Preform Permeability Research Articles

Influence of handling effects on the permeability of preforms

AbstractHandling manipulations during the process chain of dry fibre preforms starting with the production of semi‐finished products, throughout the transport and storage until the build‐up, lead to mechanical stress on the textile structure. The knowledge of the relation between the stress in preforms caused by handling processes and their effect on the quality of the subsequent composite product could be of great importance for the development of automated solutions. The reduction of permissible requirements in the handling of textiles would lead to significantly simpler and thus more cost‐effective automation solutions. Using effects of defects methodology, impacts on preforms due to handling processes and the resulting permeability will be investigated. The analysis of the preforming process chain shows various visible effects as a result of handling and storage loads on textiles. One representative effect is a spherical shaped buckling within handled textile layers. Therefore the buckling will be used as a representative imperfection in this analysis. To create measurable imperfections textile samples will be impacted with a defined and reproducible load. Followed by the measurement of permeability during vacuum infusion of imperfect textile samples the relation between handling effects and the quality of preforms will be shown. In der Prozesskette zur Herstellung von Vorformen werden textile Halbzeuge durch zahlreiche Handhabungsprozesse belastet. Von zentraler Bedeutung ist hierbei die mechanische Beanspruchung der textilen Halbzeuge bei deren Handhabung und Lagerung während des Prozesses zur Herstellung der Vorform.

The Effect of Shear Deformation on Permeability of 2.5D Woven Preform

The accurate prediction of the permeability is the key to optimizing the molding process of fiber reinforced composites, thus to improve the composite quality, and reduce the material and labor costs in the manufacturing process. In this paper, the permeability of 2.5D woven preform with shear deformation was studied by experiments and numerical simulations. The permeabilities of the samples under various shear angles were measured by the radial flow method. An RVE (representative volume element) model based on the fabric microstructure and shear deformation is developed to predict the permeability of preform and the simulation results are compared with experiments value to verify the effectiveness of this model. Using this model, the effect of the fiber volume fraction on the permeability of the 2.5D woven preform was determined. Based on the structural characteristics, experimental and simulation results of the 2.5D woven preform, an empirical equation for predicting its permeability under shear deformation was formulated. The prediction accuracy of the equation was evaluated, and the equation was used to determine the change of permeability with shear deformation for the 2.5D woven preform.

Efficient numerical simulation method for three dimensional resin flow in laminated preform during liquid composite molding processes

We propose an efficient simulation method of three-dimensional (3D) resin flow in laminated preform composed of multiple layers with different permeabilities at each layer. Because of the small thickness of each layer, a huge number of nodes are needed for 3D flow simulation if solid elements are adopted and full 3D simulation takes extremely long even by parallel computing. Instead of 3D solid elements, we propose multi-layered shell elements for 3D flow simulation with a short computing time. We describe the numerical formulation of multi-layered shell element method to consider the through-thickness flow as well as the planar flow. The accuracy and efficiency are evaluated by new dimensionless parameters defined in terms of preform permeability ratio and of the ratio of shell element size to the distance between the adjacent layers. Some simulation results are presented to demonstrate the advantages of the multi-layered shell element method for 3D flow simulation.

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.

Flow correction control with electromagnetically induced preform resting process

Resin flow correction control with electromagnetic field source, a new variation of the vacuum-assisted resin transfer molding (VARTM) process called electromagnetically induced preform resting (EIPR) for dynamical resin flow controlling is introduced to manipulate the flow front and local permeability to prevent the formation of dry spots. This paper proposes an active and real-time flow control approach that is implemented during the composite laminate infusion. The EIPR process applies an electromagnetic field source to pinch (raise) and vibrate the upper flexible mold to rest the fiber preform and increase the local permeability. Vibration action delivers the fluid through the preform. The EIPR process includes a new and creative upper flexible vacuum bag with embedded elements to lift and create local vibrations via an automated gantry system. The control methodology is performed by tracking the flow front with a real-time correction. System capability is demonstrated with three configurations of preform of different preform permeabilities in each experiment. A low permeability preform is employed in these configurations to disturb the flow pattern and cause an artificial problem or pseudo problem during the filling process. The results indicate that this system fills the mold completely and reduces the filling time without any dry spots and therefore creates no waste material.

