Permeability Of Fiber Preforms Research Articles

Characterization of effective permeability of prepreg fibers under autoclave molding process conditions using process model simulations

During autoclave molding process of prepreg composites, resin flow occurs through the fibers in the transverse direction of the layup due to applied external pressure. The extent of resin flow depends largely on the permeability of the reinforcement used in prepreg, and hence, the permeability is an important parameter for process modeling and simulations of composites fabrication processes. Most of the existing methodologies for measurement of transverse permeability of the reinforcements described in the literature are based on experimental techniques. However, the permeability measured through these experiments differs greatly from the actual permeability of the prepreg during actual fabrication process. This article focuses on this aspect and a methodology for the prediction and measurement of saturated transverse permeability of fiber preform of prepregs as encountered during fabrication is reported here. The methodology takes into account the actual process conditions and simulates them to predict permeability of the reinforcement. The permeability thus obtained will be the ‘effective permeability’ of the reinforcement. The methodology is applicable for prepregs containing any kind of resin, reinforcement and its weaving architecture.

Numerical Characterization of Permeability Tensor of Fiber Preforms for Liquid Composite Molding Applications

Composite materials are of paramount importance in aeronautics, aerospace and automotive applications. Consisting of fibers and matrix keeping the fibers together, composite materials offer high strength and low weight. In one of the most common composite manufacturing method, Liquid Composite Molding (LCM) process, reinforcing glass, carbon or Kevlar fiber preforms are placed in a mold cavity and a liquid resin is introduced to cover the remaining empty space to form a composite by curing the resin. The primary objective in LCM processes is the successful filling of the preform with resin system. In order to obtain a desired composite product, the process needs to be modeled to have an insight about the filling and curing stages of the LCM process. The fiber preform permeability plays a key role in the filling pattern of the mold, which dictates if there will be any voids in the composites. Thus, to achieve a successful model of the LCM process, one needs to characterize the permeability tensor to quantify the resistance to resin flow through fibers. Besides various experimental and numerical techniques to characterize the permeability, this study offers a new numerical methodology to characterize the permeability of a unit cell of a fiber system. The study involves the characterization of the permeability of a unit cell under various shear strains and project a permeability map from the unit cell characterizations on the macro scale including the drape effect.

A METHODOLOGY TO CHARACTERIZE FIBER PREFORM PERMEABILITY BY USING KARDAR–PARISI–ZHANG EQUATION

Permeability tensor describes the resistance to fluid flow through the fibrous porous media, which may not be spatially uniform. This nonuniformity in fiber architecture causes variation in the permeability value of the fibrous domain. The time evolution and geometry of the rough interfaces of the fluid flow in fibrous porous medium are analyzed using the concepts of dynamic scaling and self-affine fractal geometry, and they are shown to belong to the Kardar–Parisi– Zhang (KPZ) universality class. The resulting growth exponent, β is found to match the 1 + 1 KPZ values, and the roughness exponent, α, describes the standard deviation of the variation in fiber preform permeability. Additionally, this characterization is used to develop a tool to quantify the percentage and strength of defects within the fibrous porous media from flow front profile analysis.

Efficient Permeability Measurement and Numerical Simulation of the Resin Flow in Low Permeability Preform Fabricated by Automated Dry Fiber Placement

We propose a new experimental method using a Hassler cell and air injection to measure the permeability of fiber preform while avoiding a race tracking effect. This method was proven to be particularly efficient to measure very low through-thickness permeability of preform fabricated by automated dry fiber placement. To validate the reliability of the permeability measurement, the experiments of viscous liquid infusion into the preform with or without a distribution medium were performed. The experimental data of flow front advancement was compared with the numerical simulation result using the permeability values obtained by the Hassler cell permeability measurement set-up as well as by the liquid infusion experiments. To address the computational cost issue, the model for the equivalent permeability of distribution medium was employed in the numerical simulation of liquid flow. The new concept using air injection and Hassler cell for the fiber preform permeability measurement was shown to be reliable and efficient.

Use of medial axis to find optimal channel designs to reduce mold filling time in resin transfer molding

In Resin Transfer Molding (RTM) process, pre-designed channels branching out like runners from the injection port can be used for faster resin distribution and impregnation as compared to traditional single gate injection to reduce the mold filling time. To search for the optimal channel design for maximum fill time reduction, the Medial Axis (MA) of the part surface is defined. Next, a methodology is developed to search for the topology of the MA using Finite Element based mold filling simulation software. The MA location is corrected for a part with non-homogeneous fabric permeability. Finally, some case studies are presented to illustrate the effectiveness and accuracy of the channel design approach for RTM with the objective of minimizing fill time, when fabricating composite parts that contain complexities in both the geometric features such as compound curvatures and corners and in material properties such as non-homogeneous and highly anisotropic fiber preform permeability.

