The effect of fabric and fiber tow shear on dual scale flow and fiber bundle saturation during liquid molding of textile composites

In Liquid Composite Molding (LCM) processes, a fibrous reinforcement preform is placed or draped over a mold surface, the mold is closed and a resin is either injected under pressure or infused under vacuum to cover all the spaces in between the fibers of the preform to create a composite part. LCM is used in a variety of manufacturing applications, from the aerospace to the medical industries. In this manufacturing process, the properties of the fibrous reinforcement inside the closed mold is of great concern. Preform structure, volume fraction, and permeability all influence the processing characteristics and final part integrity. When preform fabrics are draped over a mold surface, the geometry and characteristics of both the bulk fabric and fiber tow bundles change as the fabric shears to conform to the mold curvature. Numerical simulations can predict resin flow in dual scale fabrics in which one can separately track the filling of the fiber tows in addition to flow of resin within the bulk fabric. The effect of the deformation of the bulk fabric due to draping over the tool surface has been previously addressed by accounting for the change in fiber volume fraction and permeability during the filling of a mold. In this work, we investigate the effect of shearing of the fiber tows in addition to bulk deformation during the dual scale filling. We model the influence of change in fiber tow characteristics due to draping and deformation on mold filling and compare it with the results when the fiber tow deformation effect is ignored. Model experiments are designed and conducted with a dual scale fabric to characterize the change in permeability of fiber tow with deformation angle. Simulations which account for dual scale shear demonstrate that the tow saturation rate is affected, requiring longer fill times, or higher pressures to completely saturate fiber tows in areas of a mold with high local shear. This should prove useful in design of components for applications in which it is imperative to ensure that there are no unfilled fiber tows in the final fabricated component.     

Compaction modelling of textile preforms for composite structures

This paper deals with modelling the compaction behaviour of dry fibre assemblies using an energy minimisation scheme. Compaction behaviour of textile preforms can significantly influence the resin permeability, fibre volume fraction and the geometry of individual tows. Tow geometry will in turn affect the elastic properties of the laminate, mode of damage initiation and progression. In this work, constitutive properties of yarns in bending and transverse compression were measured using Kawabata Evaluation System, and used for computing the potential energy stored in individual yarn segments within a preform. The compaction model has been experimentally verified for single and multi-layer 2D fabrics and 3D fabrics.     

Permeability of natural fiber reinforcement for liquid composite molding processes

We investigate the influence of liquid type on the saturated permeability of natural fabrics in liquid composite molding processes. The permeability of flax woven fabric was characterized with two different liquids which have different viscosity, wettability, and sorption characteristics with flax fiber. From the experimental data, it was observed that the saturated permeability values were different for the liquid type. The fiber swell during the mold filling process and the corresponding change of fabric microstructure were assumed to be the main reason for this dependency of saturated permeability on the liquid type. The fiber swell due to the liquid sorption was characterized as a function of time, and the corresponding change of fiber diameter was investigated. The effective fiber volume fraction of wet natural fabric was defined in terms of fiber swelling ratio. The predictions by the classical Kozeny–Carman model and by the modified Kozeny–Carman model with two model constants were compared with the experimental data. It was shown that the modified Kozeny–Carman equation considering fiber swell could predict very well the saturated permeability of natural fabrics regardless of liquid type.     

In-plane permeability of sheared fabrics

Shear is the main mode of deformation in the draping of fabrics over complex mould geometries in composites manufacturing. Hence, the measurement and prediction of the in-plane permeability of sheared fabrics is a crucial task for the design of resin transfer moulding and other composites processing techniques of complex shaped articles. A mathematical model has been developed and applied to predict the in-plane permeability in the two principal directions and the angle of the flow ellipse for sheared assemblies of bi-directional woven fabrics that are in-plane isotropic to flow when unsheared. Modelling was accompanied by in-plane permeability measurements for unsheared and sheared woven fabric assemblies, and a comparison of this experimental permeability data with the proposed model proved encouraging. A study into the change of the areal density of different woven fabrics with shear angle has also been included.     

Characterization of preform permeability in the presence of race tracking

For realistic simulation of resin flow in a stationary fibrous porous preform during Liquid Composite Molding (LCM) processes, it is necessary to input accurate material data. Of great importance in simulating the filling stage of the LCM process is the preform permeability; a measure of the resistance the preform poses to the flowing fluid. One method to measure permeability values is by conducting one-dimensional flow experiments, and matching the flow behavior to known analytical models. The difficulty is the edge effects such as race tracking disrupt the flow and violate the one-dimensional flow assumption. The new approach outlined in this paper offers a methodology to obtain accurate bulk permeability values despite any race tracking that may be present along the edges of the mold containing isotropic fabrics. Further, a method of approximate equivalent isotropic scaling is explained to extend the use of this method to determine permeability of anisotropic materials with race tracking present. Both approaches are validated with computer simulations, and then utilized in laboratory experimentation. The values calculated from this approach compare well with permeability values obtained from one-dimensional permeability experiments without the presence of race tracking.     

