A model for resin flow during composite processing: Part 1—general mathematical development

A generalized three-dimensional model for resin flow during composite processing has been developed. The model is based on a theory of consolidation and flow through a porous medium, which considers that the total force acting on a porous medium is countered by the sum of the opposing forces, including the force due to the spring-like effect of the fiber network and the hydrostatic force due to the pressure of the liquid within the porous medium. The flow in the laminate is described in terms of Darcy's Law for flow in a porous medium, which requires a knowledge of the fiber network permeability and the viscosity of the flowing fluid. Unlike previous resin flow models, this model properly considers the flows in different directions to be coupled and provides a unified approach in arriving at the solution. Comparison of numerical solutions with the closed form analytical solution shows good agreement. Resin pressure profiles show that the pressure gradients in the vertical and horizontal directions are not linear, unlike the assumption of linearity made in several previous resin flow models. The effects on the resin pressure of both linear and nonlinear stress-strain behavior of the porous fiber network were considered. The nonlinear behavior simulates a rapidly stiffening spring and the resin pressure decreases much more rapidly after a given initial period compared to the linear stress-strain behavior.     

Numerical investigation on flow through porous media in the post‐infusion process

During the vacuum‐assisted resin transfer molding (VARTM) processing, the post‐infusion behavior after complete wet‐out and before gelation of the resin is critical for the development of the thickness and fiber volume fraction distribution in the cured composite part. The pressure gradient developed during infusion results in a thickness gradient due to the flexible nature of the bagging approach. After full infusion, the resin typically bleeds into a vacuum trap, allowing redistribution of pressure and preform thickness. In this study, a non‐rigid control volume is used to formulate a set of governing equations for analysis of the post‐infusion process. The model is used to investigate the effects of processing parameters and different processing scenarios on resin flow, resin pressure, and thickness variation of the composite laminate. This work provides a tool for optimization of the VARTM process to reduce final part variability. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

界面渗透率对厚复合材料层板流动-压实过程影响的数值分析

基于有效应力原理与达西渗流定律,建立了厚复合材料层板流动-压实过程的多场耦合有限元数值模型,通过与厚单向板试验结果的对比,验证了模型的正确性。建立了含界面层的厚正交层合板流动-压实计算模型,分析了垂直于层间界面方向的界面渗透率对正交层合板流动-压实过程的影响。通过与同等厚度单向板的分析结果对比表明,当不同方向铺层层间界面渗透率高时,厚正交层合板的流动-压实过程几乎与相同厚度单向板的流动-压实过程相同。但当层间界面的渗透率低时,会阻碍内部树脂的流动,导致正交层合板内部纤维体积含量提升慢,且越靠近内部,界面渗透率的影响越明显,最终在界面处纤维含量出现明显的跳跃分布。     

A method to determine resin flow during curing of composite laminates

An experimental method is presented to determine the amount of resin flow within a composite laminate during cure. The method is analogous to the use of radioactive tracers in other applications. Heavier elements such as chlorine and bromine, which may be naturally present in small amounts in epoxy resins are used to follow resin flow and mixing. The presence and the quantity of these “tags” is determined using wavelength dispersive X-ray analysis in a scanning electron microscope. With the resins in this study, it is shown that it is possible to measure volume fractions of resin with accuracies ranging from ±0.5 to ±3 volume %. By using brominated resin in only one layer of a laminate, the degree of flow and mixing can be followed accurately. The results suggest that there is considerable resin mixing as well as flow.     

Numerical analysis of parametric effects on consolidation of angle‐bended composite laminates

In the previous study, the finite element formulation has been developed by our group based on two‐dimensional resin flow and fiber compaction model. Good agreement between simulations and experimental results was found under the one‐dimensional flow condition. In this article, the two‐dimensional model was used to simulate the consolidation of angle‐bended laminates with the convex tool in autoclave process. The effects of material properties on the consolidation were studied. It was found that the fiber bed shear modulus significantly affects the compaction behavior in the corner section of angle‐bended laminate, the fiber bed compaction property decide the laminate deformation, and the resin viscosity and fiber bed permeability affect the rate of laminate compaction and consolidation time. The angle‐bended T700/BMI QY8911‐Ilaminates were manufactured in autoclave process. The experimental data validate the numerical simulation method for the consolidation of the angle‐bended laminates. These results are greatly helpful for the optimization of processing parameters, improvement of composite parts quality, and reduction of the fabrication cost. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