A New Fiber Preform with Nanocarbon Binder for Manufacturing Carbon Fiber Reinforced Composite by Liquid Molding Process.

Carbon fiber reinforced composite has been a good candidate of lightweight structural component in the automotive industry. As fast production speed is essential to apply the composite materials for the mass production area such as automotive components, the high speed liquid composite molding processes have been developed. Fast resin injection through the fiber preform by high pressure is required to improve the production speed, but it often results in undesirable deformations of the fiber preform which causes defectives in size and properties of the final composite products. In order to prevent the undesirable deformation and improve the stability of preform shape, polymer type binder materials are used. More stable fiber preform can be obtained by increasing the amount of binder material, but it disturbs the resin impregnation through the fiber preform. In this study, carbon nanomaterials such as graphene oxide were embedded on the surface of carbon fiber by electrophoretic deposition method in order to improve the shape stability of fiber preform and interfacial bonding between polymer and the reinforcing fiber. Effects of the modified reinforcing fiber were investigated in two respects. One is to increase the binding energy between fiber tows, and the other is to increase the interfacial bonding between polymer matrix and fiber surface. The effects were analyzed by measuring the binding force of fiber preform and interlaminar shear strength of the composite. This study also investigated the high speed liquid molding process of the composite materials composed of polymer matrix and the carbon fiber preforms embedded by carbon nanomaterials. Process parameter such as permeability of fiber preform was measured to investigate the effect of nanoscale surface modification on the macroscale processing condition for composite manufacturing.

Permeability measurements and flow simulation for polyaniline carbon nanofibers modified nanopaper on glass fiber preform

Carbon nanofibers (CNFs) nanopapers have shown great potential to improve the surface of fiber‐reinforced polymeric composites, including providing electromagnetic interference shielding and erosion resistance. During typical resin transfer molding (RTM) process, the CNF nanopaper is incorporated into the fiber preform as a surface layer. To learn how resin flows through the fiber preform and nanopaper layer, permeabilities of the fiber preform and nanopaper need to be measured. As is well known, measuring the permeability of fiber preforms is experimentally challenging. Results usually exhibit large experimental variability. Measuring permeability of nanopapers is even more complicated. To improve the accuracy of results, permeability of CNF‐based nanopapers was measured using different experimental setups. In‐plane permeability of nanopaper was measured by both unidirectional microslit flow and radial flow approaches. Trans‐plane permeability was measured as well, using a trans‐plane flow cell and a flow visualization mold. In this article, we discuss the methods used and provide experimental results. We also conducted computational fluid dynamics simulations to study the detailed flow patterns of the nanopaper/RTM process and compared the simulated effect of the nanopaper on retarding the flow (length of the lag) with respect to the glass preform with flow visualization results. POLYM. COMPOS., 37:435–445, 2016. © 2014 Society of Plastics Engineers

Resin flow monitoring inside composite laminate during resin film infusion process

In this article, a novel method of measuring resin flow front under vacuum condition is presented. The in situ monitoring system with metal hollow probe based on gas flow balance can be used in resin film infusion (RFI) process, where resin film is used and transverse flow is dominated along thickness direction of fiber preform. The diameter of the probe was chosen to increase the measuring accuracy, and the reliability of the method was evaluated by comparison of visualization experiment. Experimental results demonstrate that the method is suitable for monitoring resin flow in RFI process with and without autoclave, and can obtain the information about resin filling time, nonuniform flow front, and the permeability of fiber preform. Furthermore, by means of the established monitoring system, the influences of pressure and lay‐up sequence of carbon fiber fabric on epoxy resin flow during RFI process were investigated. In addition, resin flow pattern with changing viscosity of epoxy resin was studied. POLYM. COMPOS., 35:681–690, 2014. © 2013 Society of Plastics Engineers

Characterization of 3D fiber preform permeability tensor in radial flow using an inverse algorithm based on sensors and simulation

