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Applying external pressure on the vacuum bag in heated-Vacuum Assisted Resin Transfer Molding (heated-VARTM) is a new approach for manufacturing high quality composite laminates with minimal voids. The present study focuses on the effects of external pressure and resin flushing on the microstructural features of the laminates, including spatial variation and size distribution of the voids as well as the compaction of fiber tows. The microstructural analysis performed on glass/epoxy composites reveal that external pressure reduces void content and average void size, and leads to more uniform spatial distribution of voids. Significant increase in fiber volume fraction from ∼50% up to ∼62% and a low void content of less than 1% are achieved by applying modest gauge pressure levels up to 138 kPa on laminates with various thicknesses. The increase in fiber volume fraction coupled with low void content improves the flexural properties as much as 20%, while the short-beam shear strength of the laminates remains unchanged due to low void content in all laminates.

Modeling the inhomogeneous microstructures of fibrous tows is important for analyzing the process of resin transfer molding because dual-scale pores in a preform can lead to void formation. This study focused on the development of a microscopic flow analysis method to predict the impregnation of fiber bundles. The moving particle semi-implicit method was adopted to model the microstructure of a fiber bundle explicitly and inter-particle potential force was introduced into the numerical model to take account for the capillary effect. The predicted process of impregnation and void formation agreed with empirical observations. The developed approach was applied to predict the relationship between the modified capillary number and void content to identify the optimal molding conditions to reduce microvoids. The obtained relationship reproduced the trends of a reported experiment, which indicates that the proposed approach will provide information about optimal conditions for minimizing void content.

In an RTM process the mold contains three constituents including fibres, a resin and a gas. Due to its similarity to a porous medium, the process can be modeled within the Theory of Porous Media (TPM), where the simulation of the moving resin front generates an inherent difficulty: In widely used approaches for mold filling simulations, that are element/control volume methods (Bruschke and Advani, 1990) and Level set methods (Soukane and Trochu, 2006), the moving resin front is either approximated roughly, such that an interface thickness of multiple finite elements arises, or respectively by a sharp interface with zero-thickness which requires a special numerical treatment. The present study intends to close this gap. To this end, we outline regularizations of the sharp interface such that the above mentioned difficulties can be overcome with a regularized sharp interface theory as a branch of phase-field models which go back to the classical Ginzburg-Landau equation. In this way, the sharp discontinuities between different phases, i.e. resin and gas, are approximated by smooth transitions of suitable order parameters, of which one is the resin fraction. Thus, the sharp interface topology as well as the surface energy of the interface are smeared out over a region proportional to a chosen regularization length scale. As an additional advantage, the phase-field method facilitates a thermodynamic treatment of phase interfaces rendering it more physically consistent in combination with the TPM, which also works within a thermodynamically sound framework. To this end, a coupled finite-element strategy is presented accounting for both, a Ginzburg-Landau- and Cahn-Hilliard-type regularization scheme for the interface. Two boundary value problems are formulated in the context of the TPM. They consider a multiphase-system, represented by deformable fibres interacting with the resin saturating the pores, that is coupled to one of both regularization schemes. Accordingly, two numerical examples demonstrate the capabilities of both regularized formulations by simulations of an RTM process.

The manufacturing of fiber composite materials involves a set of complex, interconnected processes that span across multiple physics and scales. The characterization of uncertainty in composite manufacturing predictions is a challenging task that involves high-dimensional, multiscale, multiphysics stochastic models. We demonstrate the use of a basis adaptation scheme within a polynomial chaos representation that permits the incorporation of a large number of stochastic variables in the analysis. We use the proposed PCE-based workflow to analyze the interplay of uncertainty through all the fiber composite manufacturing stages that comprise the Resin Transfer Molding (RTM) process. The proposed framework is centered on an integrated assessment of uncertainty in composite structures using probabilistic surrogate models for predefined QoI.

Manufacturing defects are an issue of composite manufacturing processes. Local clamping is proposed to prevent fiber washout during high pressure resin transfer molding of non-crimp fabrics. At the clamping regions the material is compacted by through-the-thickness compression. The current work investigates the mechanical performance of such regions under static and fatigue loading. Two typical manufacturing defects are intrinsic to the compacted regions: fiber undulation and resin starved/dry spots. In component tests the thereby generated damage is monitored using 3D digital image correlation and the complete processes of damage initiation and propagation are analyzed. Static and low cycle fatigue failure are dominated by fiber undulation at the edges of the compacted region, triggering matrix damage and delaminations. High cycle fatigue is dominated by decreasing fiber matrix adhesion within the compacted region, leading to longitudinal splitting and progressive fiber failure. Identification of damage initiation is supported by numerical finite element simulations.

Transverse compaction and geometric conformation of fiber reinforcements have a significant importance in the production of complex dry fiber preforms for the resin transfer molding (RTM) process. This work investigates for the first time the planar compaction and preforming of two reinforcements made from aligned flax yarns held together by a thin layer of short kraft pulp or flax fibers. Planar compaction tests have been carried out in order to evaluate the influence of temperature, pressure, humidity and number of layers of reinforcement on the creep percentage, thickness recovery and permanent deformation of reinforcements. Next, the effect of process parameters on the quality of a complex 3D preform was evaluated. It appears that temperature, pressure and humidity increase the permanent deformation while the number of layers acts inversely. In addition, hot and wet made preforms have better dimensional stability (dimensions and geometry) than preforms obtained under other conditions.

