Characterization of dual‐scale fiber mats for unsaturated flow in liquid composite molding

The drooping inlet‐pressure history during 1D filling experiments has been used in the past to detect the onset of the unsaturated flow in certain stitched mats, called dual‐scale mats, during mold‐filling in liquid composite molding. In this study, one such triaxial stitched mat was tested for unsaturated flow and manifests the characteristic inlet‐pressure droop that increases with fiber mat compression. A correlation between the previously proposed dimensionless numbers pore volume ratio (γ) and sink effect index (ψ) and the droop in the inlet pressure history was also sought. By studying the micrographs of the composite samples, γ and ψ were measured for the biaxial and triaxial stitched mats at various states of compression. The observation that the droop in the inlet‐pressure profiles increase with an increase in γ and ψ establishes the two dimensionless numbers, along with the presence of straight channels aligned with the flow direction, as good predictors of the unsaturated flow in the dual‐scale fiber mats. POLYM. COMPOS. 26:756–769, 2005. © 2005 Society of Plastics Engineers     

Fast liquid composite molding simulation of unsaturated flow in dual‐scale fiber mats using the imbibition characteristics of a fabric‐based unit cell

The use of the dual‐scale fiber mats in liquid composite molding (LCM) process for making composites parts gives rise to the unsaturated flow during the mold‐filling process. The usual approaches for modeling such flows involve using a sink term in the mass balance equation along with the Darcy's law. Sink functions involving complex microflows inside tows with realistic tow geometries have not been attempted in the past because of the problem of high computational costs arising from the coupling of the macroscopic gap flows with the microscopic tow flows. In this study, a new “lumped” sink function is proposed for the isothermal flow simulation, which is a function of the gap pressure, capillary pressure, and tow saturation, and which is estimated without solving for the microscopic tow simulations at each node of the FE mesh in the finite element/control volume algorithm. The sink function is calibrated with the help of the tow microflow simulation in a stand‐alone unit cell of the dual‐scale fiber mat. This new approach, which does not use any fitting parameters, achieved a good validation against a previous published result on the 1D unsaturated flow in a biaxial stitched mat—satisfactory comparisons of the inlet‐pressure history as well as the saturation distributions were achieved. Finally, the unsaturated flow is studied in a car hood‐type LCM mold geometry using the code PORE‐FLOW© based on the proposed algorithm. POLYM. COMPOS., 31:1790–1807, 2010. © 2010 Society of Plastics Engineers.     

In‐plane anisotropic permeability characterization of deformed woven fabrics by unidirectional injection. Part I: Experimental results

Because of the increasing use of polymer composites in a wide variety of industrial applications, the manufacturing of complex composite parts has become an important research topic. When a part is manufactured by liquid composite molding (LCM), the reinforcement undergoes a certain amount of deformation after closure and sealing of the mold. In the case of bidirectional woven fabrics, this deformation may significantly affect the resin flow and mold filling because of changes in the values of permeability. Among other considerations that govern the accuracy of numerical simulations of mold filling, it is important to predict the changes of permeability as a function of the local shearing angle of the preform. The resin flow through a fibrous reinforcement is governed by Darcy's law, which states that the fluid flow rate is proportional to the pressure gradient. The shape of the flow front in a point‐wise injection through an anisotropic preform is an ellipse. Part I of this article describes a new methodology based on the ellipse equation to derive the in‐plane permeability tensor from unidirectional injection experiments in deformed woven fabrics. Part II presents a mathematical model that predicts the principal permeabilities and their orientation for sheared fabrics from the permeability characterization of unsheared fabrics. Unidirectional flow experiments were conducted for a nonstitched, balanced, woven fabric for different shearing angles and fiber volume fractions. This article presents experimental results for deformed and undeformed fabrics obtained by unidirectional flow measurements. A comparison of the proposed characterization methodology with radial flow experiments is also included. POLYM. COMPOS. 28:797–811, 2007. © 2007 Society of Plastics Engineers.     

