Preform Permeability 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.

Prediction of the in-plane permeability and air evacuation time of fiber-placed thermoplastic composite preforms with engineered intertape channels

The two main void removal mechanisms during vacuum-bag-only (VBO) consolidation of thermoplastic composites are through-thickness diffusion and in-plane air evacuation. The automated fiber placement (AFP) process allows for the creation of preforms with an engineered intertape channel network by deliberately introducing spacing between the tapes that can facilitate air evacuation during the VBO consolidation. However, it is unclear what dimensions of the intertape channels allow for effective in-plane air evacuation. The current research presents a predictive simulation tool to optimize the intertape channel dimensions for effective in-plane air evacuation. An analytical method is developed to calculate the in-plane permeability tensors of the composite preform with uniform intertape channel dimensions. In addition, a more elaborate mesoscale method is developed that estimates the in-plane permeability tensor of the engineered intertape channel network based on the distributions in intertape channel dimensions. Finally, a finite difference model is implemented to calculate the time required for air evacuation as a function of the in-plane permeability tensors of the preform and its in-plane dimensions. The results from the models indicated that in-plane air evacuation through the engineered intertape channel network is quick, and it takes only a few minutes to evacuate 99% of the air from large preforms like fuselage panels.

Influence of Epoxy Spray Binder on Infusion and Cure in Liquid Composite Molding Processes

Binders are known to influence the different steps in liquid composite molding process chains. Most reseach focuses on powder binder and veils. Spray binders are rarely studied. Therefore, the effect of an epoxy spray binder on the infusion and cure steps of liquid composite molding processes is studied in this paper. Permeability measurements, solubility tests and measurements of glass transition temperature, resin cure time and resin viscosity show a complex interaction between binder and resin depending on the process conditions. The binder mostly increased the permeability of the tested preforms. This effect increases with increasing binder loading. At low fiber volume fractions the binder had a positive effect on preform permeability, most likely by to delaying the closure of macro flow channels or textile inhomogeneity. The binder did not dissolve in the resin until temperatures of around 120 °C, while remaining as a separate phase at 22 °C, 40 and 80 °C. This has to be considered when defining the cure profile of the resin, as the binder is expected to participate in the cure reaction. The glass transition temperature remained unchanged for the binder-resin combination used. An increase in viscosity by a factor of 1.5-2 was observed when the binder was dissolved in the resin. Samples with dissolved binder cured slightly faster than pure resin. The experimental results were theoretically transferred to real infusion processes.

LCM of thermoset-based fiber reinforced composites for large-scale applications: are process kinetics and structural properties antagonistic?

This article reviews the main methods to manufacture large-scale composite parts, with a focus on Liquid Composite Molding techniques of thermoset-based fiber reinforced structural parts. As this process relies on the impregnation of a dry textile stack, this manufacturing step is crucial in terms of part production rate, and part quality. To increase the process kinetics, a large effort has been devoted to increase the permeability of the textile preforms, while keeping a similar fiber content. An increase of almost two orders of magnitude can be attained if the textile shows a strong separation of scales between densely packed tows and large intra-two spaces. This however leads to a potential degradation in the resulting structural properties, particularly in dynamic mode due to the presence of the resin rich pockets. Alternative solutions emerge, which may help reach a cost-effective compromise.

Effect of gaps on preform and laminate made by automated dry fiber placement and resin infusion

Automated Dry Fiber Placement (ADFP) provides ease in fiber steering compared to the automated placement of prepregs and a higher material deposition rate than manual hand layup. Vacuum Assisted Resin Infusion (VARI) can be used to infuse the resin into the bed of dry fibers to make the composite. The resin flow rate depends on the permeability of the dry fiber preform, which is dependent on the preform fiber volume fraction. Automated Fiber Placement (AFP) allows the capacity to control the fiber volume fraction in terms of gaps between the tows. So, reducing the fiber volume fraction of preforms by introducing gaps into the preform pattern can increase the permeability. On the other hand, introducing gaps in layup results in a drop in the mechanical properties of the laminate (such as compressive modulus and strength). An optimum gap size may provide a significant gain in permeability with minimum loss in compressive properties. This paper investigates the effect of the gap size in the preform pattern on the preform out-of-plane permeability and compressive properties of the laminate. The out-of-plane permeability of three specimens from preforms, with varying gap sizes (0mm, 0.4mm, and 0.8mm) in all layers were measured. Also, the compressive properties of five coupons made from laminates, with and without gap sizes (0mm, 0.4mm, and 0.8mm) were determined. It was shown that introducing gaps with 0.4 mm size between fiber tapes in the layup can increase the out-of-plane permeability 17 times and reduce the compressive strength and modulus by 6 % and 7.2 %, respectively.