The epoxy nanocomposites with Fe3O4 modified graphene oxide (Fe3O4/GO) were used to influence the micro-cracks resistance of carbon fiber reinforced epoxy (CF/EP) laminate at 77 K. Fe3O4/GO with good paramagnetic properties were prepared by co-precipitation method and used to modify epoxy for cryogenic applications. Fe3O4/GO modified CF/EP laminates were also prepared through vacuum-assisted resin transfer molding (VARTM). The results show that the Fe3O4/GO can effectively improve the mechanical properties of epoxy (EP) matrix at 77 K and reduce the coefficient of thermal expansion (CTE) of EP matrix. It also can obviously improve the micro-cracks resistance of CF/EP composites at 77 K. Compared to neat EP, the CTE of Fe3O4/GO modified EP composite is decreased 51.6%. Compared to CF/EP composite, the micro-cracks density of Fe3O4/GO modified CF/EP composite at 77 K is decreased 60.0%.

The main objective of this work was an experimental investigation making it possible to monitor, as well as achieve a deeper understanding of, the structural evolution of polyamide-6 as a thermoplastic-based composite matrix. Polyamide-6 from anionic ring-opening polymerization (AROP) of e-caprolactam was chosen as a model matrix. Throughout this work, various formulations of PA with anionic polymerization from caprolactam were studied depending on different activator/catalyst pairs, concentrations and combinations. Moreover, chemorheological properties were first determined ex-situ by rheology coupled with FTIR and microdielectrometry. The extent of the reaction conversion was thus obtained and subsequently modeled. The formed PA6 matrix materials were fully characterized in terms of their viscosimetric properties and molar masses. In a second step, a hybrid extrusion resin transfer molding machine (T-ERTM) with an instrumented mold was designed for the in-situ monitoring. Thereby, specific dielectric sensors were used to follow the different processing steps for the manufacturing of complex and continuous glass fiber-reinforced parts. Furthermore, the reaction kinetics in competition with the crystallization were probed and quantified as a function of the processing parameters (mold temperature, times given for impregnation, time for demolding etc.). Calibration curves were obtained in which the ionic conductivity was correlated to the change of viscosity determined previously from ex-situ measurements. Indeed, a processing window was proposed for each PA6 system to ensure a good preform impregnation. Interestingly, the apparent conversion calculated from the online dielectric measurements corroborated the ex-situ values obtained from chemorheological studies. The viscosity and conversion evolution were then tracked in the mold. Based on the present findings and for the optimal formulation, a Time-Temperature-Transformation (TTT)-equivalent diagram was established from online measurements and related to the reactive processing to get a better handle on the PA6-based composite.

We present the development of a high-performance flame-retardant vinyl ester (FR-VE)/flax fabric (FF) biocomposite through a hybrid approach using vacuum-assisted resin transfer molding. FF was modified to form FR fibers (mFF) using surface treatment with varying amounts (5–20%) of ammonium polyphosphate (APP), and the FR-VE/mFF composites were fabricated by including flame retardants such as chitosan (CS), APP, and modified CS (NCSAPP). The optimized mFF (15% APP) exhibited enhanced thermal stability (38 wt% residue at 700 °C) and it achieved a V0 rank in the UL 94 test. In addition, the dual effects, i.e., the effects modification of the reinforcement and inclusion of the FRs into the matrix, improved the properties of the FR-VE/mFF composites, providing them with high thermal resistance (28 wt% residue at 700 °C), and fire resistance (V0 rank in the UL 94 test), and self-extinguishing characteristics with a suppressed peak heat release rate (69%), total heat release, and smoke production rate.

Liquid composite molding (LCM) simulations for processes such as resin transfer molding (RTM) and vacuum assisted resin transfer molding (VARTM) face serious challenges while modelling resin flow through dual scale fabrics such as bi-directional (BD) (stitched or woven) fabrics and triaxial fabrics as it requires characterization of two permeability values: inter-tow gap or bulk permeability and tow permeability. Conventional experimental methodologies can characterize the inter-tow gap permeability by tracking the wetting resin flow front; however, there is no closed form solution that gives an expression for effective wetting permeability due to the uncertainty in determining tow permeability explicitly. If one were to have prior knowledge of tow permeability, effective wetting permeability can be formulated that can reduce the effort of experimental characterization. The current study proposes closed form permeability model for dual scale unidirectional (UD) fabric that combines both the tow and inter-tow gap permeability while taking into account the effect of partially saturated zone during dual scale flow. Thus, it accounts for the effects of inter-tow gap permeability, tow permeability and length of partially saturated zone. Comparisons of the analytic model results with numerical simulations are presented that are encouraging.

