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On the road to real applications, although there are lots of efforts focusing on mechanical and physical features in the literature, their machining abilities were examined in a very limited manner. In this study, machining properties of pumice reinforced AA7075 syntactic foams manufactured via the newly offered sandwich infiltration technique were investigated by performing face turning. Physical and microstructural (optical and SEM works) analyses were conducted on fabricated foams to carry out sample characterization. All machining forces were measured for different cutting speeds (25, 50, and 100 m/min) and feed rates (0.05, 0.10, and 0.15 mm/rev). After the turning operation, areal surface roughness values were measured using a 3D surface profilometer and material removal rate (MRR) values were calculated. Besides, chip mixtures including pumice and metal fragments were collected to probe chip morphology in detail. The results showed that machining forces were affected by the operation parameters differently, and the lowest surface roughness was detected at the cutting speed of 100 m/min and 0.05 mm/rev feed rate. Furthermore, the shape of the metal chips changed from long/continuous characteristic to saw-tooth morphology depending on increasing cutting speed levels while pumice particles exhibited breakaway tendency as the feed rates went up.

The composite sandwich structures with foam core and fiber-reinforced polymer skin are prone to damage under local impact. The mechanical behavior of sandwich panels (glass fiber-reinforced polymer [GFRP] skin reinforced with lattice webs and syntactic foams core) is studied under crushing load. The crushing behavior, failure modes, and energy absorption are correlated with the number of GFRP layers in facesheets and webs, fiber volume fractions of facesheets in both longitudinal and transverse directions, and density and thickness of syntactic foam. The test results revealed that increasing the number of FRP layers of lattice webs was an effective way to enhance the energy absorption of sandwich panels without remarkable increase in the peak load. Moreover, a three-dimensional finite-element (FE) model was developed to simulate the mechanical behavior of the syntactic foam sandwich panels, and the numerical results were compared with the experimental results. Then, the verified FE model was applied to conduct extensive parametric studies. Finally, based on experimental and numerical results, the optimal design of syntactic foam sandwich structures as energy absorption members was obtained. This study provides theoretical basis and design reference of a novel syntactic foam sandwich structure for applications in bridge decks, ship decks, carriages, airframes, wall panels, anticollision guard rails and bumpers, and railway sleepers.

The uniform aluminum matrix syntactic foams (SFs) were prepared by the stir casting method, with alumina hollow spheres (2-3 mm and 3-4 mm) and expanded glass (2-3 mm) as reinforcements, and ZL111 aluminum alloy as matrix. The functionally graded aluminum matrix syntactic foams (FG-SFs) were obtained by superimposing two uniform aluminum matrix syntactic foams. Quasi-static compression tests were performed. The plateau stress of FG-SFs containing only hollow spheres decreased slightly with increasing volume fraction of SF containing 3-4 mm hollow spheres. The FG-SFs containing 2-3 mm hollow spheres and 2-3 mm expanded glass showed the highest plateau stress. The energy absorption behavior of all samples fluctuated in a small range. The initial position of shear band depended on the volume fraction of uniform aluminum matrix syntactic foams, reinforcement type and size. The cracks always appeared first in the uniform aluminum matrix syntactic foams containing expanded glass.

In this paper, the penetration behavior of newly designed sandwich structures consist of a 6061-T6 Al alloy sheet and two-layered plain-woven E-glass/epoxy composite laminates and syntactic foam core is investigated experimentally and numerically. Penetration tests are conducted using a single-stage gas-gun and steel conical-ended projectiles to evaluate the accuracy of the finite element model. The 3D finite element code, ABAQUS/Explicit, is used to model the penetration behavior of the sandwich structures. The Johnson-Cook models for material and damage behaviors are used to model the behavior of aluminum sheets. The progressive damage model based on the generalizations of the Hashin failure criteria in a VUMAT subroutine, and the crushable foam model associated with the Reyes fracture criterion in a VUMAT subroutine are employed to simulate the behavior of composite laminates, and syntactic foam, respectively. To validate the finite element model, the penetration behavior of sandwich structures is compared with the experimental results obtained from the experimental study and has shown accurate predictions. The suggested finite element model can predict the residual velocity and perforation energy with 11.2 % and 2.6 % errors, respectively. The effect of foam core thickness, fiber-metal laminate thickness, and impact velocity on the penetration behavior of sandwich structures is studied using the confirmed finite element model. It is observed that the failure of the sandwich structures mainly happens in the impacted area. Also, it is concluded that the effect of the Al thickness on the penetration resistance is more significant than foam and composite thicknesses. Furthermore, as the impact velocity is increased, the penetration time and loss of the velocity are decreased and the perforation energy is increased.

The objective of this work is to improve the structural characteristics of hollow glass microsphere (HGM) filled epoxy syntactic foam composites with little voids content and improved HGM dispersion in the composite. A modified degassing technique has been introduced during resin casting process of the HGM filled syntactic foam composites. The effect of HGM content volume fractions (5–25%) on the degassing techniques was examined. The syntactic foam composites were characterized by analysing structural morphology using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy(TEM), and density measurements (theoretical and experimental). Less than 5% void content has been achieved in this study. This resulted in improved tensile and dynamic mechanical properties (DMA).

