Mechanical properties of composite structures fabricated with the vacuum induced preform relaxation process

A new technique called vacuum induced preform relaxation (VIPR) can be used to improve the processing of composite parts manufactured using vacuum resin infusion methods. The VIPR process is a method for manipulating and guiding the resin filling pattern during a vacuum assisted resin transfer molding (VARTM) manufacturing process with a relatively small external vacuum chamber. This VIPR chamber can be sealed against the flexible molding surface of a VARTM mold and used to create vacuum above the preform. This causes the compressive forces compacting the fabric to decrease allowing the resin to flow faster in the effected region under the chamber. Thus the chamber can influence the resin flow pattern as well as avoid the formation of voids due to merging flow fronts. When the regulated vacuum in the chamber is applied it temporarily decreases the fiber volume fraction of the preform. It is important to investigate if this relaxation has a permanent adverse effect on the mechanical properties of the composite. The results of these tests strongly suggest that the use of the VIPR process does not compromise the mechanical properties of composite structures.     

Experimental validation of post-filling flow in vacuum assisted resin transfer molding processes

In Liquid Composite Molding (LCM) processes with compliant tool, such as Vacuum Assisted Resin Transfer Molding Process (VARTM), resin flow continues even after the inlet is closed due to the preform deformation and pressure gradient developed during infusion. The resin flow and thickness changes continue until the resin pressure becomes uniform or the resin gels. This post-filling behavior is important as it will determine the final thickness and fiber volume fraction distribution in the cured composite. In this paper, a previously proposed one dimensional coupled flow and deformation process model has been compared with the experimental data in which the resin pressure and part thickness at various locations during the post-filling stage is recorded. Two different post-infusion scenarios are examined in order to determine their impact on the final part fiber volume fraction and thickness. The effects of different venting arrangements are demonstrated. The model predictions compare favorably with the experimental data, with the minor discrepancies arising due to the variability of material properties.     

Flow sensing and control strategies to address race-tracking disturbances in resin transfer molding. Part I: design and algorithm development

In the resin transfer molding process for advanced polymer composites manufacturing, the fiber preform is placed in the mold cavity and a thermoset resin is injected into the mold to impregnate the stationary preform. The resin displaces the air in the mold through openings called vents. Once the resin emerges out of the vents, the injection is discontinued. The near net-shaped composite part can be demolded after the resin cures. Ideally, the vents should be placed at the locations where the resin arrives last to ensure the complete saturation of the preform. However, the racetracking phenomenon, in which the resin flows faster along the minuscule channels induced by imperfect fits between the preform edges and the mold walls, can dramatically change the resin infiltration process. The ramifications of racetracking are that the resin may arrive at the vents before completely impregnating the preform and create undesired dry spots, which are fiber regions devoid of resin. The racetracking strength is not repeatable and may vary from one injection trial to next. Hence, the online strategic flow control can be useful in improving the processing reliability and the parts quality by re-directing the flow to arrive last at the vents. In this article, an online strategic flow control system consisting of a flow sensing network and a flow actuation network is proposed. A flow pattern recognition technique, which is based on the dimensionless time vector collected by the flow-sensing network, is developed in order to perform the online flow characterization effectively. Flow simulations are utilized to off-line design the flow control system. An evaluation function is formulated to optimize the flow sensing network design. A multi-tier genetic algorithm is implemented to optimize the locations of vents and gates along with the necessary control actions. A numerical case study for testing the computer-generated flow control solutions is presented. It was found that there was significant improvement in the success rate (fewer dry spot regions) due to the use of the strategic flow control and the automated design approach.     

Numerical simulation of injection/compression liquid composite molding. Part 1. Mesh generation

This paper presents a numerical simulation of injection/compression liquid composite molding, where the fiber preform is compressed to a desired degree after an initial charge of resin has been injected into the mold. Due to the possibility of an initial gap at the top of the preform and out-of-plane heterogeneity in the multi-layered fiber preform, a full three-dimensional (3D) flow simulation is essential. We propose an algorithm to generate a suitable 3D finite element mesh, starting from a two-dimensional shell mesh representing the geometry of the mold cavity. Since different layers of the preform have different compressibilities, and since properties such as permeability are a strong function of the degree of compression, a simultaneous prediction of preform compression along with the resin flow is necessary for accurate mold-filling simulation. The algorithm creates a coarser mechanical mesh to simulate compression of the preform, and a finer flow mesh to simulate the motion of the resin in the preform and gap. Lines connected to the top and bottom plates of the mold, called spines, are used as conduits for the nodes. A method to generate a surface parallel to a given surface, thereby maintaining the thickness of the intermediate space, is used to construct the layers of the preform in the mechanical mesh. The mechanical mesh is further subdivided along the spines to create the flow mesh. Examples of the three-dimensional meshes generated by the algorithm are presented.     