A novel sandwich-structured composite comprising a three-dimensionally (3D) printed core and a woven carbon fiber/polyamide-6 facesheet was fabricated and characterized. By using selective laser sintering, which is a 3D printing technology, a structurally reinforced honeycomb-truss hybrid core was produced, which had superior compressive stiffness in various directions compared with conventional honeycomb cores. The interface between the core and the facesheet was enhanced by laser power control during the sintering as well as plasma surface treatment. The sandwich composite was produced by the reactive thermoplastic resin transfer molding (T-RTM) technique using anionic polymerization of e-caprolactam, which has an ultra-low viscosity in the molten state. This in-situ polymerization not only enabled manufacturing process simplification by combining fabrication and bonding processes of the facesheet, but also provided excellent resin impregnation into the fiber fabrics and core surface. Therefore, the 3D printing technology, interfacial strengthening techniques, and T-RTM process induced synergistic effects on mechanical properties of the sandwich composite by forming structural reinforcement and strong mechanical and chemical interactions. The edgewise compressive properties of the hybrid core were significantly better than those of a normal honeycomb core. In addition, the interlaminar fracture toughness, impact energy absorption, and penetration limit of the sandwich composite consisting of the structurally and interfacially strengthened hybrid core increased by 76, 77, and 125%, respectively, compared to those of a nonreinforced sandwich composite.

Void defects will inevitably occur in 3D braided composites during Resin Transfer Molding (RTM) process, which may reduce mechanical properties of 3D braided composites. Therefore, this paper focuses on void generation in 3D braided composites and optimizes its manufacturing technology. First, the basic governing equations considering flow, temperature changing and resin curing process are derived to describe the conservation of momentum, energy and mass of resin during RTM process. Second, the whole RTM process for 3D braided preform cell is simulated by means of finite element method (FEM), which indicates the distribution of resin flow speed, pressure and void content. Third, influences of resin injection flow volume, outlet pressure, volatile content in resin and curing temperature on void distributions and contents in 3D braided composite cell are investigated. The results will provide theoretical basis for analyzing void generation and evaluating the quality of 3D braided composites made by RTM technology.

This study aims at understanding the geometrical instability triggered by the residual stresses generated during the manufacturing and post-curing of composites plates. Asymmetric plates with a [0/90] configuration were manufactured by Resin Transfer Molding (RTM). Manufactured plates were reheated free standing in an convection oven to study their behavior at high temperatures. Digital image analysis was used to monitor the plates curvature evolution with time and temperature. The experimental results of this work demonstrates the impact of the resin’s viscoelastic behavior on the geometrical distortion generated by residual stresses in asymmetric plates at high temperatures.

Although vacuum-assisted transfer molding (VARTM) is preferred for manufacturing medium to large composite parts due to its simple tooling and low cost, part quality dictated by dimensional tolerances, void content and mechanical properties is usually low due to inherent limitations of the process. In this study, the conventional VARTM process was modified by external pressurization of a heated mold to increase fiber volume fraction and improve mechanical properties of laminates. During post-filling, various levels of external pressure were applied in a pressure chamber mounted on top of the mold. It was observed that pressurized VARTM led to laminates with less than 1% void content. In addition, fiber volume fraction and flexural strength were increased 25% and 13% with respect to non-pressurized VARTM, respectively which demonstrates the potential for manufacturing considerably higher quality composites by pressurized VARTM.

In Resin Transfer Molding (RTM) process, pre-designed channels branching out like runners from the injection port can be used for faster resin distribution and impregnation as compared to traditional single gate injection to reduce the mold filling time. To search for the optimal channel design for maximum fill time reduction, the Medial Axis (MA) of the part surface is defined. Next, a methodology is developed to search for the topology of the MA using Finite Element based mold filling simulation software. The MA location is corrected for a part with non-homogeneous fabric permeability. Finally, some case studies are presented to illustrate the effectiveness and accuracy of the channel design approach for RTM with the objective of minimizing fill time, when fabricating composite parts that contain complexities in both the geometric features such as compound curvatures and corners and in material properties such as non-homogeneous and highly anisotropic fiber preform permeability.

In this work, the formation of porosities in an RTM epoxy resin is investigated in the context of Resin Transfer Molding (RTM). First, differential scanning calorimetry and rheology analyses are carried out to model the cure kinetics and the viscosity, and to find the gel point. The critical time for the application of consolidation pressure is studied using an innovative experimental setup that allows visual observation of the curing resin and gelation as well as void formation and dissolution during the process. The desorption process begins with the melting of the curing agent. It is found that consolidation pressure is the key processing parameter to keep composite parts free of porosities. Applying a consolidation pressure before reaching the crossover point of the loss modulus and the storage modulus (i.e. tan δ = 1) allows dissolving all the gases into the bulk resin. In the present case, this happens when the conversion level reaches 94%.

With respect to the requirement for good dispersibility and ordered arrangement for graphene in three dimensional graphene architecture (3DGA) and the performance for epoxy-based composites, 3DGA was synthesized in various pore diameters and RTM was introduced into the fabrication of 3DGA/epoxy composites compared to the graphene/composites in direct-mixing method. Mechanical modulus and strength of 3DGA/epoxy composites increased by one or two times compared with pure epoxy. Relative to composites in direct-mixing method, the increase in compressive and flexural strength of 3DGA/epoxy composites was at least twofold to tenfold. On account of the decreased pore size for 3DGA, the compressive and flexural strength was in noticeable improvement of 41% and 78%, respectively. With distinct network connecting structure of 3DGA, over 5 orders of magnitude enhancement took place in electrical conductivities of thermal reduced 3DGA/epoxy composites. It was conducive to prepare high performance composites by adjusting the internal structure of graphene/epoxy composites.

