Permeability Of Reinforcements Research Articles

Influence of the surface energy of a basalt fiber on capillary wicking and in-plane permeability of reinforcements

This study evaluates the influence of a thermal treatment of a basalt fiber on capillary wicking tests and in-plane permeability experiments, under several pressure differences. The impact of the treatment was characterized at three scales: microscopic, to determine the fiber surface energy; mesoscopic, to estimate an equivalent capillary pressure (Pcap) of the fabric in spontaneous impregnation; and macroscopic, to determine the saturated (Ksat) and unsaturated (Kunsat) permeability of the fibrous preform at the process scale. Results at the microscopic scale showed that the thermal treatment increased the polarity of the fiber by 22% and decreased its surface roughness. Capillary wicking tests showed that the treated fabric presents a better affinity with water, increasing Pcap by 68%. At the process scale, permeability experiments showed the increase of Ksat and Kunsat after treatment. Finally, results of capillary pressure (ΔPγ) showed a dominance of capillary effects under the negative pressure difference.

An Overview of the Measurement of Permeability of Composite Reinforcements.

Liquid composite molding (LCM) is a class of fast and cheap processes suitable for the fabrication of large parts with good geometrical and mechanical properties. One of the main steps in an LCM process is represented by the filling stage, during which a reinforcing fiber preform is impregnated with a low-viscosity resin. Darcy's permeability is the key property for the filling stage, not usually available and depending on several factors. Permeability is also essential in computational modeling to reduce costly trial-and-error procedures during composite manufacturing. This review aims to present the most used and recent methods for permeability measurement. Several solutions, introduced to monitor resin flow within the preform and to calculate the in-plane and out-of-plane permeability, will be presented. Finally, the new trends toward reliable methods based mainly on non-invasive and possibly integrated sensors will be described.

Controlling resin flow in Liquid Composite Moulding processes through localized irradiation with ultraviolet light

AbstractA vacuum infusion process was implemented to produce composite specimens from a random glass filament mat and an acrylic modified polyester resin curable upon irradiation with ultraviolet (UV) light. Through localized irradiation with UV light during the reinforcement impregnation, the viscosity of the flowing resin was increased selectively. This allowed converging–diverging flow patterns with defined inclusions to be realized and racetracking along reinforcement edges to be suppressed. The approach is based on radical photopolymerisation. Here, the degree of cure and the viscosity of the resin increase under direct irradiation, such that the resin gels and the flow stalls in a matter of seconds, but remain unchanged in areas covered with an opaque mask. While this study is concerned with the feasibility of the process, potential practical applications are in flow control for Liquid Composite Moulding, that is, compensation for local variations in the fiber volume fraction and permeability of reinforcements.

Sensor Fusion for Simultaneous Estimation of In-Plane Permeability and Porosity of Fiber Reinforcement in Resin Transfer Molding.

To meet the expectation of the industry, resin transfer molding (RTM) has become one of the most promising polymer processing methods to manufacture fiber-reinforced plastics (FRPs) with light weight, high strength, and multifunctional features. The permeability and porosity of fiber reinforcements are two of the primary properties that control the flow of resin in fibers and are critical to numerical simulations of RTM. In the past, various permeability measurement methods have been developed in the literature. However, limitations still exist. Furthermore, porosity is often measured independently of permeability. As a result, the two measurements do not necessarily relate to the same entity, which may increase the time and labor costs associated with experiments and affect result interpretation. In this work, a measurement system was developed by fusing the signals from capacitive sensing and flow visualization, based on which a novel algorithm was developed. Without complicated sensor design or expensive instrumentation, both in-plane permeability and porosity can be simultaneously estimated. The feasibility of the proposed method was illustrated by experiments and verified with numerical simulations.

Simultaneous characterization of preform expansion and permeability in vacuum assisted resin infusion

AbstractVacuum assisted resin infusion (VARI) is a composite manufacturing process, in which a fibrous reinforcement is laid out in a mold, and then sealed under a vacuum bag. The preform is compressed under vacuum and a liquid polymer resin is infused into the mold cavity. A characterization of compressibility and permeability is required to get accurate predictions of infusion times and thickness of final parts. A new experimental methodology is developed to simultaneously characterize the preform expansion and permeability of fibrous reinforcements during infusion. It is implemented in a one‐dimensional rectangular workbench by impregnating the preform with silicone oil. Pressure sensors measure the liquid pressure, and the reinforcement thickness is acquired by linear variable displacement transducers (LVDTs). The flow rate is also recorded with a scale. The expansion of the wetted reinforcement and permeability can be modeled by power laws as a function of pressure and fiber volume content, respectively. For isotropic preforms, a single experiment provides all the information needed to simulate the flow and predict the infusion time, the thickness and pressure. To validate this new characterization approach, the results of two infusions are successfully compared with numerical simulations.

