High Volume Fraction Composites Research Articles

Experimental and statistical study on the effect of process parameters on the quality of continuous fiber composites made via additive manufacturing

Ongoing research in additive manufacturing towards structural and industrial application has led to the use of commingled roving as a manufacturing feedstock for printing high fiber volume fraction composites. The prospects of using this technology for high performance applications necessitates the need for a comprehensive experimental investigation into the effects of processing parameters on the quality of an additively manufactured composite printed from commingled roving feedstock. In this work, transverse flexure and void fraction matrix pyrolysis testing are both performed to evaluate composite quality. The transverse flexure test is a testing approach that evaluates the quality of the interfacial fiber-matrix bond while the void fraction test estimates the void content in the printed composite. A full observational study consisting of 27 different test combinations is done to investigate the effects of three different process parameters namely, temperature, pressure, and print speed across three different levels. Composite samples were made from commingled roving of E-glass and amorphous PET using an in-house built continuous fiber composite digital manufacturing system. Least squares regression analysis is performed to study the main, interaction and quadratic effects of process parameters. A statistical regression model having an R2 adjusted value of 80.1% is generated from the transverse flexure study, which is used to explain main and interaction effects and also predict performance. Response surface plots are also generated and are used to optimize process parameters which can subsequently be of help in scaling up composite manufacturing. Results show that all three process parameters are highly statistically significant at the 0.01 level of significance. Pressure * Temperature and Pressure * Printspeed are significant interaction terms. Pressure plays a weightier role when print speed is increased or temperature is decreased as it closes more voids that would ordinarily have been introduced because of drop in polymer melt viscosity. Micrographic analysis is also performed.

Damage behavior of high particle volume fraction composites with initial damage by finite element meso-modeling

Analyzing the deformation and damage processes of high particle volume fraction composites (HPVFCs) based on the meso-structure modeling plays an important role in their service lifetime and functional reliability. The reported mesoscopic structural models are usually ideal models without initial damage and apply to composites with the volume fraction of particles less than 80%. This work provides a new mesoscopic structure model called aggregate settlement-merging model to successfully simulate the mechanical behavior and damage mechanism of the polymer-bonded explosive (PBX) under quasi-static loads, and the macroscopic response is accurately predicted by the cohesive finite element method. This simulation commendably predicts the failure modes including interface debonding, binder filaments, and grain failure, which is consistent with the observation in micromechanical experiments. The multi-scale relationship between macroscopic mechanical properties and microscopic initial interface damage is quantitatively established in a form of an exponential function. This study shows a relationship between meso-structures and performance and provides strategy for designing next generation high-performance HPVFCs.

Dynamic behavior description and three-dimensional cutting simulation of SiCp/Al composites with high volume fraction

According to the dynamic behavior characteristics of SiCp/Al composites, the constitutive equation of high volume fraction composites is established by SHPB dynamic compression test, shear lag model and effective medium approximation method, and the calculation results are in good agreement with the experiments. The established constitutive equation is then imported into Abaqus to establish a three-dimensional cutting simulation model, which is combined with cutting experiments to analyze the dynamic removal mechanism of material during the cutting process and the cutting surface formation process. The results show that the formation and propagation of holes and cracks play an important role in the formation of chips. The failure removal mode of SiC particles is an important factor affecting the cutting surface morphology. The research results provide an important theoretical basis and research means for the study of cutting mechanism of SiCp/Al composites with high volume fraction.

Finite element modelling and simulations on effective thermal conductivity of particulate composites

In this paper, the finite element simulation approach for studying the steady-state heat transfer and predicting the effective thermal conductivity of composites has been proposed. 3D representative volume elements considering the polyhedral- and spherical-shaped inclusions are generated. Different meshing approaches for meshing these complex structured composites have been used, and the mesh quality is studied based on mesh metrics: element quality and skewness, and next, a convergence study is conducted to ensure mesh-independent results. Three different boundary conditions: mixed, constant temperature gradient and uniform flux has been used to predict the effective thermal conductivity of composites. The predicted results from simulations are in good agreement with the experimental values reported in the literature. It has been observed that the random orientation and interpenetration of particles lead to a much better heat flow and hence the higher values of effective thermal conductivity of composites. For high volume fraction composites, as the number of particles having interpenetration increases, their effective thermal conductivity increases. For low volume fraction composites, constant temperature gradient boundary condition overpredicts the effective thermal conductivity and for high volume fraction composites, uniform heat flux and mixed boundary conditions predict higher values. A design point study was conducted, considering the ratio of thermal conductivity of constituents, as a design parameter. It has been observed that by reinforcing the high thermal conductivity inclusions as an interpenetrating phase, the effective thermal conductivity of composites can be increased substantially. The reinforcement of very high conductivity material cannot yield the desired ETCs if the reinforced particles are rendered uniformly oriented with no contacts between them.