Sandwich composites consist of two rigid outer facesheets that are bonded to a lightweight internal core. Although they have a high stiffness-to-weight ratio, these structures have low interlaminar strengths, which may lead to core-to-facesheet separation when subjected to out-of-plane stresses. In this study, sandwich composites with carbon/epoxy facesheets and a 110 kg/m3 foam core, were manufactured with through-the-thickness reinforcements, or stitches, using a vacuum-assisted resin transfer molding process. A design of experiments approach was used to investigate the influence of stitch parameters (stitch density, linear thread density, and facesheet thickness) on fracture energy. Single cantilevered beam tests were performed to obtain the mode I dominant fracture energy for each treatment combination. This study reveals and discusses the unique fracture surface morphologies associated with stitch processing parameters for a sandwich composite. For increased fracture toughness, the optimum stitch parameters for different linear thread densities are captured in the response surface model.

A physics-based numerical model for prediction and compensation of the process-induced distortion of 3D woven composite (3DWC) is presented. A new tool-part interaction modelling method for the Resin Transfer Molding process is developed, in which the nonlinear material properties and non-thermoelastic distortion are taken into account. Flat and L-shaped 3DWC parts were fabricated and the process-induced distortions were evaluated. Adopting the input values determined by the warpages of flat parts, the spring-in of L-shaped parts predicted by the model is consistent with the experimental measurements. Based on the model, it is found that taking the warpage/spring-in of the uncompensated parts as the compensation value will result in a deviation between the manufactured parts and the nominal shape. An improved comprehensive compensation procedure is proposed and applied to an L-shaped 3DWC part. The result shows that the dimensional precision of the part can be significantly improved by the compensation process.

A multi-objective optimization (MOO) approach for multi-point injection of resin-transfer molding (RTM) is proposed to investigate the trade-off relationship between productivity and quality for composite structures. With this approach, the optimum gate positions for their molding properties are evaluated using finite-element analysis (FEA) with a multiple-objective genetic algorithm (MOGA), and the trade-offs are visualized with the combination of a self-organizing map (SOM) and a scatter plot matrix (SPM). We applied this approach to RTM for flat-plate and rib models. For the flat-plate model, we found a negative correlation between fill time and weld line contents, and between fill time and dry spot contents. These results imply difficulty for simultaneous reduction of cycle time and void contents. For the rib model, some tendencies agree with those of the flat-plate model, and others do not. This difference comes from complexity of structure. We also found Pareto solutions that satisfy both productivity and quality for the flat-plate and rib models (i.e., gate positions such as a combination of diagonal and quadrangle positions for the flat-plate model, and aggregation of gate positions at a beam part for the rib model). Furthermore, we conducted simple experiments for the flat-plate model to validate the simulation result. The trends acquired from MOO qualitatively agree with the experiments.

Filtration of nanofillers into porous fabric media is still an issue during the preparation of advanced fiber-reinforced composites. The assessment of resin/multiwall carbon nanotube (MWCNT) flow, MWCNT filtration, and the cure monitoring of glass fiber/carbon nanotube-polyester composites by means of the measurement of the electrical resistance was introduced. The vacuum-assisted resin transfer molding technique was used. The electrical resistances measured over the span of a composite were qualitatively correlated with MWCNT flow and the degree of MWCNT filtration. It was found that while the complexity of the fabrics could likely introduce preferential deposition of MWCNTs, their filtration is mainly affected by their dispersion state in the resin suspension. Relationships among critical parameters such as the lengths and diameters of MWCNTs, the inter- and intra-tow dimensions of glass fabrics, the dispersion level of MWCNTs, and the viscosity of nanocomposite samples are discussed and correlated to the filtration, cure, and flow phenomena. We showed that our method can also serve as an early warning to obviate defects in the resulting composite.

In Resin Transfer Molding (RTM) processes, liquid resin is injected into a dry reinforcement structure to create a composite part within given time limits. To reduce the fill time, resin may be injected into the mold through multiple gates. The minimum number of gates and their locations needs to be determined. To reduce the number of scenarios to be simulated, an iterative method is implemented for multiple-gate injection optimization. The inlet nodes on the mesh surface are used to generate a Voronoi Diagram of the mold geometry. Then the optimal Centroidal Voronoi Diagram (CVD) of the mold surface is searched iteratively. It is shown that the generation points associated with the optimal CVD correspond with the gate locations that yield the shortest fill time. The results are compared with exhaustive search and genetic algorithms results to illustrate the efficiency and accuracy of CVD method.

Volatile release during cure is a potential cause of void formation during the resin transfer molding of complex thermosetting resins. In this study, a blended benzoxazine–epoxy resin system is analyzed to determine the rate at which volatiles are evolved, as well as the dependence of that rate on process parameters. The evolution of thermophysical and thermochemical resin properties is characterized using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The identity and rate of evolution of the gaseous byproducts released during cure are determined at ambient pressure using a Fourier transform infrared spectrometer (FTIR) linked to a reaction cell. The results show that gas release during cure can be reduced but not eliminated by degassing at elevated temperature. Furthermore, the results indicate that the nature and rate of volatile release can be modified by judicious selection of cure cycle, as shown by a preliminary analysis of manufactured neat resin panels.

