Volume Fraction Of Microballoons Research Articles

The Effect of Microballoon Volume Fraction on the Elastic and Viscoelastic Properties of Hollow Microballoon-Filled Epoxy Composites.

This paper reports the study of hollow microballoon-filled epoxy composites also known as syntactic foams with various volume fractions of microballoons. Different mechanical and thermomechanical investigations were carried out to study the elastic and viscoelastic behavior of these foams. The density, void content, and microstructure of these materials were also studied for better characterization. In addition to the experimental testing, a representative 3D model of these syntactic foams was developed to further investigate their elastic behavior. The results indicate that changes in the volume percentage of the microballoons had a substantial impact on the elastic and viscoelastic behavior of these foams. These results will help in designing and optimizing custom-tailored syntactic foams for different engineering applications.

EFFECT OF MICRO-BALLOON VOLUME FRACTION ON THE TENSILE BEHAVIOR OF EPOXY SYNTACTIC FOAMS REINFORCED WITH GLASS MICRO-BALLOONS

In this study, the effects of volume fraction on the tensile behavior of epoxy syntactic foam reinforced with glass micro-balloons are investigated. Nine different samples with various volume fractions of glass micro-balloons (0%-68%) were prepared. Elastic modulus, maximum tensile stress, and strain at failure of epoxy syntactic foams reinforced with glass micro-balloons are investigated. Scanning electron microscope (SEM) images are taken from the broken samples' surfaces and used to discuss the results. Results show that the specific elastic modulus and specific maximum tensile stress (MTS) of epoxy syntactic foams increased by about 108% and 28% by increasing the micro-balloon volume fraction from 0% to 68%. However, the samples' modulus and strength are respectively decreased by about 5% and 39% with increasing the volume fraction of the glass micro-balloons from 0% to 34%. The main growth of specific strength and elastic modulus was for samples with more than 34% micro-balloon volume fraction. Moreover, the elastic modulus increased by about 11%, and the samples' maximum tensile stress (MTS) decreased by about 32%, increasing the micro-balloon volume fraction from 0% to 68%. With increasing the micro-balloon volume fraction from 0% to 68%, the strain at failure of epoxy syntactic foams is decreased by about 41%.

Densification mechanics of polymeric syntactic foams

In this paper, a fundamental understanding of the densification mechanics of polymeric syntactic foams under compressive loading is established. These syntactic foams are closed cell composite foams with thin-walled microballoons dispersed in a matrix (resin) whose closed cell structure provides excellent mechanical properties, like high strength and low density. There are several parameters that can contribute towards their mechanical properties, including, microballoon volume fraction, microballoon wall thickness, bonding between the microballoons and the matrix, and the crushing strength of microballoons. Conducting purely experimental testing by varying these parameters can be very time sensitive and expensive. Also, identification of densification mechanics is challenging using experiments only. Higher densification stress and energy are favorable properties under foam compression or crushing. Hence, the influence of key structural and material parameters associated with syntactic foams that dictate the mechanics of densification is studied here by implementing micromechanics based computational models and multiple linear regression analysis . Specifically, specific densification stresses and energy, which are densification stresses and energy normalized by weight, are evaluated which are more relevant for a wide variety of weight saving applications. Microballoon crushing strength and volume fraction are identified as the parameters that have the higher influence on densification stress and energy, and their specific counterparts, whereas the interfacial bonding has the least impact. In addition, designing aspects of syntactic foams with specified overall density are discussed by mapping microballoon volume fraction and wall thickness. The regression model allows for establishing wall thicknesses and corresponding volume fractions that result in higher densification properties for a specified overall foam density.

Warp-Knitted Spacer Fabric Reinforced Syntactic Foam: A Compression Modulus Meso-Mechanics Theoretical Model and Experimental Verification.

In this study, a new type ternary composite, called warp-knitted spacer fabric reinforced syntactic foam (WKSF-SF), with the advantages of high mechanical properties and a lower density, was proposed. Then, a meso-mechanics theoretical model based on the Eshelby–Mori–Tanaka equivalent inclusion method, average stress method and composite hybrid theory was established to predict the compression modulus of WKSF-SF. In order to verify the validity of this model, compression modulus values of theoretical simulations were compared with the quasi-static compression experiment results. The results showed that the addition of suitable WKSF produces at least 15% improvement in the compressive modulus of WKSF-SF compared with neat syntactic foam (NSF). Meanwhile, the theoretical model can effectively simulate the values and variation tendency of the compression modulus for different WKSF-SF samples, and is especially suitable for the samples with smaller wall thickness or a moderate volume fraction of microballoons (the deviations is less than 5%). The study of the meso-mechanical properties of WKSF-SF will help to increase understanding of the compression properties of this new type composite deeply. It is expected that WKSF-SF can be used in aerospace, marine, transportation, construction, and other fields.

