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.     

In-plane permeability characterization of the vacuum infusion processes with fiber relaxation

In vacuum infusion processes fiber preforms are placed onto the single molding surface and enveloped with a non-rigid polymer bag which is sealed to the molding surface. The flexible bagging film does deform during the resin infusion process thus changing the compaction of the fabric. However, one can also relax the preform by drawing a partial vacuum in a rigid chamber placed on top of the flexible bag which will increase the permeability of the fabric under the chamber. A numerical model is presented to characterize the change in permeability and describe the mold filling for such processes in which the fabrics undergo controlled relaxation by external stimuli. The predictions from the simplified model agreed reasonably well with the experiments. This characterization and resin flow front prediction with time method should prove useful in processes such as Vacuum Induced Preform Relaxation (VIPR) process which can be used to actively manipulate flow in a vacuum infusion process.     

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.     

基于混合网格方法的VARTM工艺充模仿真与实验验证

针对VARTM工艺的特点,建立了充模过程树脂流动和预成型体变形行为数学模型。提出了基于混合网格方法的VARTM充模仿真算法,在该算法中,模具型腔几何模型进行二维或三维网格划分,在每个真空袋表面单元上增加一个一维附属单元,用于在仿真过程中实时地吸收或挤出因真空袋变形而产生的局部树脂体积变化,形成混合网格仿真模型;求解过程中,对树脂流动和预成型体变形分别进行求解后,基于上述混合网格模型进行两者耦合操作,实现了仿真精度和速度的统一。搭建了VARTM充模实验平台,进行了一维充模实验,通过仿真结果与实验测量结果对比,验证了本文算法的正确性。最后,通过三维仿真算例,验证了算法对三维复杂结构和顺序浇口策略仿真的可行性。     

Flow modeling of the VARTM process including progressive saturation effects

Modeling of vacuum based liquid composite molding methods (e.g., VARTM) relies on good understanding of closely coupled phenomena. The resin flow depends on the preform permeability, which in turn depends on the local fluid pressure; the preform compaction behavior, and the membrane stresses in the vacuum bag. It has also been shown that for many preforms there is a significant unsaturated region behind the flow front, and that the flow in this region affects the overall flow behavior of the process. Studies of preform compaction have shown that the preform stiffness, as well as being non-linear and exhibiting significant hysteresis, is dependant on the fluid saturation. For this reason most researchers model the preform compaction based on the pressure-compaction behavior of saturated preforms during unloading. This assumption leads to an effective discontinuity in preform thickness at the flow front, which is not observed in actual experiments. In this paper an improved compaction model incorporating the saturation dependence of the compaction pressure in the partially saturated region, is used in a one-dimensional model of the VARTM process. The model gives physically more realistic results for the thickness in the flow front region, and an improved model for the consolidation of the preform at the end of infusion.     

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.     

A methodology to reduce variability during vacuum infusion with optimized design of distribution media

Seemann Composites Resin Infusion Molding Process (SCRIMP) is a widely used version of Vacuum Assisted Resin Transfer Molding (VARTM) in which a highly permeable layer (distribution media) is placed on top of the dry preform to distribute the resin with very low flow resistance to reduce the filling and hence the manufacturing time. The flow patterns during filling may vary from part to part due to the variability associated with the material, part geometry, and layup of the assembly, which may result in race-tracking channels. The process is considered as reliable and robust only if the resin completely saturates the preform despite changing filling patterns caused by flow disturbances.The resin flow pattern can be manipulated with a tailored distribution media layout as it does impact the flow patterns significantly. The continuous distribution media layout over the entire part surface works well for very simple geometries with no to little potential for race-tracking along the edges. In this study we address complex cases, which require placement of an insert within the assembly, which will introduce race-tracking along its edges, and hence uniform placement of distribution media over the entire top surface will fail to yield a void free part. We introduce a methodology using a predictive tool to design an optimal shape of distribution media, which accounts for the flow variability introduced due to race-tracking along the edges of the inserts. This iterative approach quickly converges to provide the placement of distribution media on selective areas of the preform surface that ensures complete filling of the preform despite the variability. This approach has been validated with an experimental example and will help mitigate risk involved in manufacturing complex composites components with Liquid Molding.     