This study investigated data assimilation through integration of visual observation with a stochastic numerical simulation of resin flow during vacuum-assisted resin transfer molding. The data assimilation was performed using the four-dimensional asynchronous ensemble square root filter and a stochastic numerical simulation by means of the Karhunen–Loeve expansion of the permeability field. Through numerical experiments of linear flow, it was verified that the estimation accuracy of the resin impregnation behavior improved compared to that when using conventional data assimilation and that the permeability field could be estimated simultaneously, although it is not explicitly related to the observation. We also investigated the applicability of the proposed method to radial-injection VaRTM by varying the model thickness. The proposed method successfully estimated the resin impregnation behavior and permeability field. Additionally, the required condition for the number of ensemble members was clarified.

This paper presents a multi-scale thermo-chemo-mechanical simulation predicting the distortion of a 3D woven composite part manufactured using the Resin Transfer Molding (RTM) process. The multi-scale numerical procedure relied on an anisotropic temperature- and degree of cure-dependent viscoelastic constitutive law whose parameters were obtained from experimental data and a blackbox optimization algorithm. Analytical (at the micro-scale) and numerical (at the meso-scale) homogenization procedures were used to compute the effective coefficients of thermal expansion and chemical shrinkage. An actual 3D woven non-flat composite specimen exhibiting variations in properties and thickness was designed and manufactured using the RTM process. The contoured geometry of the manufactured part was digitized using an industrial optical 3D scanner and compared against the developed multi-scale numerical predictions. It was found that the presented approach was capable of qualitative and, in some cases, quantitative prediction with the highest accuracy ~ 2.8 % of manufacturing induced geometrical distortions.

The process-induced residual strain and stress have a significant impact on the forming quality and service performance of 3D woven composites (3DWC). A multiscale model of 3DWC is developed to predict the residual strain and stress after the manufacturing process. Representative volume element at fiber scale and yarn scale are developed according to the geometric characteristics of 3DWC, and the modulus-development model is developed with respect to finite element based micromechanics method. An equivalent temperature load method is proposed to develop the cure shrinkage strain model. A thermal-chemical-mechanical coupling analysis of 3DWC curing process is carried out by integrating the abovementioned models, and the evolutions of temperature, degree of cure and residual strain/stress are obtained. Fiber Bragg Grating sensors are hybridized into the fabrics before Resin Transfer Molding (RTM) processing. The signal from sensors during RTM shows that the residual strain evolution is in good agreement with our modeling prediction.

Woven carbon fiber (WCF)-based polyester composites were developed via a vacuum-assisted resin transfer molding (VARTM) process in combination with CuO and graphene oxide (GO). The interlaminar resistive heating behavior and allied mechanical properties of the composites were investigated. The CuO nanoparticles were synthesized from copper nitrate and hexamethylenetetramine precursors using traditional microwave green synthesis, while the GO was synthesized by slight modification of Hummer’s method. The nanoparticle shapes and sizes were assessed via scanning electron microscopy, and the nanoparticle distributions in the composites and their chemical interactions were examined using X-ray diffraction and Fourier transform infrared spectroscopy. It was found that the composite strengths and moduli were enhanced by up to 61.2% and 57.5%, whereas the interfacial shear strength was enhanced by 89.9%. A composite filled with 120-mM CuO and 1.2-phr GO exhibited maximum performance as regards mechanical and resistive heating. Impact resistance measurements were conducted at 3-J penetration energy, and a 154.2% increase in nanofiller content was achieved. The addition of CuO nanoparticles increased the interlaminar resistive heating of the composite and, at 120-mM concentration, a 78.9% increment in the average temperature was attained. The presence of nanoparticles in the interlaminar region also decelerated the cooling process.

In composite processing of parts with varying cross-sections, homogeneous cure is sought but poses a significant challenge. Electrically heated molds for resin transfer molding (RTM) processes offer the potential to locally introduce heat and, thus, achieve more homogeneous cure and enhanced part quality. However, low conductivity of CFRP poses a risk of uncontrolled exothermic reactions. To target this potential, an appropriate and efficient numerical method is presented in this study to simulate part cure governed by resistive heated CFRP molds. A numerical control algorithm for 3D finite element cure simulations is developed, which uses the reaction flux of a temperature boundary condition to calculate the arising tool temperature field. The capability of this method to predict non-uniform tool temperatures of self-heated CFRP molds with close to thermocouple accuracy during the cure process is shown by means of numerical verification and experimental validation on a self-heated CFRP plate.

The use of phenoxy nanocomposite films as carriers of nanofillers involving multiwalled carbon nanotubes and nanoclays is successfully demonstrated for application in epoxy carbon fibers reinforced composites (CFRC) processed by RTM. Model studies on individual nanocomposite filaments embedded in epoxy precursors show that the nanofillers are passively transported by the interdiffusion gradient during heating over distance around 800 lm. A morphology gradient is generated after reaction induced phase separation and the nanofillers end up in the epoxy, despite their initial dispersion in the phenoxy. The proof of concept is extended to CFRC panels where nanocomposite phenoxy films are prepositioned between every odd carbon layer of the preform. Carbon nanotubes are filtered by the carbon fabrics, which limits their full diffusion and that of phenoxy through the preform. This has negative consequences on fracture toughness (GIc). For nanoclay, GIc is rather slightly improved although the origin is not fully clear.