Efficient Mechanical Stress Transfer in Multilayer Graphene with a Ladder-like Architecture.

We report that few graphene flakes embedded into polymer matrices can be mechanically stretched to relatively large deformation (>1%) in an efficient way by adopting a particular ladder-like morphology consisting of consecutive mono-, bi-, tri-, and four-layer graphene units. In this type of flake architecture, all of the layers adhere to the surrounding polymer inducing similar deformation on the individual graphene layers, preventing interlayer sliding and optimizing the strain transfer efficiency. We have exploited Raman spectroscopy to quantify this effect from a mechanical standpoint. The finite element method and molecular dynamics simulations have been used to interpret the above experimental findings. The results suggest that a step pyramid-like architecture of a flake can be ideal for efficient loading of layered materials embedded into a polymer and that there are two prevailing mechanisms that govern axial stress transfer, namely, interfacial shear transfer and axial transmission through the ends. This concept can be easily applied to other two-dimensional materials and related van der Waals heterostructures fabricated either by mechanical exfoliation or chemical vapor deposition by appropriate patterning. This work opens new perspectives in numerous applications, including high volume fraction composites, flexible electronics, and straintronic devices.

Optimisation of the spark plasma sintering process for high volume fraction SiCp/Al composites by orthogonal experimental design

Based on orthogonal experimental design (OED), the effects of the sintering pressure, sintering temperature and holding time on the mechanical properties of 50 vol% silicon carbide particle (SiCp)/2024Al composites prepared by spark plasma sintering (SPS) were investigated. The sintering pressure had the greatest effect on the density and bending strength of the material among these three factors, followed by sintering temperature and holding time. The optimised process conditions for producing the 50 vol% SiCp/2024Al were sintering at 550 °C for 5 min under 40 MPa, which resulted in a composite material with a density of 99.7% and good interface bonding with a comparatively high bending strength of 766.65 MPa. This work provides a promising method to produce high volume fraction composites that can meet high strength requirements.

Effect of high volume fraction reinforcement on electro-discharge machining of Al-Al2O3 MMC

Development of composites and their extraordinary properties and strength to weight ratio demands the use of modern techniques for machining of high volume fraction composites. Though Al2O3 as reinforcement is added in aluminium to increase its strength; it eventually affects its machinability. In present work, Al-Al2O3 metal matrix composites were stir cast with 10–50 percent volume fraction of Al2O3 reinforcement and subsequently machined using electro-discharge. An effect of Al2O3 volume fraction, current, voltage and T ON time was experimented to evaluate material removal rate, surface roughness, overcut and taper. Experiments were performed using L9 orthogonal array as per Taguchi's technique. Ranking of the parameters was done based on experimental findings and S/N ratio analysis. Regression analysis helped to develop mathematical equations to quantify metal removal rate and the inaccuracies produced while machining by EDM and showed statistical significance. A further in-depth study using a scanning electron microscope to observe surface characteristics can be extended.

Micromechanical modeling of thermo-mechanical properties of high volume fraction particle-reinforced refractory composites using 3D Finite Element analysis

Previously, we have developed several particle-reinforced castable ceramic composites for refractory applications with exposure to thermal shock and measured their effective thermo-elastic properties experimentally. These composites contained silicon-carbide (SiC) solid particles, zirconia (ZrO2) bubbles, and ZrO2 solid particles, dispersed in an alumina (Al2O3) matrix. The present work aims to implement representative volume element (RVE) approach and periodic boundary condition (PBC) to accurately predict those properties, namely elastic and shear modulus, thermal conductivity, and coefficient of thermal expansion (CTE), using three-dimensional (3D) Finite Element (FE) simulations while accounting for the effect of porosity. In comparison to established micromechanical schemes and two-dimensional (2D) FE predictions, 3D FE simulations specifically show more accuracy in prediction of elastic properties and thermal conductivity. This novel and thorough comparison across various thermo-mechanical properties for complex microstructures (with up to three types of inclusions) can be valuable for designing comparable high volume fraction (VF) composites.