A numerical model of the viscosity of an epoxy prepreg resin system

The development of the viscosity of a thermoset material during processing is complicated because of the dependence of the initial material state and the kinetic rate of conversion from a liquid to a solid material. Uncured thermoset materials typically have a low enough viscosity such that the consumption of energy to generate flow is relatively low. However, as the curing process advances, the flow mechanisms become hindered by the development of a network gel during crosslinking. Once the resin has reached the appropriate degree of cure for gelation, the resin system is incapable of large fluid-like deformations. In this research, the rheological properties of an epoxy resin system used in laminate processing were measured and numerically fit with a modification to the dual Arrhenius model to predict the progression of the viscosity during cure. The numerical results were compared with the experimental measurements, and it was found that the model predicts the experimental observations quite well. It was found that the initial degree of cure of the prepreg is not as significant a factor as the temperature rate dependence on the processing time between the point of flow onset and gelation. However, the minimum viscosity during processing is strongly influenced by the initial degree of cure of the prepreg system.     

Flexural Strength,Performance, and Cost Analysis of Multiaxial Warp-Knitted Preforms for Composite Reinforcements

In the scope of this study, performance analysis of industrial fabrics for composite reinforcement was evaluated from the cost point of view. For the production of composite structures, major considerations can be summarized as stiffness, strength, weight, and cost. In general, manufacturing costs of composites consist of material, labor, overhead, and development costs. The priorities and demands will determine characteristics of the material and process. Specific strength per cost relation is in favor of multiaxial warp-knitted (MWK) fabric. In order to evaluate fiber reinforcements within the group, MWK fabric laminate was compared with woven fabric laminate from the cost/performance point of view. To do so, laminate comparison was performed between MWK fabric and woven fabric reinforced composites. It was found that relative to woven reinforced composite, MWK fabric reinforced composite has lower resin weight, overall composite weight, and labor cost and higher flexural moment with slightly higher material cost.     

Nomex蜂窝夹层复合材料的成型工艺研究

针对Nomex蜂窝填充双马树脂基复合材料夹层结构在固化成型过程中易出现的蜂窝芯边缘塌陷问题进行研究。通过采用不同的成型工艺方法,以及对共固化工艺参数进行调整,研制出相应的双马来酰亚胺树脂基碳纤维/蜂窝夹层结构层板,并且对夹层结构的力学性能、内部质量以及平面拉伸性能进行测试。在此基础上分析了成型压力参数对夹层结构质量的影响。相关工艺试验表明蜂窝芯塌陷的原因主要是固化过程中蜂窝芯边缘的滑移引起的蜂窝局部失稳,通过采取分步成型、蜂窝先胶接后修型的方法能够有效地解决Nomex蜂窝夹层结构填充双马树脂基复合材料结构成型过程中的蜂窝芯塌陷问题。     

A model for resin flow during composite processing part 2: Numerical analysis for unidirectional graphite/epoxy laminates

In this paper numerical results are presented for resin flow during processing of unidirectional graphite/epoxy laminates. Resin pressure and velocity profiles, as well as resin loss, specific permeability, and resulting thickness changes, were computed to examine the effects of one and two-dimensional flow, initial laminate thickness, and various cure cycles. Input data to the model are also discussed in detail. Analysis of the input data on the stress-strain behavior of graphite fiber beds showed that the bed consolidation behavior can be divided into three regions. “Free-bleeding” (O psig and negligible bleeder resistance to flow at the boundaries) during two-dimensional resin flow leads to rapid decay in resin pressure. Comparison of the predicted results for the resin mass loss and the average final thickness per ply with experimentally determined values shows good agreement.     

Reduction of void content of vacuum‐assisted resin transfer molded composites by infusion pressure control

Vacuum‐assisted resin transfer molding is a promising technique for making large and complex composite structures. However, void formation remains a problem. Two primary contributors to void formation, non‐uniform resin flow and continuous evaporation of resin under low pressure, were experimentally studied. Improved pressure control at the vent is proposed to reduce the void content of the manufactured composite material: at the start of the resin infusion, the pressure at the vent is set to the full vacuum of the equipment, while after the resin has saturated all of the reinforcements, the pressure at the vent is increased slightly. The full vacuum at the start of infusion avoids air entrapment, and the slightly higher pressure later in the process restrains the resin evaporation. A lower void content is obtained. POLYM. COMPOS., 36:1629–1637, 2015. © 2014 Society of Plastics Engineers     