In this paper, we report the effect of introducing molybdenum disulfide (MoS2) nano-platelets: a two-dimensional metal chalcogenide, on the mechanical properties of hollow glass microballoon (HGM)–epoxy syntactic foams. MoS2 reinforced syntactic foams were prepared by mixing MoS2 nanoplatelets to epoxy containing HGMs; with the amount of MoS2 being varied from 0.01 to 0.04% v/v, while maintaining a constant total filler volume fraction of 40% for all compositions. The mechanical behaviour of reinforced syntactic foam was studied under varied loadings including compressive, tensile and flexural under different strain rate regimes. Introduction of MoS2 led to significant improvements in characteristic mechanical properties, particularly in terms of compressive strength and toughness, which suggest intercalation of MoS2 within the epoxy matrix; however, the presence of relatively larger MoS2 micro particles couldn’t be completely negated. The toughness of the foam, as indicated by the area under the compressive stress-strain curve, was found to increase by ∼21% under optimal conditions. Our results highlight the potential of the two-dimensional MoS2 sheets as a reinforcing agent in syntactic foams.

Is it possible to create low-cost functional composite metal foams? In the framework of this study, composite metal foams filled tubes were produced with lightweight expanded clay aggregate particles. Specimens were manufactured by low pressure infiltration in aluminium tubes with the wall thickness of 5 mm. The properties of the transition layer between the matrix material and the material of the tube were analysed with energy dispersive X-ray spectrometry. The importance of the research was to develop lightweight, high-capacity functional composite metal foams which have higher specific mechanical properties than ’conventional‘ metal foams, thereby achieving additional weight reduction and lower material requirements, primarily as moving machine parts or automotive components, while keeping overall costs low.

Silica fume, a by-product obtained during the smelting process of silicon and ferrosilicon alloys, is widely used as a filler material in concrete for construction purposes. This study discusses the use of silica fume (1–7% v/v) as novel fillers in epoxy glass microballoon syntactic foams. The total filler loading i.e. glass microballoons and silica fume is maintained at 40% (v/v). Epoxy syntactic foams are popularly being used in the construction industry for the fabrication of light-weight materials in the sandwiched configuration. The use of silica fume is expected to improve the mechanical properties of such syntactic foams, which otherwise suffer due to the incorporation of hollow glass microballoons. As expected, the incorporation of silica fume enhanced the mechanical properties of the base syntactic foam. Epoxy glass microballoons reinforced with silica fume (5% v/v) increased the specific tensile strength by ∼ 47 percent, flexural strength by ∼ 54 percent and compressive toughness by ∼ 23 percent, etc. The increase in properties is attributed to the homogenous dispersion of the silica fume throughout the matrix and the load-bearing ability of silica fume. The use of silica fume opens up vistas in the field of polymer syntactic foams.

In the ongoing development of light-weighted materials, syntactic foams have superior features over open-cell and closed-cell foams. Amongst the metal-based foams, syntactic foams have least material utilization and are fairly porous. Fly ash production is one of the major consequences of combustion of fossil fuels. The low density of fly ash, combined with robust properties of aluminium and magnesium can lead to the formation of light-weighted and high strength syntactic foams. This paper discusses preferable characteristics, advantageous effects and outcomes of the composite syntactic foams fabricated using fly ash as reinforcing material within the aluminium and magnesium matrices, on the basis of proceeding investigations carried out across the globe. By using fly ash as a reinforcement material, avoidance of challenges such as inhomogeneity, sudden deleterious reactions, change in alloy chemistry and reinforcement gradients is attained. Hence, syntactic foams with fly ash as reinforcement material finds wide applications in compressive and high temperature conditions. Effect of using fly ash in aluminium and magnesium syntactic foams, considering change in alloy matrix and dimensional changes of reinforcement on mechanical and thermal properties have been summarized. Already assessed and further possible applications have also been enlisted.

The purpose of innovative engineering inventions in our modern world is not necessarily to solve a new problem, but to implement and optimize an existing solution in a more energy-efficient and cost-effective manner. This can be achieved by increasing the load-bearing capacity or reducing the weight. Using these two methods simultaneously we can produce so-called syntactic metal foams. The mechanical properties of syntactic foams are highly dependent on the filler materials’ volume. Therefore, in the course of our research, we produce gradient syntactic metal foams (GSMFs) with gradient filler volume within one metal foam. Mechanical analysis as compressive tests and microstructural analysis were performed to determine the properties of the differently filled syntactic metal foams.

Owing to the expanding world of materials, there appears an intelligible and timely need for the development of materials that are easily fabricable, light in weight and have greatly enhanced properties in comparison with the conventional ones. A quick glance on foams, alluding a cellular closed structure, depicts remarkable combination of optimized material utilization for high strength and low density. Aluminium, being commonly available and light weighted, under controlled processing, offers substantial advantages for the evolution of improved materials. Aluminium Syntactic Foams (ASFs) are the composites having aluminium matrix with usually pre-formed inorganic hollow spherical reinforcements arranged in an orderly fashion with constituents of either metallic, polymeric or ceramic nature, or superfluous byproducts. These spherical reinforcements impart high strength to the syntactic foams and compensate the lower hardness and stiffness of aluminium. Several researches have been done for developing lightweight materials. Liquid route has proven highly effective over the solid-state methods for fabrication of lightweight materials. Dispersion mechanisms are adopted by various liquid route fabrication methods offering uniform distribution of reinforcements within the aluminium matrix. With focus on Aluminium Syntactic Foams (ASFs), these fabrication methods are broadly classified in the current study and, their constructive effects on properties are a summarized and scrutinized on the basis of ongoing explorations being conducted across the world.