Effect of Zn and mechanical vibration agitation on the microstructure evolution in brazed joint of SiC particle-reinforced aluminum matrix composites

In order to broaden SiC particle-reinforced aluminum matrix composites in the applications of high-temperature and high-power electronic devices for electronics packaging, three types of the zinc-containing fillers and a low-frequency/low-amplitude mechanical vibration agitation technology were introduced for brazing of 10 vol.% and 70 vol.% SiCp/Al-MMCs. The effect of Zn element and mechanical vibration agitation on the microstructure of the joints was investigated. The increasing aluminum concentration and the decreasing zinc concentration in the bond seam were confirmed by the microstructure images, weight loss measurements, and EDS results and were attributed to zinc penetration and volatilization. Vaporization of zinc in the liquid filler increased the voids and number of vacancies, which destroyed the arrangement structure of liquid filler, namely, short-range order and long-range disorder, and facilitated the mutual diffusion of other atoms. The joints of 70 vol.% SiCp/A356-MMCs with a shear strength of 16.6 MPa were produced at 480 °C and 0.5 MPa using Zn-25Al-10Ga-9Mg-1Ti interlayer. For 10 vol.% SiCp/A356-MMCs, sound joints with a shear strength of 119 MPa were obtained at 520 °C using the Zn-5Al-4Mg foil. Cracks are initiated from the particle/metal (P/M) interface and subsequently propagated to the matrix/metal (M/M) interface. Mg-Si-rich phases were observed in the bond seam and at the interface of 70 vol.% SiCp/Al-MMCs with different Mg content interlayers, which was also a weak interface for the joint. The results also indicated that the active element Mg in the filler should be controlled to an appropriate content for brazing of high volume fraction composites.

Improved approximation of transverse and shear stiffness for high volume fraction uniaxial composites

With uniaxial composites, the matrix material properties dominate the overall transverse and shear composite stiffness. Knowledge of these quantities are of practical importance in composite structures such as large wind turbine blades, which despite having a predominantly uniaxial layup, have both their torsional response and buckling behavior as critical design considerations. Accurate estimations of transverse and torsional rigidity for high volume fraction composites are currently dependent on extensive experimental work due to the low reliability and precision of transverse/shear micromechanical models. The Continuous Periodic Fiber Model is proposed to improve these predictions by considering fiber-fiber interactions through the use of periodic boundary conditions. Experimental data and standard models are used to validate the results and quantify the predictive improvements relative to traditional methods. The Continuous Periodic Fiber Model offers greatly improved accuracy for transverse stiffness estimations, reducing the average difference from the experiments by a factor of 1.7 (an average difference with experiment of 14% compared to the next best approach value of 24%) as well as modest improvements for the shear moduli.

A new approach to rapidly generate random periodic representative volume elements for microstructural assessment of high volume fraction composites

An algorithm based on event-driven molecular dynamics theory was developed to rapidly generate periodic representative volume elements (RVEs) with nonuniform distributions for both unidirectional fibre-reinforced and spherical particle-reinforced composites. Detailed statistical analyses were conducted for assessing the ability to generate RVEs with nonuniformly dispersed microstructures and either constant or random inclusion sizes for a wide range of volume fractions. The generated microstructures were directly compared with available microstructural optical images of a composite material, showing excellent statistical correlation and providing validation for the developed RVE generation approach. For further validation, finite element analysis was conducted using the generated RVEs in order to evaluate volume averaged elastic constants. The expected isotropic characteristics of the RVEs were correctly calculated, and excellent correlations with experimental data from the literature provided additional support for the algorithm accuracy. The versatile algorithm can rapidly generate RVEs with realistic reinforcement dispersions and high volume fractions up to 80%, which is advantageous compared to other algorithms. The proposed algorithm can be used as a design tool to accurately evaluate and tailor the mechanical properties of distinct composite material systems, and for their microstructural assessment including local damage predictions.

Three-dimensional microscopic assessment of randomly distributed representative volume elements for high fiber volume fraction unidirectional composites

Generating realistic non-uniformly distributed three-dimensional representative volume element (RVE) for modeling the response of high fiber volume fraction composites is challenging. A novel rapid approach employing a random event-driven molecular dynamics simulation (EDMD) was proposed in this study for this purpose. Distributions of continuous unidirectional fibers in the RVEs were assessed using two approaches, namely Voronoi tessellations and radial distribution functions. The developed method was highly efficient in generating RVEs with realistic fiber dispersions. Furthermore, applying the corresponding periodic boundary conditions (PBCs) for finite element modeling is also arduous. A technique was developed within the commercial finite element software ABAQUS to rapidly produce required matching mesh patterns on opposite surfaces of the RVE, and to apply the corresponding PBCs using custom scripts. By coupling the EDMD approach and custom scripts within ABAQUS to rapidly generate and mesh RVEs with high fiber volume fractions, evaluation of the RVE response was greatly simplified. In- and out-of-plane elastic properties of unidirectional glass/epoxy composite in a range of 10–80% fiber volume fractions were determined and compared with analytical models, and available experimental data, with good correlation.