High velocity impact on composite sandwich panels with nano-reinforced syntactic foam core

Abstract In this work, an experimental investigation of the high velocity impact resistance of syntactic foam core composite sandwich panel has been undertaken. The syntactic foams filled with ceramic microballoons of three different sizes and three different volume fractions. Also, to enhance the matrix strength of the foam core, some specimens reinforced with nano particles. The high velocity impact tests have been carried out using a light gas-gun and a 10 mm blunt-head steel projectile and the ballistic limit was measured. The Impact behavior and the failure mechanisms are thoroughly investigated and also the absorbed energy during projectile perforation was correlated to the damaged area of specimens. The test results show that the ballistic resistance decreases with increasing the volume fraction of microballoons and the panel with 40% volume fraction of microballoons has the highest specific perforation energy. Moreover, nano-reinforcing the syntactic foam core increased the ballistic limit velocity up to 10%.

The Effect of Carbon Nanofiber on the Dynamic and Mechanical Properties of Epoxy/Glass Microballoon Syntactic Foam

The aim of this study was to research the effect of carbon nanofiber (CNF), the microballoon (MB) volume fraction and its density on both the vibration properties and impact behavior of carbon nanofiber – reinforced syntactic foam. Twelve different types of syntactic foam were produced with two different CNF contents, two different MB densities and two different MB volume fractions. Neat epoxy, syntactic foams with and without carbon nanofiber were manufactured for characterization by comparing each other in terms of mechanical and dynamic properties. With the free vibration tests, natural frequencies and damping ratios were investigated. Besides vibrations tests, low-velocity impact and tensile tests were conducted on the syntactic foams. Analyses of the results were performed on the effect of the volume fraction of additives and MB density on the vibration, impact and tensile behavior of the syntactic foams. In addition, SEM was used to understand the microstructure of the samples tested. Vibration results presented that the presence of carbon nanofiber enhanced strength but was not effective on damping, while low-velocity impact, and tensile test results also demonstrated the same trend.

Rheokinetic studies and compressive response of high performance polybenzoxazine syntactic foams

ABSTRACTPolybenzoxazines are finding increasing usage in demanding applications where high temperature stability is required, especially in the field of aerospace. In this work, thermally stable bisphenol F‐based polybenzoxazine [poly(BF‐a)] syntactic foams containing varying volume fractions (30–60%) of hollow glass microballoons (HGMs) were prepared and their mechanical response in the quasi‐static regime was established. The effect of introducing glass microballoons on the curing profile of benzoxazine resin was studied using both nonisothermal differential scanning calorimetry and rheometry. Temperature‐sweep experiments were performed to arrive at the optimal processing window of the benzoxazine‐glass microballoons formulations, particularly in terms of viscosity, gelation temperature, and time. Thermally accelerated ring‐opening polymerization of the benzoxazine resin led to complete curing of the syntactic foam formulations, as assessed by calorimetric studies. The thermal degradation behavior of the poly(BF‐a)/HGM was studied using thermogravimetric analysis. As expected, the density of the syntactic foam specimens decreased with increasing microballoon content. Maximal increase in the specific compressive properties of the poly(BF‐a)/HGM samples was observed in formulations containing 40% volume fraction of glass microballoons. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47234.

A crack healable syntactic foam reinforced by 3D printed healing-agent based honeycomb

In this study, a novel grid skeleton (i.e., the 3D printed healing-agent based honeycomb structure) was embedded in a syntactic foam matrix to enable the composites to own crack closing and healing capabilities. There is always a trade-off between crack healing efficiency and overall structural properties. The new type grid skeleton was proposed here to make syntactic foam composites owning excellent crack healing ability without sacrificing overall mechanical properties. The volume fraction of glass microballoons within syntactic foam matrix remained constant at 30%. The reinforcing honeycomb cell sizes varied with a ratio from 3.8, to 5.4 and 7.3, which resulted in various physical and mechanical properties, such as different density and compressive strength. The thermal properties of the syntactic foam composites were tested through Differential Scanning Calorimetry (DSC). The interfaces between the syntactic foam matrix and PCL grid skeleton (i.e., printout) was investigated by the Fourier Transform Infrared (FTIR) spectrum. It displayed that the interaction between those phases were completely in a physical manner. Nano-indentation tests were conducted to study the composite component mechanical properties. Three-point bending tests were conducted to initiate structural level cracks to examine the crack healing capability. Healing efficiency was obtained according to the comparison between specific flexural strength before and after the crack healing event. Under free constraint condition, the 3D printed PCL honeycomb reinforced syntactic foam exhibited promising crack healing performance with healing efficiencies above 80%. This study provided an understanding on the interface property and crack healing mechanisms in the 3D printed honeycomb reinforced syntactic foam system.

