Determination of in‐plane permeability of fiber preforms by the gas flow method using pressure measurements

A method is described for measuring the in‐plane permeability of orthotropic fibrous preforms using gas flow. The method is based on an optimization process between computed and measured pressures at various locations in the mold during steady state gas flow through the enclosed preform. The computed pressure is obtained by the control volume finite element method (CVFEM). This method was demonstrated by using a specially designed mold with multiple ports for gas injection and pressure measurement and it was shown that it can be implemented easily and yields consistent and reliable results.     

A gas flow method for determination of in‐plane permeability of fiber preforms

Resin flow plays a crucial role in many composite manufacturing processes. The most important parameters used in modeling and designing mold filling are the permeability of the fibrous preform, which is a kind of flow conductance and a property of the reinforcement, and the viscosity of the resin. The extent reaction, or degree of cure, is also important and causes change of chemical during mold filling. To determine the permeability of fiber preform searchers have been using liquid flow analysis. In this study, a new scheme for determining permeability using gas flow is proposed. In conventional liquid flow methods, radial propagation of the polymer into a porous medium is measured and used to determine permeability, whereas in the gas flow method, the several different preform geometries is measured and used. The effectiveness of the gas flow method was verified by comparing it with conventional methods.     

An improved self-consistent method for estimating the permeability of a fiber assembly

This paper presents an improved self-consistent method for estimating the permeability of an aligned fiber assembly, in both longitudinal and transverse directions. In this method an insertion is assumed to include open space surrounded by densely packed fibers. This improvement allows us to describe effectively the permeability of dense structures containing distributed voids. As used in self-consistent methods, the insertion is placed into a homogeneous medium with an unknown permeability. Stokes flow and Darcy flow are then considered, respectively, at different regions. Boundary and interface conditions as well as two consistency conditions, including the total amount of the flow and the dissipation energy, are applied accordingly. The permeability is solved from these considerations. This improved permeability model captures the flow characteristics of a fiber bundle. In the longitudinal flow case, the openings within a bundle due to disturbance dominate the flow path. In the transverse flow case, the gaps between neighboring fibers govern the flow resistance. The derived expression for the transverse permeability contains two variables, the averaged fiber volume fraction and the maximum packing efficiency, which adequately describe the status of a fiber bundle. These two variables can also be measured experimentally. The predictions agree with available data reported. The result for the longitudinal flow shows not only the influence of these two parameters, but also the very strong effect of the openings within the bundle on the permeability. This explains the significant differences between the data of idealized packings, such as square and hexagonal packing, and those measured from real fiber bundles. The comparison also provides an estimation of the average opening sizes within a fiber bundle as a function of fiber volume fraction. Numerical simulation results of previous studies are also used to verify this approach.     

Vacuum‐assisted resin transfer molding using tackified fiber preforms

A low cost composite fabrication process—tackified SCRIMP—is described for fabricating aerospace‐grade composites based on tackification and vacuum‐assisted resin transfer molding (VARTM). Tackification based on a commercial tackifier (FT 500 from 3M) was used to make the net‐shape fiber preform. It was found that tackifier concentration and application conditions play important roles in governing the moldability of tackified fiber preforms. An epoxy resin (PR 500 from 3M) was used in the VARTM process‐SCRIMP at high temperatures. Experimental results show that composites with high fiber content (> 60% by volume) can be manufactured at low cost using tackification. Effects of tackification methods on composite dimension control, void content and mechanical properties were investigated and compared in both RTM and SCRIMP.     

Numerical simulation of unsaturated flow in woven fiber preforms during the resin transfer molding process

In this paper, the unsaturated flow encountered in the woven or stitched fiber mats used in RTM is simulated using an adaptation of the Finite Element Method/Control Volume (FEM/CV) technique. The movement of resin through such fiber mats is modeled as flow through dual scale porous media and the mass balance in such media creates a sink term in the equation of continuity of the macroscopic flows. Combining this equation with Darcy's law leads to a non-homogeneous non-linear elliptic partial differential equation for pressure that is solved iteratively. First the simulation is used to study simple flows encountered during the characterization of preforms, such as the constant injection pressure 1-D flow and the constant flow rate radial injection flow. Previously observed experimental results of relatively flatter pressure histories for the latter type of flows in wove fiber mats are replicated, both numerically and analytically, by the pressure equation with the sink term. A quantity called pore volume ratio is shown to play an important role in such flows. Finally, the unsaturated flow in a typical RTM mold, packed with woven fiber mats, is simulated numerically, and inlet pressures, fill times, and mat saturation are studied.     