The effect of CuO nanowires on the improvement of the mechanical properties of woven carbon fiber (WCF)-based polyester resin composite was studied. The composite was manufactured by the vacuum-assisted resin transfer molding (VARTM) process. CuO nanowires were grown on woven carbon fiber sheets in subsequent steps of seeding followed by growth. Scanning electron microscopy (SEM) showed the growth of CuO nanowires on the surface of the carbon fibers; this growth increased with the number of seeding cycles and the length of the growth time. The concentration of the growth solution did not have a significant effect. The maximum amount of growth occurred for 8 seeding cycles with a 60 mM growth solution and a growth time of 8 h. An analysis of the percent weight change, along with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, supported the above findings. The crystalline peak height of the CuO nanowires increased with the nanowire growth. The new absorption peaks arising in the FTIR spectra also indicated growth of CuO nanowires on the WCF. The mechanical properties in terms of tensile strength, modulus, and impact resistance improved significantly after the growth of nanowires on the carbon fibers: the modulus and strength improved by up to 33.1% and 42.8%, while the impact energy absorption increased by 136.8% relative to bare WCF.

Carbon nanotube (CNT)-grafted carbon fibers (CFs) have emerged as new reinforcements for improving the mechanical properties of CF-reinforced composites but such enhancement in macroscale composites has not been realized. This paper reports a facile method for preparing CNT-grafted CFs and improving the tensile strength of their composites. A CNT/polyacrylonitrile solution was sprayed onto the surface of the CF woven fabrics, and the CNTs were grafted by a thermal treatment at 300 °C. CNT-grafted CF composites were fabricated using the CNT-grafted CF woven fabrics using a vacuum-assisted resin transfer molding process with epoxy resin. The CNT-grafted CF composite exhibited 22% enhancement in the tensile strength compared to that of the pristine CF composite. Fracture surfaces of the CNT-grafted CF composites showed that the grafted CNTs obstructed the propagation of micro-cracks and micro-delamination around the CFs and also yarn boundaries, resulting in improved tensile strength of CNT-grafted CF composites.

In resin transfer molding, void type defect is one of common process problems, it degenerates the mechanical performances of the final products seriously. Void content prediction has become a research hotspot in RTM, while the void formation when the flow direction and the tow direction are not identical or the fabric is sheared has not been studied to date. In this paper, based on the analysis of the resin flow velocities inside and outside fiber tows, a mathematical model to describe the formation of micro- and meso-scale-voids has been developed. Particular attention has been paid on the influence of flow direction and fabric shear on the impregnation of the unit cell, so their effects on the generation and size of voids have been obtained. Experimental validation has been conducted by measuring the formation and size of voids, a good agreement between the model prediction and experimental results has been found.

Present paper investigated the effect of mercerization and silane treatments on the transverse thermal conduction properties of unidirectional abaca fiber–epoxy composite fabricated by resin transfer molding. As indicated by FTIR, XRD and SEM, the changes in chemical composition, crystalline and lumen structure of abaca fibers were introduced by chemical treatments. Transverse tensile test showed that the weakest linkage of unidirectional composite changed from interface between abaca fiber bundle and epoxy resin for untreated abaca fiber to interface between elementary fibers treated by mercerization and silanization. With the increasing of weakest linkages strength and the decreasing of void content, the transverse thermal conductivity (TCC) of the composite presents increasing trend. The changing of interfaces, cell wall and lumen derived from chemical treatments are the mainly factors affecting TTC. It was concluded that the abaca fiber composite with controllable transverse thermal conduction property can be designed by proper chemical treatment.

Because slight differences in the wrinkle of a vacuum bag and other inherent variations in the preforms may cause unexpected resin flow in vacuum-assisted resin transfer molding (VaRTM), flow control is strongly required to prevent dry spots. We propose an active flow control scheme by forecasting resin flow from the monitored time to the filling ends using numerical flow simulation and taking corrective action using dielectric heating at a specific targeted location to decrease the viscosity of the resin. Because dry-spot configuration can be forecasted early, the flow can be actively controlled before the occurrence of an adverse flow front. This method can be extended to forecasting load-bearing performance as the critical dry spot can be selectively prevented based on applied stress distribution information. We demonstrate the validity of the proposed method to improve uniaxial compressive buckling of plate structures by conducting virtual experiments based on a multifunctional interdigital electrode array film.

To study the effect of the microstructure of natural fiber on the transverse thermal conductivity of unidirectional composite, abaca and bamboo fibers were unidirectionally aligned to fabricate epoxy composites by a resin transfer molding (RTM) technique. The transverse thermal conductivity of these two types of composites was measured in a steady-state platform. X-ray diffractometer and scanning electron microscopy were applied to analyze the microstructure and morphology of both fibers and composites. The results indicated that the transverse thermal conductivity showed two types of tendencies with fiber content increasing: increasing for bamboo fiber composites, and decreasing for abaca fiber composites. The microstructure and theoretical analysis suggest that the lumen structure plays a great role rather than crystal structures and chemical compounds on the transverse thermal conductivity of unidirectional composites, which is useful for further development and design of natural fiber reinforced composites with better thermal insulation property for people’s daily life.

