Through-thickness Permeability Research Articles

Three-Dimensional Permeability of Thick-Section Glass Fabric Reinforced Polymer Composite by Vacuum-Assisted Resin Infusion Molding

Abstract Determination of permeability of thick-section glass fabric preforms with fabric layers of different architectures is critical for manufacturing large, thick composite structures with complex geometry, such as wind turbine blades. The thick-section reinforcement permeability is inherently three-dimensional and needs to be obtained for accurate composite processing modeling and analysis. Numerical simulation of the liquid stage of vacuum-assisted resin infusion molding (VARIM) is important to advance the composite manufacturing process and reduce processing-induced defects. In this research, the 3D permeability of thick-section E-glass fabric reinforcement preforms is determined, and the results are validated by a comparison between flow front progressions from experiments and from numerical simulations using ansys fluent software. The orientation of the principal permeability axes were unknown prior to experiments. The approach used in this research differs from those in literature in that the through-thickness permeability is determined as a function of flow front positions along the principal axes and the in-plane permeabilities and is not dependent on the inlet radius. The approach was tested on reinforcements with fabric architectures which vary through-the-thickness direction, such as those in a spar cap of a wind turbine blade. The computational simulations of the flow-front progression through-the-thickness were consistent with experimental observations.

Concurrent characterization of through-thickness permeability and compaction of fiber reinforcements

Flow-induced fiber-bed compaction has considerable effects on through-thickness permeability due to resulting changes in fiber-volume-fractions. This study introduces a novel method for the simultaneous characterization of compaction and unsaturated out-of-plane permeability of engineering textiles in one single experiment. The measurement technique relies on visually tracking flow-front positions and compaction during through-thickness impregnation using a camera and a rigid tool equipped with a pressure sensor and a transparent window. This strategy allows flow-induced fiber-bed compaction to be considered in unsaturated permeability measurements, providing permeability at different fiber-volume-fractions, whilst also providing dry and wet compaction curves. This new measurement tool also readily verifies any race-tracking that may occur during impregnation, which drastically influences flow behavior. The method is successfully validated through testing of glass fiber NCF and woven fabrics, where permeability results are compared to analytical solutions, while compaction results are compared to those obtained from a universal testing machine.

Process robustness and defect formation mechanisms in unidirectional semipreg

Out-of-autoclave/vacuum-bag-only (OoA/VBO) composite processing has emerged as an alternative to autoclave cure, addressing economic, environmental, and production flexibility limitations associated with autoclave production. VBO processing can produce defect-free components under ideal processing conditions; however, adverse process conditions (e.g. poor vacuum) commonly encountered in manufacturing environments result in unacceptably high scrap rates, preventing more widespread adoption of such techniques. This work explores how modifications to prepreg format can increase process robustness. A unidirectional (UD) prepreg was produced with a customized, discontinuous resin distribution, henceforth referred to as semipreg. The semipreg exhibited through-thickness permeability orders of magnitude greater than conventional hot-melt VBO prepregs. The semipreg also was less sensitive to variations in process conditions than conventional VBO prepreg. In situ process monitoring allowed observation and identification of two defect formation mechanisms arising during cure of the custom prepreg. Resin feature topography played a critical role in these mechanisms, indicating its importance to the design of next generation VBO semipregs.

Development of innovative flow visualisation methods to investigate the stages of Wet Compression Moulding (WCM) process

The WCM is a novel production technique for fibre-reinforced polymer components. This technique is increasingly adopted in industry, typically in the automotive sector for medium to large-scale production. This process reduces the cost per part significantly by reducing the number of tools and processing steps, relative to Resin Transfer Moulding. To date, the processing variables, their relations and effect on the composite quality have been established through the knowledge gained by technologists. The drive of this research is to develop a fundamental understanding of the impact of process parameters (key process timings and cavity thicknesses, mould closing speed, edge clamping frame dimensions) and material properties (fibre reinforcement compaction response, permeability, and wettability together with test fluid viscosity and surface tension) on textile wetting, impregnation, and resulting composite part quality. In this study, a 480g/m2 non-crimp unidirectional glass fibre reinforcement has been chosen and characterised for compaction response, in-plane, and through-thickness permeability. For ease of handling, motor oil (DTE heavy VG100) has been characterised and used as a test fluid. Two experimental flow visualisation setups have been developed at the CACM to investigate the influence of process and material parameters on initial resin application and textile wetting, and the subsequent compression flow stage.