A study on the determination of in-plane permeability of fiber preforms

One of the parameters most frequently used in modeling and designing the mold filling process is the permeability of fibrous preforms. To obtain radial propagation of a viscous polymer into a homogeneous orthotropic porous medium, an approximate solution is derived and its results are compared with numerical ones obtained from boundary element method (BEM). A simple and direct procedure incorporating approximate solution with experimental data has been proposed to determine the principal in-plane permeabilities of the reinforcements. A scheme is also suggested in order to increase the accuracy in the determination of degree of anisotropy. The effect of resin injection type on permeability is investigated through the experiments.     

Random discontinuous carbon fiber preforms: Experimental permeability characterization and local modeling

Injection experiments indicate that for random discontinuous carbon fiber preforms, increasingly uneven flow fronts develop with increasing fiber bundle length and filament count. While at high propensity for fiber bundle splitting, the preform permeability increases continuously with increasing fiber length, no trend can be identified at low propensity. No clear influence of the virgin bundle filament count on the preform permeability was observed. Types of sizing used on the fibers and bundle cross‐sectional shapes may vary and affect the intrinsic filamentization behavior, thus dominating the preform permeability. In a model for local preform permeability, interbundle voids, distributed randomly across the preform thickness, are approximated via a regular void structure. Simulated filling patterns are qualitatively similar to those observed experimentally, showing more pronounced features than those derived from a model based on local through‐thickness homogenization of the filament distribution. A model based on an alternating arrangement of fiber bundles and voids allows prediction of global preform permeability values from series of injection simulations, showing quantitatively better agreement with corresponding experimental results than the homogenization model. For global permeability, agreement between simulated and experimental mean values improves with increasing fiber volume fraction, whereas calculated coefficients of variation show no strong dependence on the fiber volume fraction. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Preform permeability predictions by self-consistent method and finite element simulation

The self-consistent method and finite element simulations have been used to estimate the permeability of an aligned fiber bundle. The self-consistent method gives out formulas for both longitudinal and transverse permeabilities as functions of the fiber volume fraction. The finite element simulation presents the solutions of various periodic fiber packings. The results for square and hexagonal fiber packings are found coincident with the literature results. It is shown that the permeability is not only related to the fiber volume fraction of porosity, but is also greatly influenced by the packing structure or micro-level disturbance. A unified empirical model is proposed, which uses as variables ultimate fiber volume fraction in addition to fiber volume fraction. The model predications agree with numerical simulation results in different cases.     

Numerical investigation of three-dimensional fiber suspension flow by using finite volume method

Fiber suspension flow is common in many industrial processes like papermaking and fiber-reinforcing polymer-based material forming. The investigation of the mechanism of fiber suspension flow is of significant importance, since the orientation distribution of fibers directly influences the mechanical and physical properties of the final products. A numerical methodology based on the finite volume method is presented in the study to simulate three-dimensional fiber suspension flow within complex flow field. The evolution of fiber orientation is described using different formulations including FT model and RSC model. The pressure implicit with splitting of operators algorithm is adopted to avoid oscillations in the calculation. A laminate structure of fiber orientation including the shell layer, the transition layer and the core layer along radial direction within a center-gated disk flow channel is predicted through a three-dimensional simulation, which agrees well with Mazahir’s experimental results. The evolution of fiber orientation during the filling process within the complex flow field is further discussed. The mathematical model and numerical method proposed in the study can be successfully adopted to predict fiber suspension flow patterns and hence to reveal the fiber orientation mechanism.     