This research focuses on the mechanical investigations of in-situ produced AlSi12 aluminium alloy matrix syntactic foam-filled tubes. The samples were manufactured by low-pressure infiltration in a tube made from AlMgSi0.5. As second phase 3.5–4 mm average diameter lightweight expanded clay aggregate particles were used. The mechanical analysis extends to compressive tests of three different type of samples: empty tubes, metal matrix syntactic foams, and foam-filled tubes. The results show the best specific mechanical energy absorbing capacity in the case of foam-filled tubes, since the matrix creates a durable boundary layer with the tube wall.

In this paper, experimental study on drilling of cenosphere reinforced epoxy composites was carried out to investigate drilling characteristic using tungsten carbide drill bits. The influence of drilling process variables, such as feed rate, drill diameter, % weight of reinforcement and spindle speed on Specific Cutting Coefficient (SCC) has been studied. The composite samples were prepared with varying concentration of cenosphere (filler) by varying the content from 10% to 60% in 25% increment in the epoxy resin matrix. The plan for the drilling experiments was based on full factorial design (FFD). The drilling characteristic was analyzed through mathematical equation based on Response Surface Methodology (RSM). The experimental investigation on drilling characteristic clearly revealed that the SCC decreases with an increase in the feed rate and % of fly ash cenosphere in epoxy resin matrix.

The incorporation of multi-walled carbon nanotubes (MWCNTs) into syntactic foams (hollow glass microspheres bonded to a polymer matrix) can enhance the mechanical performance of syntactic foams. In this study, several syntactic foam specimens reinforced with MWCNTs were manufactured and tested under static and low-velocity impact loads, and the mechanical properties of these syntactic foams were compared to those of reference syntactic foams without MWCNTs by focusing on the impact resistance and optimal energy absorption. Moreover, the influences of the applied impact energy and microsphere and MWCNT contents on the impact responses of syntactic foam panels were discussed, and the vibration equation of a panel with four clamped edges was determined based on the energy principle. An analytical solution for the displacement responses of the syntactic foam panels under sinusoidal impact loads is proposed, in which Young’s modulus of the syntactic foam is expressed as a function of the moduli of the fillers and binders and contents of the fillers. The predicted maximal displacements agree with the experimental values well.

In recent years, there has been an increasing interest in the study of lightweight syntactic foams (SFs) because of their tailorable properties, quality characteristics and suitable applications in diverse technology industry. A considerable amount of literature has been published on SFs with most of these studies focusing on particular aspect like the matrices types (polymer, metal, ceramics, bio and hybrids); filler types (microparticles/microballoons/microspheres); nanoadditives types (nanotubes, nanofibers, nanoparticles); types of production techniques (powder metallurgy, infiltration and casting, mechanical molding and chemical deposition); and industry applications (aerospace, marine, building and construction, automotive and electrical and electronics). However, much of the literatures up to now has been research articles and few review articles. And there has been no attempt to comprehensively discuss SFs touching on the main research interest in a robust manner. Thus far, this study, brings a comprehensive insight toward the isolated research areas in SFs and then, compares and summarizes outcomes and inference from previous and current studies. And recommends on future research.

In this study a FE model is prepared for drop weight impact hammer testing of polymeric syntactic foam. The foam is modelled using crushable foam material and hammer is modelled using bilinear material model of LS-DYNA®. A series of simulation is performed by varying density of foam and impact velocity of hammer. Based on the prepared FE model and the force-displacement relation, energy absorption of the foam is computed and compared for three densities and three velocities. A comparative study is presented based on the displacement, reaction force-time history, and forcedisplacement behaviour.

A method for the reuse of aluminum (alloy 5754) sawing chips as a starting material for manufacturing aluminum matrix syntactic foams by powder metallurgy is described. In the tested samples, spherical Ni-superalloy hollow particles were used to increase the closed porosity of specimens up to 50 wt.%. Spark plasma sintering was used to consolidate the specimens. Only the sample with 30% hollow particles showed a clear maximum corresponding to a compression strength of 130 MPa. For all the samples, the plateau stress was over 120 MPa. The densification strain was over 40%, reaching 61% for the sample containing 30% hollow particles. All the samples showed high energy absorption capabilities and good absorption efficiency.

Syntactic foams are composite materials consisting in the association of hollow particles, called “microspheres” and a polymer matrix. The use of soft shell microspheres confers to the foam interesting properties but in return increases significantly its compressibility. Therefore, understanding and predicting the relationship between pressure and volume change is a crucial issue for the development of this type of material. The present study focuses on a high void fraction syntactic foam made with soft shell polymer microspheres embedded in a polyurethane matrix. Compression tests are performed using a capillary rheometer and a PVT accessory for the hydrostatic compression, and a more conventional apparatus for the confined compression. The experimental results are compared with De Pascalis’s pressure/volume model predictions, using Fok and Allwright’s model to determine the critical buckling pressure of the microspheres. The model proves to be fairly accurate at low pressure and high pressure, despite a notable deviation in the mid-pressure range. The influence of key model parameters such as microsphere size distribution and microsphere and matrix elastic properties is investigated. It is shown that the reinforcement of the matrix seems to be the only efficient way to limit the compressibility of such a syntactic foam.