In Liquid Composite Molding (LCM), the fiber preform is placed in a mold and resin is injected through a gate to fill the empty spaces within the mold. LCM processes are modeled as resin flow through fibrous porous media in which if one knows the permeability values, one can determine the arrival times of the resin at any location. A mold with radial injection with 192 flow arrival detection sensors along 16 radial lines are mounted flush with the top and the bottom mold surface with an additional sensor on the top surface opposite to the injection hole of the bottom surface. The inverse problem addressed here is from the recorded resin arrival times at sensor locations, how accurately can one determine the permeability of the preform? The proposed method uses correlation between the experimentally recorded resin arrival times and 3D flow simulation of the experiment. The optimization routine varies the permeability components in the simulation to achieve the best possible match with the experimental arrival times at all the sensor locations. Currently, all in-plane permeability components and through-the thickness permeability are characterized from a single experiment with the potential to evaluate the cross-thickness off-diagonal terms as well. In addition, the technique also demonstrates how one can reduce the variation in through thickness permeability by the use of a distribution media at the injection hole to avoid blockage of the inlet by the fiber tows. The optimization routine sequentially optimizes the values of the individual components of the permeability tensor using golden search method, and then repeats the entire sequence until the best match is found. The validation and sensitivity of this method is explored and it has been shown that this technique is promising for characterization of all permeability components from a single radial flow experiment.

Statistical characteristics of out‐of‐plane permeability for plain‐woven structure

AbstractSince out‐of‐plane permeability of fiber preforms is a function of the number and arrangement of stacked layers, either many layers of preforms or numerous experiments are required to obtain an exact out‐of‐plane permeability experimentally. The reason is that there exist nesting and phase shifting when the preforms are laid up. From a statistical viewpoint, the effect of the number of preform layers on the out‐of‐plane permeability was analyzed by adopting an analytical model proposed in this study. Numerical simulation for a unit‐cell constructed based on geometry of the preform was carried out to validate the analytical model as well as experimental measurements of the permeability. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

Statistical characterization and robust design of RTM processes

Abstract Resin transfer molding (RTM) process has been gaining extensive attention for its capability of producing complex structural parts. Researchers have made significant progress in understanding the process. These achievements have resulted in a rapid growth of the number of RTM applications. However, this process is still underutilized relative to its potential. One major barrier is that the reproducibility of the finished parts is often lower than expected. This, in turn, results in high cost of RTM components. In RTM process, major quality problem comes from unbalanced resin flow in mold filling process. Fiber preform permeability is the key parameter influencing the resin flow behavior. On the other hand, due to the variations existed in the raw materials and processing, the preform permeability varies across the regions within a part and from part to part. This variability results in reduced reproducibility of the composite components made by RTM. It is imperative to understand the statistical behavior of the permeability variations of preform in typical RTM processes. In this research, a comprehensive study was conducted to investigate the statistical property of the typical flow disturbance due to race-tracking in RTM processes. It was found that the ratio of the permeability values caused by race-tracking over average value for a rectangular mold could be represented as Weibull variables. With the statistical analysis results, a robust RTM process design method was introduced and illustrated. This new process design approach can help design RTM processes to be insensitive to preform permeability variations coming from materials and processing. Therefore, high quality composite components can be made consistently.

Gate Effectiveness in Controlling Resin Advance in Liquid Composite Molding Processes

In Liquid Composite Molding (LCM) processes, the flow pattern of the resin in the mold cavity during mold filling dictates the quality of the composite part. Disturbances such as uncertainty in the fiber preform permeability, variations in the resin viscosity, and racing of the resin along the mold walls may create unexpected and unanticipated flow patterns that could result in dry spots in the preform. Process control with sensor feedback can potentially provide the ability to make fully impregnated parts despite various disturbances by manipulating the flow front movement during resin impregnation through modification of the injection conditions at the gates. Studies have shown that under certain conditions, the flow front shape is not influenced by even large changes in the flow rate or the pressure at the injection gates. This study investigates gate effectiveness in modifying the flow front shape by analyzing the flow dynamics and conducting a parametric study in a simulation environment.

Determination of three-dimensional permeability of fiber preforms by the inverse parameter estimation technique

A method is presented for measuring the permeability of three-dimensional orthotropic fiber preforms using the inverse parameter estimation technique. The method is based on an optimization procedure that minimizes differences between computed and measured times of arrival of the flow front at various locations in the mold during resin transfer molding. The arrival time is computed by numerical simulation using the control volume finite element method. The validity of this method was examined experimentally. It was shown that the proposed method can be implemented in a straightforward manner and yields reliable results.

Determination of in‐plane permeability of fiber preforms by the gas flow method using pressure measurements

AbstractA method is described for measuring the in‐plane permeability of orthotropic fibrous preforms using gas flow. The method is based on an optimization process between computed and measured pressures at various locations in the mold during steady state gas flow through the enclosed preform. The computed pressure is obtained by the control volume finite element method (CVFEM). This method was demonstrated by using a specially designed mold with multiple ports for gas injection and pressure measurement and it was shown that it can be implemented easily and yields consistent and reliable results.