A Two-layer Model for the Simulation of the VARTM Process with Resin Distribution Layer

Vacuum assisted resin transfer molding (VARTM) is one of the important processes to fabricate high performance composites. In this process, resin is drawn into the mold to impregnate the fiber reinforcement to a form composite. A resin distribution layer with high permeability was often introduced on top of the fiber reinforcement to accelerate the filling speed. Due to the difference of the flow resistance in the resin distribution layer and the reinforcement as well as the resulting through thickness transverse flow, the filling flow field is intrinsically three-dimensional. This study developed a two-layer model with two-dimensional formulation to simulate the filling flow of the VARTM process with a resin distribution layer. Two-dimensional flow was considered in each layer and a transverse flow in the thickness direction was estimated between the two layers. Thermal analysis including the transverse convection was also performed to better simulate the temperature distribution.     

Post-filling flow in vacuum assisted resin transfer molding processes: Theoretical analysis

For rigid mold filling processes such as resin transfer molding, the resin flow stops when the preform is fully saturated with the resin. However, in vacuum assisted resin transfer molding process (VARTM), due to preform deformation the resin flow continues after the filling stage is complete as it does take a finite time for the pressure field to become uniform during this post-filling period. In this paper, the post-filling flow in the VARTM process with and without the membrane is examined. The governing equations for post-filling flow, in which the preform is allowed to deform, are developed with simplifying assumptions. A one-dimensional flow and deformation coupled process model is developed to simulate the time dependent pressure distribution during the post-filling stage. The model is implemented using finite differences, both in time and space, and utilizes the explicit time integration which is found to be conditionally stable. The change in pressure inside the mold during the post-filling stage is predicted for three different injection scenarios. The influence of the pressure distribution at the end of filling on the dwell time for the pressure to equilibrate and on the final thickness of the part is discussed. The effects of change in preform permeability and compliance on the dwell time and thickness are demonstrated and the extension of the model to more complex geometries and systems is outlined.     

嵌入加强筋的缝合泡沫夹芯结构复合材料VARTM工艺树脂充填模拟及验证

在缝合泡沫夹芯结构复合材料的泡沫中嵌入轻质的加强筋板,可以在不增加缝合密度并且在只增加较少质量的前提下,增强复合材料制品整体的强度和刚性。文中对真空辅助树脂传递模塑成型(VARTM)工艺树脂在嵌入加强筋的缝合泡沫夹芯结构复合材料预成型体中充填过程进行模拟和验证研究。采用一种矩形流道模型代替沿加强筋与泡沫空隙间的树脂流动,并对其等效渗透率及孔隙率进行计算;通过PAM-RTM软件模拟了嵌入加强筋板的缝合泡沫夹芯结构VARTM工艺的树脂充填过程,并建立了流动可视化实验装置与模拟对比,结果表明模拟与实验相当吻合。而模拟与实验的结果均表明加强筋的引入可以在局部加强树脂沿厚度方向的流动,但是会延缓树脂对整个预成型体的充填。     

Simulation of the reinforcement compaction and resin flow during the complete resin infusion process

Resin infusion (a.k.a. VARTM) is one of the LCM processes, for which liquid resin is drawn into dry reinforcements. Significant cavity thickness changes occur during processing, due to the flexibility of the vacuum bag used as one side of the tool, and the complex stress balance within the laminate. While the magnitude of thickness change is often small, the influence is significant on reinforcement properties. Changes in permeability during filling and post-filling have the potential to significantly affect the process. To simulate this behaviour, it is important to accurately model compaction and unloading of reinforcement in dry and wet states. A series of tests were completed to determine compaction behaviour of an isotropic glass fibre mat. From these tests several non-linear elastic compaction models have been determined, and applied within a resin infusion simulation which addresses pre-filling, filling and post-filling. This simulation was then used to assess different post-filling strategies.     

A model for thermoplastic melt impregnation of fiber bundles during consolidation of powder-impregnated continuous fiber composites

Continuous fiber thermoplastic matrix composites were fabricated using a novel powder-impregnation process that combined vacuum assisted resin transfer molding (VARTM) with compression molding. The resulting composite has an average fiber volume fraction of 65%. A model has been developed for the consolidation phase to predict the void fraction of the resulting composite. This model takes into account the fabric unit cell dimensions and material properties and assumes that tow permeability remains constant. The model is compared to experimental values for void fraction for samples prepared using a range of consolidation pressures and dwell times.     