复合材料层压板的变形:树脂基体、增强纤维与非对称正交铺层变形的关系

主要研究了复合材料层压板的变形情况。分别选取环氧、苯并嗯嗪为基体,以特定型号的预浸料为材料,选取不对称铺[0／0／90／90]条件,用ANSYS软件模拟复合材料层压板变形情况,与实际不对称层合板变形情况对比,分析树脂基体、增强纤维和非对称铺层与复合材料变形之间的关系。结果表明,不同的树脂基体材料和不同的铺层设计对非对称正交铺层复合材料层压板的变形情况有影响。     

5428/T700体系制造大厚度构件的工艺适宜性研究

对一种航空用改性双马来酰亚胺树脂/碳纤维(5428/T700)复合材料体系制造大厚度构件的工艺适宜性进行研究,通过差示扫描量热法研究了5428树脂的固化反应特性,并测量了5428/T700厚板在固化过程中的温度场。发现5428树脂固化反应放热缓慢而均匀,有利于制造大厚度构件;5428/T700厚板在固化过程中其厚度方向上的温度分布均匀,没有出现明显的温度突增现象;并通过热压罐技术制造出了质量良好的厚板试件。结果表明,5428/T700体系适合于制造大厚度复合材料构件。     

Determination of prepreg tack

Prepreg materials — fibre-reinforcement preimpregnated with uncured resin — are widely used for the manufacture of large composite material components. Current commercially available prepreg materials often show significant variations in tack strength, from point to point within a sheet and from sheet to sheet. Such inconsistencies can lead to void formation in the final composites laminate. This paper describes the techniques and apparatus developed for the investigation of the surface tack strength of adhesives in general and of prepreg materials in particular, as a function of contact time and pressure and of rate of separation. It is hoped that the more detailed knowledge of prepreg tack will enable the production of more consistent material and hence the manufacture of improved quality composite laminates.     

Online monitoring and analysis of resin pressure inside composite laminate during zero‐bleeding autoclave process

Resin pressure is one of the most important parameters in manufacturing composites during autoclave process. It not only greatly influences resin flow behavior, but also has effects on void formation and elimination. Online monitoring resin pressure can provide an important guidance for the optimization of the processing parameters and the control of the quality of composites. In this study, a resin pressure online measuring system for autoclave process was established based on the principle of pressure transfer in liquid, and the size of the measuring probe of the system was optimized to increase the accuracy of measured resin pressure. The results indicate that the accuracy and the dynamic response of the system can meet the requirements of resin pressure measurement during autoclave process. Furthermore, by means of this proposed resin pressure measuring system and the measurements of compaction properties of the fabric stacks, the resin pressures inside carbon fiber fabric/epoxy resin and glass fiber fabric/epoxy resin prepreg stacks during autoclave process were investigated, especially for the zero‐bleeding process which is prevailing for aircraft composite structures. It is demonstrated that during zero‐bleeding process, the resin pressures, which conform to the spring and piston model, uniformly distribute along through‐thickness and in‐plane directions. In addition, the resin pressure profile is significantly influenced by the fiber volume fraction of the prepregs, indicating that fiber content of prepreg should be optimized for achieving free defects and uniform fiber distribution. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Dual polymer bonding of thermoplastic composite structures

This paper describes a process that facilitates fusion bonding of thermoplastic composite components without the need for complex fixtures and without disrupting the fiber alignment in the component laminates. The dual polymer bonding process, Thermabond, requires that an interlayer polymer be fused to the surface of each laminate prior to bonding. The characteristics of the interlayer polymer allow for joining of the components at a temperature below the softening/melting point of the reinforced polymer in the composite laminates. This leads to significant processing advantages without significant loss in mechanical performance. Discussions of resin compatibility, the effect of process conditions on mechanical performance, and the application of the APC-2/PEI Thermabond system to various structural components are included.     

A structure and property based process window for void free thermoset composites

A process window providing guidelines to minimize internal stress levels and to prevent void formation during cure of thermoset composite materials is presented. A model taking into account the applied pressure and the level of stress borne by the fiber assembly was introduced to calculate the hydrostatic internal stress state in the resin during cure. Based on the fundamental mechanisms of matrix shrinkage and evolution of viscoelastic properties under the given processing conditions, the internal stress in the resin was calculated as a function of fiber volume fraction, fiber stacking sequence, applied pressure and resin conversion. This level of stress is compared to a criterion for void initiation in the resin. A process window was hence constructed for preventing void formation during cure. Composite laminates with different stacking sequences and fiber volume fractions were cured with different applied pressures within and out of the process window boundaries. The composite void contents were measured and correlated perfectly with the process boundaries. This process window construction taking into account the material vis‐coelastic properties and the composite architecture is a unique tool for determining optimum process condition of composite laminates.     

