Micromechanical discrete element modeling of fiber reinforced polymer composites

An analytical model of mechanical behavior of carbon fiber reinforced polymer composites using an advanced discrete element model (DEM) coupled with imaging techniques is presented in this article. The analysis focuses on composite materials molded by vacuum assisted resin transfer molding. The molded composite structure consists of eight‐harness carbon fiber fabrics and a high‐temperature polymer. The actual structure of the molded material was captured in digital images using optical microscopy. DEM was developed using the image‐based‐shape structural model to predict the composite elastic modulus, stress–strain response, and compressive strength. An experimental case study is presented to evaluate the accuracy of the developed analytical model. The results indicate that the image‐based DEM micromechanical model showed fairly accurate predictions for the elastic modulus and compressive strength. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Modeling tensile response of fiber‐reinforced polymer composites using discrete element method

An advanced discrete element method (DEM), coupled with imaging techniques, of the tensile response of carbon fiber‐reinforced composite materials is presented in this article. DEM was developed using the image‐based shape structural model to determine the composites' elastic modulus, stress–strain response, and tensile strength. The developed model utilizes the microfabric micromechanical discrete element modeling technique. Clusters of very small bonded discrete elements were used to model the two composite constituents (matrix and reinforcement). The microparameters of each discrete element were determined from the macrocharacteristics of each constituent. The results from the developed model were compared with the results from an experimental case study. The results obtained from DEM simulations are within the coefficient of variation of the experimental values. The comparison indicates that the image‐based DEM micromechanical model accurately determines the elastic modulus and tensile strength of the molded carbon fiber‐reinforced polymer composite. POLYM. COMPOS., 34:877–886, 2013. © 2013 Society of Plastics Engineers     

Process development and characterization of spraying carbon nanofibers over fabrics for reinforcing polymer composites

Process development and characterization of spraying carbon nanofibers (CNF) over carbon fiber fabrics for reinforcing polymer composites are presented in this study. The molded composite structure consists of a high‐temperature polymer reinforced with carbon fiber fabrics sprayed with different dosages of carbon nanofibers. The materials were molded using vacuum assisted resin transfer molding process. Tensile testing and scanning electron microscopy (SEM) were used to characterize the molded materials. The results show that the tensile strength and modulus were both improved over the molded materials without CNF. Spraying CNF with a dosage of an 8 µg/mm² of the used fabrics helped to increase the tensile strength by 12%. The tensile modulus increased by 28% with a CNF dosage of 16 µg/mm². Uniform distribution of CNF was observed under SEM in the molded composites. POLYM. COMPOS., 35:1629–1635, 2014. © 2013 Society of Plastics Engineers     

风电叶片用真空灌注型环氧树脂及其复合材料性能研究

本文针对两种自制的风电叶片用真空灌注型环氧树脂体系EP-1和EP-2,研究了树脂的工艺性和力学性能,并选取单轴向和三轴向玻璃纤维织物,采用真空灌注工艺制备了复合材料层板,考察了复合材料在室温和高低温下的力学性能。结果表明,EP-2体系浸润性、流动性和韧性更好,但强度、模量和耐热温度略低;常温及-45℃下两种树脂基复合材料的力学性能相近,纤维/树脂界面粘结较强;50℃环境下,复合材料的压缩强度降低,受玻璃化转变温度偏低的影响,EP-2复合材料压缩性能降低更为明显;两种环氧树脂的工艺性和力学性能优异,与纤维匹配性好,满足风电叶片对树脂的性能要求。     

Resin transfer molding of BMIs and polymides

Bismaleimdes (BMI) and experimental polyimides were resin transfer molded into carbon fiber fabrics using a custom-built injection mold placed within a vacuum hot press. This represents the first time that the latter type of materials has been resin transfer molded. This is a critical stage in developing materials and processing methods for future aerospace applications, such as the High Speed Civil Transport (HSCT).     

