is an important engineering problem. One can encounter this problem in many fields. For example in fiber reinforced thermoplastics produced in both injection and compression molding, fiber orientation affect final part properties. Fiber orientation models are constructed for short fibers in a simple shear flow case and though this case is important it is not the general case. In this work we extract rotational friction coefficients from Jeffery's model, create a general case long‐flexible fiber orientation model, and apply it in a simple shear flow. POLYM. COMPOS., 37:2425–2433, 2016. © 2015 Society of Plastics Engineers     

Prediction of elastic properties for curved fiber polymer composites

The objective of this article is to study the effect of fiber curvature on the elastic properties of a long‐fiber composite. The study was carried out using a unit cell homogenization approach and micromechanical modeling. In the first approach, a unit cell with a fiber bundle was defined and used in the analysis. Appropriate boundary conditions were prescribed to extract the elastic stiffness components. The second approach made use of the Eshelby‐Mori‐Tanaka model to compute the stiffness of the aligned fiber composite. Fiber curvature was then accounted for through the variation of fiber orientation within a prescribed range that corresponds to a given degree of fiber curvature. It was found that curved fibers significantly affect the composite properties since they lead to a significant stiffness reduction in the longitudinal direction while relatively small increase in stiffness is achieved in the transverse direction in the plane containing the fiber tow. POLYM. COMPOS., 2008. Published 2008 Society of Plastics Engineers     

Structurally different short aramid fiber–reinforced thermoplastic polyurethane

The reinforcing effect of two structurally different Aramid short fibers, Technora and Twaron on the mechanical, dynamic mechanical, and thermal properties of an ester‐based thermoplastic polyurethane (TPU) was investigated. A fixed fiber length of 6 mm is used by varying the fiber loading ranging from 3 to 10 phr. The Young's modulus and the low strain modulus of Technora–TPU composite was found three times higher than that of Twaron–TPU composite at all ranges of fiber loading. Optical microscopic analysis revealed that a severe processing‐induced fiber breakage of Twaron is the primary reason behind the inferior properties shown by these fiber‐reinforced TPU composite. A brittle kind of failure has been observed during tensile testing in both the composite at a fiber loading of 10 phr. To solve this problem, an economic pretreatment with maleic anhydride‐grafted polybutadine (PB‐g‐MA) has been applied on the Aramid fiber surface before mixing it with the TPU matrix. A good quality of fiber dispersion with significant improvement in mechanical properties could be achieved with the addition of only 5 phr of PB‐g‐MA. Morphological analyses on the tensile‐fractured and cryogenically fractured surfaces of these composites offer strong evidences for the dispersing and coupling action of PB‐g‐MA with these Aramid fibers and the TPU matrix. POLYM. COMPOS., 35:1767–1778, 2014. © 2013 Society of Plastics Engineers     

Investigation of fiber orientation states in injection‐compression molded short‐fiber‐reinforced thermoplastics

Injection‐compression molding (ICM) has received increased attention because of its advantages over conventional injection molding (CIM). This article aims to investigate the effects of five dominating ICM processing parameters on fiber orientation in short‐fiber‐reinforced polypropylene (SFR‐PP) parts. A five‐layer structure of fiber orientation is found across the thickness under most conditions in ICM parts. This is quite different from the fiber orientation patterns in CIM parts. The fibers orient orderly along the flow direction in the shell region, whereas most fibers arrange randomly in the skin and the core regions. Additionally, the fiber orientation changes in the width direction, with most fibers arranging orderly along the flow direction at positions near the mold cavity wall. The results also show that the compression force, compression distance, and compression speed play important roles in determining the fiber states. Thicker shell regions, in which most fibers orient remarkably along the flow direction, can be obtained under larger compression force or compression speed. Moreover, the delay time has an obvious effect on the fiber orientation at positions far from the gate. However, the effect of compression time is found to be negligible. POLYM. COMPOS., 31:1899–1908, 2010. © 2010 Society of Plastics Engineers.     