Experimental analysis and numerical modeling of flow channel effects in resin transfer molding

Composite manufacturing by Liquid Composite Molding (LCM) processes such as Resin Transfer Molding involve the impregnation of a net‐shape fiber reinforcing perform a mold cavity by a polymeric resin. The success of the process and part manufacture depends on the complete impregnation of the dry fiber preform. Race tracking refers to the common phenomenon occurring near corners, bends, airgaps and other geometrical complexities involving sharp curvatures within a mold cavity creating fiber free and highly porous regions. These regions provide paths of low flow resistance to the resin filling the mold, and may drastically affect flow front advancement, injection and mold pressures. While racetracking has traditionally been viewed as an unwanted effect, pre‐determined racetracking due to flow channels can be used to enhance the mold filling process. Advantages obtained through controlled use of racetracking include, reduction of injection and mold pressures required to fill a mold, for constant flow rate injection, or shorter mold filling times for constant pressure injection. Flow channels may also allow for the resin to be channeled to areas of the mold that need to be filled early in the process. Modeling and integration of the flow channel effects in the available LCM flow simulations based on Darcian flow equations require the determination of equivalent permeabilities to define the resistance to flow through well‐defined flow channels. These permeabilities can then be applied directly within existing LCM flow simulations. The present work experimentally investigates mold filling during resin transfer molding in the presence of flow channels within a simple mold configuration. Experimental flow frot and pressure data measurements are employed to experimentally validate and demonstrate the positive effect of flow channels. Transient flow progression and pressure data obtained during the experiments are employed to investigate and validate the analytical predictions of equivalent permeability for a rectangular flow channel. Both experimental data and numerical simulations are presented to validate and characterize the equivalent permeability model and approach, while demonstrating the role of flow channels in reducing the injection and mold pressures and redistributing the flow.     

Model resin permeation of fiber reinforcements after shear deformation

In liquid composite molding processes such as resin transfer molding and structural reaction injection molding, fiber reinforcements are formed with automated processes to conform to the complex shape of the mold cavity. Deformation of the fiber reinforcement during the forming operation can be characterized by factors such as the local surface curvature of the mold and the type of reinforcement. For bidirectional fiber fabrics, simple shear is the major deformation mode in the forming process. Deformation of the fiber reinforcement after being formed to the mold cavity shape results in variations of local fiber content. In addition, the network structure of the fiber reinforcement is also rearranged. This may cause some significant effects on the fiber permeability and result in a mold filling pattern quite different from that expected. Therefore, a good understanding and measurement of the permeabilities for the deformed fiber reinforcements is of great importance. In the flow simulation of the filling process, the success of the prediction depends greatly on the correct values of in-plane permeabilities. A change of the in-plane permeability of the fabric after shear deformation must be well understood before an accurate flow simulation can be obtained. This article investigates the permeability of fiber reinforcements in relation to different shear angles. Several flow experiments were conducted on bidirectional woven roving fabrics at different shear angles. Two relevant factors—the ratio of principal permeabilities and the direction of principal axes with respect to the orientation of the fabric—are studied to investigate their variations with respect to shear deformation of the fiber reinforcements. It is found that the angle shift of the principal axes increases with the shear angle. At the same time, the in-plane permeability ration may decrease with the shear angle.     

Micro scale flow behavior and void formation mechanism during impregnation through a unidirectional stitched fiberglass mat

Liquid composite molding (LCM) processes such as resin transfer molding (RTM) and structural reaction injection molding (SRIM) have been perceived as high potential processes for the near-net-shape manufacturing of composite parts. This paper addresses two major issues in LCM technology: fiber wetting and void formation during mold filling. Flow visualization experiments were carried out to develop a better understanding of the flow induced voids. The formation and elimination of voids were studied using several liquids and a unidirectional stitched fiberglass mat. Void formation was correlated to capillary number and liquid-fiber-air contact angle.     