Correlation of void distribution to VARTM manufacturing techniques

Vacuum assisted resin transfer molding (VARTM) is a liquid composite molding (LCM) technique used to manufacture large scale composite structures. Fiber preforms are placed on a tool surface and covered by a flow enhancement layer and a plastic bag. A vacuum is drawn on the system to infuse the resin. When the resin does not fully saturate the empty regions in between the fibers, voids are created. The fiber tows in woven and stitched preforms have a much lower permeability as compared to the bulk permeability of the fabric. Hence, fiber tows saturate with resin later than the pores between fiber tows and are more prone to voids.This study explores the impact of extended resin bleeding time and additional flow resistance at the vent on the void content within fiber tows both experimentally and by numerical simulation. Samples were machined from each of the manufactured panels and analyzed using image analysis techniques to obtain a relative void content. The experimental results were compared to results obtained by numerical simulation.The experimental void distribution showed that if resin is not allowed to bleed or if no external resistance is attached at the vent, the void content over the length of the part is not uniform. All void levels reduced when resistance was added or bleeding was allowed. The discrepancy between experimental and numerical results was addressed by including deformable distribution media in numerical model to capture the continuation of resin flow after the injection gate is closed.     

Investigation on the spring-in phenomenon of carbon nanofiber-glass fiber/polyester composites manufactured with vacuum assisted resin transfer molding

Process-induced residual stress arises in polymer composites as a result of mismatched resin contraction and fiber contraction during the cure stage. When a curved shell-like composite part is de-molded, the residual stress causes the spring-in phenomenon, in which the enclosed angle of the part becomes smaller than the angle of its mold. In this paper, a new approach is presented to control and reduce the spring-in angle by infusing a small amount of carbon nanofibers (CNFs) together with liquid resin into the glass fiber preform using vacuum assisted resin transfer molding (VARTM) process. The experimental results showed that the spring-in angles of the L-shaped composite specimens were effectively restrained by the CNFs. An analytical model and a 3-D FEA model were developed to predict the spring-in phenomenon and to understand the role of CNFs in reducing the spring-in angle. The models agreed with the experimental results reasonably well. Furthermore, the analytical model explains how the CNF-enhanced dimensional tolerance control is accomplished through the reductions in the matrix’s equivalent coefficient of thermal expansion and linear crosslinking shrinkage.     

Variation of part thickness and compaction pressure in vacuum infusion process

In vacuum infusion (VI), it is difficult to manufacture a composite part with small dimensional tolerances, since the thickness of the part changes during resin injection. This change of thickness is due to the effect of varying compaction pressure on the upper mold part, a vacuum bag. In this study, random fabric layers with an embedded core distribution medium is used. The thickness of the composite part and resin pressure are monitored using multiple dial gages and pressure transducers; the results are compared with the model developed by Correia et al. [Correia NC, Robitaille F, Long AC, Rudd CD, Simacek P, Advani SG. Analysis of the vacuum infusion molding process: I. Analytical formulation. Composites Part A: Applied Science and Manufacturing 26, 2005. p. 1645–1656]. To use this model, two material characteristics databases are constructed based on the process parameters: (i) the thickness of a dry/wet fabric preform at different compaction pressures, and (ii) the permeability of the preform at different thicknesses. The dry-compacted preform under vacuum is further compacted due to fiber settling in wet form after resin reaches there; the part thickens afterwards as the resin pressure increases locally. The realistic model solution can be achieved only if the compaction characterization experiments are performed in such a way that the fabric is dry during loading, and wet during unloading, as in the actual resin infusion process. The model results can be used to design the process parameters such as vacuum pressure and locations of injection and ventilation tubes so that the dimensional tolerances can be kept small.     