Analysis of vacuum bag resin transfer molding process

An analytical model is developed to analyze the resin flow through a deformable fiber preform during vacuum bag resin transfer molding (VBRTM) process. The force balance between the resin and the fiber preform is used to account for the swelling of fiber preform inside a flexible vacuum bag. Mold filling through multiple resin inlets is analyzed under different vacuum conditions. The formation of dry spots is demonstrated in the presence of residual air. Molding of a three-dimensional ship hull with lateral and longitudinal stiffeners is simulated to demonstrate the applicability of the model.     

Dry fiber automated placement of carbon fibrous preforms

The superior material properties of carbon fiber-reinforced composites make them especially attractive for applications in aeronautics and aerospace industries. Cost reduction and time saving are continuously driving industry, leading to new industrial challenges which include manufacturing composite structures with optimal mechanical performances using the potential of advanced processes using robotics.To produce complex part shapes, technologies implying fabric draping in a mold imply large waste amount, fabric structure variability and uncertainties concerning local fiber volume fraction amount and thus final mechanical properties. To overcome such issues and comply with cost and time efficiency, automated dry fiber placement for preform manufacturing is proposed. This approach allows to integrate many functions in a complex part thank to the ability of the robot to steer fiber tows at specific locations. The final composite part is obtained by injecting the produced preform with resin using RTM (Resin Transfer Molding) or infusion process.The presented project aims to define the influence of the process driving parameters during fiber placement on the final preform properties range. Preforms were produced using a lab-scale automated placement demonstrator. Three preforms configurations were tested to highlight the influence of the preform structure on permeability and mechanical parameters through characterization of the compression behavior and permeability of the produced preforms. Choice of configuration will affect mechanical properties on the manufactured preforms, whereas creation of open channels to enhance the flow propagation during manufacturing does not necessarily increase the preform permeability.     

Flow analysis during compression of partially impregnated fiber preform under controlled force

Compression resin transfer molding (CRTM) is an alternative solution to conventional resin transfer molding processes. It offers the capability to produce net shape composites with fast cycle times making it conducive for high volume production. The resin flow during this process can be separated into three phases: (i) metered amount of resin injection into a partially closed mold containing dry fiber preform, (ii) closure of the mold until it is in contact with the fiber preform displacing all the resin into the preform and (iii) further mold closure to the desired thickness of the part compacting the preform and redistributing the resin. Understanding the flow behavior in every phase is imperative for predictive process modeling that guarantees full preform saturation within a given time and under specified force constraints.     

基于VOF/PLIC界面追踪方法的RTM工艺充模过程数值模拟

针对基于Darcy定律的树脂传递模塑(RTM)工艺的充模过程数值模拟的局限性,将纤维预制体内的充填流动作为两相流(树脂相和空气相)处理,在动量方程中考虑了惯性项和粘性项,采用有限体积方法(FVM)离散控制方程,并与VOF/PLIC界面追踪方法相结合,发展了求解树脂在纤维预制体内非稳态流动问题的数值模拟方法.在此基础上开发了RTM工艺的充模过程数值模拟程序,其算例的数值模拟结果与解析解或实验结果吻合良好,验证了此数值模拟方法的有效性和可靠性.     

In situ measurement and monitoring of whole-field permeability profile of fiber preform for liquid composite molding processes

For liquid composite molding (LCM) processes, such as resin transfer molding (RTM), the quality of final parts is heavily dependent on the uniformity of the fiber preform. However, the conventional permeability measurement method, which uses liquid (oil or resin) as its working fluid, only measures the average preform permeability in an off-line mode. This method cannot be used to create an in situ permeability profile because of fiber pollution. Further, the conventional method cannot be used to reveal preform's local permeability variations. This paper introduces a new permeability characterization method that uses gas flow to detect and measure preform permeability variations in a closed mold assembly before resin injection. This method is based upon two research findings: (1) resin permeability is highly correlated with air permeability for the same fiber preform with well-controlled gas flow, and (2) the whole-field air permeability profile of a preform can be obtained through measuring the pressure field of gas flow.In this study, first the validity of the gas-assisted, in situ permeability measurement technique was established. Then the technique was demonstrated as effective by qualitatively detecting non-uniformities and permeability variations in fiber performs. Finally, a two-dimensional flow model, based on the finite difference scheme, was developed to quantitatively estimate the whole-field preform permeability profile using predetermined pressure distribution. The efficacy of the new method was illustrated through experimental results.     