Preliminary Design and Experimental Investigation of a Novel Pneumatic Conveying Method to Disperse Natural Fibers in Thermoset Polymers.

Natural fibers can be attractive reinforcing materials in thermosetting polymers due to their low density and high specific mechanical properties. Although the research effort in this area has grown substantially over the last 20 years, manufacturing technologies to make use of short natural fibers in high volume fraction composites; are still limited. Natural fibers, after retting and preprocessing, are discontinuous and easily form entangled bundles. Dispersion and mixing these short fibers with resin to manufacture high quality, high volume fraction composites presents a significant challenge. In this paper, a novel pneumatic design for dispersion of natural fibers in their original discontinuous form is described. In this design, compressed air is used to create vacuum to feed and convey fibres while breaking down fibre clumps and dispersing them in an aerosolized resin stream. Model composite materials, made using proof-of-concept prototype equipment, were imaged with both optical and X-ray tomography to evaluate fibre and resin dispersion. The images indicated that the system was capable of providing an intimate mixture of resin and detangled fibres for two different resin viscosities. The new pneumatic process could serve as the basis of a system to produce well-dispersed high-volume fraction composites containing discontinuous natural fibres drawn directly from a loosely packed source.

Experimental characterization of voids in high fibre volume fraction composites processed by high injection pressure RTM

Replacing autoclave processes is a well-known industry drive in the composites community. One of the most recognized candidates for this replacement is high injection pressure resin transfer moulding (HIPRTM), because it is both an out of autoclave process and because the high processing pressures can, hypothetically, reduce the size of voids, thereby reducing void content. In order to clarify this issue, this paper presents our results on the size distribution and total void fraction of composites containing high fibre volume fractions (>60%) composites produced by HIPRTM. To substantiate this work we present a comparative study considering both autoclave and RTM at lower pressure/fibre volume fractions. Results show that HIPRTM is able to produce high fibre volume fraction parts at very low void content (<0.05%) and is comparable to autoclave results. Future work should study the mechanical properties of these laminates in order to clarify further the limits of HIPRTM.

Effect of SiC volume fraction and size on dry sliding wear of Fe/SiC/graphite hybrid composites for high sliding speed applications

In this work we investigated the friction and wear properties of Fe/SiC/graphite hybrid composites using a sub-scale dynamometer disk brake testing system. Two particle size ranges (1–30μm and 150–180μm) and three particle volume fractions (10%, 15% and 20%) of SiC were considered. The sliding speed conditions considered in this study (25–35m/s) were comparable to that experienced by brake materials in high speed braking applications in aircrafts, race car and high speed trains. We examined the effect of coating the SiC particles with BaSO4 to improve interfacial properties and prevent potential undesirable interfacial reactions. The wear loss was found to decrease with increasing volume fraction of SiC for all particulate sizes. At low sliding speeds the composites with large particle sizes and high volume fractions were found to be more effective in controlling wear. On the other hand, at higher sliding speeds the high volume fraction composites were found to be more effective in controlling wear for all particle sizes. This is attributed to a transition in the wear mechanism at higher sliding speeds.

Multifunctional Nanoprepregs Based on Aligned Carbon Nanotube Sheets

ABSTRACTThe unique properties of carbon nanotubes (CNTs), such as high strength and stiffness, extremely large specific surface area, low density and high electrical and thermal conductivities, open up opportunities for a wide range of applications. This has led to widespread research on the synthesis of multiscale CNT-based polymeric composites transferring the properties from nanoscale to macroscale. Currently known conventional methods for producing CNT composites rely on dispersing short nanotubes in polymer matrices or infiltrating CNT “buckypaper” with resins. Unlike conventional methods, in this work, super-aligned long CNTs were directly drawn from the vertically aligned CNT arrays and assembled into high volume fraction (~60 vol%) composites. These critical structural features allowed the CNT composites to reach tensile strengths over 1.8 GPa and electrical conductivity ~700-800 S/cm. Thermoplastic and thermosetting polymers were utilized for the fabrication and the results were compared. Our observation suggests that the strengthening and conducting mechanism is derived from the synergistic effect of high degree of CNT alignment, long CNT length, high volume fraction and uniform distribution of CNTs in the polymer matrix produced by spray-winding.