Characterization of anisotropic permeability from flow front angle measurements

Textile permeability is a generally anisotropic material property, which characterizes the ease of establishing a resin flow through the fibrous reinforcement in Liquid composite molding (LCM) processes. Unidirectional injection experiments are commonly performed to determine in‐plane permeability. Effective permeability values have to be measured along three different textile directions to calculate the full in‐plane permeability tensor. This article presents a strategy to reduce the number of the required unidirectional experiments to two or even one by considering the angle that the flow front forms with the measurement direction. The relationship between this flow front angle and the permeability tensor elements was derived theoretically and verified by both simulations and experiments with various textile reinforcements. In addition, two methods were investigated to measure the flow front angle and the effective permeability during the experiments: a standard approach based on visual observations and a new method that relies on three pressure sensors, applicable also in the case of nontransparent tooling. The results show that: (I) the two methods provide consistent measurements and are substantially equivalent; (II) the strategy devised to characterize permeability by measuring the flow front angle is effective and accurate; (III) the proposed procedure allows reducing considerably the time and the material samples required for permeability characterization by unidirectional experiments. POLYM. COMPOS., 37:2037–2052, 2016. © 2015 Society of Plastics Engineers     

Calibration of one‐dimensional flow setup used for estimating fabric permeability using three different reference media

Experimental estimation of the permeability of reinforcement fabrics is very important in conducting accurate mold‐filling simulations for liquid composite molding (LCM) processes employed to manufacture polymer matrix composites. In this study, the one‐dimensional (1D) flow based permeability measuring setup was calibrated for the first time using three different reference media: an aluminum block with drilled parallel holes, a lattice of 3D unit cells created using rapid prototyping, and carbon fabric used in the recently concluded permeability bench‐mark study [Vernet N, et al. Compos Part A, 61, 172 (2014)]. The steady‐state permeability was estimated for all the three cases while the transient permeability was estimated only for the lattice‐type and carbon fabric media. The carbon‐fabric results were presented as the transient and steady‐state permeabilities for three different directions of 0°, 45°, and 90°, and the in‐plane principal permeability components were calculated using the correlations for anisotropic fabrics. The results for the aluminum‐plate and lattice‐like media were compared with the previous numerical and experimental studies and good agreement was observed. To validating the carbon‐fabric results, the experimental permeability was compared with two different analytical permeabilities for dual‐scale porous media [Papathanasiou, Int. J. Multiphase Flow, 27 ( 8 ), 1451 (2001) and K.M. Pillai and S.G. Advani, Transport Porous Med., 21 ( 1 ), 1 (1995)], and a good agreement with experimental results established the accuracy of our 1D flow setup. The study raises some important questions on the permeability benchmark study conducted recently [Vernet N, et al. Compos Part A, 61, 172 (2014)]. POLYM. COMPOS., 37:925–935, 2016. © 2014 Society of Plastics Engineers     

RTM一维单向流动模型的理论概况及研究

本文研究了牛顿流体在铺置纤维预制件的RTM平板模具中,一维单向恒流和恒压流动的渗流模型.通过求解析解,求出模型的各个可测量物理量之间的关系,对传统的解析方程进行了拓展和改进,提供了在不透明模具中测定渗透率的方法.即在恒流条件下,只需利用压力传感器和恒流泵的读数即可测定预制件的一维渗透率;或者在恒压条件下,利用数字流量计和压力表的读数测得一维渗透率.而这些解析解的关系式也可用于实验预估模具中的压力分布、充模时间等.     