Micro Computed Tomography based stochastic design and flow analysis of dry fiber preforms manufactured by automated fiber placement

The effective design of channels in dry tape preforms is crucial for achieving desired preform permeability for successful resin injection for composites manufacturing using Automated Fiber Placement (AFP) process. Achieving target gaps and their locations in the AFP layup is extremely challenging. This work investigates the correlation between the spatial variability of the preforms and the in-plane permeability using an X-ray Computed Tomography (XCT) based characterization framework. The tomographic images of two different dry carbon tape preforms with different tape widths were used to generate realistic and XCT based stochastic models to be used for numerical permeability predictions. The variability in the tape placement by the robotic head and its effect on preform permeability was also examined through stochastic geometric modeling of the laid preform. A benchmark transient permeability measurement set-up was utilized to obtain experimental in-plane preform permeability through 2D radial mold filling. The in-plane numerical permeability values showed significant scatter, with a coefficient of variance of 75%–130%, which deviated from the experimental measurements by approximately one order of magnitude. These findings strongly re-affirm that the experimental permeability measurement technique based on transient mold filling of dry fiber AFP preforms is complex however, the XCT based stochastic modeling technique is an effective way to estimate the permeability of dry fiber AFP preforms virtually.

Effects of the Interlayer Toughening Agent Structure on the Flow Behavior during the z-RTM Process.

In this paper, interlayer toughening composites were prepared by the z-directional injection RTM process (z-RTM), which has the advantage of increasing the interlaminar toughness and shortening the filling time and completely impregnating the fibers. The nonwoven fabrics and dot matrix structure material were used as ex situ interlayer toughening agents. The effect of the interlayer toughening agent structure on the resin flow behavior during the z-RTM process was investigated. The macro-flowing and micro-infiltration behaviors of the resin inside the preforms were deduced. The permeability of the fabric preforms with different toughening agents was investigated. The results show that the introduction of the nonwoven structure toughening agent makes the macro flow slow, and the flow front more uniform. The toughening agent with a dot matrix structure promotes the resin macro flow in the preforms, and shortens the injection time. The z-directional permeability of the preform with a dot matrix structural toughening agent is one order of magnitude lower than that of the non-toughened preform, while being higher than the preform toughened by the nonwoven fabric preforms, which is helpful for the further applicability of the z-RTM process. Furthermore, the mode II interlaminar fracture toughness of composites was evaluated.

A dynamic data driven application system for real-time simulation of resin transfer moulding processes

This paper presents a numerical solution to optimize RTM processes based on a DDDAS methodology, taking permeability as stochastic. The model proposed in this investigation allows real-time accurate prediction of the filling rate by adjusting the parameters with the on-line captured process data. The suitability of the proposed model was tested and validated under different process parameters variations like pressure and preform permeability.

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.

Prediction of permeability of five-harness satin fabric by a modified Kozeny constant determined from experiments

Permeability is a critical parameter not only in flow simulation analysis but also in liquid composite molding process. When a liquid resin is infused into a dry preform, the impregnation is mainly characterized by the permeability. The permeability of a dry preform can be obtained through theoretical and experimental methods. In the theoretical estimation of permeability, the effects of fiber arrangement as well as fabric type and form for various types of preforms are not sufficiently reflected in the calculation. Thus, there is a gap between the theoretical and experimental permeability. Recently, experimental determination has been gaining considerable attention as a mean to obtain accurate permeability values; however, it requires a number of trials. In this study, the permeability of the Hexforce G0926 5HS (five-harness satin) carbon fabric preform is estimated using representative theoretical prediction models, the Gebart and Kozeny–Carman equations. In addition to the Kozeny–Carman permeability (using the Kozeny constant values from literature), the Kozeny constant obtained through experiments was used to obtain a modified Kozeny–Carman permeability. All three calculated permeabilities were compared and verified with the fabric manufacturer’s reference value. The results showed that the modified Kozeny–Carman permeability using the experimentally determined Kozeny constant was closest to the reference value at 57% fiber volume fraction. Further, the predicted permeability was compared with other experimental permeability values from literature over the 40%–65% range of fiber volume fraction. We found that the modified Kozeny–Carman permeability once again came closest to the literature values. Finally, an optimized fitting equation was proposed to replace the Kozeny–Carman equation for predicting the permeability of Hexforce G0926 5HS carbon fabric over the 40%–65% fiber volume fraction range.