Effect of drawing on the molecular orientation and polymorphism of melt‐spun polyvinylidene fluoride fibers: Toward the development of piezoelectric force sensors

Thin piezoelectric polyvinylidene fluoride fibers containing a high piezoelectric β‐phase content of up to 80% were developed in this work using a melt‐spinning process. After crystallization from the melt, the fibers were subsequently stretched unidirectionally at 120°C between 25 and 75% of their original length. The effects on the molecular orientation, polymorphism and tensile properties of the fibers were investigated. Polarized infra‐red spectroscopy and X‐ray diffraction results show that the conversion of α‐phase to β‐phase occurred during the stretching process as a result of molecular alignment and creation of a dipole induced by the CF₂ groups normal to the fiber direction. These fibers were then integrated into various weave architectures in order to design flexible two‐dimensional textile‐based piezoelectric force sensors. The piezoelectric responsiveness of these materials, tested under impact (70 Newton force, 1 Hz frequency) was very promising, with a maximum output voltage of up to 6 V and an average sensitivity of up to 55 mV/N measured. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Natural fiber suspensions in thermoplastic polymers. I. Analysis of fiber damage during processing

The final properties of the composites materials are strongly dependent on the residual aspect ratio, orientation, and distribution of the fibers, which are determined by the processing conditions. Present work is a systematic study of the influence of natural fiber concentration on its damage during all the steps involved in the composite compounding. The system under study is cellulose fiber‐reinforced polypropylene. The fiber geometrical parameters—length, diameter, and aspect ratio—are measured, and their statistical distributions are assessed for different concentrations. It is found that the higher the fiber concentration, the lower the fiber damage. These results evidence a difference in behavior between the damage of flexible natural fiber and rigid ones. The results are analyzed in terms of fiber concentration regimes, fiber–fiber interaction, flexibility, and entanglements. Two competitive mechanisms of the fiber interaction are proposed for explaining the fiber damage behavior during the flow of the flexible natural fiber suspensions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2501–2506, 2007     

Mechanical property enhancement in short-fiber composites through the control of fiber orientation during fabrication

The mechanical properties of plastics and elastomers reinforced by short fibers are generally dictated by the selection of matrix and reinforcement. However, the high tensile properties attainable in these systems through laboratory processing techniques are frequently not obtained in conventional fabrication operations. To gain a wider latitude in meeting economic and performance constraints, the control of composite structure in the fabrication step should not be overlooked. Tool geometry and processing conditions can be manipulated specifically to control the fiber orientation distribution in the product. A study of fiber orientation in composite compounds during flow through runners, gates, and dies leads to recommendations for optimizing the directional strength and stiffness according to a kinematic model. Performance data on parts fabricated from various short-fiber composite materials bear out these projections.     

Rheology of LDPE‐based semiflexible fiber suspensions

Molten LDPE suspensions containing fibers of different flexibilities have been investigated in simple shear and small and large amplitude oscillatory shear (LAOS) flow. The suspensions exhibited viscosity and normal stress overshoots in stress growth experiments, and the magnitude and width of the overshoots became larger as the fiber flexibility increased. LAOS was used to help understanding the relationship between stress growth and fiber orientation. For all composites, the stress signal decreased with time in LAOS, and this behavior was more pronounced in the case of the more rigid fibers. The energy dissipated per LAOS cycle was evaluated for each composite, and it showed that less energy was dissipated as fiber flexibility decreased. In addition, the dissipated energy decreased with time and this has been interpreted in terms of a reduction of fiber contacts. The first normal stress difference showed a nonsinusoidal periodic response, and fast Fourier transform analysis indicated the presence of a first harmonic corresponding to the applied frequency for the fiber‐filled systems, in addition to the second harmonic observed for the neat LDPE. It resulted in asymmetrical strain‐normal force Lissajou curves for the suspensions, with this asymmetry being more pronounced in the case of the more rigid fibers. This has been attributed to a more extensive fiber orientation for the latter. POLYM. COMPOS., 31:1474–1486, 2010. © 2009 Society of Plastics Engineers     

A Wide-angle X-ray diffraction method of determining chopped fiber orientation in composites with application to extrusion through dies

A method of determining fiber orientation in composites using wide-angle X-ray diffraction (WAXS) is described. Oriented crystalline fibers are suspended in an amorphous polymer matrix. The WAXS reflects characteristics of the fiber are used to determine the mean orientation and orientation distribution of the crystallographic axes representing the polymer chain relative to preferred axes located in the test specimen. The chain direction crystallographic axis is taken as representing the fiber axis, and the orientation of this axis to represent the orientation of the fibers. Experimental studies were carried out using Kevlar (poly(p-phenylene terephthalamide)) fibers suspended at a 20 volume percent loading in a polymethyl methacrylate matrix. The Kevlar fibers had Hermans orientation factors of 0.92. Specific attention is given to how through circular dies. We have examined both extrudates and the material frozen-in when the composite in the reservoir and die is cooled to room temperature. Fiber orientation factors, corresponding to Hermans orientation factors, 0.3 to 0.38 were obtained for the extrudates. Orientation factors for fibers within the die is about 0.45. Specially prepared completely oriented samples had orientation factors of 0.93, which closely corresponds to the orientation of the fiber.