Experimental Determination of the Transversal and Longitudinal Fibre Bundle Permeability

In the resin transfer moulding process, fabrics are very often used as reinforcement. These fabrics consist of fibre bundles. In this context, both the permeability of the macroscopic fabric and the permeability of the fibre bundle are the key parameters to accurately predict the impregnation of the fabrics, the impregnation time and resulting void content. The fibre bundle permeability can be either predicted theoretically or experimentally. Whereas the theoretical determination lacks on accuracy as the realistic packing of the fibres in the bundles is not integrated in the models. In this work, we present an experimental setup to measure the longitudinal and transversal permeability of fibre bundles. The results are compared to model predictions.     

Influence of Tow Architecture on Compaction and Nesting in Textile Preforms

Transverse compression response of tows during processes such as vacuum infusion or autoclave curing has significant influence on resin permeability in fabrics as well as the laminate thickness, fibre volume fraction and tow orientations in the finished composite. This paper reports macro –scale deformations in dry fibre assemblies due to transverse compaction. In this study, influence of weave geometry and the presence of interlacements or stitches on the ply-level compaction as well as nesting have been investigated. 2D woven fabrics with a variety of interlacement patterns - plain, twill and sateen- as well as stitched Non-crimp (NCF) fabrics have been investigated for macro-level deformations. Compression response of single layer and multilayer stacks has been studied as a function of external pressure in order to establish nesting behaviour. It appears that the degree of individual ply compaction and degree of nesting between the plies are influenced by tow architectures. Inter-tow spacing and stitching thread thickness appears to influence the degree of nesting in non-crimp fabrics.     

Assessment of semi-impregnated fabrics in honeycomb sandwich structures

Semi-impregnated fabrics, or semipregs, are fabrics alternating dry and resin impregnated areas along the fibre bed surface. Due to their increased initial through thickness permeability to gas flow, these could constitute an alternative to prepreg in the skins of vacuum-bagged honeycomb sandwich structures, reducing the pressure in the honeycomb. The semipreg through thickness air permeability before cure is measured and is approximately three orders of magnitude higher than that of a unidirectional prepreg impregnated with the same resin. A model is proposed for the air permeability change during cure, as dry areas get infiltrated. Due to resin pouring inside the honeycomb cells, this type of semipreg is viable as a skin only if combined with a material that has low permeability to resin, e.g., a prepreg.     

A semi-analytical model for simulating fluid transport in multi-layered fibrous sheets made up of solid and porous fibers

Direct simulation of fluid transport in fibrous media consisting of swelling (i.e., fluid-absorbing) and non-swelling (i.e., solid) fibers is a challenge. In this work, we have developed a semi-analytical modeling approach that can be used to predict the fluid absorption and release characteristics of multi-layered composite fabrics made up of swelling and non-swelling fibrous sheets. The simulations presented here are based on a numerical solution of Richards’ equation. Two different fibrous sheets composed of non-swelling (PET) and swelling (Rayon) fibers with different Solid Volume Fractions (SVFs) and thicknesses were arbitrarily chosen in this study for demonstration purposes. The sheets’ capillary pressure and relative permeability are obtained via a combination of numerical simulations and experiment. In particular, the capillary pressure expression for non-swelling media is obtained from the analytical expressions that we previously developed via 3-D microscale simulations, while the capillary pressure for swelling media is obtained via height rise experiments. The relative permeability expressions for both swelling and non-swelling media are obtained from the analytical expressions previously developed via 3-D microscale simulations, which are also in agreement with experimental correlations from the literature. On the macroscale, simulation results are reported for fluid transport in bi-layered composite fabrics, and comparison is made between the performances of these fabrics in terms of the order in which the layers are stacked on top of one another. A higher rate of absorption was observed when the layer in contact with the fluid is that comprised of swelling fibers. A similar study was conducted for motion-induced fluid release from the composite fabrics when partially-saturated with a fluid. It was shown that less fluid release is expected when the swelling sheet is placed in contact with the surface.     

Woven fabric permeability: From textile deformation to fluid flow mesoscale simulations

A two-step methodology is proposed in order to estimate from numerical simulations the permeability of deformed woven fabrics. Firstly, the shear deformation of a glass plain weave until the shear locking is studied from a mesoscale analysis achieved with a representative volume element (RVE) of the periodic plain weave. Simulations have been carried out within the scope of large transformations, accounting for yarn–yarn contacts, and assuming that yarns behave as hypoelastic materials with transverse isotropy. From the simulated deformed solid RVE, a complementary periodic fluid RVE is then built and the slow flow of an incompressible Newtonian fluid within it is investigated. This allows to compute, in a second step, the permeability of the deformed plain weave. The role of the shear deformation on the permeability of multi-layers or single layer preforms is discussed.     