Polymeric foam core sandwich composites are the nascent materials used in marine and aerospace structural applications for its low-density characteristics. A special class of foam called syntactic foam is one of the promising core material having high specific properties. However, these polymeric foam core sandwich composite structures may encounter problems in correlation with temperature and fire. Damages that happen due to the variation in temperature and by the catch of fire are sometimes imperceptible which may lead to the deterioration of load carrying ability or catastrophic failure of these composite structures. Present investigation is focused on the possibilities of reducing the extent of damages due to variation in temperature and by the catch of fire by enhancing its thermal stability and flame resistance characteristics. This was achieved from the development of syntactic foam by embedding hollow micro-spherical particles in phenolic resin for fire containment or fire isolation. Result of the experimentation reveals that the phenolic syntactic foam core was thermally more stable than glass/epoxy face skins up to a temperature of 450°C. The minimum concentration of oxygen required for burning was found to be 30%, in which phenolic syntactic foam core helps in flame isolation, whereas E-glass/epoxy face skins contribute to flame spread in the event of burning of sandwich composites. Improved thermal stability and fire resistance characteristics of developed sandwich composites are attributed to the phenolic syntactic foam core and by its orientation.

Alumina hollow spheres with a diameter range of 1–2 mm were filled in 6061 Al alloy by gravity infiltration casting to synthesize Al-matrix syntactic foams. The effects of infiltration temperature and heat treatment on microstructure, compressive properties, and energy absorption properties of the syntactic foams were studied. The results show that high quality syntactic foams could be synthesized in the infiltration temperature range 770–830 °C and the particles seamlessly bonded with the matrix. The average density and porosity of the syntactic foams were around 1.66 g/cm3 and 37.47%, respectively. The highest energy absorption capacity and specific energy absorption of the as-cast syntactic foams occurred at infiltration temperature 770 °C, reaching 41.11 MJ/m3 and 23.63 kJ/kg, respectively, while these parameters for the heat-treated samples showed the maximum values for the solution-aging treated samples, reaching 48.92 MJ/m3 and 31.36 kJ/kg respectively. Extensive literature survey is presented in this work by extracting the compressive properties of a variety of Al-matrix syntactic foams and comparing them with the results obtained in the present work. It is observed that the syntactic foams synthesized by gravity infiltration method equal or exceed the properties demonstrated by many Al-matrix syntactic foams synthesized by other methods reported in recent years. As a major advantage, the gravity infiltration method can be scaled up to industrial production level.

The syntactic foams based on polymethylphenylsiloxane resin filled by hollow glass microspheres were developed for using in different applications of construction industry. Thermophysical properties of the developed syntactic foams were analyzed in this work. According to the study results, with an increase of polymethylphenylsiloxane content in the syntactic foam, the thermal conductivity and the specific heat capacity increased. The coefficient of thermal linear expansion was dependent on the ratio of the components, reaching the lowest value for the composites with a minimum binder content.

Syntactic foams are synthesized by using pre-fabricated hollow microspheres. Syntactic foams are used as energy absorption sandwich core for several applications like marine, automotive and aerospace. Properties of syntactic foam can be tailored by addition of filler material. In this work hybrid syntactic foam is fabricated by addition of two types of filler materials that are hollow glass microballoons and fly ash cenosphere into vinyl ester matrix at different concentration. The volume fraction of filler content with respect to matrix is fixed to 50% and composition of two fillers varied. Hybridization of two different types of ceramic microballoons in vinyl ester matrix gives maximum 9.58% improvement in impact strength with respect to plane hollow glass microsphere syntactic foam. Enhancement in flexural strength and Flexural modulus observed is 27.7% and 27.16% respectively as compared to plane hollow glass microsphere syntactic foam

In the present study, the tribological behaviour of the syntactic foams manufactured using the fly ash cenospheres as the reinforcements and the epoxy as the matrix were studied for their tribological performance. The effect of the reinforcement concentration on the tribological properties like wear rate and the coefficient of friction were observed by using the pin on disc tribo tester. Four types of samples were prepared by varying the cenosphere volume fraction from 10% to 40% and tribological tests were conducted at constant sliding speed of 1 m/s and constant applied load of 50 N for a sliding distance of 2000 m. The increase in cenosphere volume fraction reduced wear rate and increased friction coefficient. The reduction in the wear rate may be due to the reduction in the effective contact area due to the pores created by crushed cenospheres. The increment in the coefficient of friction may be due to the increase in the cenosphere shells in the wear debris which act as obstacles. The worn surfaces of the foams were observed using the scanning electron microscope to determine the dominant wear mechanism.

The present paper deals with manufacturing and characterization of aluminium syntactic foam which was fabricated through stir casting technique. The developed foam was made of 2014 aluminium alloy and 25 vol% of cenosphere particles. The quasi-static compression behavior was studied experimentally at a strain rate of 0.001/s. The fabricated foam shows high strength and a plateau region under compression behavior. The young’s modulus, yield stress, plateau stress and densification strain were quantified and presented. The fabricated foam was also characterized in terms of microstructure to show the cenosphere distribution in aluminium matrix. The microstructure results confirms uniform distribution of cenosphere particle.

Products made of syntactic foams are replacing many conventional materials due to several desirable properties and commendable low density. To overcome its limitation in application it has been hybridized with CNT in this work to achieve multifunctional, mechanical and thermo-mechanical enhanced composites. It is found that when HGM is added to epoxy/CNT matrix compressive strength increases till 5% to a maximum value of 176MPa at 5wt% of HGM and beyond this volume of HGM strength decreases considerably. Storage modulus increases with increase in HGM volume due to increased restriction in polymer chain mobility of epoxy matrix. Microstructure images of fractured surface of the composites evident the good dispersion of HGM and CNT in the epoxy matrix. It can also be seen in SEM images that many microcracks generated are discontinuous and hence increasing the stability of composite in plastic region

Syntactic foams are light-weight novel composites that have gained importance over conventional materials due to their low weight and high specific mechanical properties. They are two- phase composites consisting of a matrix embedded with hollow glass microspheres. Various micromechanics-based approaches exist to evaluate the bulk properties of the composite. In this study, a generalized self- consistent three-phase method is used to determine the bulk properties of the composite (elastic, shear moduli and the Poisson’s ratio) analytically. Parametric studies are carried out to study the effect of microsphere wall thickness and radius on the bulk properties. The inaccurate assumption of a perfectly elastic matrix is replaced by introducing viscoelasticity in the matrix and evaluating the shear response of the resultant composite. The hysteresis response of the composite and the energy dissipation factor tan( δ ) are studied at different frequencies and volume fractions.