Analysis of the specific properties of glass microballoon-epoxy syntactic foams under tensile and flexural loads

Syntactic foam samples of varying volume fractions were fabricated using glass microballoons (GMB) as the filler in an epoxy matrix. The tensile and flexural properties of the foams were obtained from experiments for microballoon volume fractions up to 0.5. Special attention was given to their specific (per unit density) properties which established the means to compare materials of different densities. Even though the actual properties of the foams show a decreasing trend due to the hollow nature of the fillers, the specific properties, especially the specific modulus, show an increase in the 0.3 to 0.5 microballoon volume fraction regime. The foams were also analyzed numerically by creating a suitable representative volume element (RVE) and analyzing it using a commercial software. The RVE was a multi-particle unit cell having microballoons in the same size distribution and volume fraction as the foam itself. The effective overall properties were obtained over it and compared with the experimental results.

Comparative Study on Microstructural Characteristics and Compression Deformation Behaviour of Alumina and Cenosphere Reinforced Aluminum Syntactic Foam Made Through Stir Casting Technique

In order to examine the effect of microballoons type on microstructure and compressive deformation behaviour of aluminum syntactic foam, alumina reinforced and cenosphere reinforced aluminum syntactic foams have been made through stir-casting technique. Alumina microballoons reinforced aluminum syntactic foam (AMRASF) has been developed using stir casting technique. Volume fraction of alumina microballoons in AMRASF varies in the range of 0.39–0.74. The compressive deformation behavior of these AMRASF is compared with that of cenosphere reinforced aluminum syntactic foam (CPRASF). The AMRASF does not exhibit clear plastic collapse stage as observed in case of CPRSAF. It is further noted that AMRASF shows the existence of work hardening phenomena after yielding but CPRASF does not show any significant work hardening after yielding even at higher relative densities of foam. This is explained on the basis of characteristics of alumina microballoons and cenospheres and interface characteristics between matrix and respective microballoons used. Effect of individual microballoons’ characteristics on the foam properties has also been explained.

The effect of nano-additive reinforcements on thermoplastic microballoon epoxy syntactic foam mechanical properties

Syntactic foams comprising glass microballoons have gained considerable attention over the past several years due to mechanical and thermal properties that are advantageous for use as a core material in naval and aerospace applications. Recent advancements in the production of thermoplastic microballoon syntactic foams have allowed for an increase in microballoon volume fraction (up to 0.9 volume fraction), with correspondingly lower densities but reduced mechanical properties. In this work, carbon nanofibers and halloysite nanotubes were incorporated in thermoplastic microballoon-based syntactic foam to enhance the mechanical properties and the relative effects of these two nanoscale reinforcements were compared. X-ray micro-computed tomography was employed to analyze the microstructure of the materials produced, and scanning electron microscopy was used to assess the dispersion of nano-additives within the resin. Compressive strength and modulus enhancements as large as 180% and 250% respectively were achieved with a 0.25 wt% addition of carbon nanofiber and increases of 165% and 244% respectively were achieved with a 0.5 wt% addition of halloysite nanotube. Tensile strength and modulus enhancements as large as 110% and 165% respectively were achieved with a 0.125 wt% addition of carbon nanofiber and increases of 133% and 173% respectively were achieved with a 0.125 wt% addition of halloysite nanotube.

