Fiber Orientation State Research Articles

Simulation of Fibrillation of PC/LCP/Kevlar Blends and Its Characterizations

AbstractKevlar fiber was fluorinated and oxy‐fluorinated directly in presence of undiluted fluorine and fluorine gas mixture and processed with Polycarbonate and LCP at 320 °C under 20 rpm in a twin‐screw extruder. The composites were then injection molded into dumbbell shaped specimens under different conditions like various mold temperatures, injection temperatures, injection speeds and mold filling rates. Various physico‐chemical characterizations have been performed under definite processing parameter. Orientation of fibers under different injection parameters was evaluated using mold flow simulation technique. Most injection molded or extruded structures however, exhibit non‐uniform fiber orientation across the final parts, with a diverging variety of different local fiber orientation states. Distinct skin and core regions were observed in the injection molded parts and it has also been found out that fiber orientation is different in skin and core region for both unmodified and modified derivative, which affects the flow behavior. Processing parameters significantly affect the fiber orientation pattern in the skin and core region for all blended materials. It is worth mentioning that the maximum fiber orientation occurred during the extrusion process at the wall but different extent of fiber orientation is observed during the injection molding depending on the shape of the dumbbell specimen. This fibrillation has been corroborated by the SEM study in both the skin and core region.

Study of the rotational diffusivity coefficient of fibres in planar contracting flows with varying turbulence levels

The Fokker–Planck equation is solved by describing the evolution of a 3D fibre orientation state along a planar contraction. A constant value of the effective rotational diffusion coefficient was determined for four different turbulent flow cases in planar contractions, reported experimentally in the literature. Two hypotheses for the non-dimensional rotational diffusivity are presented, each based on two different turbulent time scales, i.e. the Kolmogorov time scales and the time scale associated with large energy bearing eddies. These hypotheses are dependent on either the Reynolds number, based on the Taylor micro-scale, and/or a non-dimensional fibre length. The hypothesis, based on the assumption of long fibres, L f / η ≳ 25 , compared to the Kolmogorov scale and in the limit of large Re λ seems to capture the basic trends presented in the literature. This hypothesis has also the feature of predicting effects of varying fibre length within certain limits. Accordingly, by modeling the variation of turbulent quantities along the contraction in a CFD analysis, local values of rotational diffusivity can be evaluated with the mentioned hypothesis, based on either Kolmogorov time scale or Eulerian integral time scale.

Measurement of fiber orientation angle in FRP by intensity method

Abstract As fiber reinforced polymeric composites consist of reinforcements and matrixes, the mechanical characteristics of materials are determined by these two properties. Naturally, the property of composites depends on the property of matrices but fiber length, fiber aspect ratio, fiber content ratio, fiber orientation state of reinforcements directly affect the mechanical characteristics of composites such as tensile strength, fatigue strength, elasticity coefficient, thermal and fracture characteristics. Since the measurement of fiber orientation distribution state within fiber reinforced polymeric composites is very important for deciding the molding conditions of the products or for verifying the dynamic characteristics of molded products, a reliable method of measuring fiber orientation angle distribution should be established firmly. As the measurement of fiber orientation angle is very important, it is correct to measure each fiber orientation angle directly but it is not desirable as a measurement method since it requires lots of efforts. Therefore, researches are required on non-destructive simple measurement method. The fiber orientation angle can be measured precisely as much as the fiber aspect ratio is small when measuring the fiber orientation angle placed on the x–y plane from the result of this study. This method can be applied by image processing the photographs obtained from soft X-ray composites, which was currently used as products.