Syntactic foams with fly ash cenospheres or commercial microballoons as fillers have been widely used in various applications ranging from aerospace to marine fields and the automotive industry. However, these two extensively adopted fillers possess multiple shortcomings, such as variations in the composition, material degeneration and distinct structural heterogeneity, which will inevitably hamper accurate prediction of the structure-property relationship and the corresponding design of the syntactic foams, reducing material utilization. Here, we present a microfluidic-based approach integrated with a subsequent heat treatment process to engineer syntactic foam fillers with a predefined composition, specified dimensional scope and reduced structural heterogeneity. These fillers are fully guaranteed by the synergy of the flexible and controllable generation of droplet templates with hydrodynamic regulation and rational selection of the nanoparticle dynamic response with respect to the heating temperature. In addition, two distinct surface morphologies have been observed with a temperature demarcation point of 1473 K, further endowing the fillers with multiplicity and optionality, simultaneously laying the foundation to regulate the properties of the syntactic foams through the diversity of the filler selection. Then, we fabricated a syntactic foam specimen by mold casting, and the integrity of the fillers inside was verified using an elaborate buoyancy comparison experiment, exhibiting its potential value in lightweight related applications. As the fillers derived from our approach show significant advantages over conventional ones, they will provide considerable benefits for the regulation and improvement of syntactic foam fillers in many practical applications.

Sandwich composites and syntactic foams independently have been used in many engineering applications. However, there has been minimal effort towards taking advantage of the weight saving ability of syntactic foams in the cores of sandwich composites, especially with respect to the impact response of structures. To that end, the goal of this study is to investigate the mechanical response and damage mechanisms associated with syntactic foam core sandwich composites subjected to dynamic impact loading. In particular, this study investigates the influence of varying cenosphere volume fraction in syntactic foam core sandwich composites subjected to varying dynamic impact loading and further elucidates the extent and diversity of corresponding damage mechanisms. The syntactic foam cores are first fabricated using epoxy resin as the matrix and cenospheres as the reinforcement with four cenosphere volume fractions of 0% (pure epoxy), 20%, 40%, and 60%. The sandwich composite panels are then manufactured using the vacuum assisted resin transfer molding process with carbon fiber/vinyl ester facesheets. Dynamic impact tests are performed on the sandwich composite specimens at two energy levels of 80 J and 160 J, upon which the data are post-processed to gain a quantitative understanding of the impact response and damage mechanisms incurred by the specimens. A qualitative understanding is obtained through micro-computed tomography scanning of the impacted specimens. In addition, a finite element model is developed to investigate the causes for different damage mechanisms observed in specimens with different volume fractions.

Syntactic foams are widely used in damage tolerance and low-density applications. In present work compressive behavior of 3D printed three-phase syntactic foams under quasi-static strain rates (0.001, 0.01 and 0.1 s−1) are investigated. Extruded filaments of High density polyethylene (HDPE) with environmentally pollutant fly ash cenospheres (0, 20, 40 and 60 vol%) are used for 3D printing. Micrography reveal that syntactic foam filament and 3D printed samples are three phase systems comprising matrix, cenosphere and porosity. Matrix porosity of about 7% makes these foams lightweight and suitable for buoyant applications. The compressive properties are extracted from the stress-strain plots. It is observed that modulus and specific modulus increases with strain rate and cenosphere content. Specific compressive strength increases with strain rate and decrease with cenosphere content.

In the present study, the thermal behavior of pure Al6061 and fly ash reinforced Al6061 with varying particulate sizes were investigated for the first time. Metal matrix syntactic foams of ceramic fly ash (4 – 16 wt. %) embedded in aluminum (Al6061) matrix have been fabricated by using stir casting technique with different fly ash particulate sizes of size below 50µm and 53µm-100µm, respectively. The microstructural characteristics were studied by using scanning electron microscopy (SEM). It has been observed that there is a uniform distribution of lower particulate size fly ash as compared to higher particulate size in the aluminium matrix respectively. For all the cases, the accumulation of fly ash in Al6061 decreases the thermal conductivity and increases the specific heat carrying capacity of a hybrid composite. Increase in weight percentage of fly ash in Al6061 decreases thermal conductivity and thermal diffusivity respectively.

In this article, the co-continuous hollow glass microspheres (HGMs)/epoxy resin syntactic foams were prepared by filling the HGM preforms with epoxy resin. The effects of HGM wall thickness and solid loading on the density, open porosity, and morphology of HGM preform and HGM/epoxy resin syntactic foams were studied in detail. The results show that the HGM wall thickness had no effect on the open porosity of HGM preforms. However, the density of preforms increased with the true density of HGMs. The density of HGM/epoxy resin syntactic foams decreased with the contents of HGMs, and the measured density was essentially equal to the theoretical density. The compressive strength decreased with the contents of HGM, regardless of the HGMs wall thickness. However, the variation in compressive modulus was related to the HGMs’ wall thickness. As the HGM solid loading increased, the compressive modulus of composites, prepared using thin-walled HGMs, decreased, while the composites prepared using thick-walled HGMs showed the opposite trend. The co-continuous HGM/epoxy resin syntactic foams possessed low density (0.567–1.002 g/cm3), and excellent compressive strengths (30.32–131.60 MPa). These properties were due to the elimination of matrix pores, and are a type of promising material in deep sea exploration area.