A methodology is presented to determine the saturation curve of a resin/glass fabric system, during infiltration in a transparent mould under constant flow rate. Video acquisitions are transformed by image analysis into saturation level versus position and time, and coupled to inlet pressure measurements. A numerical multiphase flow model is then used to simulate the infiltration for various combinations of drainage curve parameters. The numerical parameters to describe the saturation and relative permeability are determined by response surface optimization. The drainage curve and relative permeability equations determined at one time are shown to adequately describe the entire injection process, and to be flow-rate dependent. (C) 2011 Elsevier Ltd. All rights reserved.

The electrical properties of sisal fiber reinforced polyester composites fabricated by resin transfer molding (RTM) have been studied with special reference to fiber loading, frequency and temperature. The dielectric constant ( e ′), loss factor ( e ″), dissipation factor (tan δ ) and conductivity increases with fiber content for the entire range of frequencies. The values are high for the composites having fiber content of 50 vol.%. This increment is high at low frequencies, low at medium frequencies, and very small at high frequencies. The volume resistivity varies with fiber loading at lower frequency and merges together at higher frequency. When temperature increases the dielectric constant values increases followed by a decrease after the glass transition temperature. This variation depends upon the fiber content. Finally an attempt is made to correlate the experimental value of the dielectric constant with theoretical predictions.

The study investigates the effect of carbon nanotubes (CNTs) on the damage development in a woven carbon fiber/epoxy composite under quasi-static tension in the bias direction. The composite is produced by the resin transfer molding and contains 0.25 wt.% of CNTs in the matrix. The tensile tests are carried out till different strain levels and are accompanied with acoustic emission (AE) registration. The nano-modified composite possesses a higher stiffness and strain-to-failure. It also exhibits a significantly increased AE activity, both in terms of the number of events and the energy level, but reveals a lower crack density. The combined analysis of the AE data and X-ray images indicates that in the nano-modified composite cracks progress through the material in smaller jumps than in the virgin composite. The crack faces in the composite with CNTs also display a fine web of secondary fractures, which is not detected in the virgin composite.

Structural composite manufacturing in automotive industry tends towards short cycle times for being competitive with other material solutions. Development towards resin transfer molding process can be considered when highly reactive resin with short curing cycle is used. Mixing of the resin system components is then held at the mold inlet to reduce catalyzed resin life time. Cure kinetics and viscosity changes induced during filling are thus to be taken into account when injection process simulation is considered. In the case of on-line mixing of resin components, viscosity and degree of cure of the first injected resin droplets are not the same than for the following ones. This topic will be addressed here for mixed constant flow rate and pressure injection schemes. A solution that could be implemented in a RTM simulation package was developed and validated with an analytical solution and on a complex shape automotive part demonstrator.

Unidirectional orientation of natural fibers in a polymer composite ensures the highest efficiency of reinforcement. Flax fiber reinforcement is discontinuous due to limited fiber length and heterogeneous due to the presence of elementary fibers and their bundles. In order to assess the upper limit of tensile strength of such slightly misoriented, nominally UD natural fiber composite, a statistical strength model of continuous UD fiber reinforced composites is applied. It is found that the experimental strength of UD flax composites, produced from rovings or manually aligned fibers, approaches the theoretical limit only at relatively low fiber volume fraction ca. 0.2, being markedly below it at higher fiber content.

Most conventional methods for flow control during resin transfer molding have varied the applied pressure using pressure regulators. However, with this method it is difficult to control the flow far from the inlet or outlet points. The present study proposes a flow control scheme by monitoring and actuating flow/temperature using a thin multifunctional interdigital electrode array film. The film is used for estimation of resin impregnation and temperature distribution, and increases the resin temperature by dielectric heating at arbitrary positions on the film, thereby increasing the resin velocity at the required positions. Foam cored GFRP structures were impregnated with resin by VaRTM, and the effectiveness of the flow control scheme assessed. The occurrence of dry spots was effectively prevented.

The manufacturing of fibre reinforced plastics using Liquid Composite Moulding (LCM) techniques requires compaction of fibrous reinforcements and flow of liquid resins through them before curing. In the modelling of LCM processes, an accurate description of compaction response and resin flow resistance through the fibrous media (also described as permeability) is required. Permeability and compaction response affect a number of important parameters, such as required tooling forces and mould fill-times. To study the influence of yarn diameter and length, on permeability and compaction characteristics of natural fibre reinforcements, six different types of flax yarn mats were manufactured. Compaction tests were carried out on both dry and saturated samples. Average in-plane permeability was also measured at different fibre volume fractions. It was found that increasing the yarn diameter reduced compaction forces and increased permeability. Long yarn length mats demonstrated higher permeability and compaction response than short yarn length mats.

The spring-in phenomenon is a common dimensional infidelity problem that occurs when a curved composite part is released from the mold after full consolidation during a Vacuum Assisted Resin Transfer (VARTM) process. In this study, a multi-stage curing (MSC) technique, which is a typical resolution to cure thick composite parts, is applied to the parts manufactured via VARTM for investigating the induced improvement in the spring-in problem. The experimental results evidently show that the MSC technique significantly mitigates the spring-in problem. A new MSC spring-in model that accounts for the slipping effect between the stage-laminates of each curing stage is also developed for predicting the spring-in behavior during the MSC process. The predictions show a good agreement with the experimental results and suggest that the MSC technique would be a useful method for controlling the spring-in of composite parts.