Analysis of non-isothermal mold filling process in resin transfer molding (RTM) and structural reaction injection molding (SRIM)

In this paper, we present a modeling and numerical simulation of a mold filling process in resin transfer molding/structural reaction injection molding utilizing the homogenization method. Conventionally, most of the mold filling analyses have been based on a macroscopic flow model utilizing Darcy's law. While Darcy's law is successful in describing the averaged flow field within the mold cavity packed with a porous fiber preform, it requires experiments to obtain the permeability tensor and is limited to the case of porous fiber preform-it can not be used to model the resin flow through a double porous fiber preform. In the current approach, the actual flow field is considered, to which the homogenization method is applied to obtain the averaged flow model. The advantages of the current approach are: parameters such as the permeability and effective heat conductivity of the impregnanted fiber preform can be calculated; the actual flow field as well as averaged flow field can be obtained; and the resin flow through a double porous fiber preform can be modelled. In the presentation, we first derive the averaged flow model for the resin flow through a porous fiber preform and compare it with that of other methods. Next, we extend the result to the case of double porous fiber preform. An averaged flow model for the resin flow through a double porous fiber preform is derived, and a simulation program is developed which is capable of predicting the flow pattern and temperature distribution in the mold filling process. Finally, an example of a three dimensional part is provided.     

Resin impregnation during composites manufacturing: Theory and experimentation

An experimental investigation into the area of resin impregnation during the manufacturing of composite materials is undertaken. The study is specifically directed at furthering the degree of understanding of the resin transfer molding and resin film stacking manufacturing processes. An additional goal of the present work is the testing of a previously developed numerical model for simulating such processes. The experimentation performed consists of the fully monitored impregnation of resin into a thin mold containing a three-dimensional reinforcement fiber composite preform material. Experimental results for two cases involving highly anisotopic resin impregnation are compared to corresponding numerical results. Reasonable agreement between the two sets of results is found to exist and suggestions are made as to future work which could further increase the technical understanding of such processes.     

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

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

Simulation and experimental validation of flow flooding chamber method of resin delivery in liquid composite molding

A new method of resin delivery, which we refer to as the flow flooding chamber (FFC), is investigated to improve infusion time and reduce material waste associated with the Vacuum Assisted Resin Transfer Molding (VARTM) process. The FFC method uses a rigid chamber that rests on top of the bagging material and a vacuum drawn inside the chamber stretches the bag to take the shape of the chamber above the fiber preform. Resin is then drawn into this chamber unimpeded, and once the chamber is full of resin, the release of the vacuum results in application of atmospheric pressure on top of the bag that drives the resin into the fiber preform. The distribution media and other subsequent materials for its removal are not needed in this modified VARTM process. This process is mathematically modeled using a two event model that couples them by using the output conditions from the first event to the input conditions of the second event. The model is implemented in a numerical simulation so one can track the movement of the resin into the chamber and the preform. Experiments using the FFC process are conducted in complex geometries containing inserts and the flow fronts and fill times are recorded. The results compare very well with the predictions validating the assumptions made in the model to describe the flow.     

Predicting dynamic in-plane compressive properties of multi-axial multi-layer warp-knitted composites with a meso-model

Proper prediction of material microstructure from known processing conditions and constituent material properties is a critical step to determine the bulk properties of the composite. This paper reports a meso-structure model of multi-axial multi-layer warp-knitted (MMWK) composites from an elastic–plastic material model considering the strain rate effect for the components of the MMWK composite. The representative unit cell (RUC) of fiber tow is created to obtain the elastic–plastic parameters of the fiber tow. The 3D meso-structure model of the MMWK composite is based on an idealized geometrical model according to the preform structure of the MMWK fabric. The model is used to investigate the effect of the volume fraction of the knitting yarn on the dynamic in-plane compressive properties. The results show that the fiber tow failure at large extent is mainly caused by the micro cracking of the matrix, and the effects of the knitting yarn on the mechanical properties of MMWK composite are very limited. Particularly, MMWK composites could be considered as laminates when the volume fraction of the knitting yarn is low, such as below 1.5%. Experiments were also conducted to validate the results from the simplified meso-structure model of the MMWK composite. The material is found to be strain rate sensitive, and the experimental and predicted results agree well with respect to the compressive strength and modulus of the composite. This confirms that the meso-structure MMWK composite model proposed is capable of capturing the essential features for the response of the composite under different strain rate conditions at the meso-level.     