THE EFFECTS OF CLOSURE MODEL OF FIBER ORIENTATION TENSOR ON THE INSTABILITY OF FIBER SUSPENSIONS IN THE TAYLOR–COUETTE FLOW

An analysis for the linear stability of fiber suspensions in the Taylor–Couette flow is conducted. The orientation state of fibers is determined using the linear, quadratic and composite HL-I closure model and the model of the fiber contribution to the total stress is established based on the slender-body theory. The linear stability equation is derived and solved with the Chebyshev spectral method. The results show that the suppression effect is most predominant for the composite HL-I model, while least predominant for the linear closure model. The longer fibers and suspensions with high volume fraction have a more predominant influence on suppressing the instability. The contribution of the suspending fluid to the change rate of the disturbance energy dominates the energy dissipation. While the contribution of the fiber shear stress increases monotonously with increasing fiber aspect ratio for the three closure models, the contribution based on the composite HL-I closure model is the largest, and that based on the linear closure model is the smallest. The effect of fiber aspect ratio on the change rate of the disturbance energy caused by the fiber shear stress is more predominant than that of the fiber volume fraction. The positive energy caused by the fiber shear stress consumes the gross energy of the fluid system and leads to a suppression of the flow instability.

Comparison of Intensity Method and Counting Method in Measurement of Fiber Orientation Angle Distribution Using Image Processing

The fiber reinforced composites has high specific strength and stiffness than metallic material and are an anisotropic material whose mechanical properties, such as strength and elasticity, change with their fiber orientation state, the fiber length, the fiber aspect ratio, fiber mat structure, etc. Above all, the fiber orientation angle distribution state of fiber reinforced composite is fundamental element of mechanical properties. So, many researches on this element have been conducted by means of nondestructive method currently. The fiber distribution state is measured by intensity difference of pixel using image processing and these methods are intensity method by calculating of intensity value of pixel and counting method by calculating of fiber quantity. In this research, the fiber orientation simulation picture was constructed by plotter according to change of fiber’s diameter, length and orientation. The fiber orientation distribution state was measured by this intensity information. The fiber orientation angle distribution state measured by intensity method and counting method was compared with fiber orientation function calculation value.

Effect of Screw-Axial Vibration on Structure and Properties of Short Glass Fiber Reinforced High-Density Polyethylene Composites

Interfaces between resins and glass fibers perform concernful roles in mechanical and physicochemistry properties in composites. Additionally, the increase of the glass fiber orientation can drastically strengthen the mechanical qualities of composites. Here we employ the screw-axial vibration force field (VFF) via the self-devised electromagnetic dynamic plastics injection-molding machine to demonstrate the effects of the orientation and distribution state of short glass fibers (SGFs) and the distribution state of interfacial modifiers, compared with steady-state injection molding. We thus fabricated the samples of SGF reinforced high-density polyethylene with graftomer, plasticizer and other additives under screw-axially superposing vibration force field. The composite samples experience tensile testing, impact testing, scanning electronic micrograph techniques and the mathematical calculation of orientation of short glass fibers. The results betray that VFF with the vibrational frequency f = 3 Hz and amplitude A = 0.2 mm considerably enhances the orientation of SGFs along the melt flow direction, even at the core, compared with steady-state injection molding. Likewise, VFF dramatically improves the distribution state of short glass fibers and interfacial modifiers. Therefore, the tensile strength and impact strength of specimens are both significantly heightened, in contrast with those of conventional injection molding specimens.

Effect of packing pressure on fiber orientation in injection molding of fiber‐reinforced thermoplastics

AbstractInjection molding of fiber‐reinforced polymeric composites is increasing with demands of geometrically complex products possessing superior mechanical properties of high specific strength, high specific stiffness, and high impact resistance. Complex state of fiber orientation exists in injection molding of short fiber reinforced polymers. The orientation of fibers vary significantly across the thickness of injection‐molded part and can become a key feature of the finished product. Improving the mechanical properties of molded parts by managing the orientation of fibers during the process of injection molding is the basic motivation of this study. As a first step in this direction, the present results reveal the importance of packing pressure in orienting the fibers. In this study, the effects of pressure distribution and viscosity of a compressible polymeric composite melt on the state of fiber orientation after complete filling of a cavity is considered experimentally and compared with the simulation results of Moldflow analysis. POLYM. COMPOS. 28:214–223, 2007. © 2007 Society of Plastics Engineers

Fiber Orientation State Depending on the Injection Mold Gate Variations during FRP Injection Molding