Liquid flow in molds with prelocated fiber mats

The mechanism associated with mold filling in the manufacture of structural RIM (SRIM) and resin transfer molding (RTM) composites is studied by means of flow visualization and pressure drop measurements. To facilitate this study, an acrylic mold with a variable cavity was constructed and the flow patterns of nonreactive fluid flowing through various layers, types, and combinations of preplaced glass fiber reinforcement mats were photographed for both evacuated and nonevacuated molds. The pressure drops in the flow through a single type of reinforcement (e.g., a continuous strand random fiber mat) and also a combination of reinforcement types (e.g., a stitched bidirectional mat in combination with a random fiber mat) were recorded at various flow rates to simulate high-speed feeding processes (e.g., SRIM) and low-speed feeding processes (e.g., RTM). By changing the amount of reinforcement placed into the mold, the permeabilities of the different types and combinations of glass fiber mats were obtained as a function of porosity. It is shown that partially evacuating the mold cavity decreases the size of bubbles or voids in the liquid, but ultimately increases the maximum pressure during filling. The results also show that glass fiber mats exhibit anisotropic permeabilities with the thickness permeability, K_z, being extremely important and often the determining factor in the pressure generated in the mold during filling.     

In-plane permeability measurement and analysis in liquid composite molding

This work presents methods to measure and analyze in-plane permeabilities of various fabric reinforcements. The principal flow directions need to be determined first by conducting flow visualization. From the flow front pattern, the ratio of the permeabilities in the two principal directions can be determined. The pressure and the flow rate relationship from both radial and unidirectional flow measurement methods are then used to calculate the values of the permeabilities. By the use of the unidirectional flow measurement method, the edge flow effect can also be estimated.     

Experimental analysis and simulation of flow through multi-layer fiber reinforcements in liquid composite molding

Liquid composite molding (LCM) is a process in which a reactive fluid is injected into a closed mold cavity with preplaced reinforcement. Combined layers of different permeabilities are often used in LCM, which creates through thickness and inplane porosity and permeability variations. These inhomogeneities may influence the flow front profile in the thickness direction. To investigate the effect of the through thickness inhomogeneities, mold filling experiments were performed using preforms containing layers of two different fiber architectures. Aqueous corn syrup solutions were injected into a tempered glass mold containing the reinforcement stack. The progress of the flow front at various locations within the reinforcement was measured by an electrical conductivity technique based on the insertion of small wires between the reinforcement layers. Experimental data reveal the details of the flow front shape as the fluid penetrates the preform. Using these data, a model is proposed to calculate the overall in-plane permeability of the preform. Numerical simulations of the flow front progression performed with the computer software RTMFLOT developed in our laboratory are compared to the experimental flow front for various stacking arrangements. Results show good agreement between simulations and experiments and demonstrate the capability of the software to simulate multi-layer flow process.     

Trans-plane fluid permeability measurement and its applications in liquid composite molding

This work discusses tow independent methods to measure and analyze the trans-plane fjuid permeability of various fiber reinforcements. In the unidirectional flow method, the measured injection pressure and flow rate, together with a one-dimensional Darcy's law were used to calculate the trans-plane permeability of fiber mats was independent of flow rate only at low injection pressure. Flow-induced fiber mat permeability change occurred when the injection pressure exceeded the clamping pressure. Measured permeability in conjunction with a three-dimensional mold filling computer program was used to simulate the effect of stacking sequence for a combination of different fiber mats on the mold filling pattern. Finally, a method is proposed to simplify the simulation of a three-dimensional flow through the fiber perform.     

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%.     

RTM工艺中玻纤增强材料渗透率的测量与分析

通过对树脂传递模塑成型工艺中广泛使用的纤维增强材料——玻纤连续毡渗透率的测量和分析，建立了该增强材料在模具中的纤维体积含量与渗透率之间的关系，考察了纤维增强材料的渗透率与注模时间的关系，分析对比了纤维增强材料的结构型式对注模时间和纤维浸透性的影响。结果表明；随着纤维体积含量的增大，渗透率迅速下降，对于玻纤连续毡，其渗透率ｋ与纤维体积含量ｖｆ的关系可以表示为一个多项式；在恒定的压力下，渗透率大，注模需要的时间越短，体积含量相同的玻纤连续毡和玻纤方格布比较，玻纤连续毡的渗透率约大一倍，而所需注模时间约为玻纤方格布的１／２；玻纤方格布中存在渗透率相差特别大的两大区域是造成其浸透性差的主要原因。     

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.     

