Internal stress control in epoxy resins and their composites by material and process tailoring

The development of internal stress during cure of epoxy and hyperbranched polymer-modified epoxy resins was characterized, taking into account the evolving viscoelastic properties, the volumetric shrinkage due to the chemical reaction, and the thermal expansion. A criterion for void formation during cure in a constrained mold was proposed, providing guidelines for the construction of a process window for manufacturing of void-free composites. It was shown that the internal stress development in epoxy resins during cure is strongly influenced by the presence of hyperbranched polymer modifiers. The role of these modifiers was illustrated for the case of autoclave processing of glass fiber/epoxy composites. This study showed that higher fiber volume fractions could be used with hyperbranched polymer-modified resins than with unmodified resins, for producing void-free laminates. It also appeared that by suitable tailoring of the process cycle, a fully stress-free laminate could be obtained after cure, using the modified resin.     

The Influence of Temperature and Pressure During the Curing of Prepreg Carbon Fiber Epoxy Resin

The physical and hence mechanical properties of carbon fiber reinforced epoxy resin are affected by the curing conditions used in their manufacture. The relationship between the cure temperature and pressure and the density, fiber volume fraction, and the void content of cured laminates, was investigated. For the unidirectional 914C prepreg material used, an optimum cure temperature was found which gave maximum fiber volume fraction and composite density, and minimum void content. This behavior is related in the paper to resin flow and cure characteristics. A linear relationship between cure pressure and fiber volume fraction is reported and explained by reference to the void content of the laminates. It is concluded that in-house trials are required to determine the optimum size of the processing window for specific systems and components.     

Stress‐initiated void formation during cure of a three‐dimensionally constrained thermoset resin

During cure of epoxy resins, polymerization induces an increase in mechanical properties, which is accompanied by a volumetric shrinkage. When the resin is cured in a constrained mold to which it adheres, tensile stresses will hence develop, which may exceed the stremgth of the resin at a given curing stage. Voids will then form. The origin and governing parameters of void formation are studied using an epoxy resin cured in a three‐dimensionally constrained glass mold following isothermal cure cycles. Two types of voids are shown to appear during cure, one early in the process and a second around the gelation point. A viscoelastic analysis of the material stress state over the whole range of cure is performed. Both the viscoelastic modulus obtained from a time‐cure‐temperature superposition and the volumetric shrinkage, which was continuously measured by density change, are taken into account. A value for the critical internal stress at void initiation is thus proposed. This criterion can be used to provide guidelines for tailoring the material properties toward an increase of the critical stress for void initiation. Also, since during theprocessing of composite materials, cases may arise where the resin cures within the interstices left between consolidated fibres that do not move, this critical stress failure criterion can be of use in the eastablishment of a process window providing guidelines for the production of void free composites.     

Formation of voids in composite laminates: Coupled effect of moisture content and processing pressure

Void formation as a result of prepreg moisture content and processing pressure during cure was experimentally investigated in thermosetting composite laminates. This was achieved by determining the void contents of eight‐ply laminates fabricated from TenCate^® BT250/7781 E‐glass/epoxy prepreg at processing pressures of 1.7, 3.0, 4.4, and 5.8 atm. At each processing pressure, three types of laminates were fabricated using: (i) unconditioned prepregs (direct from the storage bag); (ii) prepregs conditioned at 25% relative humidity; and (iii) 99% relative humidity. Dynamics of prepreg moisture uptake during conditioning was measured using a moisture analyzer and was shown to exhibit Fickian diffusion behavior. The void contents of the cured laminates were found to vary from 1.6% to 5.0% depending on humidity environment the prepregs were exposed and the pressure applied during fabrication. The void contents of all laminates were observed to approach an asymptotic value of ∼1.6% as pressure was increased. The experimental results indicated the processing pressure applied during fabrication was increasingly carried by the fiber bed, reducing resin pressure during cure. Therefore, an enhanced void formation model was proposed through the addition of a pressure reduction factor and an asymptotic void content term. The proposed model was found to accurately predict the void content of laminates made of prepregs exposed to constant/varying humidity environments and fabricated at a wide range of processing pressures. POLYM. COMPOS., 36:376–384, 2015. © 2014 Society of Plastics Engineers     