On‐line mixing during injection and simultaneous curing in liquid composite molding processes

Liquid composite molding (LCM) processes such as resin transfer molding (RTM) and vacuum assisted RTM (VARTM) are used to manufacture high quality and net‐shape fiber reinforced composite parts. All LCM processes impregnate fiber preforms packed in a mold cavity with a thermoset resin. After the preform is fully saturated, the injection is discontinued but the resin continues to cure. Once the curing step is complete, the part is de‐molded. The resin has to be mixed with a curing agent to cure. Typically, the resin and the curing agent are mixed together in a pressure pot before the injection. This has several disadvantages, such as storage of large amounts of hazardous polymerizing resin, wastage, and cleaning of cured resin from the injection line. This paper proposes the implementation and calibration of an alternative to this technique. The approach is to mix the curing agent with the resin as the resin enters the mold through a separate system featuring two feed‐lines. Such a system will enable one to maintain a uniform gel time throughout the part by varying the mixing ratio of resin and the catalyst during the injection. An experimental study of such on‐line mixing to obtain simultaneous curing and to reduce the overall curing time is conducted and presented in this paper. Implementation of a control scheme that varies the curing agent during injection and its effect on cure time is benchmarked with the process in which the percentage of curing agent is held constant. The gel time for the fabricated parts was reduced by 20–25% by continuously varying the percentage of curing agent during injection. POLYM. COMPOS., 26:74–83, 2005. © 2004 Society of Plastics Engineers     

Process induces fiber orientation: Numerical simulation with experimental verification

The performance of short fiber molded composite structures is determined uniquely by the properties of the molding material and the process induced fiber orientation. Consequently, the capability to accurately predict the fiber orientation distribution is of fundamental importance in computer-aided mold design. Methodology for the numerical prediction of fiber orientation during the mold-fill process is presented for a short glass fiber thermoset (57 percent phenolic resin, 10 percent calcium carbonate filler, and 33 percent glass fiber by volume). On the basis of a finite element flow characterization, Jeffery's orientation equation is numerically integrated along streamlines to calculate fiber orientation. Correlation of experimental and numerical results for an end-gated bar with a molded-in hole is reasonably good.     

Recyclability of a continuous e-glass fiber reinforced polycarbonate composite

Fiber reinforced plastics are multi-component materials for which physical properties are strongly dependent on fiber and resin structure. Despite the disruptive nature of recycling methods on such structures, these materials nevertheless can be recycled. In this report, the recyclability of a fiber-reinforced cyclic BPA polycarbonate has been studied. It is found that ground up composite is recyclable and possesses properties as good as or better than a comparable commercial composite. The processing techniques investigated herein are injection, extrusion compression, and compression molding. As expected, processing technique and parameters are important in determining the mechanical properties of the molded regrind. Our results show that injection and extrusion compression molding yield recycled composites with good tensile properties, though the impact strengths are relatively low. This is due to high fiber orientation and fiber bundle dispersion. On the other hand, compression molded samples, which show random fiber orientation and low fiber bundles dispersion have relatively low tensile properties, but excellent impact strength. Results are discussed in terms of microstructural details, which include resin molecular weight and fiber length and orientation.     

Viscoelastic evaluation of effects of fiber size and composition on cellulose‐polypropylene composite of high filler content

The dispersion of filler in resin is an important factor which determines the mobility of a compound in a molder and also the mechanical properties of molded products in wood/plastic composite, especially with high wood filler content. In this report, the dispersion of components in the melt‐mixture of a compound in response to the size of cellulose filler and resin content was examined for compounds with a high content of cellulose in polypropylene by the evaluation of viscoelasticity using a cone rheometer, and the following conclusions were obtained: (1) The viscoelasticity measured by a cone rheometer was dependent on the dispersion state of the compound, and it was confirmed that the presented evaluation method of viscoelasticity affords information relevant to the dispersion state of filler in a compound; (2) On the basis of the viscoelastic evaluation of compounds with different resin contents, a model for the distribution of cellulose and resin is proposed in terms of the tangle of fibers; (3) Viscoelastic analysis showed that cellulose size has an influence on the tangle of fibers; (4) The results of viscoelasticity and mechanical properties of molded products suggested that an important factor determining the tangle of fibers is the average aspect ratio. The proposed analysis of viscoelasticity presents a simple technique for estimating the dispersion state of a compound with high filler content, which is hardly possible by conventional fluid evaluation methods. POLYM. ENG. SCI., 48:168–176, 2008. © 2007 Society of Plastics Engineers     

Short‐term flexural creep behavior and model analysis of a glass‐fiber‐reinforced thermoplastic composite leaf spring