Flow control using localized induction heating in a VARTM process

Voids formed during the mold filling stage of the vacuum assisted resin transfer molding (VARTM) process become defects in the fabricated parts. Active flow control is one way to eliminate these defects by guiding the flow along a desired path during the mold filling stage of the process. Building upon previous work of the authors [Johnson R, Pitchumani R. In: Proceedings of the thirty-fourth international SAMPE technical conference, MD, USA, vol. 34(1); 2002, p. 250–61; Johnson R, Pitchumani R. Enhancement of flow in VARTM using localized induction heating. Compos Sci Technol 2003;63(15):2202–15; Johnson R., Pitchumani R. In: Proceedings of the fourteenth international conference on composite materials, CA, USA; 2003, Paper# 0861; Johnson R, Pitchumani R. Simulation of active flow control based on localized preform heating in a VARTM process. Compos Part A-Appl Sci Manuf, in press doi:10.1016/j.compositesa.2005.09.007], this paper presents implementation of an active flow control using induction heating as a means of locally reducing viscosity to counteract the effects of nonhomogeneity in the permeability of preform layups in a prototype VARTM process. Feedback of flow front locations during the filling stage of the process is used together with a numerical process model to arrive at decisions on the trajectories of the induction coil and the coil voltage, so as to maintain a uniform flow progression without exceeding a prescribed maximum temperature limit. A flow front following control strategy is implemented in a lab-scale experimental setup and tested on several preform layups exhibiting spatial permeability variation, as well as in the case of preforms with mold inserts. Results of these studies demonstrate that active flow control is capable of reducing the fill time, improving the flow front uniformity throughout the duration of the mold fills, and eliminating dry spot formation.     

Mathematical Analysis of Resin Flow through Fibrous Porous Media

Resin flow through fiber preforms was analyzed mathematically. Closed form solutions for fiber volume fraction distribution and pressure field during resin infusion into fiber preforms were suggested, and a new effective permeability was defined. The effect of preform compressibility on the fiber volume fraction and pressure distributions in resin-saturated region was investigated analytically. The findings show that the compaction behavior of preforms has significant impact on the resin infusion process. The solutions derived analytically in this study can provide insight into a liquid composites molding (LCM) process.     

Resin flow analysis with fiber preform deformation in through thickness direction during Compression Resin Transfer Molding

Resin flow during Compression Resin Transfer Molding (CRTM) can be best described and analyzed in three phases. In the first phase, a gap is created by holding the upper mold platen parallel to the preform surface at a fixed distance from it. The desired amount of resin injected into the gap quickly flows primarily over the preform. The second phase initiates when the injection is discontinued and the upper mold platen moves down squeezing the resin into the deforming preform until the mold surface comes in contact with the preform. Further mold closure during the final phase will compact the preform to the desired thickness and redistribute the resin to fill all empty spaces. This paper describes the second phase of the infusion. We assume that at the end of phase one; there is a uniform resin layer that covers the entire preform surface. This constrains the resin to flow in through the thickness direction during the second phase. We model this through the thickness flow as the load on the upper mold forces the resin into the preform, simultaneously compacting the preform. The constitutive equations describing the compaction of the fabric as well as its permeability are included in the analysis. A numerical solution predicting the flow front progression and the deformation is developed and experimentally verified. Non-dimensional analysis is carried out and the role of important non-dimensional parameters is investigated to identify their correlations for process optimization.     

Infusion design methodology for thick-section,low-permeability preforms using inter-laminar flow media

The vacuum-assisted resin transfer molding (VARTM) process is used to fabricate large-scale composite structures that can have substantial laminate thicknesses. For thick-sections and preforms of low permeability, surface distribution media based VARTM processes such as SCRIMP require long infusion times because the resin has to flow through the complete laminate thickness limiting the process to very long gelation time resin systems. The processing time can be greatly reduced if multiple high-permeability fabrics are integrated periodically between fabric layers reducing the apparent infusion thicknesses of the preform. This paper outlines the infusion challenges of thick-section, low-permeability preforms and illustrates an injection design methodology using inter-laminar flow media. The design is applied on a 7.5 cm thick glass part and includes material characterization, flow simulation and fabrication of the component using an instrumented setup to validate the flow behavior and design. Overall, the methodology can be used to optimize the flow setup of thick-section composites.     

Application of natural fiber reinforced composites to trenchless rehabilitation of underground pipes

Although various trenchless technologies for repairing worn-out underground pipes have been developed and tried, they have not been widely used due to their drawbacks such as high cost, inconvenience of operation, and long construction time. The new rehabilitation process with glass fabric preform based on the vacuum assisted resin transfer molding (VARTM) could reduce cost and construction time much, but the entrapped air during resin transfer process often produced interior defects in the repaired composite.Since the glass fabric preform was not suitable for drain pipes or water pipes because of the health hazard of small broken glass fibers when the underground pipe is worn out, in this work, natural fibers were used for the trenchless rehabilitation of underground pipes. The permeability and strength of natural fiber reinforced composites were measured. The developed process with the natural fiber composites was found to satisfy both the required structural strength for rehabilitation and shorter processing time without interior defects.