Numerical computation of the fiber preform permeability tensor by the homogenization method

AbstractThis paper uses the homogenization method to formulate governing equations and boundary conditions to predict the permeability of anisotropic fibrous porous media. An approach is also put forth for addressing dual porosity media with this method. It is applicable to a wide range of porous materials, which may be considered with approximately periodic structure. The permeability components are calculated by solving a boundary value problem in a periodic cell. The application of the method is demonstrated on a periodic cell modeling the geometry of woven fiber performs used in composites manufacturing. The resulting equations are solved using CFD package FIDAP. The influence of dual porosity and nesting of adjacent fabric layers on the macro permeability was investigated and the results were compared with experiments.

An Experimental Method to Continuously Measure Permeability of Fiber Preforms as a Function of Fiber Volume Fraction

An Experimental Method to Continuously Measure Permeability of Fiber Preforms as a Function of Fiber Volume Fraction

An Experimental Method to Continuously Measure Permeability of Fiber Preforms as a Function of Fiber Volume Fraction

Liquid composite molding processes have the potential to produce complex shaped parts utilizing tailored fiber reinforcements and inserts. Existing simulation tools have the capability to predict the flow front advancement as the resin impregnates the fiber preform to saturate it, thus minimizing cost intensive trial and error approaches to successful mold filling. An important input to such simulations is the permeability of fiber preforms, which describes its resistance to flow. Permeability is a function of the fiber architecture and fiber volume fraction. Traditional experimental methods to measure permeability allow one to obtain a single value at a prefixed fiber volume fraction; hence, if one wants to obtain permeability over a wide range of fiber volume fractions, a multitude of experiments are necessary. This paper introduces a novel approach to determine permeability over a wide range of fiber volume fractions in one continuous experiment using the same preform. Repeatable results for woven glass fabrics over a range of fiber volume fraction are presented.

An Experimental Method to Continuously Measure Permeability of Fiber Preforms as a Function of Fiber Volume Fraction

Liquid composite molding processes have the potential to produce complex shaped parts utilizing tailored fiber reinforcements and inserts. Existing simulation tools have the capability to predict the flow front advancement as the resin impregnates the fiber preform to saturate it, thus minimizing cost intensive trial and error approaches to successful mold filling. An important input to such simulations is the permeability of fiber preforms, which describes its resistance to flow. Permeability is a function of the fiber architecture and fiber volume fraction. Traditional experimental methods to measure permeability allow one to obtain a single value at a prefixed fiber volume fraction; hence, if one wants to obtain permeability over a wide range of fiber volume fractions, a multitude of experiments are necessary. This paper introduces a novel approach to determine permeability over a wide range of fiber volume fractions in one continuous experiment using the same preform. Repeatable results for woven glass fabrics over a range of fiber volume fraction are presented.

A gas flow method for determination of in-plane permeability of fiber preforms

Resin flow plays a crucial role in many composite manufacturing processes. The most important parameters used in modeling and designing mold filling are the permeability of the fibrous preform, which is a kind of flow conductance and a property of the reinforcement, and the viscosity of the resin. The extent of chemical reaction, or degree of cure, is also important and causes change of resin viscosity during mold filling. To determine the permeability of fiber preforms, many researchers have been using liquid flow analysis. In this study, a new scheme for determining permeability using gas flow is proposed. In conventional liquid flow methods, radial propagation of the polymer into a porous medium is measured and used to determine permeability, whereas in the gas flow method, the flow rate for several different preform geometries is measured and used. The effectiveness of the gas flow method was verified by comparing it with conventional methods.

Statistical characterization of fiber permeability for composite manufacturing

AbstractResin transfer molding (RTM) is considered one of the most promising composite fabrication techniques because of its relatively low equipment and tooling costs, short cycle times, and excellent design flexibility. Yet several technical issues impede wider application for this process. One of those issues is the measurement and characterization of fiber preform permeability, which plays a key role in process design and control. In this study, an automatic permeability measurement apparatus with automatic data acquisition and processing capabilities has been developed to improve measurement accuracy and reliability. Four major factors and their effects on permeability for a knitted and a woven fabric performs are investigated. Design of experiments, ANOVA analysis, and distribution characterization techniques are used to analyze the permeability‐influencing factors and statistical properties of fiber preform permeability. It is found that the fiber preform permeability values vary significantly for different mold shapes and fiber handling conditions. It is also revealed that permeability values for both fabrics follow normal distributions, even though their means and variances are different. These results can be used to better understand the behavior of the RTM process and to improve process design and control.