Pressure window analysis for thin laminated composites in autoclave process

The curing temperature, pressure, and curing time have significant influence on finished thermosetting composite products. The external pressure and the time of pressure application are two major factors affecting the laminate thickness, fiber volume fraction, and void content. Based on the resin flow/fiber compaction model and corresponding program developed by our group, the genetic algorithm is accepted to design the pressure window for the consolidation of thin laminate. The objective of the optimization is to find the time of pressure application that achieves the desired average fiber volume fraction under given pressure. The pressure windows are analyzed for S‐2 glass fiber/5228 and T700S/5228 laminates with unidirectional and bidirectional lay‐up. It is found that no special viscosity region can be defined as pressure window for many factors affecting the consolidation process. The fiber and lay‐up type largely affect the time of pressure application. For laminates with the same fiber and lay‐up type, the fiber distribution is not much influenced by pressure cycle. The uneven degree of fiber distribution is larger for the fiber bed having higher deformation properties. With the genetic algorithm optimization system, the time of pressure application can be gotten quickly. It is helpful for the improvement of composite parts quality, reduction of the fabrication cost. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Void formation model and measuring method of void formation condition during hot pressing process

For resin matrix composites, voids are common defects that can seriously deteriorate the properties of the composite parts. Thus, the elimination of voids is a crucial element in controlling the manufacturing process of composite parts. This article focuses on void formation originating from hygroscopic water for resin matrix composite laminates prepared with hot pressing process. The Kardos void formation model was developed to analyze the critical resin pressure for the initiation of voids, and the influencing factors were investigated experimentally to validate the modified model. It is found that resin pressure and gel temperature are the two key parameters to control void defects and that entrapped air in prepreg stacks must be considered in the void formation model. Furthermore, a simple method was established to measure the relationship between porosity and the processing parameters, and the void formation conditions of the resin and the prepreg stack were also studied. The theoretically predicted void formation conditions and the experimental results were compatible for the studied cases. These results are valuable for eliminating void defects, optimizing processing parameters, and enhancing the performance of composite parts. POLYM. COMPOS., 31:1562–1571, 2010. © 2009 Society of Plastics Engineers     

Effects of compaction and UV exposure on performance of acrylate/glass‐fiber composites cured layer by layer

With an aim to reducing manufacturing costs, in general and specifically to provide a solution to the thick laminate curing depth issue for composite materials, UV curing technology was combined with a fiber placement process to fabricate acrylate/glass‐fiber composites. A novel layer‐by‐layer UV in situ curing method was employed in this article and interlaminar shear strength (ILSS) tests and SEM were used to evaluate the effect of processing parameters, including compaction force and UV exposure dose, on ILSS. The SEM images from short‐beam strength test samples and the results of ILSS showed that the fibers' distribution was uniform in the cured matrix resin resulting from the compaction forces and that beneficially influenced the ILSS of the composite greatly. However, the matrix resin produced large shrinkage stresses when it reached a high degree of conversion (DC) in one‐step, which resulted in poor interlaminar adhesion. In addition, the fast curing speed of UV on the composite resulted in poor wetting between fiber and resin, and accordingly resulted in lower ILSS. To overcome these problems and obtain high ILSS value composites, an optimized compaction force and UV exposure dose were determined experimentally. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Pultrusion die pressure response to changes in die inlet geometry

Pultrusion is a process of manufacturing composites that requires a high resin pressure rise in the tapered die inlet region. A sufficiently high pressure rise is important for a good quality pultruded product, thereby necessitating a study of the mechanisms affecting the die inlet pressure rise. Various process control parameters affect the resin pressure rise in the die inlet. The geometry of the tapered die inlet region can have a significant effect on the pressure rise in the pultrusion die. In this study a finite element model was developed to predict this pressure rise as a function of the tapered die inlet geometry. The composite matrix being pultruded was modeled on the assumptions of Darcy's laws for flow in porous media. A Galerkin's weighted residual based finite element technique was used to solve the governing equations. The pressure rise in the tapered inlet region of the die, as well as in the straight portion of the die, is predicted by this finite element model. Circular, parabolic, and wedge shaped die inlets have been modeled to compare their shapes on the resin pressure rise in the pultrusion die. Different angles for wedge shapes, different radii for circular shapes, and different foci for parabolic shapes were modeled to predict the influence of varying key geometrical parameters for each die inlet contour on pressure rise. The finite element model developed provides insight as to how to design the die inlet to produce a suitable pressure rise in the pultrusion die inlet.