Experimental investigation of flow through multi-layered preforms

This research investigates resin flow phenomena through preforms composed of multiple layers of reinforcement material. Two complete studies were performed: the first investigates the effect of varying the order of lay-up of a fixed number of plies, and the second studies the impact of varying the thickness of the individual layers of a thick preform. In each case, a set of compression experiments were first performed to characterize the preforms so that in-situ layer thicknesses can be calculated. Next, flow experiments were performed upon the preforms to determine the permeability and flow front behavior. In the first study, it was found that the weighted average scheme provides a reasonable estimate for the effective permeabilities of the preforms, with errors ranging from 14.2% to 23.8%. In the second study, however, the weighted average scheme was found to provide only a rudimentary estimate of the permeability, with an error that was seen to increase in magnitude during the course of each experiment. In addition, computer simulations were performed for the second study, which show the possibility of deriving transverse permeability values without direct measurement, although significant errors in inlet pressures were once again evident.     

异形聚酯纤维束芯吸效应的分析

通过建立纤维束单元模型,采用自行开发的纤维束截面数学模拟软件,模拟了圆形、方形、三角形和双十字形4种不同截面的异形聚酯纤维束内毛细空隙当量半径值的大小和分布状态,计算了液体在纤维束内的统计流量。双十字形截面的纤维束当量半径值多分布在较小的区域内,有利于液体爬升,当量半径累计值和统计流量大于其他3种纤维束,具有较好的芯吸效应。     

In-plane permeability measurements on fiber reinforcements by the multi-cavity parallel flow technique

This report discusses the advantages and drawbacks of the multi-cavity parallel flow technique for permeability measurements. An experimental series with repeated measurements on material from the same roll shows that the repeatability of the technique is very good considering the manufacturing variability of the fabric. The measured standard deviation in the repeatability study is about 10%. It is, however, shown that the permeability can vary considerably- between reinforcements of similar geometry. Furthermore, computer simulations were used to estimate the errors when highly anisotropic materials are oriented at an angle to the material principal direction in the parallel flow technique. The conclusion based on the simulations is that the length to width ratio of the cavity should be larger than the anisotropy of the reinforcement for an acceptable error.     

Control of flow in resin transfer molding with real-time preform permeability estimation

Variabilities in the preform structure in situ in the mold are an acknowledged challenge to achieving reliable preform saturation in liquid molding processes. Physical models offer an effective means of deriving real-time process control decisions so as to steer the resin flow in a desired manner, which ensures complete preform saturation. An important parameter influencing the fidelity of the simulations is the preform permeability, which is a strong function of the preform microstructure. A model-based control strategy that incorporates the ability to determine and utilize local permeability information in real-time is of much value, and forms the focus of the paper. An intelligent model-based controller is developed that uses virtual sensing of permeability to derive optimal decisions on controlling the injection pressures at the mold inlet ports in a resin transfer molding (RTM) process. The controller employs an artificial neural network, trained using process simulation data, as an on-line flow simulator, and a simulated annealing algorithm to optimize the injection pressures on-the-fly during the process. Preform permeability is virtually sensed during the process, based on the flow front velocities and the local pressure gradient along the flow front, estimated using a fuzzy logic model. The controller, implemented on an RTM process, is shown to be able to accurately steer the flow fronts through various preform configurations.     

Numerical computation of the fiber preform permeability tensor by the homogenization method

This paper uses the homogenization method to formulate governing equations and boundary conditions to predict the permeability of anisotropic fibrous porous media. An approach is also put forth for addressing dual porosity media with this method. It is applicable to a wide range of porous materials, which may be considered with approximately periodic structure. The permeability components are calculated by solving a boundary value problem in a periodic cell. The application of the method is demonstrated on a periodic cell modeling the geometry of woven fiber performs used in composites manufacturing. The resulting equations are solved using CFD package FIDAP. The influence of dual porosity and nesting of adjacent fabric layers on the macro permeability was investigated and the results were compared with experiments.     