Conventional mould casting technique for cell foam production demonstrated trapped air bubbles in the cell foam which are undesirable stress concentration points in the matrix. This study devise a modified process technique of fabricating syntactic foam by eliminating interstitial porosity. Varying epoxy hollow spheres and matrix stoichiometry ratio was involved in the fabrication of syntactic foam. Analysis through characterization of the mechanical properties, macrostructural morphology, cell size distribution, wall thickness and curing conditions was determined. The influence of fabrication techniques on performance of the syntactic foams was also evaluated. From the results the hollow sphere size distribution prevented the interstitial spaces, enhanced density and create foam compactness. The matrix stoichiometry ratios impacts the foam morphology, the optimal formulation was 1:1 because it enhances cross-linking within the matrix and produced a smooth, strong surface microsphere coating with structural rigidity. The relative density, compressive modulus and compressive strength of the syntactic foam are 377.11 kg/m3, 556.39 MPa, 10.40 MPa and 334.61 kg/m3, 37.05 MPa, 4.11 MPa, for vacuum assisted mould filling and conventional mould casting method, respectively. Crack propagation test showed failure started at the center and spread through the edges of the syntactic foam because of good interfacial bonding between sphere and matrixes. Keywords: Syntactic foam, Microsphere, Mechanical properties, Vacuum assisted mould technique

Steel matrix syntactic foam is a promising lightweight and energy absorbing material. In this study, an infiltration casting technology is developed to prepare syntactic foams, which brings in alum...

In this paper, novel syntactic foams (CPSFs) composed of the rigid polyurethane foam (PUR) and the fly ash cenospheres were prepared through the pressure infiltration approach. The aluminum honeycombs were proposed to improve the mechanical response of CPSFs by internally restricting the lateral deformation. The influences of the cenospheres size and the reinforcement on the dynamic behavior of the CPSFs were specified. The dynamic compression tests were carried out for the plain and Al honeycomb reinforced CPSFs with the strain rate ranging from 0.001/s to 2000/s. The results shown that the dynamic strength of the CPSFs containing large cenospheres was 35%–77% higher than that under quasi-static compression. But, the plateau stress was insensitive to the strain rate because the main failure mechanism of the CPSFs at different stages was dominated by different material compositions. The dynamic compressive strength and plateau stress of the plain CPSFs containing small cenospheres shown obvious strain rate sensitivity. The failure mechanism of the two plain CPSFs both transformed from shear failure into longitudinal cracking with the loading condition changed from the quasi-static to high-velocity impact. For the reinforced CPSFs, the dynamic mechanical properties including the strength and the plateau stress were sensitive to the strain rate. The failure mode of the reinforced CPSFs was dominated by the gradual compressive deformation. It was suggested that the Al honeycombs can not only enhance the mechanical properties, but also improve the dynamic response of the CPSFs. As a consequence, the dynamic energy absorption of the CPSFs varied in the range of 7–14 MJ/m3, and was about 23%–32% higher than that under static compression. This study expects to provide a potential approach in disposing the industrial waste and an innovation to adopt the internal constraints to improve mechanical behaviors of syntactic foams.

Deformation of metal foams under high rate of loading is a complex phenomenon due to the effects of various parameters involved therein. In the present investigation, cenosphere-filled aluminum alloy syntactic foam is studied under high rate of loading in comparison with their quasi-static behavior. The experiments, for high strain rates, are carried out using split Hopkinson pressure bar and full stress–strain curves of foam are developed under such rate of loadings. Foams with three different cenosphere sizes at three different high rates of loadings are investigated for their mechanical behavior. Compressive behavior and energy absorption capacity are reported considering the effect of high loading rates and cenosphere sizes. It is observed that, increase in loading rate results in higher strength of foams by an amount of 16-32%. Further, it is observed that energy absorption is improved with the increase in strain rates and cenosphere sizes and this improvement is observed in the range of 80-182%. Based on deformation modes and failure damage study, using SEM microstructure, deformation mechanism is observed to be almost independent of strain rate increment particularly at higher strain rates considered in this investigation. Moreover, there exists a limiting value of strain rate beyond which there is an insignificant increase in compressive strength of foams considered in the present investigation.

Glass microspheres are considered to be suitable fillers in syntactic foams due to their high specific strength, however, it's evidently easy to react with Al-Mg alloy or Mg alloy. A surface protection coating MgO were successfully prepared on microspheres using sol-gel method. The crystallization process and coating effect of MgO with different molar ratio acetic acid complexant in sol compositions were studied. The dried gels were dehydrated and polycondensed to form magnesium acetate and finally turned to be stable MgO after annealing at 400 °C. The MgO coatings, with small particles after annealing, successfully modified the surface of microspheres when the sol composition (molar) magnesium acetate hydrate/acetic acid/dry ethanol was 1:6:29. The MgO coating has inhibited the excessive interfacial reaction between the glass microspheres and the Al-Mg alloy matrix in syntactic foam which the reasons have been discussed.