A combined experimental–numerical approach for generating statistically equivalent fibre distributions for high strength laminated composite materials

A technique is presented where actual experimental distributions, measured from a high strength carbon fibre composite, are considered in the development of a novel method to generate statistically equivalent fibre distributions for high volume fraction composites. The approach uses an adjusted measure of nearest neighbour distribution functions to define inter-fibre distances. The statistical distributions, characterising the resulting fibre arrangements, were found to be equivalent to those in the actual microstructure. Finite element models were generated and used to determine the effective elastic properties of the composite and excellent agreement was obtained. The algorithm developed is simple, robust, highly efficient and capable of reproducing actual fibre distributions for high strength laminated composite materials. It does not require further heuristic steps, such as those seen in fibre stirring/shaking algorithms, in order to achieve high volume fraction microstructures and provides a useful alternative to both microstructure reproduction and random numerical models.

Promoting the adhesion of high‐performance polymer fibers using functional plasma polymer coatings

AbstractPlasma‐copolymerized functional coatings of acrylic acid and 1,7‐octadiene were deposited onto high strength, high modulus, poly‐p‐phenylene benzobisoxazole (PBO) fibers. X‐ray photoelectron spectroscopy (XPS) with trifluoroethanol derivatization confirmed that the PBO fibers were covered completely with the plasma copolymer and that the coating contained a quantitative concentration of carboxylic acid groups. Microdebond single filament adhesion and interlaminar shear strength (ILSS) tests were used to evaluate the interfacial strength of epoxy resin composites containing these functionalized PBO fibers. Both the interfacial shear strength (IFSS) obtained from single filament tests, and the ILSS of high volume fraction composites were a function of the surface functionality of the fibers so that there was a good correlation between ILSS and IFSS data. The tensile strengths of single fibers with or without coating were comparable, demonstrating that the fiber surface was not damaged in the plasma‐coating procedure. Indeed, the statistical analysis showed that Weibull modulus was increased. Therefore, plasma‐polymerized coatings can be used to control the interfacial bond between PBO fibers and matrix resins and act as a protective size for preserving the mechanical properties of the fibers. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

Determination of interface integrity in high volume fraction polymer composites at all strain levels

In fiber-reinforced composites, the mechanical performance is strongly dependent upon the quality of fiber/matrix interface. In the present work, a novel technique is presented whereby all important interface parameters of high volume fraction composites are determined in situ on standard tensile coupons. The method involves the introduction of a small surgical cut to the upper ply of a unidirectional aramid/epoxy laminate prior to vacuum bagging. During the curing procedure and subsequent consolidation in an autoclave, the resin penetrates the area of the cut and thus no effect to the integrity of the composite is observed. The stress transfer profiles during mechanical loading emanating from the fiber break(s) can be obtained by means of the technique of laser Raman microscopy. The technique allows the determination of the maximum interfacial shear stress at all far-field strain levels and therefore presents the only alternative for the accurate estimation of interface integrity in high volume fraction polymer composites.

Assessment of particle–matrix debonding in particulate metal matrix composites using ultrasonic velocity measurements

Particulate metal matrix composites are a class of materials that have evoked keen interest, largely due to the promise of improved properties over conventional metals and alloys. However, the effective realization of the improved properties of these composites is contingent upon the presence of good bonding between the reinforcement particles and the metal matrix. Therefore, a quantitative measure of the particle–matrix bonding efficiency would be particularly useful in helping to fully realize the potential of these materials. It is known that particle–matrix debonding degrades the elastic modulus of the composite considerably while having practically no effect on its density. Based on this, a model has been developed and a quantitative measure of debonding has been proposed. In conjunction with measurement of ultrasonic velocity, this model is capable of giving quantitative estimates of the debonding factor of the composite material. In the present study, aluminum alloy A356–SiC p composites containing between 3 and 15 vol.% SiC particles have been synthesized by the melt stirring method. Debonding factors as high as 0.29 have been estimated in low SiC volume fraction composites. In the case of the high SiC volume fraction composites, the estimated debonding factor was found to be as low as 0.10, corresponding to the lesser particle–porosity interaction observed in these composites. Elastic modulus measurements on composite specimens also confirm the observation that particle–porosity association is seen to a greater extent in the low SiC volume fraction composites. Elastic modulus deviation of up to 14% was observed in the case of the low SiC volume fraction composites while the high SiC volume fraction composites showed a maximum modulus deviation of less than 10%. It was also seen that measurement of debonding factor before and after hot rolling of the composites provides a method of evaluating the effectiveness of the rolling operation in improving the integrity of the composites.