A micromechanical model to study failure of polymer-glass syntactic foams at high strain rates

Syntactic foams are lightweight composite materials that find extensive application as core materials for sandwich panels in marine and aerospace structures. While several models have been proposed to analyze the elastic response and failure of these composites at small strain rates, the understanding of syntactic foam behavior at high strain rates remains elusive. In this work, we simulate the response of polymer-glass syntactic foams under high strain rate compressive loading conditions, by using a three-dimensional micromechanical model consisting of fifty hollow spheres randomly dispersed in the matrix material. The mechanical response of the matrix is described by generalizing a phenomenological viscoplastic constitutive model from the literature to the three-dimensional stress state. The filler behavior is assumed to be linear elastic until brittle failure, which is predicted on the basis of a structural criterion for glass microballoons. The collapse of the first glass microballoon is hypothesized to trigger the failure of the whole composite. Such a micromechanical model is implemented in the commercial finite element code ABAQUS. We focus on glass-vinyl ester syntactic foams and perform a parametric study to elucidate the roles of strain rate, microoballoon density, and microballoon volume fraction on the compressive modulus, strain energy, and effective strength. Comparisons between model findings and available experimental data are presented to assess the accuracy of the proposed numerical model. Our results enable the study of syntactic foam behavior at high strain rates, for a wide range of strain rates, microballoon densities, and microballoon volume fractions. This knowledge is expected to aid in the design of lightweight composite materials subjected to high strain rate compressive loading.

Production and characterization of epoxy syntactic foams highly loaded with thermoplastic microballoons

Glass microballoon syntactic foams consisting of 60–70 vol% hollow glass microballoons and epoxy resin matrix have gained considerable attention in recent years due to their unique combination of mechanical properties and low density, with applications in the naval and aerospace industries. An important limitation of these materials is the volume fraction ceiling (∼0.74) and subsequent density limit (0.36 g/cm3). Utilizing thermoplastic microballoons, syntactic foams were produced with densities as low as 0.067 g/cm3, achieved by developing a method that produces epoxy/microballoon compositions comprising an unusually high volume fraction of microballoons (0.75–0.95). The resulting morphology features microballoons which, having expanded in a restricted volume, are deformed into irregular shapes that efficiently pack together and are encapsulated by a thin coating of epoxy. The compressive yield strength, tensile strength and initial modulus of these highly loaded syntactic foams exhibit a non-linear decrease with increasing microballoon volume fraction to values typical of highly porous polymers, but display a high degree of recovery, or rebound, from large compressive strain compared with glass microballoon syntactic foams.

Elastic behaviour and failure mechanism in epoxy syntactic foams: The effect of glass microballoon volume fractions

A representative elementary volume (REV) in epoxy syntactic foams was generated to incorporate randomly distributed glass microballoons that followed a log-normal size distribution. Finite element modelling of the REV foam was developed and experimentally validated to investigate the elastic behaviour and failure mechanism in the foams with different microballoon volume fractions (V). The localised stresses concentrate in various zones within the foam, and can cause the vertical splitting fracture of microballoons and the micro-crack formation in the matrix. Dependent on the microballoon volume fraction, micro-cracks can propagate to join adjacent micro-cracks and voids left by fractured microballoons, and finally develop into a macro-crack either in the preferred longitudinal (for low V) or diagonal (for high V) directions. This is consistent with the macroscopic observations of the fracture process in the foam specimens. It was also found that elastic characteristics of the foam vary with microballoon volume fractions.

Quasi-static and dynamic compressive properties of ceramic microballoon filled syntactic foam

The compressive behaviour of epoxy based syntactic foams filled by ceramic microballoons is experimentally investigated in this study. Nine different types of syntactic foams are fabricated with three different microballoon sizes and three different microballoon fractions. All of the syntactic foam specimens are tested at various strain rates from quasi-static to high strain rates. Analysis of the results is carried out on the effect of the volume fraction, microballoon size and strain rate on the compressive behaviour of syntactic foams. Also, scanning electron microscopy is used to understand the fracture mechanisms of tested specimens. The results show that as the microballoon volume fraction increases the compressive strength, compressive modulus, failure strain and plateau stress decreases for all types of syntactic foams at all strain rates. Although, this decrease is slight for 20% and 40% volume fraction, it is considerable for 60% microballoon volume fraction syntactic foam. The results indicate that reducing the microballoon size or increasing the strain rate of testing would enhance the compressive strength.

Mechanical properties of graphene platelets reinforced syntactic foams

Graphene platelets (GPs) are two-dimensional thin plates containing few layers of graphene sheets. Compressive and tensile behaviors of epoxy-based syntactic foams with pristine GPs as additives are discussed in this article. Four sets of syntactic foams containing 0, 0.1, 0.3, and 0.5vol.% of GPs were fabricated and tested. The volume fraction of microballoons in all syntactic foam samples was kept constant at 30%. Results indicated that the compressive and tensile moduli of the syntactic foams were significantly improved as compared to samples that did not contain GPs. The addition of GPs also enhanced the tensile strength while the compressive strength was only slightly increased. Optimal property improvements were obtained for very low filling fraction of approximately 0.3vol.%. Samples with higher volume fraction of GPs (0.5%) showed deterioration in mechanical properties when compared to other GP containing samples. Transmission microscopy study indicated formation of voids enclosed by undispersed GPs in the samples which could explain the decline of the properties. High matrix porosity could also play an important role in this observation. Utilizing surface modified GPs could allow incorporation of higher volume fraction of GPs homogeneously, thus improving the mechanical properties of the syntactic foams.