In-plane virtual permeability characterization of 3D woven fabrics using a hybrid experimental and numerical approach

A robust platform in the form of a hybrid experimental-numerical framework is proposed for reinforcement characterization with minimal material consumption and labor costs. In this hybrid approach, X-ray micro computed tomography (XCT) images of a 3D orthogonal fabric at different levels of compaction were acquired through a non-destructive experimental setup. The XCT images were reconstructed to generate 3D models from which computational unit cells were extracted for numerical solutions of boundary value problem using governing equations of fluid dynamics. The flow field data from the numerical solution were used to compute the virtual preform permeability, which was found to be in very good agreement with benchmark experimental results. Geometrical measurements taken from XCT images were used to quantify variabilities within the preform architecture. A modified permeability model has been validated for the numerical permeability predictions. The flow field analysis and pressure drop in the flow direction suggest that the z-binder yarn poses a major obstruction to in-plane flow. The sizes of the inter-yarn channels, as well as the shape of the z-binder yarn, in the orthogonal fabric play a vital role in determining the overall in-plane permeability values. The inter-yarn gaps were found to be larger in the top and bottom layers relative to the middle layers, which results in a dominant flow regime in these outermost layers. The inter-yarn areal gaps in weft direction were found to be greater than in the warp direction. The results presented here highlight the versatility of the proposed hybrid characterization method over traditional experimental techniques.

Experimental evaluation of temperature effect on the transverse permeability of a fibrous preform

Study of the transverse permeability of fibrous reinforcements became increasingly important when liquid molding processes started being more widely employed to obtain more complex-shaped or thicker parts. In this paper, a system specially built to measure transverse permeability of fabrics based on the correlation between pressure difference and flow rate was used to evaluate the effect of temperature on the measurements of the transverse permeability of an E-glass fiber fabric. Three fiber volume fractions (38-61%) and three temperatures were studied (20-63°C). The temperature showed no significant effect on transverse permeability of the tested fabric, even though it greatly alters the working fluid viscosity.

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.

Dynamic pressure control in VARTM: Rapid fabrication of laminates with high fiber volume fraction and improved dimensional uniformity

The compaction pressure on the fibrous preform is one of the most critical parameters in vacuum assisted resin transfer molding (VARTM), which significantly affects the preform permeability, mold filling time, and final thickness of the fabricated composite. In this study, the compaction pressure on the vacuum bag was controlled during and after the mold filling to achieve rapid impregnation and improve the fiber volume fraction of the laminate. It was shown that the dynamic pressure control (1) enabled the manipulation of the fabric permeability and faster distribution of the resin to decrease the mold filling time, (2) improved the dimensional uniformity of the laminate by reducing the thickness variation, and (3) increased the fiber volume fraction by further consolidating the preform and removing the excess resin. One of the most essential and prominent features of the process was shown to be the resin removal from the inlet by applying external pressure, which reduced the thickness variation in laminates from 15 to 1%. The mold filling time was reduced by 48% compared with conventional VARTM, while achieving a high fiber volume fraction up to 64% and a low void content of below 1%. POLYM. COMPOS., 40:2482–2494, 2019. © 2018 Society of Plastics Engineers

Impact of stitching on permeability and mechanical properties of preforms manufactured by dry fiber placement

Dry fiber placement (DFP) is a technology that enables outstandingly high mechanical performance of composite parts, as preform structures with load‐related fiber orientations can be created with minimum of fiber crimp and undulation. Yet, the poor impregnation behavior of DFP preforms during liquid composite molding processing methods remains a main challenge. Encouraging an out‐of‐plane impregnation, for example, by using a vacuum assisted or compression resin transfer molding process, reduces the flow length within the preform and is a first step towards reducing cycle times. However, further improvements are required in order to apply DFP preforming methods in serial production. In a previous work, the influence of various process parameters as well as post‐treatment steps on out‐of‐plane permeability have been investigated with the conclusion, that tufting/stitching led to the highest increase in out‐of‐plane permeability (more than two decades). For further understanding, this study deals with the influence of a stitching step on the out‐of‐plane permeability of DFP preforms as well as the mechanical properties of the finished laminate depending on a variety of stitching parameters such as thread tension during stitching, denier of the used thread, stitch distance, seam distance, seam displacement, and treated DFP layup. It is shown that the biggest impact on out‐of‐plane permeability results from the achieved stitching density (as a product of stitch distance and seam distance) and denier while thread tension and seam displacement show negligible effects. For the mechanical properties, it can be stated that the presence of stitching channels in thickness directions leads to a minor decrease of tensile properties while bending properties are more significantly reduced by about 30%. POLYM. COMPOS., 40:1631–1642, 2019. © 2018 Society of Plastics Engineers