大丝束碳纤维产业发展现状及面临的问题

简要介绍了大丝束碳纤维的特点、应用前景以及目前产业发展现状,并分析了大丝束碳纤维实际生产和应用中面临的问题,以及相关解决方案。指出大丝束碳纤维采用成本较低的民用聚丙烯腈丝作为原丝,具有较高的性能价格比,广泛应用于休闲体育用品、基础设施、工业应用等领域,需求前景广阔;我国碳纤维生产以小丝束碳纤维为主,国内大丝束碳纤维应用在展纱工艺和浸润的效果方面仍待技术突破;今后大丝束碳纤维的发展应从制定大丝束碳纤维生产标准、改进展纱设备、改善浸渍树脂体系黏度和大丝束碳纤维相容性等方面着手,从而提升产品质量。     

Estimation of hydrogen mobility of coal tar pitch for carbon fiber using a tritium tracer method

Three coal tar pitches for high performance carbon fiber (HPCF) and general performance carbon fiber (GPCF) were hydrogenated by tritium labelled gaseous hydrogen in the absence of both catalyst and vehicle solvent under conditions of temperature 300–400°C, nominal reaction time 0–300 min and initial hydrogen pressure 5.9 MPa. The relative rate constants and apparent activation energies for hydrogen transfer from gas phase to the pitches have been estimated by tracing tritium. The apparent activation energies for hydrogen addition from the gas phase to the three pitches were very close (ca. 9–10 kcal/mol), but apparent activation energies for hydrogen exchange between the gas phase and the three pitches were different from each other (HP, 15.4; GP-A, 2.9; GP-B, 3.7 kcal/mol). Especially, apparent activation energy of hydrogen exchange of the raw pitch for HPCF was significantly larger than those of the raw pitches for GPCF. These results of kinetic parameters for hydrogen transfer can be used to evaluate the distribution and mobility of hydrogens in component molecules of pitches.     

A model for the transverse permeability of bi‐material layered fibrous preforms

We consider viscous flow across unidirectional hexagonal arrays of porous tows of elliptical cross section. Using the lubrication approximation we develop an analytical model for the hydraulic permeability of layered systems, in which the permeability of the porous tows making up each alternating layer is allowed to differ. Extensive validation of this model is carried out through numerical computations using the Computational Fluid Dynamics package FIDAP. Good agreement between model predictions and numerically calculated permeabilities is found in the interesting region of small porosity (ϕ = 0.20 or ϕ = 0.30). Using this model we also address the issue of the inaccuracies that may be introduced if the overall permeability of a fibrous preform is calculated using an arithmetic average permeability instead of the actual permeability of each layer. We find that use of an average permeability will result in overestimation of the overall permeability of the system. This overestimation will increase as the porosity is reduced and/or as the difference in permeabilities increases.     

钓鱼竿用大丝束碳纤维预浸料的研制

本文简要介绍了钓鱼竿用大丝束(60K)碳纤维预浸料熔胶膜法浸渍工艺，并着重就影响预浸料质量的几个主要因素进行了讨论。     

Accurate permeability characterization of preforms used in polymer matrix composite fabrication processes

The permeabilities of fabrics composed of carbon and glass fibers have been determined by utilizing both simple 1-dimensional and 2-dimensional radial flow measurements using silicone oil and motor oil as permeants. The carbon fabric is typical of that used in fabrication of aerospace grade polymer matrix composites, while the glass fabric is a 3-dimensional woven fabric that has been proposed as a standard reference material for permeability characterization. Our results indicate that reliable permeability data for fiber preforms with varying architectural complexity can be obtained provided that the experiments are performed with utmost care and that appropriate equations are used to analyze the data. In-plane permeabilities for the carbon fiber preforms from transient unidirectional constant flow rate and constant pressure experiments agreed within 5%, regardless of the preform orientation to the flow direction. Steady-state results on the same preforms showed agreement within 2% between constant flow rate and constant pressure experiments. The capillary pressure effect was shown to be negligible for the transient experiments. The maximum difference between the transient and steady state permeability values was 3%. The maximum difference between a permeability measured with unidirectional flow and the same permeability measured with radial flow is less than 10%.