Characterization and simulation of electromagnetically induced preform resting (EIPR) process

Electromagnetically induced preform resting (EIPR) process is a new version of vacuum-assisted resin transfer molding (VARTM) process which allows manipulation of resin flow during the filling step. The EIPR process enhances the permeability of preform locally in case of undesirable flow front situations. This technique ensures the perfect filling despite the existence of permeability variation. To utilize the EIPR process in a better way, its comprehensive characterization to find the optimum value of key factors and filling simulation of it is necessary. Key factors of the EIPR process (i.e., amplitude, frequency of vibration) and permeability of preform as a material index are recognized as independent factors that must be considered to characterize the process. These factors are considered to establish a mathematical model for the permeability of preforms. Maximum and minimum values of the frequency and amplitude are determined based on the observations in acceptable composite material manufacturing and in-plane permeability characterization. Response surface methodology is used to model the permeability of EIPR process to find the optimum response values of the key factors for selected preforms. To assess the process numerically, EIPR process simulation with the optimum values is conducted on two case studies. These case studies involving two different permeability zones are designed in order for evaluation. In each case, a low permeability preform is employed in the middle of high permeability one to create an artificial problem. Simulation results demonstrate an acceptable accuracy.

Improvement of impregnation quality on out-of-autoclave processed CFRP aircraft wing spar through resin flow simulation

Vacuum-assisted resin transfer molding (VaRTM) is becoming one of the most robust alternatives for autoclave processes. VaRTM, which applies the resin injection method in a vacuum environment, generally uses fiber reinforcement and a polymer matrix separately in the process. The VaRTM is mainly dominated by the characteristics of constituent materials, such as preform permeability and resin viscosity. Among them, process design with the arrangement of resin inlet/outlet locations is closely related to process defects, and inappropriate inlet/outlet layouts cause voids, etc, which has a decisive effect on quality degradation. Therefore, in this study, a highly curved and twisted spar structure was fabricated by the VaRTM, and both flow simulations using Programs for Applied Mechanics–Resin Transfer Molding software and experimental test parts built with three different inlet/outlet line conditions were performed and compared to predict and improve impregnation quality. There was good agreement between the simulation and built test specimen for the three cases that the shorter inlet and outlet length resulted in improved impregnation quality. It was verified that impregnation and inner quality could be improved through flow simulation analysis during the VaRTM process.

Modeling the reactive PA6 flow for LCM processes

Fiber reinforced thermoplastic composites have shown to be attractive for industry as they can be reused, reshaped, welded and repaired, while keeping mechanical properties on par with thermoset composites. Since thermoplastics usually have high melt viscosities unsuitable for liquid composite molding processes, in-situ synthesis of PA6 from ε-caprolactam is considered. Its reactive mix has low viscosity which allows impregnation. However, the coupled crystallization and polymerization affects the resin viscosity and its flow is altered by the dual-scale permeability of the fiber preform. Thus, to predict the local differences in the thermoplastics properties, a coupled polymerization crystallization model needs to be integrated in the LCM processing simulation at representative scales. This study aims to propose a reliable simulation of the resin flow through a fibrous preform. Hence, viscosity measurements on the reactive mix are achieved using a rheometer with parallel-plate geometry, aiming to associate a viscosity model with the Hillier coupled polymerization-crystallization model previously determined by Vicard. The full chemorheological model will then be integrated into a simulation of LCM process in OpenFOAM®, an open source CFD software in order to follow the extent of the synthesis in the resin flow during the process. As a future work, simulations including microscale tow information extracted from a real textile specimen will permit to investigate the effect of permeability and double scale porosity in fibrous preforms on the final polymerization rate and crystallinity.

In-plane permeability changes of plain weave glass fabric induced by tufting

The presence of the tufts has direct influence on the permeability of textile preforms. In this paper, new insights are presented into the mechanisms responsible for changes to the in-plane permeability of tufted preforms according to three different tufting patterns relative to flow direction with a constant tufting distance. The saturated permeabilities are measured based on the rectilinear flow in a tool constituted by a steel base, a transparent PMMA top, and steel supports. Beforehand, the permeability measurements of textile preform composed of eight layers of plain weave glass fabric without tufting are conducted at three different cavity heights. The results show that the in-plane permeability values for the tufted preforms are lower than the permeability values of the preform without tufting. The paper reveals the significant influence of the tufting patterns on the permeability values for three different fibre volume fractions.