The properties regarding thermal stability of syntactic foam such as coefficient thermal expansion, alpha (CTE) had been investigated in this paper. Determination of CTE in composite materials is commonly used for application in heat exchange transformation. The CTE comparison using kinetic theory model has not really been investigated yet for composite materials. The syntactic foam prepared by mixing fused borosilicate or glass microballoon with matrix resin were considered in this study. The different weight percentage of glass microballoon varied from 2wt.% to 10wt.% were analysed using thermo-mechanical analyser (TMA). The results showed that CTE value became up to 53-63% lower than the pure resin after mixing with different weight percentages (2wt.%-10wt.%) of glass microballoon. Experimental results showed that the CTE decreases when glass microballoons are added into the composites measured at different temperatures ranging from 30°C to 70°C. The CTE from the experimental results were also compared with Turner’s modification model for composites for its suitability for thermal expansion of syntactic foams. The results indicate that Turner’s modification model exhibits a close correlation with the reduction up to 80% of CTE based on experiment.

It is generally accepted that matrix pores in the hollow glass microspheres (HGMs)/resin-based composites prepared using stir-casting method (SCM) seriously affect its performance. In this paper, H...

This paper investigates the effect of temperature on the microstructure, failure mechanism and compressive mechanical properties of newly developed ZA27 syntactic foams. Two different types of filler particles are considered, i.e. expanded perlite (P) and expanded glass (G). Metallic syntactic foam (MSF) has been produced via a counter-gravity infiltration process of packed particle beds, followed by controlled thermal exposure. Quasi-static compressive tests were carried out on cylindrical samples at five different in-situ testing temperatures between 25 °C and 350 °C. At all considered temperatures, P-MSF exhibits superior mechanical properties compared to G-MSF. The mechanical properties of both foam types decrease significantly with increasing testing temperature. For comparison, solid ZA27 samples were compressed at the same testing temperatures. Due to microstructural changes, a significant strength degradation of solid ZA27 was observed starting at 100 °C. Comparison of results indicates that the temperature-dependent mechanical properties of P-MSF and G-MSF are strongly controlled by the matrix material. However, the addition of particles decreased the relative reduction of plateau stress and volumetric energy absorption of ZA27 MSF at elevated temperatures.

The influence of types and parameters of hollow microspheres in the composition of syntactic foams on their structure and coefficient of thermal conductivity has been studied. By using structural and thermal analysis it has been found that the volume concentration and the size of the ceramic and glass hollow microspheres have a strong impact on the density and thermophysical properties of the thin syntactic foams coatings. It has been shown that the best heat insulating properties belong to syntactic foam with composition of 60 vol. % ceramic microspheres with particle size of 1 - 40 μm (k = 0.029 W/m·K, R = 0.008 (m2·K)/W) and with composition of 80 vol. % glass hollow microspheres with particle size of 9 - 25 μm (k = 0.087 W/m·K, R = 0.008 (m2·K)/W). The results demonstrate that application of syntactic foams as thin insulating coatings is appropriate and they are an energy efficient material with number of benefits compare with the common thermal insulators.

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 nano...

Quasi static compression (10−1, 10−2 and 10−3 s−1 strain rates) and dynamic mechanical analysis (temperature sweep of 30–175 °C) of cenosphere/epoxy syntactic foams are investigated. Effect of cenosphere content (20, 40 and 60 vol %) and surface modification are presented. Quasi-static tests reveal lower modulus for neat epoxy samples as compared to all the syntactic foams. With increasing cenosphere content and strain rate, elastic modulus increases for all the tested conditions. Foams reinforced with surface modified cenosphere exhibit higher modulus as compared with untreated ones and neat epoxy. Energy absorption of samples increases with increasing cenosphere content and surface modification. Storage modulus of untreated and treated syntactic foams register higher values with increase in cenosphere content and are higher than the neat epoxy samples. Loss modulus of syntactic foams at room temperature are lower as compared with pure epoxy while damping of untreated and treated foams registered higher values as compared with neat resin. Scanning electron microscopy of the samples are performed for structure property correlations. Finally, property map for quasi-static compression is presented by comparing results of present work with the extracted values from literature.

This paper presents a systematic study on the interaction between particle strength, matrix ductility and the deformation mechanism in metallic syntactic foam (MSF). Packed beds of near-spherical NaCl particles (o2-2.8 mm) were infiltrated with liquid metal to produce the required samples. The brittle zinc alloy ZA-27 and ductile Aluminium (99% Al) were used as matrix materials. NaCl particles were retained inside half of these samples to replicate MSF containing high-strength particles. NaCl was leached from the remaining samples to investigate MSF with weak particles. This procedure ensured that all samples had a near-identical matrix geometry and microstructure, thus isolating the effect of particle strength. Quasi-static compression tests were conducted and significant changes in the deformation behaviour were observed. Samples containing NaCl particles deformed in a shear-band dominated mode, whilst the ductile foam underwent layer-by-layer deformation.

Friction and wear behaviour of magnesium/glass microballoon (GMB) foams synthesized by Disintegrated Melt Deposition (DMD) were investigated under dry sliding conditions. The coefficient of friction (μ) decreases with increasing GMB content. Mg-25wt.% GMB exhibits ∼13% lower μ pure compared to magnesium. Wear resistance of magnesium showed a significant enhancement (∼2.5 times) post GMB addition. Abrasion and oxidation were identified as dominant wear mechanisms post worn-surface analysis. Delamination wear, which has traditionally limited the advantages of composites with discontinuous reinforcements in sliding wear conditions for structural and biomedical applications can be effectively addressed by the development of these proposed syntactic foams.