Bayesian inversion algorithm for estimating local variations in permeability and porosity of reinforcements using experimental data

A novel Regularising Ensemble Kalman filter Algorithm based on the Bayesian paradigm was applied to RTM processes to estimate local porosity and permeability of fibrous reinforcements using measured values of local resin pressure and flow front positions during resin injection. The algorithm allows to detect locations of defects in the preform. It was tested in virtual experiments with two geometries, a two-dimensional rectangular preform and a more complex 3D shape, as well as in laboratory experiments. In both the virtual and laboratory experiments, it was demonstrated that the proposed methodology is able to successfully discover defects and estimate local porosity and permeability with good accuracy. The algorithm also provides confidence intervals for the predictions and estimations of defect probabilities, which are valuable for analysis of the process.

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.

Experimental investigation of transverse permeability applied to liquid molding

The study of transverse permeability of fibrous reinforcements has become increasingly important when liquid‐molding processes started being more widely employed to obtain more complex‐shaped or thick parts. In this paper, a system has been specially built to measure transverse permeability of fabrics with different characteristics based on the correlation between pressure difference and flow rate. Influence of preform thickness and working fluid on permeability was initially evaluated to validate the measurements. Next, the effect of fiber type and fiber volume fraction on permeability was investigated. In‐plane permeability of the reinforcements was also measured to study the relative importance between in‐plane and transverse permeabilities within a fiber content range, which varied differently for the various reinforcements. The results were discussed with the aid of fiber compaction experiments and micrographs. POLYM. COMPOS., 40:3938–3946, 2019. © 2019 Society of Plastics Engineers

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.

Influence of textile parameters on the in‐plane permeability characteristics of non‐crimped fabric preforms

Material parameters such as the permeability of dry reinforcing textiles are key variables for modern composite production using liquid composite molding (LCM) technique. Nowadays numerical filling simulations are required for predicting the mold filling behavior. Inaccurate predictions can lead to a high risk of air inclusion and corresponding need for cost‐intense revision of the mold design. Permeability values of the textiles used in the process are basic requirements for a numerical filling simulation, since the permeability is directly linked to the filling behavior. Nevertheless, the permeability values of non‐crimped fabrics (NCF) which are used for aerospace and automotive structures are rare. In this study the influence of textile parameters of NCF on the in‐plane permeability has been investigated using a capacitive in‐plane permeability measurement technology. The results show the influence of the roving filament number as well as the used stitch length on the in‐plane permeability. It is confirmed that the textile grammage is not affecting the in‐plane permeability of NCF reinforcements. The results of this study are valuable for textile selection with specific permeability data as well as for numerical filling simulations. POLYM. COMPOS., 37:1854–1863, 2016. © 2015 Society of Plastics Engineers

Experimental measurement of transversal micro‐ and macro permeability during compression molding of PP/Glass composites

In this work, a new experimental method is presented, aimed to measure the transversal permeability of fabric reinforcements for composite production. Through‐thickness impregnation of a glass woven fabric with polypropylene matrix was studied during compression molding experiments. The composite thickness was continuously measured during compression molding at different temperatures and pressure levels. The measured composite thickness was used to evaluate the molten front advancement during fabric impregnation. The existence of two different mechanisms of impregnation was highlighted by changes in the slope of the molten front advancement. Optical microscopy confirmed the occurrence of these two different mechanisms: macro‐scale impregnation, associated to void reduction between the bundles (inter‐bundle) occurring at lower times, and micro‐scale impregnation, associated to the flow of the matrix inside each bundle (intra‐bundle), occurring at longer times. Each of the two processes is characterized by a different value of the permeability, calculated according to Darcy law. The intra‐bundle and inter‐bundle permeability were used for calculating the transversal global permeability applying the Papathanasiou model. These values were compared with transversal permeability results evaluated using a low viscosity test fluid. POLYM. COMPOS., 35:105–112, 2014. © 2013 Society of Plastics Engineers

Continuous measurement of fiber reinforcement permeability in the thickness direction: Experimental technique and validation

It is an important topic to measure the through-thickness permeability of fiber reinforcements as the resin flow in the thickness direction is widely employed in many composites manufacturing techniques. Continuous techniques for the permeability measurement by simultaneous fabric compaction and liquid flow have been recently proposed as an alternative way to the tedious and laborious conventional permeability measurement techniques. In spite of their efficiencies, these continuous techniques have some limits if the fabric compaction speed or flow rate is relatively great. To address this issue, a new equation for the permeability estimation is proposed. Parametric studies are performed to investigate the influences of the experimental conditions on the validity of the continuous technique. A dimensionless number is proposed as a measure of the relative error of the continuous technique.