Influence of Non-Reactive Epoxy Binder on the Permeability and Friction Coefficient of Twill-Woven Carbon Fabric in the Liquid Composite Molding Process

In the liquid composite molding process, a binder is used to fix the preform. In this study, the influence of a non-reactive epoxy binder was investigated. To allow the measurement of permeability, the preform specimen was produced under three preforming conditions: neat fabric preform, binder-treated fabric preform without heat treatment, and binder-treated fabric preform with heat treatment. The in-plane directional permeability, K1 (having maximum flow velocity), and K2 (having minimum flow velocity) of the binder-treated fabric preform decreased approximately 80% compared to the neat fabric preform. The permeability in the out-of-plane direction decreased approximately 80% in the binder-treated fabric preform without heat treatment and about 98% in the binder-treated fabric preform with heat treatment. This decrease occurred because the treated binder on the fiber hindered resin impregnation. The effect of the binder on the friction coefficient of carbon fabric was also investigated. The friction coefficient was high when the binder was on the friction surface and increased 40–200% at 110 °C, compared to 25 °C.

Out-Of-Plane Permeability Evaluation of Carbon Fiber Preforms by Ultrasonic Wave Propagation.

Out-of-plane permeability of reinforcement preforms is of crucial importance in the infusion of large and thick composite panels, but so far, there are no standard experimental methods for its determination. In this work, an experimental set-up for the measurement of unsaturated through thickness permeability based on the ultrasonic wave propagation in pulse echo mode is presented. A single ultrasonic transducer, working both as emitter and receiver of ultrasonic waves, was used to monitor the through thickness flow front during a vacuum assisted resin infusion experiment. The set-up was tested on three thick carbon fiber preforms, obtained by stacking thermal bonding of balanced or unidirectional plies either by automated fiber placement either by hand lay-up of unidirectional plies. The ultrasonic data were used to calculate unsaturated out-of-plane permeability using Darcy’s law. The permeability results were compared with saturated out-of-plane permeability, determined by a traditional gravimetric method, and validated by some analytical models. The results demonstrated the feasibility and potential of the proposed set-up for permeability measurements thanks to its noninvasive character and the one-side access.

Relaxation-Compression Resin Transfer Molding under Magnetic Field

Relaxation-compression resin transfer molding under magnetic field is a new variant of VARTM (“vacuum assisted resin transfer molding”) process, which uses a flexible magnetic membrane controlled by a magnetic force, in order to govern the relaxation and compression phases by changing the permeability of the fabric preform. Thus permits to the resin to enter easily into the mold and to increase the resin impregnation velocity and the fiber volume fraction. This innovation is based on the application of the TRIZ theory (“the theory of inventive problem solving”), which allows us to answer to the shortcomings and the conflict links exist inside the VARTM processes. The objective of this paper is to present this new process and to study the effect of the current intensity and the separated gap between the flexible magnetic membrane and solenoid on the permeability of the preform.

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

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

Modelling the permeability of random discontinuous carbon fibre preforms

A force-directed algorithm was developed to create representative geometrical models of fibre distributions in directed carbon fibre preforms. Local permeability values were calculated for the preform models depending on the local fibre orientation, distribution and volume fraction. The effect of binder content was incorporated by adjusting the principal permeability values of the meso-scale discontinuous fibre bundles, using corresponding experimental data obtained for unidirectional non-crimp fabrics. The model provides an upper boundary for the permeability of directed carbon fibre preform architectures, where predictions are within one standard deviation of the experimental mean for all architectures studied.

Permeability Analysis of Natural and Artificial Fiber Textiles for Liquid Composite Molding Process

In liquid composite molding (LCM) processes a catalyzed resin is forced to flow through a dry fibrous preform, opportunely arranged within a mold. The pressure gradient, positive or negative depending on the particular process, drives the uncured liquid flow at a flow rate which relies on fluid viscosity and preform porosity and permeability. Nowadays, sustainability reasons are pushing the market to the wider usage of natural fibers, such as hemp, jute, flex among others, as an interesting alternative to traditional glass and carbon artificial textiles. This manuscript describes an experimental/numerical approach to analyze the preform permeability and the impregnation flow in the LCM process using different textiles, based on glass, carbon and natural fibers. Unidirectional flow tests were performed using a laboratory scale LCM setup, constituted by a sensed mold in order to dielectrically monitor impregnation and saturation flows. The position of the unsaturated flow front was detected also by means of a camera monitoring the resin flow though milled eyelet sealed using transparent material. The permeability was also numerically inferred using the FlowTex software for all the investigated cases. Numerical and experimental outcomes were finally compared and discussed.