Present work investigated the short‐term flexural creep performance of fiber reinforced thermoplastic injection molded leaf springs. Unreinforced polypropylene, 20 wt % short and 20 wt % long glass fiber reinforced polypropylene materials were injection‐molded into constant thickness varying width mono leaf spring. Short‐term flexural creep tests were performed on molded leaf springs at various stress levels with the aid of in‐house developed fixture integrated with the servo‐hydraulic fatigue machine. Spring rate reduction is reported as an index for the accumulated damage. Experimental creep performance of molded leaf springs for 2 h was utilized to predict the creep performance with the aid of four parameter HRZ model and compared with 24‐h experimental creep data. Test results revealed that HRZ model is sufficient enough to predict short‐time flexural creep performance of engineering products over wide range of stress. Test results also confirmed the suitability of long fiber reinforced thermoplastic material for creep application over other considered materials. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

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     

An analytical approach to evaluate the coefficients of thermal expansion of textile composite materials

An analytical approach is developed to evaluate the coefficients of thermal expansion (CTE) of textile reinforced composites. At the micro level, a cylindrical composite model is employed to model the fiber/matrix thermal and mechanical interactions. The effects of voids and fiber coating on the thermal expansion coefficients of composites are considered at this level. The cylindrical model was then embedded in a macro hybrid finite element solutio structure to calculate the value of the CTE for textile composites. AS‐4/epoxy balanced plain weave textile composites were manufactured. Five different fiber volume fractions were tested for CTE. Evaluatio of the thermal expansion coefficients using the current model was compared to experimental data for in‐plane and out‐of‐plane directions.     

Structure–property relationships in glass‐reinforced polyamide,part 1: The effects of fiber content

We present the results of an extensive study of the performance of injection‐molded glass‐fiber reinforced polyamide 66 with glass content between 0 and 40% and based on two chopped glass products both sized with polyamide compatible sizing. Mechanical properties generally improved with increasing glass content, modulus linearly, strength with a maximum at 40–50% glass content, and impact showing an initial decrease from the resin value with a minimum at 4% glass content before increasing at higher glass contents. Residual fiber length decreased linearly with increasing glass content. Interfacial strength was found to be in the range of 30–36 MPa, and no significant differences in dry as molded performance was found between the 123D and 173X sizings. Conditioning these composites in either boiling water or water/glycol mixtures leads to a dramatic drop in both tensile modulus and tensile strength. This is most likely due to the high level of matrix plasticization. After conditioning, the 173X sized glass delivered a significantly higher level of tensile elongation at all fiber contents. Excellent agreement was obtained between the experimental data and the theoretical predictions of the rule of mixtures model for modulus and the Kelly‐Tyson model for strength over the range of fiber concentrations studied. POLYM. COMPOS. 27:552–562, 2006. © 2006 Society of Plastics Engineers     

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.     

Simultaneous optimization of the mechanical properties of postconsumer natural fiber/plastic composites: Phase compatibilization and quality/cost ratio

In this work, a simultaneous optimization by phase compatibilization of four mechanical proprieties (flexural and tensile moduli, impact strength, and tensile stress at yield) of natural fiber/plastic composites was performed with respect to raw materials cost. In particular, a recycled resin of postconsumer origin (blend of high density polyethylene and polypropylene) with flax fibers was extruded with an additives package: a coupling agent (maleic anhydride grafted polypropylene) and an impact modifier (maleic anhydride grafted ethylene octene metallocene copolymer) to improve the interface between each phase. Then, the compounds were injection molded and tested. The analysis was performed according to a Box‐Behnken experimental design to study the effect of fiber concentration, total additives concentration, and impact modifier fraction in the additives package. The optimization process required three steps: to model the relationships between mechanical properties and selected factors by a multiple linear regression analysis, to identify the potentially optimum conditions using the desirability function approach (Derringer–Suich and Ch'ng et al.), and to determine the best composite composition (optimum condition) by a comparative analysis of the material quality/cost ratios. POLYM. COMPOS., 35:730–746, 2014. © 2013 Society of Plastics Engineers     

Resin transfer molding with powder-coated preforms

Incomplete fiber wetting in a resin transfer molded composite may result in poor surface finish, high void content, and reduced mechanical properties. This work studied the use of tows that are precoated with a powdered version of the liquid molding resin (towpregs). The goal is to see if such preforms improve the final part properties because of better fiber wetting. Hercules 12K AS-4 fibers and PR500 (liquid) and PS500 (Powder) resins (3M) were used to make fabrics from towpregs containing 50 wt% total resin (liquid and powder combined). The powder fractions were 0, 13, 21, 50 wt%. Samples were resin transfer molded from preforms made from the towpreg fabrics. Results showed that samples molded with powder-coated preforms had improved surface finishes and reduced void contents (1.4 vs. 5%), but that the mechanical properties were not improved (transverse moduli of ∼ 7.8 Gpa and axial moduli of ∼ 100 Gpa), probably because of defects inherent in the hand-woven towpreg fabric that was used.     