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     

基于材料物性参数时变特性的复合材料层合板固化残余应变应力数值模拟

热固性树脂基复合材料层合板成型过程形成的残余应力是影响材料质量的重要因素。针对复合材料固化过程建立了基于复合材料物性参数时变特性的复合材料固化过程的三维多场耦合计算模型。该模型包含经典的热-化学模型、树脂固化动力学模型、残余应力模型;在此基础上将材料物性参数时变特性引入多场耦合计算模型中,模型计算结果通过与文献中实验结果比较,验证所建立的固化模型的可靠性;在此基础上,对AS4/3501-6复合材料层合板的固化残余应变应力进行数值模拟,研究了固化过程中残余应变/应力的变化规律,分析工艺参数对应力应变的影响。通过与光纤光栅应变试验比较,验证其正确性。研究结果表明:模型可以很好地仿真复合材料固化过程;温度、树脂体积分数、铺层角度对层合板应力/应变都有较为显著的影响,为正相关关系,其中树脂的体积分数影响最为显著。     

Processing‐interphase‐property relationship in fiber‐reinforced thermosetting‐matrix composites

Fabrication of thermosetting‐matrix composites is based on a critical step of cure, which involves applying a predefined temperature cycle to a fiber‐resin mixture. Several temperature‐dependent mass transport processes occur in the vicinity of the reinforcement fiber, leading to the formation of an interphase region with different chemical and physical properties from the bulk resin. The cure cycles applied on the macroscopic boundaries of the composite govern the microscopic cure kinetics near the fiber surface, which in turn determines the interphase and composite properties. A predictive approach to directly linking the cure cycles and final composite properties is not presently available and is established for the first time in this paper. A multiscale thermochemical model is developed to predict the concentration profile evolution with time near fiber surfaces at various locations across the composite thickness. The concentration profiles at the gelation time are mapped to modulus profiles within the interphase region, and a finite element analysis is used to determine the overall composite modulus in terms of the constituent interphase, fiber, and matrix properties. Relevant numerical results are presented for the first time where the composite modulus is directly linked to the cure cycle and interphase formation parameters without assumed structures or properties of the interphase. The results provide useful information for selecting material components and cure cycles parameters to achieve desired interphase and composite properties. POLYM. COMPOS., 26:193–208, 2005. © 2005 Society of Plastics Engineers     

一种双酚A型氰酸酯树脂流变特性及其改性研究

本文采用平板法研究了一种双酚A型氰酸酯树脂的流变特性,通过恒温预固化的方法对其进行改性,并借助红外光谱、差示扫描量热法(DSC)和凝胶渗透色谱法(GPC)等手段分析了预固化对氰酸酯树脂化学结构的影响.在此基础上,针对S-2玻璃纤维/氰酸酯复合材料层板,采用热压机工艺考察了加压时机和预固化对层板成型质量的影响.研究结果表明,该氰酸酯树脂反应活性低且流动性高,热压工艺中树脂流动不易控制;预固化可使氰酸酯树脂发生一定程度的自聚反应,从而降低其流动性,可明显改善氰酸酯树脂的工艺性,提高了层板的成型质量.     

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     

A model for the consolidation of warp‐knitted reinforced laminates

A model for the consolidation of thermoplastic composites reinforced with a warp‐knitted fabric was developed. The evolution of void content during composite consolidation achieved via film stacking was related to the processing parameters and the material properties. Since the knitting process results in local variations of fiber volume fractions even within a tow, the flow mechanisms are not identical to those occurring in homogeneously dispersed fibers. Two different types of micro‐scale porosities were determined and a unit cell geometry was proposed for modeling. The reduction of the effective pressure due to the gas entrapment was also accounted for. Predictions of the variation of the void content as a function of time, pressure and temperature were compared to experimental data. As a good correlation was found between experiments and predictions, this approach can thus be applied to the consolidation of textile reinforcements in which the tows are locally compressed.     