Design and application of discontinuous resin distribution patterns for semi-pregs

Vacuum-bag-only (VBO) prepregs fabricated with discontinuous resin (semi-pregs) on a unidirectional fiber bed reportedly exhibit high through-thickness permeability and yield high-quality laminates, even under adverse process conditions, such as poor vacuum or long out-times. In this work, semi-pregs were fabricated using fiber beds of various weaves, fiber types, and areal weights (200–670 GSM). Flat and curved laminates were produced and characterized, confirming that porosity-free parts can be manufactured from a range of constituent materials using VBO semi-pregs. Contoured laminates were produced with negligible porosity, although a slight increase in bulk factor of prepreg plies was observed (Δ∼0.1). In addition, design considerations and limitations for the fabrication of semi-pregs were presented. The findings demonstrate that polymer film dewetting can be used effectively to produce semi-pregs that yield porosity-free laminates via VBO processing, imparting robustness to out-of-autoclave cure of prepreg laminates.

Effects of debulk temperature on air evacuation during vacuum bag-only prepreg processing

Air removal prior to cure is critical for limiting porosity during vacuum bag-only (VBO) processing of prepregs. In this study, the effects of pre-cure dwell temperature (debulk) on air evacuation were investigated for both plain weave (PW) and unidirectional (UD) prepregs. In situ observations revealed that increasing dwell temperature promoted inter-ply air evacuation (by as much as 2x for UD prepregs). Through-thickness gas permeability increased with increasing temperature and decreased with increasing number of plies. The decrease in in-plane permeability during heated debulk was attributed to increased tow impregnation. The findings provide guidelines for cure cycle optimization. Heated debulk enhanced air evacuation in PW laminates, particularly as laminate width/thickness ratios exceed a threshold value. However, warm debulks were less effective, particularly for thicker laminates (>8 plies).

Effects of resin distribution patterns on through-thickness air removal in vacuum-bag-only prepregs

Prepregs with discontinuous resin patterns facilitate air removal and impart robustness to vacuum-bag-only processing of composites. However, optimal pattern characteristics have not yet been identified. A geometric model was developed to guide the fabrication of prepregs with various discontinuous patterns and laminates with different orientations and ply counts. The model was used to evaluate metrics related to gas transport: projected surface area exposed, sealed interfaces, and tortuosity. Statistical analysis revealed that single layer surface area exposed and ply count had the greatest effect on projected surface area exposed; orientation had the greatest effect on sealed interfaces and tortuosity. From these insights, prototype prepregs were fabricated to measure through-thickness permeability. Prepregs with a large percentage of sealed interfaces and high tortuosity exhibited lower permeability. The study demonstrated a methodology to differentiate/screen patterns for gas transport efficiency. The model can guide prepreg design and support robust production of composites via out-of-autoclave manufacturing.

Experimental study on the influence of cyclic compaction on the fiber-bed permeability, quasi-static and dynamic compaction responses

Fiber-reinforcements’ permeability and compressibility are crucial parameters in Liquid Composite Molding Processes, as they represent the main textile properties that influence the behavior of the injected resin. Before designing LCM molds and optimizing manufacturing processes, permeability and compaction characterizations are performed; however, the results obtained often show considerable deviations. One reason for this behavior is the variable compaction histories of fiber-beds, which can be altered during storage or handling. This work presents a novel method to normalize compaction histories through cyclic compaction, thus increasing the reproducibility of compaction and through-thickness permeability responses. It also shows how fiber-bed’s viscoelasticity can be reduced with minor influence on permeability. It is greatly beneficial in developing processing routes for vacuum assisted processes, and for future compaction or permeability benchmarks, in defining optimal characterization methods. It provides also solid basis for the comparison and validation of compaction and through-thickness permeability models with reduced measurement errors.

Non-destructive evaluation of through-thickness permeability in 3D woven fabrics for composite fan blade applications

In this study, we propose a non-destructive modeling framework, with an ability to obtain computational models for numerical prediction of resin flow through 3D fabrics (commonly used in engine fan blade applications) at multiple fiber volume fractions using a single experiment. The work involves extracting real-time geometrical features of two types of 3D reinforcements during an in-situ compaction experiment using micro CT. The 3D image slices obtained at four different thicknesses were converted to computational models. The fiber tows and inter-tow gaps were modeled as distinct phases. The gap analysis and geometrical measurements revealed significant differences in the preform microstructure investigated at different thicknesses. This was followed by a detailed flow analysis using two different commercial software packages. The exported mesh was analyzed in GeoDict® and Ansys Fluent® for through-thickness permeability computations. The governing fluid dynamics equations were solved to obtain the flow field within the inter-tow gaps for representative volume elements. The predicted through-thickness permeability values were in very good agreement with the experimental data.