A general model for the permeability of fibrous porous media based on fluid flow simulations using the lattice Boltzmann method

Fluid flow analyses for porous media are of great importance in a wide range of industrial applications including, but not limited to, resin transfer moulding, filter analysis, transport of underground water and pollutants, and hydrocarbon recovery. Permeability is perhaps the most important property that characterizes porous media; however, its determination for different types of porous media is challenging due its complex dependence on the pore-level structure of the media. In the present work, fluid flow in three-dimensional random fibrous media is simulated using the lattice Boltzmann method. We determine the permeability of the medium using the Darcy law across a wide range of void fractions (0.08 ⩽ ϕ ⩽ 0.99) and find that the values for the permeability that we obtain are consistent with available experimental data. We use our numerical data to develop a semi-empirical constitutive model for the permeability of fibrous media as a function of their porosity and of the fibre diameter. The model, which is underpinned by the theoretical analysis of flow through cylinder arrays presented by [Gebart BR. Permeability of unidirectional reinforcements for RTM. J Compos Mater 1992; 26(8): 1100–33], gives an excellent fit to these data across the range of ϕ. We perform further simulations to determine the impact of the curvature and aspect ratio of the fibres on the permeability. We find that curvature has a negligible effect, and that aspect ratio is only important for fibres with aspect ratio smaller than 6:1, in which case the permeability increases with increasing aspect ratio. Finally, we calculate the permeability tensor for the fibrous media studied and confirm numerically that, for an isotropic medium, the permeability tensor reduces to a scalar value.

Permeability of Hybrid Reinforcements and Mechanical Properties of their Composites Molded by Resin Transfer Molding

The aim of this study was to carry out RTM experiments to determine in-plane permeability of glass mats, non-woven polypropylene flow media (PP) and hybrids (glass + PP), with different stacking sequences, and to compare these with various sisal mats and hybrids (glass + sisal). RTM composites were also molded. Permeability decreased for higher fiber content and, for the same fiber volume content, the permeability of the sisal mats was much higher than that of glass mats and also higher than that of the PP non-woven core, often used as an infiltration medium. Besides, a tendency of increasing permeability with sisal fiber length (up to 30 mm) was noticed. The use of the sisal mat as a flow medium increased the permeability of the hybrid reinforcement and slightly improved the mechanical properties of the composite. Hence, the sisal mat may be indicated in engineering applications as a substitute for commercial flow media, widely used in the process called RTM light.

Permeability of textile reinforcements: Simulation, influence of shear and validation

The permeability of textile reinforcements is a crucial input for the simulation of the impregnation stage of a composite material fabrication process. In this paper, we present a fast and accurate simulation method for the permeability of a textile reinforcement, based on a finite difference discretisation of the Stokes equations. Results for single layer, multi-layer and sheared models are discussed. The influence of intra-yarn flow and periodic or wall boundary conditions are considered. We compare the numerically computed permeability values with experimental data.

Effect of Multi-Scale Porosity in Local Permeability Modelling of Non-Crimp Fabrics

The influence of multi-scale porosity of fibre reinforcements on local permeability is investigated, in order to determine the possibility of simplifying permeability models for more efficient permeability calculations. Unit cell models of a biaxial Non-Crimp Fabric are developed and used to investigate, whether or not the porous bundles can be excluded, when modelling the local permeability. Numerical accuracy of calculations is controlled to guarantee the quality of the results and the conclusions drawn from them. It is found that fibre bundles with high fibre density can be excluded from permeability models, while bundles with low fibre volume fractions need to be included. A new method to model the local permeability of multi-scale reinforcements is developed and verified for low fibre density in the bundles. In this method, the effects of the flow inside the fibre bundles are included through modifications of the boundary conditions of a single-scale model representing the interbundle regions. The local permeability of multi-scale reinforcements can, therefore, be calculated by models with simplified fluid domains for all fibre bundle porosities, instead of being calculated by models consisting of the entire multi-scale geometry.