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.

Transverse permeability of dry fiber preforms manufactured by automated fiber placement

This work presents a correlation between the transverse permeability of a preform and the process variability of the automated dry fiber placement manufacturing technique. In this study, an experimental and numerical analysis of the dry tape preform, with a focus on its through-thickness permeability, has been undertaken. Geometric models, containing flow channels of two different width dry tape carbon preforms, have been created in the TexGen modeller. A Computational Fluid Dynamics (CFD) simulation has been undertaken to obtain the predicted through-thickness permeability of the dry tape preform. A parametric study on the effect of different dry tape gap sizes on the permeability of the preform is presented. An in-situ compaction study, carried out in an X-CT machine, revealed that the gap sizes were irregular throughout the manufactured preforms. In addition, an experimental investigation of the through-thickness permeability, which is based on a saturated flow condition at a thickness corresponding to full vacuum pressure, is also presented. The permeability prediction based on the X-CT re-constructed geometric model has been validated using the experimental data. A further parametric study has revealed that the process variablity in automated dry fibre placement influences the through-thickness permeability by a factor of upto 5.

Geometry modeling and permeability prediction for textile preforms with nesting in laminates

In the multilayer textile preform which is usually used in the resin transfer molding (RTM) process, the penetration phenomenon existing between two neighboring layers is referred to as nesting, which has important influence on the permeability of the preforms. Predicting the permeability of the textile preforms with nesting accurately is the foundation of mold‐filling simulation and process optimization for RTM. In this article, based on the analyses of the penetration and local deformation behaviors between two neighboring layers, an assumption on constant area of yarn cross‐section is established, and a long axis compensation method is proposed to build the geometry model of the multilayer textile preform with nesting in laminates. The governing equations of the resin flow in textile preform are built, and a high resolution total variation diminishing difference scheme is constructed based on Adams–Bashforth scheme and Chorin projection method to numerically solve the above equations and predict the permeability of the preform. According to the comparison between simulation predictions and experimental measurements, the validities of the above geometry modeling method and permeability prediction algorithm are proved. POLYM. COMPOS., 39:4408–4415, 2018. © 2017 Society of Plastics Engineers

Complete simulation process chain for the manufacturing of braided composite parts

A complete simulation process chain has been used to predict the production and subsequent injection of over-braided textile preforms. A range of mandrel geometries and braiding configurations were used to illustrate how these factors affect the resin injection of the part. Braiding simulations were first completed, predicting the geometry of the braided textile throughout the mandrel. Following this, a range of multi-layered unit-cells were modelled, capturing the variations in geometry. These virtual stacks were produced with both no and maximum in-plane ply shift so as to capture the range of stacking configurations possible. Following a compaction simulation of these stacks, their in-plane permeability tensor was predicted and used to identify the permeability of the braided preform at different regions. This was used to predict the propagation of the resin flow front, highlighting the effects that the mandrel geometry, braiding process parameters and stacking method have on the resulting resin injection.