In-plane permeability characterization of the vacuum infusion processes with fiber relaxation

In vacuum infusion processes fiber preforms are placed onto the single molding surface and enveloped with a non-rigid polymer bag which is sealed to the molding surface. The flexible bagging film does deform during the resin infusion process thus changing the compaction of the fabric. However, one can also relax the preform by drawing a partial vacuum in a rigid chamber placed on top of the flexible bag which will increase the permeability of the fabric under the chamber. A numerical model is presented to characterize the change in permeability and describe the mold filling for such processes in which the fabrics undergo controlled relaxation by external stimuli. The predictions from the simplified model agreed reasonably well with the experiments. This characterization and resin flow front prediction with time method should prove useful in processes such as Vacuum Induced Preform Relaxation (VIPR) process which can be used to actively manipulate flow in a vacuum infusion process.     

Improving the resin transfer moulding process for fabric-reinforced composites by modification of the fabric architecture

The use of resin transfer moulding (RTM) as an economic and efficient means of producing high-performance fibre-reinforced composites is critically limited by the permeability of the fabrics employed. Commercial fabrics are available where the architecture of the reinforcement is designed to cluster the fibres giving higher permeabilities than conventional fabrics. This has been shown to improve processing times, but there is evidence that such clustering is detrimental to the mechanical performance of the resulting composite material.

The objective of this work was to relate variations in permeability, and in the laminate mechanical properties, to differences in microstructure. A series of experimental carbon fibre fabrics woven to incorporate a novel flow enhancement concept (use of 3K tows in a 6K fabric) were used to manufacture plates by RTM in a transparent mould. The progress of the resin front was recorded to computer disc during injection, thus allowing the permeabilities of the fabrics to be calculated.

The manufactured plates were subsequently sectioned for mechanical testing (moduli and strengths in tension and compression) and automated image analysis. Relationships were sought between measured permeabilities, mechanical properties and microstructures using a Quantimet 570 automatic image analyser to determine fractal dimensions from polished sections. It has been shown that variations in the microstructures can be related to the permeability and mechanical property values obtained. Further the deterioration of mechanical properties for the novel fabrics with reduced fibre volume fractions is less than has been reported for fabrics with clustered flow-enhancing tows at constant fibre volume fraction.     

Effect of Frictions on the Ballistic Performance of a 3D Warp Interlock Fabric: Numerical Analysis

3D interlock woven fabrics are promising materials to replace the 2D structures in the field of ballistic protection. The structural complexity of this material caused many difficulties in numerical modeling. This paper presents a new tool that permits to generate a geometry model of any woven fabric, then, mesh this model in shell or solid elements, and apply the mechanical properties of yarns to them. The tool shows many advantages over existing software. It is very handy in use with an organization of the functions in menu and using a graphic interface. It can describe correctly the geometry of all textile woven fabrics. With this tool, the orientation of the local axes of finite elements following the yarn direction facilitates defining the yarn mechanical properties in a numerical model. This tool can be largely applied because it is compatible with popular finite element codes such as Abaqus, Ansys, Radioss etc. Thanks to this tool, a finite element model was carried out to describe a ballistic impact on a 3D warp interlock Kevlar KM2? fabric. This work focuses on studying the effect of friction onto the ballistic impact behavior of this textile interlock structure. Results showed that the friction among yarns affects considerably on the impact behavior of this fabric. The effect of the friction between projectile and yarn is less important. The friction plays an important role in keeping the fabric structural stability during the impact event. This phenomenon explained why the projectile is easier to penetrate this 3D warp interlock fabric in the no-friction case. This result also indicates that the ballistic performance of the interlock woven fabrics can be improved by using fibers with great friction coefficients.     

织物形式对苎麻纤维渗透率及其复合材料力学性能的影响

采用树脂传递模塑工艺(RTM)研究了三种典型苎麻纤维织物结构(平纹、 斜纹和缎纹)对树脂流动性的影响, 并研究了三种苎麻纤维织物结构对其增强酚醛树脂复合材料的拉伸性能和层间剪切性能的影响。结果表明, 苎麻纤维织物树脂渗透率主要受纤维屈曲和流道面积的影响。斜纹和缎纹苎麻织物的纤维屈曲较小且流道面积较大, 其织物的树脂渗透率较大, 同时, 较小的纤维屈曲使其增强的复合材料拉伸性能也较优。然而, 不同织物形式对苎麻纤维织物/树脂复合材料的层间性能影响不大。     