A simple method for the measurement of compaction and corresponding transverse permeability of composite prepregs

AbstractProcess simulation is of great importance in the development of processes for cost‐effective fabrication of composite structures, particularly for thermoset matrix composites. For the simulation of autoclave or hot press process, it requires knowledge of the compaction behavior and the saturated transverse permeability of fiber reinforcements. In this paper, a simple method without any sophisticated equipment is shown, which can simultaneously measure the compaction curve and the saturated transverse permeability as a function of fiber volume fraction. The method was used to measure the properties of S‐2 glass rovings and T700S carbon rovings prepregs. The effects of the impregnating fluid variety, the initial fiber volume fraction of prepreg, and the lay‐up type on the compaction behavior were investigated. The transverse permeability was also studied as a function of fiber content for various lay‐up types. The results indicate that Gutowski's compaction model and the modified Kozeny‐Carman equation proposed by Gutowski, which are important input parameters for the resin flow model, can be used to adequately fit the experimental data. POLYM. COMPOS. 28:61–70, 2007. © 2007 Society of Plastics Engineers

A note on permeability simulation of multifilament woven fabrics

A conventional approach for modeling permeability of multifilament fabrics is to consider their warps and wefts to be individual thick filament made of homogeneous porous media and solve the flow equations for such monofilament fabrics. In this work, for the first time, the full 3-D geometry of an idealized multifilament woven fabric, wherein the filaments are packed in hexagonal arrangement, is generated to compute its permeability and compare with the homogeneous anisotropic lumped model of Gebart (1992. Permeability of unidirectional reinforcements for RTM. Journal of Composite Materials 26(8), 1100–1133). While a relatively good agreement is obtained, our results indicate that Gebart's model slightly underestimate the permeability of multifilament fabrics even at high yarn's solid volume fractions.

A prediction method on in-plane permeability of mat/roving fibers laminates in vacuum assisted resin transfer molding

Vacuum assisted resin transfer molding (VARTM) is one of the promising manufacturing techniques for large-scaled composite components with complex geometry, such as yachts or fishing vessels. To reduce the failure risk of production, numerical simulation of resin infusion process before manufacture is helpful. In general, basic characteristics of perform, such as permeability, need to be measured by experiments in practice. However, this experimental approach sometimes may be costly because specific types of fibers as well as preform with different layer numbers need individual experiments. This study first introduces the experimental procedure of measuring the permeability of reinforcements via Darcy's Law. On the basis of experimental observation of permeability of different layer order, we assumed that the change of the permeability in different experiments is mainly affected by the space provided by the fiber. Accordingly, an efficient prediction method based on the idea of “total porous space of the reinforcement” is proposed. It is shown that this method can give reference between prediction and experiments of the mat/roving fiber preform. Though the resin flowing is complex, this prediction gives a simple, macroscopic reference way for the injection characteristic of large-sized ships, and consequently facilitates the numerical design work of composite structures manufactured by VARTM technique. POLYM. COMPOS., 27:665–670, 2006. © 2006 Society of Plastics Engineers

Multipurpose carbon fiber sensor design for analysis and monitoring of the resin transfer molding of polymer composites

The resin transfer molding (RTM) process is taking an ever-growing place among the manufacturing technologies of polymer composite parts because of its numerous advantages. However, consistent production of high-quality parts is difficult to achieve and requires better understanding of the process and good control of the raw materials. Part-to-part variations are inevitable as a result of uncarefully controlled molding environments and unidentifiable or undetected disturbances that cannot be completely eliminated or accounted for. Despite of many efforts to understand and model the fundamental physical and chemical behavior of materials during processing, there is no reliable system able of predicting the optimum processing parameters to manufacture high-quality parts in a productive way. In this context, this work aims to develop systems allowing the monitoring of the whole RTM process (from preforming to resin curing) and that are reliable, cheap, and easy to use on production line. We have chosen to investigate the potential of the electric and dielectric carbon fiber sensors, which have already proved to be suitable for in service damage monitoring and preventive maintenance without any integration issues. However, the development of the continuous electric sensor has been limited by the polarization of the resin under the direct current. The flow front and the cure monitoring of the resin has been achieved with the dielectric sensor, energized by an alternative current preventing the polarization. Additionally, the ability of this carbon fiber sensor to evaluate the thickness of dry reinforcements and to measure online the actual unsaturated permeability of reinforcements has been demonstrated. POLYM. COMPOS., 26:717–730, 2005. © 2005 Society of Plastics Engineers