Short‐beam and three‐point‐bending tests for the study of shear and flexural properties in unidirectional‐fiber‐reinforced epoxy composites

The bending properties of composite materials are often characterized with simply supported beams under concentrated loads. The results from such tests are commonly based on homogeneous beam equations. For laminated materials, however, these formulas must be modified to account for the stacking sequence of the individual plies. The horizontal shear test with a short‐beam specimen in three‐point bending appears suitable as a general method of evaluation for the shear properties in fiber‐reinforced composites because of its simplicity. In the experimental part of this work, the shear strength of unidirectional‐glass‐fiber‐reinforced epoxy resin composites was determined in different fiber directions with the short‐beam three‐point‐bending test. Also, the elastic constants and flexural properties of the same materials were determined from bending experiments carried out on specimens in the 0, 15, 30, 45, 60, 75, and 90° fiber directions with high span–thickness ratios. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 63–74, 2004     

BaTiO3 incorporation effect on the dielectric properties of polymer from aqueous emulsion: An enhanced dispersion technique

An economic and environment friendly process was adapted to synthesize new dielectric composite materials. Using ethylene vinyl acetate (EVA)/vinyl ester of versatic acid (VeoVa) terpolymer as an aqueous emulsion provides a homogenous dispersion of BaTiO₃ (BT) particles, due to the high viscosity and polarity of the vinyl resin (VR). Composites films were obtained from these dispersions by water evaporation. The evolution of the dielectric properties as a function of the BaTiO₃ content, was correctly fitted by a Maxwell‐Garnett model. This fitting of the experimental curve shows a good dispersion of filler in the vinyl resin and the particles separation by a layer of resin as expected for the preparation method used in this study. The VR/BT composites show good synergy between the dielectric properties of the different phases of the composites due to the formation of macrodipoles and to the strong interactions between polar EVA/VeoVa groups and the BaTiO₃ particles surface. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44333.     

Compressibility and relaxation of fiber reinforcements during composite processing

In many reinforced composite manufacturing processes it is necessary to compact the fiber materials to obtain the desired fiber/resin ratio in the finished part. Detailed knowledge of applied surface force versus material fiber volume is particularly important in processes such as pultrusion, resin transfer molding, and compression molding. The force required to compact a stack of reinforcing material is strongly dependent on the type of fiber used and its material form. Complicated interactions are possible, particularly when mixtures of unidirectional, oriented cloth and random fiber mats are used. This paper will present results of an experimental and analytical investigation of the response of various dry reinforcing materials subjected to compressive forces applied normal to their principle plane. Experiments were conducted by applying up to 8.6 MPa normal force to thick stacks of E-glass, graphite cloth, mat and unidirectional material and combinations of two different fiber orientation. Pressure versus fiber volume data were generated for both individual materials and various combinations. Experimental results were compared to analytical predictions. Data showed that the force versus deformation is very strongly dependent on the details of the fiber form or forms being used. There is structural relaxation during fiber compression. Relaxation is very related to fiber orientation, span length, and fiber breakage during compaction. Relaxation behavior decreases with fiber alignment. Random mats and 0/90 cloth show much more relaxation than unidirectional fibers. Data of relaxation is very well fitted with a Maxwell-Wiechert viscoelastic model.     

Empirical nonlinear viscoelastic model for injection molded thermoplastic composite

Injection molded components made from fiber reinforced thermoplastics exhibit strong viscoelastic behavior. In the present study, the bending creep tests show that the 43-wt% glass fiber reinforced polyamide 66 is highly stress-dependent and requires a nonlinear viscoelastic representation. However, such representation is complex for isotropic materials and is even more arduous for composite materials. In order to overcome this complexity, an empirical approach is used herein to develop a viscoelastic model based on a simple power law with stress-dependent parameters. The proposed model demonstrates high stress sensitivity and agrees with experimental data over a wide range of applied stress.