Carbon nanofibers (CNFs) and halloysite nanotubes (HNTs) were incorporated in syntactic foams containing a 90% by volume homogeneous mixture of (20/80 wt%) glass/thermoplastic microballoons to enhance the mechanical and impact response properties. Tensile, compressive, and impact tests were employed to comparatively characterize the effect of nano-additive reinforcement on mechanical response properties. Compressive strength and modulus enhancements as large as 39% and 18%, respectively, were achieved with a 0.125 wt% addition of CNF and increases of 61% and 7%, respectively, were achieved with a 0.125 wt% addition of HNT. Tensile strength and modulus enhancements as large as 107% and 68%, respectively, were achieved with a 0.125 wt% addition of CNF and increases of 104% and 70%, respectively, were achieved with a 0.125 wt% addition of HNT. Impact analysis data were used to show that measured peak force increased and build-up time to peak force decreased with increasing CNF or HNT weight percentage due to stiffening of the matrix. The smallest increase observed in peak force was 20% for a 0.125 wt% addition of CNF and 17% for a 0.125 wt% addition of HNT.

Epoxy syntactic foams with different compositions of cenosphere were fabricated and characterised. The effect of loading cenosphere in epoxy syntactic foams was analysed. Good thermal stability of cenosphere–epoxy syntactic foams was established from the thermogravimetric analysis results. The completion of cure reaction at ambient temperature conditions was ascertained from differential scanning calorimetry results. Dynamic mechanical analysis revealed 114°C as the glass transition temperatures ( Tg) for neat epoxy sample, which increased to 132°C with 50% loading of cenosphere. Cenosphere-filled epoxy syntactic foams had low density and low water absorption values when compared to the neat epoxy sample. Homogeneous distribution of the cenosphere particles was confirmed using scanning electron microscopy. The compression studies confirmed brittle failure of the syntactic foams. This was also supported by the scanning electron microscopic images. The incorporation of hollow cenosphere particles led to a decrease in the flexural strength. Syntactic foams with 30% loading of cenosphere exhibited best specific modulus and specific strength. The specific strength increased by 24% for T30 sample and specific modulus increased by 36% for T30 samples when compared to the neat epoxy sample. As the need for strong but lightweight thermally stable products is continually increasing, there is a great possibility for the utilisation of these cenosphere–epoxy syntactic foams as lightweight core for sandwich composites.

In this paper, cenosphere particles embedded in AA2014 aluminium matrix are used to fabricate syntactic foam by stir casting method. The particle size is about 100 µm and foam density is about 1990...

Effect of glass microballoon (GMB) wall thickness and cutting parameters (cutting speed, feed and drill diameter) on thrust force (Ft), surface roughness (Ra), specific cutting coefficient (Kf), cylindricity (CYL ), circularity error ( Ce-Exit) and damage factor (Fd-Exit ) in drilling of GMB/epoxy syntactic foam is presented. CNC vertical machining centre is utilised for conducting experiments based on full factorial design . Significant process parameters are identified through response surface methodology . Wall thickness significantly affects the Ce-Exit and CYL of the drilled hole. Increasing wall thickness significantly reduces the Ra (30%), CYL (41%) and Ce-Exit (56%) due to the increased thermal stability of syntactic foams. This observation is very crucial for the syntactic foams used in structural applications pertaining to structural stability. Drill diameter is observed to be significant for Ft, Ra, CYL and Fd-Exit; while Kf is governed by feed. Furthermore, grey relation analysis (GRA) is used to identify the specific combination of process parameters to obtain good quality drilled hole. Combination of higher particle wall thickness and feed, lower cutting speed and drill diameter produces a sound hole quality as observed from GRA. Hole quality is highly influenced by drill diameter followed by cutting speed and GMB wall thickness. The present study offers guidelines for the industries (structural applications) to produce quality holes in GMB reinforced epoxy matrix .

In this study, the flame-resistant epoxy syntactic foams with different content of hollow glass microspheres (HGM) were prepared by using aluminum diisobutylphosphinate (AlPBu) as flame retardant. Scanning electron microscope (SEM) images of the fracture surface of different epoxy syntactic foams showed that AlPBu particles had good compatibility with the matrix due to their similar polarities. And the limiting oxygen index, UL 94 test, cone calorimeter and thermogravimetric analysis were applied to characterize the flame retardation and thermal behavior of the foams. These results showed that AlPBu improved the flame retardancy of epoxy syntactic foams, while the HGM exhibit a complicated influence on the flame retardation, although both AlPBu and HGM enhanced the char residues and decreased the peak heat release rate as well as the total heat release of composites. In addition, swollen char layers mainly containing the HGM were generated after adding AlPBu. Then, the char residues of the foams after combustion were observed by SEM, and the results were applied to reveal the different fire behaviors of foams with different densities.

High-density polyethylene and its syntactic foams reinforced with 20 vol.% and 40 vol.% glass microballoons were 3D printed using the fused filament fabrication method and studied for their compressive response. The three-phase microstructure of syntactic foams fabricated in this work also contained about 10 vol.% matrix porosity for obtaining light weight for buoyancy applications. Filaments for 3D printing were developed using a single screw filament extruder and printed on a commercial 3D printer using settings optimized in this work. Three-dimensional printed blanks were machined to obtain specimens that were tested at 10−4 s−1, 10−3 s−1, 10−2 s−1 and 1 s−1 strain rates. The compression results were compared with those of compression-molded (CM) specimens of the same materials. It was observed that the syntactic foam had a three-phase microstructure: matrix, microballoons and air voids. The air voids made the resulting foam lighter than the CM specimen. The moduli of the 3D-printed specimen were higher than those of the CM specimens at all strain rates. Yield strength was observed to be higher for CM samples than 3D-printed ones.