Axial and transverse permeability of unidirectional fiber-reinforced composites

Unidirectional fibrous structures are of interest in composite materials processing for applications in which fluid is injected into preforms that have arrays of parallel fibers and their axes are oriented either perpendicular to or in line with the direction of resin flow. The stumbling block in the liquid molding of composites is the resin infiltration of the fibrous preform. Permeability of the fibrous preform determines the extent of resin infiltration. Despite conduction of considerable researches on various aspects of permeability, measurement and interpretation of this key property is still in its infancy. In this work, permeability of unidirectional fibrous performs in axial and transverse directions which, respectively, correspond to flow along and perpendicular to the axis of fibers were determined. This was achieved by generation of virtual randomly unidirectional fibrous structures using an in-house developed Matlab®-based program. Permeability was computed by numerical solution of incompressible creeping Newtonian flow through the porous micro-structures. The results were compared with the published experimental and analytical models. The results were also compared with the experimental data and those of micro-computed tomography-based model of authors. Acceptable agreement between the results and previously published findings was observed. It was also established that randomly unidirectional structures exhibit high anisotropic behavior, possessing an approximate axial to transverse permeability ratio of 4:1.

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.

Characterization of 3D fabric permeability with skew terms

Flow simulations can predict resin flow behavior and void formation locations in a preform. One important parameter for simulation is the preform permeability. For thick parts with distribution media on the surface, resin flow is three dimensional and through the thickness permeability is required for simulation. If the fabric is a 3D preform or unbalanced, the through the thickness (Kzz) and two skew components (Kxz and Kyz) must be characterized. The skew terms influence the flow behavior and hence the void formation. In this study, we present a measurement station that provides all six independent components of the permeability tensor from one experiment. The methodology uses the location data of the flow front with time and then couples it to an optimization algorithm and our flow simulation tool, LIMS (Liquid Injection Molding Simulation). The process is automated and experimental results are superimposed on the simulation results to confirm fidelity of the values determined.

A pore-scale, two-phase numerical model for describing the infiltration behaviour of SiCp/Al composites

We propose a method of modelling the infiltration process of a liquid aluminium matrix into a porous silicon carbide particle (SiCp) preform based on porous medium geometry obtained from scanning electron microscopy (SEM) images. Our simulation illustrates that via a pore-scale, two-phase flow model, an improved description of the flow structure in the porous medium can be obtained compared with the description offered by traditional methods, such as those based on the capillarity and wetting conditions of porous media and Darcy’s law, the Brinkman equations or Richards’ equation. Simultaneously, we validate the proposed model by comparing simulated and experimental data. In particular, the simulation results successfully capture several important features of a pore-scale, two-phase flow: the evolution of the flow front during the infiltration process, the most probable locations of potential defects, the preferential flow paths, the pressure distribution, the velocity field and the permeability of the porous SiCp preform.

Compaction behavior and permeability property tests of preforms in vacuum-assisted resin transfer molding using a combined device

A device was designed to study compaction behavior and permeability of preforms in vacuum-assisted resin transfer molding, consisting of a pressure control module, a pressure test module, a thickness test module, and an experimental plate. It can also be used to test the effects of resin flow and the post-filling process on the thickness variation of preforms. Four tests were conducted with the device: (1) compaction tests on preforms of 1, 15, 20, and 25 layers in dry and wet states; (2) in-plane permeability tests on preforms of 1, 5, 10, and 20 layers; (3) transverse permeability tests for 20 layers preforms; (4) determination of the effects of resin flow and post-filling time on preform thickness variation with two liquids. The results show the in-plane permeability decreases as the number of layers increases, and the value for 20 layers is about 62% of one layer. Preform thickness is affected by the liquid viscosity and post-filling time, and the averaged thicknesses of edible oil and Cycom890 resin were 2.12 and 2.03mm, respectively, at post-filling times of 30 and 40min, which represent a decrease by 1.9% and 2.9%, respectively, compared with the cases in which there was no post-filling process.

Identification of the effect of typical curvatures encountered in RTM parts on localized permeability of fibrous preforms

Filling is a critical stage in resin transfer molding (RTM) manufacturing; it is associated to the formation of impregnation imperfections which can lead to defects. Simulations are typically employed to predict the flow behavior, which however need the preform permeability as an input. Significant work has been done in the past in in-plane permeability identification. This study focuses on the determination of the permeability of unidirectional curvatures typically encountered in RTM parts. A model for analytical calculation and a numerical optimization approach for inverse determination have been developed and applied in an omega-shaped RTM part where material-embedded flow sensors were employed for the essential flow measurements. The differences found between the experimentally derived permeability of the curvature and theoretical predictions were discussed critically and associated to flow time disagreements between simulations and reality.