Modeling formability of multiaxial warp knitted fabrics on a hemisphere

The aim of this paper is to describe a model for the prediction of the formability of a multiaxial warp knitted (MWK) fabric to a 3D surface. For this purpose, we first characterized in detail the forming behavior of MWK fabrics containing two bias inserting yarns (TBMWK fabric). Through experimental observation, it was found that the two bias inserting yarns always tend to gather along the weft direction. The angle between the two bias yarns has a linear relationship with the perpendicular distance from the measured points to the longitudinal axis of the hemisphere during forming process. The slope of this linear relationship is also linear with the magnitude of radius of the pressing hemisphere provided that the radius is larger than 7 cm.Based on the above finding, a mathematical model is established for predicting the deformations of TBMWK fabrics during the hemisphere-forming process. The shape of flat TBMWK fabric that can yield the corresponding hemisphere during the forming process as well as local deformations can be calculated through this model. The hemisphere-forming experiments show that the present model is workable and accurate. The results from both the model and experiments suggest that the shape of flat TBMWK fabric that can yield the corresponding hemisphere which is close to a rectangular, not to a square as presented by woven fabric.The method developed in the paper has laid a foundation for further modeling of the forming behavior of MWK fabrics onto other 3D surfaces. More importantly, it is of great value to find that the two bias inserting yarns always tend to gather along the weft direction of the fabric which is a starting point for modeling of the forming behavior of MWK fabrics.     

The transport properties of polymer membrane-fabric composites

Breathable waterproof fabrics used in this study were prepared by polyurethane coating and polytetrafluoroethylene (PTFE) laminating processes. Outer garments manufactured from these materials can improve wearer comfort by increasing the transport properties of the fabrics. Water vapor permeability, water resistance, water repellency, air permeability, and the other characteristics were measured to evaluate the changes of transport properties with various finishing methods. In the case of the coated fabrics, wet coating A type had high water vapor permeability and low waterproof value, but dry coating A type showed opposite results. Air permeability at low pressure and airflow rate with differential pressure up to 350 kPa were different according to finishing methods. Significant differences in transport properties appeared as the coating methods or amounts of coating solution added to base fabrics were changed. Water vapor permeability and air permeability decreased, and water resistance increased with the amount of coating dope. On the other hand, the transport properties of laminated fabrics were relatively uniform regardless of two or three layers although some tactile properties might have been changed with laminated layers.     

Influence of fabrics’ design parameters on the morphology and 3D permeability tensor of quasi-unidirectional non-crimp fabrics

This research addresses the effects of quasi-UD non-crimp fabric (NCF) design parameters on the fabric architecture and on the permeability tensor. These fabrics are designed for the Liquid Resin Infusion (LRI) of large and thick composite parts. Three fabrics’ parameters intended to bring a flow enhancement to the NCF are investigated: the stitch spacing, the stitch pattern and the weft tow lineal weight. Image analysis is undertaken to characterize the morphology of non-crimp fabric composite. A new continuous permeability measurement method based on compressive tests is proposed to relate the permeability of the quasi-UD NCF to the design parameters during the infusion process. The latter are proven to influence significantly both the fabric architecture and the permeability tensor coefficients.     

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.     

Characterization of a novel natural cellulose fabric from Manicaria saccifera palm as possible reinforcement of composite materials

Identifying novel natural fibers/fabrics with proper properties as reinforcement material is a new challenge in the field of bio-composites. Hence, the aim of this paper is to study the possibility of using a natural fabric extracted from Manicaria saccifera palm as a novel reinforcement in composites. This fabric was extensively characterized by chemical composition analysis, infrared spectroscopy (FTIR) analysis, morphological studies (SEM), thermo-gravimetric analysis (TGA) and physical /mechanical properties studies. From SEM analysis it was identified globular protrusions spread uniformly over the fiber which could help the mechanical interlock with the resin. As well, Manicaria fabric showed good thermal stability, low density, low moisture content and good tensile properties. Further, their properties are comparable to most natural cellulose fabrics and some synthetic fabrics, such as fiber glass fabrics. Manciaria saccifera fabric showed to be a suitable candidate as natural reinforcement material for the development of bio- composite.     

纺织复合材料预制件多层机织布内气泡形成机理

以多层机织布为对象,首先基于两个分别对应于两种典型叠加模式的单元结构模型,通过理论分析,建立了多层机织布纵向切面内气泡形成的分析模型;然后采用有限元-控制体积方法对单元结构内树脂的扩散及气泡形成的过程进行了数值模拟。理论分析和数值模拟的结果符合一致,均表明:不同局部区域差异很大的渗透系数是气泡形成的最根本原因,而纬线的轴向渗透系数和经线的横向渗透系数之比对多层机织布纵向切面内气泡的形成及尺度起着决定性的作用。