Because of orientation and separation, injection molded products are heterogeneous and anisotropic. These heterogeneousness and anisotropy have a vast influence on mechanical properties of molding material and product characteristics. It is well known that fiber orientation state in fiber-reinforced polymeric composite has a profound effect on dynamic qualities like intensity, rigidity, and etc. To measure the fiber orientation of injection molded product's weld part, we first X-rayed moldings and recognized this photo by using an image scanner. Then, image processing method, which uses intensity difference in measuring fiber orientation state, is applied. Through these procedures, we can analyze the influence of molding's fiber orientation state according to mold gate changes. Fiber orientation is related mainly with the mold gate positions than with fiber content or mold temperature. When the distance from the gate increases, by matrix and reinforcement's flow speed differences, fiber orientation occurred. As diversion flow occurred at the end of fluid flow, fiber oriented at a right angle to the flow, and this is the same effect of weld line formation.

Production and characterization of polycarbonate composite sheets reinforced with vapor grown carbon fiber

Abstract This investigation describes fabrication of polymer composite sheets incorporated with vapor grown carbon nanofibers by a solution-evaporation and a rolling method, and a procedure to determine fiber dispersion, orientation state and electrical and mechanical properties. Microscopic analysis has shown a homogeneous dispersion of VGCFs in a polycarbonate matrix with no agglomerates and no shortening of the fibers. Higher amounts of VGCFs also were relatively well dispersed. The electrical resistivity and the tensile strength were decreased in both materials; the porosities were increased by the addition of the fibers. The rolling process improved the orientation of nanofibers and enhanced its mechanical properties. Thin composite sheets showed a high tensile modulus due to the compressor-processing effect.

Study on mechanical properties and material distribution of sandwich plaques molded by co-injection

In the sandwich injection molding process (co-injection), two different polymer melts are sequentially injected into a mold to form a part with a skin/core structure. Sandwich molding can be used for recycling, improving barrier and electrical properties, or producing parts with tailored mechanical properties. In this study the evaluation of flexural modulus and impact strength of co-injected plaques have been investigated. Virgin and short glass fiber reinforced (10 and 40%) polypropylene were used in six different combinations of sandwiched layers. The skin and core thicknesses were measured by optical microscopy and used to calculate the theoretical flexural modulus, which was compared to the experimentally measured modulus. Fiber orientation states were also observed by scanning electronic microscopy (SEM) at some specific locations and their effect on mechanical properties discussed. The experimental results indicate that an important improvement in transverse modulus, near the gate, is obtained when the virgin polypropylene (PP) is used as a skin and 40% short glass fiber polypropylene (PP40) as core. When both skin and core are made of PP40, the flexural moduli are slightly higher than conventionally injected PP40. POLYM. COMPOS. 26:265–275, 2005. © 2005 Society of Plastics Engineers.

Coupling of flow simulation and structural analysis for glass‐filled thermoplastics

AbstractIn the automotive industry, glass‐filled thermoplastics are used in air intake manifolds, radiator tanks, and many other parts. Plastic parts have many advantages over metal parts including mass, design flexibility and parts consolidation. However, widespread application of glass‐filled thermoplastic materials has been limited in many cases by the inability to accurately predict performance and durability. Fiber‐filled injection‐molded parts contain complex fiber orientation patterns. This fiber orientation state affects material properties including elastic modulus and strength and part properties including shrinkage and warpage. Tremendous amounts of time and money can be saved if one can predict the moldability and mechanical properties of a part at the design stage. In this work, we present a method where commercially available injection molding and structural analysis software may be coupled together with appropriate material property data to improve prediction of structural performance for parts molded from short glass fiber‐filled plastics. In addition, we compare the experimental and predicted performance based on simple equations and complex finite element calculations. Two major conclusions may be drawn from this work. In general, the assumption of geometric nonlinearity must be made. Also, an orthotropic material model is generally more robust and accurate than an isotropic analysis. Polym. Compos. 25:343–354, 2004. © 2004 Society of Plastics Engineers.

Global Stiffness of a SMC Panel Considering Process Induced Fiber Orientation

A material model, that translates into a stiffness matrix, the second order fiber orientation tensor, described by Advani and Tucker, and the stiffness matrix of a composite with aligned ellipsoidal inclusions, has been implemented in a FE programme and validated. The stiffness of a SMC panel with known state of fiber orientation is calculated using FEM. The influence of process induced fiber orientation is analysed. The fiber orientation for a realistic charge pattern for the panel has been obtained through mould filling simulation in a separate project. It is found that the fiber orientation has a rather small impact on the global stiffness. Only 0.8% lower stiffness compared to isotropic material model is obtained taking into account the fiber orientation distribution. The main reason for the low impact of the process induced fiber orientation is that the charge is symmetrically placed in the mould leading to a symmetric fiber orientation distribution.