Polymer film dewetting for fabrication of out-of-autoclave prepreg with high through-thickness permeability

Polymer film dewetting on a substrate (independent of fiber bed architecture) was explored, developed, and demonstrated as a method to produce out-of-autoclave, vacuum bag-only (OoA/VBO) prepregs with high transverse permeability and process robustness. The dimensions of the surface openings created by dewetting were measured,and thepercent surface area exposed was calculated. Prepregs were fabricated with continuous and dewetted (discontinuous) films to producetrial laminates. The laminates were cured under both standard and sub-optimal conditions, and were characterized before, during, and after cure.Laminates fabricated with dewetted resin consistentlyachieved near-zero porosity. In contrast,laminates with continuous filmdisplayed high levels of porosity,particularly during sub-optimal cure. The findings demonstrate that dewetting can be used effectively to produce OoA prepregs with high through-thickness permeability, whichcan yieldporosity-free laminates via VBO processing. Furthermore, these results elucidate aspects of resin dewetting that are critical in thecreation ofrobust OoA prepregs.

Effective permeability averaging scheme to address in-plane anisotropy effects in multi-layered preforms

In Liquid Composite Molding (LCM) processes, fabric layers are stacked in a mold which may be a few meters long and wide to build up a thickness of not more than a few millimeters. Resin is introduced to fill all the empty spaces between the fibers. As the in plane dimensions are a few orders of magnitude larger than through the thickness, flow of resin through the preform can be modeled using the two-dimensional Darcy’s law, neglecting the through-thickness velocity and assigning the preform an arithmetic averaged permeability from the layers. However, there are situations in which the through-thickness flow is significant where this assumption is no longer valid or justified. To address such cases, a modified averaging scheme was proposed by Calado and Advani (1996) to account for the transverse flow between adjacent layers of a preform and consequently derive an homogenized one-dimensional value of effective permeability. In the current work, such a model is extended to account for the effect of anisotropic off-axis layers in the stack. The result is a generalized scheme for effective permeability averaging layers of heterogeneous preforms, capturing both through-thickness and in-plane effects into a one-dimensional permeability value. This methodology was validated and a parametric study was conducted with different combinations of in-plane and through-thickness permeability values to identify the influence of preform in-plane dimensions and thickness and to define a criteria that relates the material and geometric parameters to the transverse flow.

Effect of Nesting in Laminates on the Through-Thickness Permeability of Woven Fabrics

The layer nesting phenomenon of multilayer fabric has a great influence on the through-thickness permeability, which is a key parameter for the simulation of the through-thickness LCM (Liquid Composite Moldling) processes. In this paper, based on the analyses of the formation reason and characterization parameters of layer nesting, the geometry models of fabric unit-cells with nesting are established. The through-thickness flow in the unit-cell is analyzed to built the governing equations of the resin flow. The inter-yarn and intra-yarn regions of the unit-cell model are discretized uniformly, then the governing equations of the through-thickness flow are numerically solved based on Adams-Bashforth scheme and Chorin projection method, so the through-thickness flow parameters is obtained and the through-thickness permeability of the fabric with nesting can be predicted. The verification of the above method is implemented by comparisons with the available experimental results. A series of simulation experiments are carried out to investigate the nesting behaviors under different layer shifts, and the effects of nesting on the total thickness and through-thickness permeability of woven fabric are researched in detail.

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.

Effect of prepreg format on defect control in out-of-autoclave processing

Prepreg format plays a key role in part quality for composites produced using vacuum bag only (VBO) techniques. To date, however, VBO prepregs have been produced by modifying existing autoclave formats. In this work, we introduce USCpreg, a prepreg format designed specifically for out-of-autoclave cure, featuring through-thickness permeability. We describe the fabrication and analysis of laminates processed with USCpreg, as well as laminates fabricated from traditional VBO prepreg formats. The through-thickness pathways for air transport in USCpreg result in near-zero internal porosity and defect-free surfaces in parts cured under VBO conditions, even under challenging processing conditions. Results highlight the fact that surface and internal porosity depend on prepreg format, and that through-thickness permeability is critical to achieving high quality parts in non-ideal manufacturing scenarios.

Through-thickness permeability of woven fabric under increasing air pressure: Theoretical framework and simulation

Many technical applications of woven fabric are subject to increasing high pressure from air transport through the fabric. The through-thickness permeability (TP) of woven materials exhibits a dynamic response to increased air pressure. This paper presents an analytical model for predicting the steady TP of woven fabric. The approach was based on Darcy’s law and the Poiseuille equation, using the flow boundary of an idealized plain-weave unit cell. The unit cell model consists of a gradual converging-diverging (GCD) duct with a rectangular cross-section. Further, the dynamic TP of the GCD duct was established analytically as a function of increasing pressure, which correlates to the separation of air flow from the GCD duct wall. Air flow separation from the duct wall led to a quadratic relationship between the increasing pressure and air flow velocities. This dynamic TP and air flow nonlinearity were simulated numerically in the computational fluid dynamics solver CFX. Five GCD ducts under increasing air pressure were analyzed numerically and analytically. The comparison showed good agreement between the proposed analytical model and the CFD simulation, with a maximum error up to 12%. A sensitivity study showed that an increase in porosity or a decrease in the thickness of weave materials could result in a larger dynamic TP value.

