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Magnesium-lithium (Mg-Li) alloys are emerging lightweight materials that can find applications in the aerospace, automobile, and electronics fields. However, the widespread use of Mg-Li alloys is limited due to their low strength and processing challenges via metal casting due to the unpredictable loss of lithium by oxidation. That said, adding micron-sized high-strength hollow spherical glass microballoon (GMB) particles can improve the mechanical and damping response of Mg and its alloys, besides reducing the density. In the current study, Mg-Li syntactic foams reinforced with GMB particles were reported for the first time. Magnesium-lithium-based syntactic foams of density 1.45-1.66 g/cc were synthesized via disintegrated melt deposition technique and hot extrusion. The effect of GMB reinforcement on the mechanical, thermal, and damping characteristics of lightweight Mg-Li alloy is studied. The developed Mg-Li/GMB composites displayed improvements in the mechanical and damping response while maintaining an ~ 12% lower density than Mg. Microstructure-property correlations of the developed composites are discussed. [ABSTRACT FROM AUTHOR]

Polymer matrix syntactic foams have received a lot of interest in recent years because of their advantages, such as their lightweight and energy-absorbing properties. The purpose of this study is to produce representative elementary volume of syntactic foams with a random filling of hollow glass microspheres from neat epoxy to 0.4 v/v filling in epoxy resin. Different forms of syntactic foams have been evaluated. ANSYS software (2020, R1) was used to investigate the numerical results. The impact of the microsphere volume fraction on the elastic mechanical properties of syntactic foams was investigated. The relative modulus, density and Poisson's ratio values of foams fall as the microsphere volume fraction rises but specific modulus values of composites increases, confirming that the weight of a component can be reduced. The relative modulus, Poisson's ratio and density of syntactic foam decreases with increasing volume fraction of microballoon, dropping to approximately 4.5, 8.9 and 30 percent respectively at 0.4 volume fraction compared to neat epoxy. Parameters for beam and plate design ensuring that with same bending and axial stiffness for beam and bending stiffness for plate, weight of a component could well be reduced. As a consequence, utilizing syntactic foams in structural applications may result in significant weight reductions. [ABSTRACT FROM AUTHOR]

Bimodal composite metal foams made from Al99.5 aluminium and quasi-eutectic Sr-modified AlSi12 matrix were investigated, where the bimodality was introduced by two alumina hollow sphere sets with nominal diameters of Ø7.0 and Ø2.4 mm in various volume concentrations. The research aimed to determine the effect of the cell structure (especially the unintended porosity) on the first compressive stress maximum, depending on the applied matrix material. The various features of the cell structure were quantitatively measured in composite metal foams by filtering raw computed tomography analysis data. The casting porosity, the wetting porosity, the porosity from the various-sized hollow spheres, and the partially matrix-filled hollow spheres have been successfully distinguished based on volumetric and geometrical conditions, such as sphericity and compactness. The unintended porosity (as the sum of the casting porosity and the wetting porosity) influences the first compressive stress maximum of the Al99.5 matrix composite metal foams. In contrast, for the AlSi12Sr bimodal foams, the results correlated with the total porosity values. The first compressive stress maximum could be estimated well from the compressive yield strength of the matrix and the total porosity of the foam.

Polymer matrix syntactic foams made of hollow glass microspheres (HGMs) and epoxy resin are widely used in underwater vehicles providing buoyancy due to their advantages of low density and high mechanical strength. When HGMs is strong enough, surface modification of HGMs is an effective method to improve the compressive strength of syntactic foams. However, the effects of different surface modifiers on the physical properties of HGMs and the compressive strength of syntactic foams have rarely been systematically studied. It is of great significance to enhance the interfacial strength of syntactic foams. Herein, the T40 HGMs was treated by different modifiers (HCl, NaOH, Li2SiO3, Me3SiONa, KH550, KH560, KH570, and Volan), physical properties of modified HGMs were studied, including true density, angle of repose, isostatic compressive strength, specific surface area, and contact angle. Furthermore, the effects of modified HGMs on the compressive strength of syntactic foams were also studied. The results showed that the compressive strength depend largely on the interface bonding between HGMs and resin matrix. This work can provide some references for mechanical property reinforcement of syntactic foams with high filling ratio and plenty of interface zones.