A numerical simulation of the resin film infusion process

A numerical analysis was conducted for the resin film infusion (RFI) process using semi-cured thermosetting resin films. Mathematical models were developed for the compression of fiber and the viscosity of resin. The force balance between the fiber preform and the resin was considered to account for the deformation of fiber preform and the swell of fiber during the infusion. In an effort to locate the optimal process conditions such as the mold temperature, the fiber volume fraction, and the infusion pressure, a parametric study was carried out for the progression of resin and the infusion time for different process conditions. The numerical code developed in this study was found to be useful in determining the maximum height of vertical sections that can be infused by squeezing the liquefied resin film from the base panel.     

基于“离位”增韧技术Z向注射RTM成型的浸润研究

针对"离位"增韧技术和Z-RTM成型技术,引入饱和度参数修正Darcy定律,建立描述树脂在纤维预制件中非稳态流动的偏微分方程,研究恒流注射过程中体积流量、树脂黏度和纤维预制件渗透率等工艺参数对非稳态浸润过程注入压力的影响,模拟树脂在层间未增韧和增韧纤维预制件束内和束间的流动。结果表明:数值模拟结果具有可靠性;随着注射时间的增加,纤维预制件内部各点的压力增加;随着体积流量、树脂黏度的增加,注入压力线性增加,而随着纤维渗透率的增加,注入压力减少,符合Darcy定律;实现了树脂在纤维预制件细微观层次浸润的可视化,这种可视化结果为预测树脂在预制件中的宏观流动提供了重要补充,并为实际工艺提供了一定指导作用。     

Numerical and Experimental Study of the Bleeder Flow in Autoclave Process

In the autoclave process, resin flow is a primary mechanics for the removing of excess resin and voids entrapped in the laminate and obtaining a uniform and void free composite part. A numerical method was developed to simulate the resin flow in the laminate and the bleeder, and the effects of ‘bleeder flow’ on the resin flow and fiber compaction were conducted. At the same time, fiber distribution in the cured laminates was investigated by both experiments and simulations for the CF/Epoxy and CF/BMI composites. The data of the experiments and simulations demonstrated that fibers consolidated and reconsolidated in the laminate and it was impacted by the viscosity and gel time of the resin system. Compared to the post study in which only resin flow in the laminate are considered, these results will deepen the understanding of the consolidation process, resin pressure variation and void control during the autoclave process, which is valuable for the study of the performance of composite parts, provided that fiber distribution does affect some properties of composite material.     

Experimental investigation of high fiber tow count fabric unsaturation during RTM

High fiber tow count fabrics have been developed by fibers and fabrics suppliers to meet automotive cost and performance requirements when manufacturing structural automotive composite parts at high production rates by RTM. Impregnation of these large fiber clusters may lead to local incomplete saturation of fabrics. Mechanical softening, early failure, or part rejection because of high voids content may be expected. A new experimental method has been proposed to measure the air volume entrapped within the wetted part of the fabric at any given time and to quantify air entrapment kinetics. An important observation is that the whole unsaturation grows linearly with time for 1D flow. The modified capillary number has been correlated to the amount of air entrapped during the injection process. However, results of this study show that it cannot account for void mobilization and elimination. A critical pressure for the onset of void mobilization has been identified for one fluid/preform combination. This experimental work carried out with proper calibration provided the evidence that for high modified capillary number, a decrease in void content is to be expected.     

Analysis and minimization of void formation during resin transfer molding process

In the resin transfer molding process, residual air in the pores of fiber preform results in dry spots and microvoids in the finished product. The dry spots are usually formed due to irregular permeability of fiber mat and improper injection locations. The microvoids result from non-uniform microarchitecture of the fiber preform, and they are transported through the gap between fiber tows during infiltration of the resin. In this study, a real-time simulation/control method was proposed to actively control the formation and the transport of air voids during the mold filling. The flow equations were solved in real time to predict the change of the flow front shape. The flow front was detected by optical sensors and the control actions were taken based on the sensor signals. Through this automated simulation/control scheme, a real-time control of resin flow could effectively avoid the dry spots and minimize the formation of microvoids by modulating the injection pressure.