Characterization of 3D woven reinforcements for liquid composite molding processes

Compaction and permeability characterization of fibrous reinforcements is fundamental to liquid composite molding (LCM) processes, given that these fabric properties determine the part thickness, fiber volume content and mold filling time and patterns. In this study, the permeability of three different 3D woven carbon fiber reinforcements (orthogonal, angle interlock and layer-to-layer) was studied, each having a different weave style and z-binder pattern. For all reinforcements, single-cycle and multiple cycle compaction experiments were conducted on dry and saturated samples. The orthogonal preforms were more difficult to compact to the target fiber volume fraction of 0.65, with peak stresses reaching up to 2.3 MPa. Cyclic compaction tests were conducted to highlight the importance of permanent deformation in the reinforcements, when higher fiber volume content is desired via manufacturing using an LCM process. Unsaturated in-plane radial and saturated through-thickness permeability data were obtained at several fiber volume fractions. The orthogonal and layer-to-layer fabrics exhibited greater levels of anisotropy in the plane of flow. The rate of change of in-plane permeability with increasing fiber volume content was lower compared to through-thickness values. The effect of cyclic compaction on permeability was greater in the orthogonal and angle interlock reinforcements. Micrographs of infused samples showed significant permanent deformation of the z-binder yarns, an effect that reduced permeability at high fiber volume contents.

In-plane and through-thickness permeability models for three-dimensional Interlock fabrics

An analytical model has been created to estimate the in-plane and through-thickness permeability of three-dimensional Interlock fabrics. The model is based on the calculation of the pressure drop in the mesoscopic pores, i.e. in the inter-tow spaces of the fabric. Firstly, the dimensions and distribution of the mesopores are determined from the weaving parameters. A mesoscopic permeability model can then be inferred from the fabric structural data. The contribution of the microscopic flow inside the tows is also taken into account by introducing the concept of dual scale porosity. This additional feature is shown to improve the permeability model, which is validated by comparison with the experimental values of principal permeability measured for five different three-dimensional Interlock fabrics.

Through-thickness air permeability of woven fabric under low pressure compression

Through-thickness permeability (TTP) is one primary property of technical textiles used in air-related applications, such as filtration and protection. The TTP depends on the textile geometrical factors and usually varies according to the test conditions. In this article, the effect of low air pressure compression (LPC) on TTP of woven fabric was investigated. Nine woven fabrics were measured for the relationships of LPC and thickness, LPC and fabric in-plane dimensions, air pressure drop (APD) and air velocity, as well as LPC and fabric TTP. A dramatic decrease of woven fabric thickness was found below the APD value of 200 Pa and less decreased thickness was observed with a continued increase of APD. The variation of fabric in-planar dimensions was found to be negligible during LPC. The plot relationship of the APD and measured air velocity was presented in linearity for most fabric samples. The fabric TTP showed a linear proportion to the fabric thickness, indicating the fabric to be more permeable with the increase of thickness. A sensitivity study showed an evident difference between using fabric constant and decreased (LPC) thickness in calculating TTP, disclosing the importance of compression in fabric TTP evaluation.

Through-thickness permeability study of orthogonal and angle-interlock woven fabrics

Three-dimensional (3D) woven textiles, including orthogonal and angle-interlock woven fabrics, exhibit high inter-laminar strength in addition to good in-plane mechanical properties and are particularly suitable for lightweight structural applications. Resin transfer moulding (RTM) is a cost-effective manufacturing process for composites with 3D-woven reinforcement. With increasing preform thickness, the influence of through-thickness permeability on RTM processing of composites becomes increasingly significant. This study proposes an analytical model for prediction of the through-thickness permeability, based on Poiseuille’s law for hydraulic ducts approximating realistic flow channel geometries in woven fabrics. The model is applied to four 3D-woven fabrics and three 2D-woven fabrics. The geometrical parameters of the fabrics were characterized by employing optical microscopy. For validation, the through-thickness permeability was determined experimentally. The equivalent permeability of inter-yarn gaps was found to account for approximately 90 % of the through-thickness permeability for the analysed fabrics. The analytical predictions agree well with the experimental data of the seven fabrics.