Experimental measurements and numerical modeling of dynamic compression response of an interpenetrating phase composite foam

A 3D interpenetrating phase composite (IPC) foam made by infusing syntactic foam (SF) into an open-cell aluminum scaffold is studied under high-strain rate conditions. The SF is prepared by dispersing hollow glass microballoons into an epoxy matrix. Dynamic compression characteristics, including strength and failure progression of pure SF and IPC foam made of four different volume fractions of microballoons in SF measured using a split Hopkinson pressure bar (SHPB) apparatus are reported. The results show that in general IPC foams outperform SF in terms of compressive strength. An idealized 3D elasto-plastic unit-cell based finite element (FE) model is proposed for studying the IPC foam. The 3D geometry of the aluminum constituent in the cubic unit-cell is modeled as a tetrakaidecahedron, whereas the SF constituent is modeled as the occupant of the rest of the unit-cell. The computational model incorporates infinite elements to represent the far-field regions surrounding the unit-cell. Comparisons of computational and experimental results for different microballoon volume fractions of SF are reported.

Characterization and modeling of compression behavior of syntactic foam-filled honeycombs

Processing and compression behavior of lightweight syntactic foam-filled aluminum honeycomb composites is presented. Different foam-filled honeycomb composites are prepared by varying the volume fraction of microballoons in the syntactic epoxy foam while keeping the volume fraction of the metallic network the same. Uniaxial compression tests are then carried out on syntactic foam and foam-filled honeycomb composites. The latter shows improvement in elastic modulus and plateau stress values by 26—31% and 36—39% when compared to the syntactic foam with the same volume fraction of microballoons. The maximum increase in energy absorption for syntactic foam-filled honeycomb composite is found to be approximately 48%. The compression response and stress-strain characteristics of the foam-filled honeycombs are also examined relative to a 3D variant, namely, an interpenetrating aluminum-syntactic foam composite. Elasto-plastic finite element models are developed to simulate experiments performed on syntactic foam...

Energy Absorption of Integrated Functionally Gradient Syntactic Foam

Functionally gradient syntactic foams (FGSFs) are fabricated using a layer-over-layer integrated technique in five different layer sequences. Four different types of glass microballoons, with similar outer diameters but different inner diameters are used to fabricate individual layers in integrated FGSFs (IFGSFs). The volume fraction of microballoons in each IFGSF layer is kept constant at 60% to maintain uniformity in the structure. The different layers of IFGSFs are integrated before major solidification takes place. IFGSFs are investigated for flat-wise and edge-wise compression properties on a MTS-810 servo hydraulic machine. Compared to plain syntactic foams with a similar density, it is found that the energy absorption of flat-wise and edge-wise IFGSFs increase by more than 200% and 20%, respectively. Also, the IFGSFs show a dramatic increment in compressive properties compared to adhesively bonded FGSFs. Furthermore, IFGSFs are compared to one another to examine the effect of layer sequencing on flat-wise and edge-wise compressive behavior. The layer sequencing effect on crack propagation behavior of IFGSF is carefully monitored and analyzed with an optical microscope (OM) and a scanning electron microscope (SEM).

Syntactic foams from copolymers based on epoxidized soybean oil

Syntactic foams are been increasingly used as core of sandwich panels due to their light weight and good mechanical properties. This investigation evaluates the compressive, flexural and thermo-mechanical properties of syntactic foams made by embedding randomly dispersed hollow glass microspheres in bio-based resins obtained by partial substitution of diglycidyl ether of bisphenol A (DGEBA) with epoxidized soybean oil (ESO). Volume fraction of glass microballoons was 0.55 in all foam formulation. Flexural and compressive strength values decreased simultaneously with increasing ESO content. Similar trend was observed for the flexural and compressive modulus and glass transition temperature. The work further showed that mechanism of failure mainly depended on the fracture of microballoons regardless the ESO content in the formulation. Results reported herein suggest that large fractions of DGEBA can be replaced by ESO with minor effect on mechanical and thermal properties.