A kinetics model for interphase formation in thermosetting matrix composites

During the cure of thermosetting polymer composites, the presence of reinforcing fibers significantly alters the resin composition in the vicinity of the fiber surface via several microscale processes, forming an interphase region with different chemical and physical properties from the bulk resin. The interphase composition is an important parameter that determines the micromechanical properties of the composite. Interphase development during processing is a result of the mass‐transport processes of adsorption, desorption, and diffusion near the fiber surface, which are accompanied by simultaneous cure reactions between the resin components. Due to complexities of the molecular‐level mechanisms near the fiber surface, few studies have been carried out on the prediction of the interphase evolution as function of the process parameters. To address this void, a kinetics model was developed in this study to describe the coupled mass‐transfer and reaction processes leading to interphase formation. The parameters of the model were determined for an aluminum fiber/diglycidyl ether of bisphenol‐A/bis(p‐aminocyclohexyl)methane resin system from available experimental data in the literature. Parametric studies are presented to show the effects of different governing mechanisms on the formation of the interphase region for a general fiber–resin system. The interphase structure obtained from the model may be used as input data for the prediction of the overall composite properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3220–3236, 2003     

Tow impregnation during resin transfer molding of bi-directional nonwoven fabrics

A model has been developed for analyzing resin impregnation of fiber tows during resin transfer molding of bi-directional nonwoven fiber performs. The model is based on the existence of two main regions of resin flow: the macropore space formed among fiber tows and the micropore space formed among individual fiber filaments within a tow. The large difference in permeability between these two regions of flow leads to the potential for void formation during resin transfer molding. The model was formulated for both constant flow rate and constant pressure mold filling. For ambient pressure mold filling, the model predicts a difference in the size of the voids and distribution between axial tows (oriented along the flow direction) and transverse tows (oriented in the transverse direction). When vacuum is imposed on the mold, the model predicts the same resin impregnation behavior for both axial and transverse tows. Furthermore, given sufficient time, voids generated under vacuum mold filling will eventually collapse because of the absence of an opposing internal void pressure. In addition to insights on void formation, the model also provides a basis for the study of the relationship between resin transfer molding parameters and the resin impregnation process.     

In situ monitoring of residual strain development during composite cure

Internal (residual) stresses build up in a thermosetting composite as the matrix shrinks during cure, and again as the composite is cooled to ambient from its elevated processing temperature. These stresses can be significant enough to distort the dimensions and shape of a cured part as well as initiate damage in off‐axis plies, either during fabrication or under the application of relatively low mechanical loads. The magnitude of these stresses depends on a number of factors including constituent anisotropy, volume fraction and thermal expansion, ply orientation, process cycle, and matrix cure chemistry. In this study, embedded strain gauges were employed to follow, in situ, the buildup of residual strains in carbon fiber‐reinforced laminates during cure. The data were compared to those from volumetric dilatometer studies to ascertain the fraction of resin shrinkage that contributed to residual stress buildup during cure. Based on earlier studies with single‐fiber model composites, the process cycle in each case was then varied to determine if the cycles optimized to minimize residual stresses for isolated fibers in an infinite matrix were applicable to the reduction of residual stresses in conventional multifiber composites. The results of these studies are reported here.     

Defect reduction strategies for the manufacture of contoured laminates using vacuum BAG‐only prepregs

Complex structures manufactured using low‐pressure vacuum bag‐only (VBO) prepreg processing are more susceptible to defects than flat laminates because of complex compaction conditions present at sharp corners. Consequently, effective defect mitigation strategies are required to produce structural parts. In this study, we investigated the relationships between laminate properties, processing conditions, mold designs, and part quality in order to develop science‐based guidelines for the manufacture of complex parts. Generic laminates consisting of a central corner and two flanges were fabricated in a multipart study that considered variation in corner angle and local curvature radius, the applied pressure during layup and cure, and the prepreg material and laminate thickness. The manufactured parts were analyzed in terms of microstructural fiber bed and resin distribution, thickness variation, and void content. The results indicated that defects observed in corner laminates were influenced by both mold design and processing conditions and those optimal combinations of these factors can mitigate the defects and improve quality. POLYM. COMPOS., 38:2016–2025, 2017. © 2015 Society of Plastics Engineers     

A process for prestaging thermosetting towpreg

A method for prestaging thermosetting towpreg consisting of passing the material through a tunnel oven was developed to address the difficulties inherent in making thick-walled thermosetting composite parts: aging, moisture uptake, and low resin viscosity during lay-up; and residual stresses, warping, thermal degradation, resin gradients, and excessive void nucleation during the final cure. AS-4 carbon fiber in a 3501-6 epoxy matrix was used to test the continuous prestaging process. With prestaging, significant reductions in towpreg bulk, voids, and total enthalpy were achieved. Cure shrinkage also was achieved. The final towpreg product had good drape and greatly reduced tack. These changes addressed many of the aforementioned difficulties. A relation between residence time at 200°C (the process temperature) versus percent degree of cure (%DOC) was developed. Manufacturing characteristics such as drape, tack, and bulk were related to %DOC. A processing window of 15 to 22 %DOC yielded towpreg with optimal characteristics. Prestaged towpreg would be well suited to manufacturing with advanced fiber placement (AFP) type equipment. Initial trials in industry have been promising.     