Fibre orientation structures and their effect on crack resistance of injection moulded transverse ribbed plate

An extensive study of the fibre orientation structures developed in a transverse ribbed plate during injection moulding, and the use of these structures to investigate the effect of local fibre orientation state on crack initiation resistance, is reported. The fibre orientation results for the ribbed plate, measured using large area image analysis system developed at Leeds University, showed that after an initial settling down period, the central core region, where the fibres are aligned perpendicular to the flow direction, decreased in size monotonically, with an associated monotonic increase in the outer shell regions, where the fibres are aligned preferentially along the injection direction. Interestingly, the level of orientation in the two regions remained almost constant: only the proportions of the two regions were found to change with flow length. Across the plate, close to the gate, the central core region was found to have a lens-like shape, while at the other end of the plate the core was thinner and also consistent in thickness across the sample width. The transverse rib was found to cause little disturbance to the fibre orientation of the base plate. The different proportions of the shell and core regions at different locations over the ribbed plate provided an ideal case to test the proposition of Friedrich that the crack resistance of a short fibre reinforced material depends on the number of fibres that are perpendicular to the crack tip. The impact test results gathered in this way confirmed this hypothesis of Friedrich.

On the solution of Fokker–Planck equations in steady recirculating flows involving short fiber suspensions

Numerical modeling of non-Newtonian flows typically involves the coupling between equations of motion characterized by an elliptic character, and the fluid constitutive equation, which is an advection equation linked to the fluid history. Thus, the numerical modeling of short fiber suspensions flows requires a description of the microstructural evolution (fiber orientation) which affects the flow kinematics and that is itself governed by this kinematics. There are different ways to describe the microstructural state. The use of orientation tensors, whose evolution is governed by advection equations, is widely considered. Other possibility is to describe the fiber orientation state from its probability density whose evolution is described by the Fokker–Planck equation (which does not involve any closure relation). The numerical treatment of advection problems is not a simple matter. Moreover, many industrial flows show often steady recirculating areas which introduce some additional difficulties, because in these flows neither boundary conditions nor initial conditions are known. In some of our former papers, we have proposed accurate techniques to solve the linear and non-linear advection equations governing the evolution of the second-order orientation tensor, in steady recirculating flows. These techniques combine a standard treatment of the non-linearity with a more original treatment of the associated linear problems imposing the periodicity condition. In this paper, we generalize those numerical strategies to compute the steady solution of the Fokker–Planck equation (which governs the evolution of the fiber orientation probability distribution) in steady recirculating flows.

Measurement and numerical simulation of three‐dimensional fiber orientation states in injection‐molded short‐fiber‐reinforced plastics

AbstractTo determine three‐dimensional fiber orientation states in injection‐molded short‐fiber composites, a confocal laser scanning microscope (CLSM) is used. Since the CLSM optically sections the specimen, more than two images of the cross sections on and below the surface of the composite can be obtained. Three‐dimensional fiber orientation states can be determined by using geometric parameters of fiber images obtained from two parallel cross sections. For experiments, carbon‐fiber‐reinforced polystyrene is examined by the CLSM and geometric parameters of fibers on each cross‐sectional plane are measured by an image analysis. In order to describe fiber orientation states compactly, orientation tensors are determined at different positions of the prepared specimen. Three‐dimensional orientation states are obtained without any difficulty by determining the out‐of‐plane angles utilizing fiber images on two parallel planes acquired by the CLSM. Orientation states are different at different positions and show the shell–core structure along the thickness of the specimen. Fiber orientation tensors are predicted by a numerical analysis and the numerically predicted orientation states show good agreement with measured ones. However, some differences are found at the end of cavity. They may result from the fountain flow effects, which are not considered in the numerical analysis. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 500–509, 2003

Orientation Averaging for Stiffness and Thermal Expansion of Short Fiber Composites

It is fairly common in practice that during injection moulding, the mold filling process results in non‐uniform fiber orientation distributions in the final injection molded short fiber reinforced composite part. It would be very attractive to employ an orientation averaging scheme to form predictions for the design of short fiber reinforced composite parts. Here, the authors use a finite‐element‐based numerical procedure developed by themselves and, for the first time, directly predict the stiffness and thermal expansion of several hundred multi‐fiber computer models with a variety of different predefined fiber orientation states.