Flow channels/fiber impregnation studies for the process modeling/analysis of complex engineering structures manufactured by resin transfer molding

The success of resin transfer molding (RTM) depends upon the complete wetting of the fiber preform. Effective mold designs and process modifications facilitating the improved impregnation of the preform have direct impact on the successful manufacturing of parts. Race tracking caused by variations in permeabilities around bends, corners in liquid composite molding (LCM) processes such as RTM have been traditionally considered undesirable, while related processes such as vacuum assisted RTM (VARTM) and injection molding have employed flow channels to improve the resin distribution. In this paper, studies on the effect of flow channels are explored for RTM through process simulation studies involving flow analysis of resin, when channels are involved. The flow in channels has been modeled and characterized based on equivalent permeabilities. The flow in the channels is taken to be Darcian as in the fiber preform, and process modeling and simulation tools for RTM have been employed to study the flow and pressure behavior when channels are involved. Simulation studies based on a flat plate indicated that the pressures in the mold are reduced with channels, and have been compared with experimental results and equivalent permeability models. Experimental comparisons validate the reduction in pressures with channels and validate the use of equivalent permeability models. Numerical simulation studies show the positive effect of the channels to improve flow impregnation and reduce the mold pressures. Studies also include geometrically complex parts to demonstrate the positive advantages of flow channels in RTM.     

Permeability characterization. Part 2: Flow behavior in multiple-layer preforms

Within the resin transfer molding (RTM) process, flow is generally characterized by the progression of a distinct, nonuniform flow front into the preform as a function of time. The flow front progression introduces unsaturated characteristics into RTM flow fields. As a result, the definition of an effective in-plane permeability (K_eff) is used to determine the permeability of actual preforms as they fill with fluid. This K_eff expression expands upon the original definition of Darcy's law by generalizing its applicability to unsaturated creeping flows. Results of experimentally obtained K_eff for flow in single-layer preforms have been detailed for two common RTM materials, a random mat and a 3-D weave, in Part 1. In this paper (Part 2), we characterize the unsaturated and saturated permeabilities of multiple-layer preforms constructed from the random mat and 3-D weave materials characterized in Part 1. This work identifies the apparent permeability characteristics of a specific unsaturated multiple-layer flow that demonstrates behavior inherent to this important class of heterogeneous flows. Also, parallels are drawn between the unsaturated permeability behavior of complex 3-D weave materials and multiple-layer preforms.     

RTM二维径向流动模型的理论概况及研究

综述了牛顿流体在放置了各向同性和各向异性预制件的RTM平板模具中二维径向恒流和恒压渗流的流动模型.对各个模型的可测量物理量之间的关系通过求解其解析解进行了必要的拓展补充推导和研究分析,对传统的二维解析方程进行了拓展和改进,提供了在不透明模具中测定渗透率的方法.即在恒流条件下利用压力传感器和恒流泵测定预制件的二维渗透率;或者在恒压条件下利用数字流量计和压力表测得二维渗透率.这些解析解的关系式也可用于实验预估模具中的压力分布及充模时间等.     

操作参数对玻纤毡增强复合材料流动成型的影响

研究了玻纤毡增强热塑性复合材料(GMT)流动成型过程中模腔压力和材料流动方向对制品力学性能和纤维取向的影响。研究表明：随着压力提高，弯曲强度和模量同步增加；但压力超过18MPa后，性能变化不明显。随布料面积分数减小，单轴拉伸流动成型将导致沿流动方向和垂直流动方向强度及模量差异变大，纤维取向严重；但双轴拉伸流动成型得到的制品各方向性能较均匀，基本呈现各向同性。