Mechanical characterization of carbon fiber reinforced phenolic resol and epoxy condensate plastic

A matrix resin for carbon fiber reinforced composite was developed that consisted of resol type phenolic and difunctional epoxy resin (PR-EP) condensate or adduct. Carbon fiber reinforced composite with fiber volume fraction of 0.6 was prepared with PR-EP matrix containing 0, 50, 100, 150, and 175 parts of epoxy resin per hundred parts of phenolic resin (php), especially a synthesized resol type. One-shot and prepreg techniques have been adopted and the study of loss of volatiles has indicated the superiority in terms of favorable processability of prepreg technique over the other. FTIR spectroscopic analysis confirmed the PR-EP adduct formation at the prepreg preparation stage. The improvement in properties such as tensile strength and elongation at break was observed in resin matrices with epoxy and phenolic resin; however, the flexural strength and modulus remained more or less unaltered. The prepreg technique of composite preparation has resulted in substantial improvement in mechanical properties and the same was attributed to the formation of PR-EP adduct and low volatiles during cure. Composites of carbon fiber reinforced PR-EP matrix developed here are likely to meet the requirement of aerospace structures in view of the realization of a wide spectrum of properties.     

The effect of microwave resin preheating on the quality of laminates produced by resin transfer molding

Cycle times in resin transfer molding (RTM) of an unsaturated polyester have been reduced significantly using an in-line microwave resin preheating system. Microwave preheating lowers the resin viscosity during injection and modifies the thermal age of the resin, potentially influencing the quality of RTM laminates. The tensile properties of RTM laminates have been measured with regard to improved fiber wet-out by the lower viscosity resin. Degree of cure measurements have been included to establish the effect of microwave preheating on resin conversion within the laminate. Local pressure that develops within the mold during the cure phase can lead to mold deflections. Variations in the laminate thickness associated with these deflections are presented, and the use of microwave resin preheating to reduce these variations is discussed.     

Properties of carbon fiber composite laminates fabricated by coresin film infusion process for different prepreg materials

A new cocured process called coresin film infusion (co‐RFI) process, which combines RFI process and prepreg/autoclave process, was introduced and four kinds of commercial carbon fiber prepreg material systems and a kind of resin film were applied to fabricate co‐RFI laminates. The compatibility between the resin film and the prepreg matrix and the application of co‐RFI process were investigated based on the resin flowability, glass transition temperature of cured resin, processing quality of laminate, and variation in resin modulus on cocured interphase region measured by nanoindentation. Furthermore, mode I (G_IC), mode II (G_IIC) delamination fracture toughness, and flexural strength and modulus were measured to evaluate the mechanical properties of cocured laminates with different prepreg materials. The experimental results show that thickness and fiber volume fraction of co‐RFI laminates with the four kinds of prepreg materials are similar to those of prepreg laminates and RFI laminate with acceptable differences. In addition, there are no obvious defects in co‐RFI laminates. Moreover, the reduced modulus of resin at cocured interface and glass transition temperature values of the mixed resin reflect good compatibility between prepreg matrix resin and RFI resin. The G_IC, G_IIC values, and flexural performances of cocured laminates lie between and even exceed those of prepreg laminates and RFI laminates, indicating no weakening effect in the cocured interface. Therefore, the co‐RFI process is believed to effectively fabricate composite with low cost and it can be applied using various prepreg systems. POLYM. COMPOS., 34:2008–2018, 2013. © 2013 Society of Plastics Engineers     

Cure temperature effects on cryogenic microcracking of polymeric composite materials

A model prepreg system was used to evaluate the effect of cure temperature on microcracking in polymeric composite materials exposed to cryogenic cycling. Symmetic and unsymmetric carbon fiber/epoxy laminates were fabricated to examine the development of thermal stresses and microcracks at cryogenic temperatures. The residual strains and theoretical curvatures of the laminates were calculated from the composite properties and correlated with the microcrack density and experimentally observed curvatures. Higher cure temperatures resulted in higher stress free temperatures and residual strains in the laminates, which corresponded directly to increased levels of microcracking.