Numerical studies of fiber suspensions in an axisymmetric radial diverging flow: the effects of modeling and numerical assumptions

Until recently, most simulations of fiber suspensions have been developed using the Hele–Shaw approximation. Therefore, the fluid behavior around the melt-front, solid walls, weldlines, gates, ribs, and sudden thickness changes could not be predicted accurately. In this study, a finite element method (FEM) analysis of an axisymmetric three-dimensional geometry was performed using the pseudo-concentration method (PCM) as a melt-front capturing technique for fiber suspensions. The non-Newtonian governing equations were solved using a streamline upwind Petrov–Galerkin (SUPG) FEM to maintain the stability of the convection-dominated problem. Second-order orientation tensors with a closure model were adopted to predict the evolution of fiber suspensions. A center-gated disk geometry was used as a benchmark test problem because of the complicated nature of its flow history. The predictions were compared with experimental data, and the coupling effects between the fluid and fibers, the effects of fountain-flow, initial conditions (sprue effect), closure approximation, and processing conditions were observed. Fountain-flow effect on the fiber orientation state has been found to be most significant of these effects.

Characterization of fiber orientation in short fiber reinforced composites with an image processing technique

An experimental method is developed to measure the three-dimensional fiber orientation in short fiber reinforced composites by utilizing an image processing technique. The second order orientation tensor can be calculated with geometrical data that were obtained from two parallel planar cross-sections. The orientation state of individual fibers is determined from the geometry of the elliptical cross-sectional shape on the polished surface. The basic concept in determining the three-dimensional fiber orientation tensor is to slice the composite sample twice in the same direction within a small distance. The tensor is determined by using a digital image processing technique and a computational code which calculates the tensor from the geometrical characteristics obtained for the elliptical fiber cross-sections. Experiments are performed to measure the three-dimensional orientation tensor of composite specimens and good results are obtained by using the method proposed in this study

A study on fiber orientation in the compression molding of fiber reinforced polymer composite material

It is important to predict the fiber orientation state in controlling the mechanical and physical properties of the final parts in the compression molding process. A computer simulation has been performed to predict the fiber orientation state during compression molding. In the flow analysis, the generalized Hele–Shaw (GHS) model and the control volume finite element method (CVFEM) were employed, and second order tensor representation was used in the fiber orientation analysis. The conventional fiber orientation model has been based on the assumption of homogeneous suspension, i.e. the fiber content is uniform everywhere in the charge. However, in the actual compression molding process where long fibers are used and the fiber content is high, the homogeneous suspension assumption is violated since the fiber separation phenomenon occurs and a fiber concentration gradient is developed. In this study, a new fiber orientation model considering the fiber separation effect is suggested. The results of the analysis are compared with those of experiment and they show good agreement.

Confocal microscopy measurement of the fiber orientation in short fiber reinforced plastics

To determine three-dimensional fiber orientation states in injection-molded short fiber composites a CLSM (Confocal Laser Scanning Microscope) is used. Since the CLSM optically sections the composites, more than two cross-sections either on or below the surface of the composite can be obtained. Three dimensional fiber orientation states can be determined with geometric parameters of fibers on two parallel cross-sections. For experiment, carbon fiber reinforced polystyrene is examined by the CLSM. Geometric parameters of fibers are measured by image analysis. In order to compactly describe fiber orientation states, orientation tensors are used. Orientation tensors are determined at different positions of the prepared specimen. Three dimensional orientation states are obtained without the difficulty in determining the out-of-plane angles by utilizing images on two parallel planes acquired by the CLSM. Orientation states are different at different positions and show the shell-core structure along the thickness of the specimen.