For multilayer preforms from three different triaxial carbon fibre braids with bias angles 45°, 60° and 70° at fibre volume fractions of 0.54 and 0.59, the in-plane permeability was characterised experimentally. A finding of high practical relevance is that, at a bias angle of approximately 55°, the principal flow direction switches from the preform 0° direction to the preform 90° direction. For different zones of characteristic yarn arrangement in the braid unit cell, the local permeability was modelled as a function of bias angle, global fibre volume fraction, and geometrical yarn parameters. The global braid permeability was derived from numerical flow simulation based on simplified 2D models of the textile architecture and local permeabilities, taking into account effects of nesting between adjacent layers. Calculated trends for the permeability as a function of the braid angle reproduce the experimental results qualitatively well. Quantitatively, agreement depends on the value of a geometry parameter, which decreases with increasing fibre volume fraction.

This paper focuses on the mechanical and water absorption properties of kenaf fibre reinforced polyester laminates manufactured by resin transfer moulding. Varying processing conditions were considered as alternatives to fibre treatments, thereby potentially avoiding additional cost and complexity in the manufacturing process. Laminates were produced by altering fibre moisture content, mould temperature and mould pressure following injection. Tensile, flexural, impact and water absorption tests were conducted. Processing conditions were found to have little effect on properties except for pressurisation which increased tensile and flexural strength and decreased water absorption at low fibre volume fractions. Examinations using a scanning electron microscope showed that all the laminates failed by fibre pull-out.

A room-temperature cure epoxy consisting of diglycidyl ether of bisphenol-F (DGEBF) and diethylene triamine (DETA) was modified with 26.5 wt.% and 63.5 wt.% octaglycidyl polyhedral oligomeric silsesquioxane (POSS) to investigate elevated temperature thermomechanical performance. Composites fabricated using vacuum assist resin transfer molding (VARTM) were compared to vinylester and epoxy standards. POSS modified matrices were low viscosity of 0.25–0.40 Pa s. Although Tg was 20% lower than vinylester, we observed an increase of >300% in 150 °C storage modulus, >50% in tensile modulus, >35% in flexural modulus, and the complete elimination of a heat distortion temperature (HDT) up to 200 °C. The matrices demonstrated an excellent balance of flow, wetting, and pot-life behaviors making them attractive alternatives for ambient cure marine applications.

Carbon nanotubes (CNTs) were used as an in situ sensor to detect the initiation of micro-cracks and their accumulation in fiber-reinforced polymer composites. The breakage of the electrically conductive networks formed by CNTs throughout the polymer matrix when dispersed in composites enables the micro-cracks to be sensed. This methodology was applied to three-dimensional (3D) braided composites with the aim of investigating the feasibility of detecting their matrix failure and analyzing their damage behavior. Tensile specimens were prepared using 3D braided ultra-high molecular weight polyethylene (UHMWPE) preforms and vinyl ester containing multi-walled CNTs (0.5 wt%) via vacuum-assisted resin transfer molding (VARTM). The electrical resistance of the composites was then measured during tensile testing, while their internal structures were analyzed using X-ray computer tomography (CT), demonstrating that the CNTs dispersed in the matrix enable micro-cracks to be sensed and the damage modes of the 3D braided composites to be analyzed. Finally, four critical strain levels that can classify the damage modes were identified from the change of the electrical resistance of the 3D braided composites.

A finite element model of a 3D woven angle interlock fabric undergoing compaction based on the multi-element digital chain technique has been developed. The aim was to create a kinematic model to predict the internal architectural features in a commercial off-the-shelf code. A statistical analysis of yarn crimp and resin channel size was carried out on sections from the model at increasing levels of compaction and compared to laboratory-manufactured samples with the same weave style. Results show a good correlation between overall mean crimp values in the warp, weft and weaver yarns as well as reasonable accuracy in the frequency distribution of local crimp angles. The trend in resin channel size with respect to increasing levels of compaction was also good but significant discrepancies in the absolute dimensions of a resin channel were present due to limitations in controlling the yarn bundle internal interactions in the model.

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.

This paper describes the development of an electromagnetic interference shielding material using recycled carbon fibre. Fibre recycled from a fluidised bed process was transformed into a non-woven veil and was moulded into a glass-fibre reinforced polymer plaque to provide shielding. Factors affecting shielding performance were established using a virgin fibre and the result was compared with veil made of the recycled fibre. Shielding performance increased with veil areal densities. The influence of fibre length on shielding seemed insignificant provided the fibre was distributed evenly. Sandwiching the plaque between fibre veils enhanced the shielding performance. A shielding value of 40 dB was attained from a layer of 80 g/m 2 recycled fibre veil and it was increased to 70 dB when the plaque was sandwiched between two layers of 30 g/m 2 recycled fibre veil. The correlation between veil formation and shielding effectiveness was established and found that shielding effectiveness increased with the degree of fibre dispersion.