In this study, the free vibration analysis of syntactic foam curved beams with two simply supported ends was numerically investigated by the Differential Quadrature Method (DQM). The displacement field was expressed according to classical beam theory, and motion equations were derived by Newton's laws of motion. For this purpose, a DQM code was developed using MATLAB, and the natural frequencies of structures were obtained to examine the effects of material parameters such as microballoon (MB) volume fractions, MB densities, as well as geometric parameters like curvature angle and curvature radius on the free vibration analysis of epoxy/glass microballoon-based syntactic foam curved beams. The results obtained from the research indicate that MB volume fractions, densities, and the geometric characteristics of curved beams significantly influence the vibration behavior of syntactic foam curved beams. [ABSTRACT FROM AUTHOR]

Aluminium smelter waste (ASW) is a big contributor to landfills, and its recycling has been of great interest. This study investigates the tribological properties of aluminium matrix syntactic foams manufactured using an Al 6082 alloy and ASW. Ball-on-disc tests were conducted under both dry and lubricated conditions. Under dry sliding conditions, the coefficient of friction (COF) had an initial sharp increase, followed by a gradual decrease and finally a steady state as the sliding distance increased. The wear surfaces showed the presence of adhesive, abrasive and oxidative wear, with some presence of delamination. Syntactic foams containing small ASW particles led to a decrease in surface roughness, decrease in the average COF and decrease in specific wear. Heating large ASW particles before manufacturing the syntactic foams enhanced overall wear properties because the particles are hardened due to a compositional change. The T6 treatment of the syntactic foams enhanced the wear properties due to the hardening of the Al matrix. The average COF of the ASW syntactic foams was higher than that of the E-sphere syntactic foam, which was predominantly abrasive wear. The specific wear of the ASW syntactic foams can be higher or lower than the E-sphere syntactic foam, depending on the ASW particle size. Under lubricated sliding test conditions, the wear was reduced significantly, and the type changed from predominantly adhesive to predominantly abrasive. The porous ASW particles acted as lubricant reservoirs and provided a constant supply of lubricant, further improving the lubrication effect. [ABSTRACT FROM AUTHOR]

Syntactic foams consisting of vinyl ester resin hollow glass microspheres (microballoons) are used in many marine or air structures due to their low density, high stiffness/density ratio, and ability to absorb energy. In this research, pure vinyl ester resin with three different volume percentages of 20%, 40%, and 60% of glass microballoons was produced by molding and casting method. The effect of the percentage of microballoons on the density, compressive, and tensile properties of foams was evaluated. The density measurement of the foams showed that with the increase in the percentage of glass microballoons, from 0% to 60% by volume, the density decreased from 1.04 g/cm3 to 0.563 g/cm3. The results showed that with the increase in the percentage of microballoons, the compressive and tensile strength of the foams decrease, but the specific compressive strength (compressive strength to density ratio) of foams increases. The highest tensile strength is related to the vinyl ester resin without microballoons with a tensile strength of 40 MPa. The specific compressive strength of foams increased from 76 MPa.cm3/g to 98 MPa.cm3/g by increasing the percentage of glass microballoon from 0% to 60%. [ABSTRACT FROM AUTHOR]

One of the biggest problems the maritime industries confront is the erosion of composite structures brought on by interaction with sludge and sea debris. In the current study, erosion behaviour of cenosphere reinforced syntactic foam is examined in relation to specimen rotation angle, slurry concentration, and rotational speed. Moulding method was used to create the specimens. Sea sand particles were used as the erodent material in experiments carried out in a slurry pot erosion test rig. The impact of particular parameters on erosion is examined using the Taguchi L9 array. A scanning electron microscope was used to analyse the morphology of the eroded surfaces in order to analyse the erosion behaviour. Maximum erosion was seen when the Cenosphere content is least in the syntactic foams and the erosion process may be classified as semi-ductile in nature. It has been found that Cenosphere content have a major impact on erosion behaviour. The erosion damage in the matrix diminishes as the volume % of cenospheres rises, indicating that cenospheres boost the material's resistance against erosion.

Abstract In this study, a sandwich composite structure was fabricated with core syntactic foam composites (SFC) reinforced with face-sheets of combined kenaf and glass fibers. The core SFC was made from hollow glass microsphere (HGM)/epoxy resin mixture. The focus of this study is to design a suitable material for marine, automobile, and structural applications by investigating the water absorption capacity, hardness, buoyancy, and acoustic resistance level of the sandwich composite. Four sequences of face-sheet reinforcement [e.g. kenaf-kenaf(KK); glass-glass(GG); glass-kenaf(GK), and kenaf-glass(KG)] were adopted. The results show that the KK sandwich absorbed more water, with buoyancy level of 0.329% than other sequence. GK shows the highest sandwich in hardness strength with 58Hv, and 35% more than the KK, while there was no significant difference in the sound pressure level of the sandwich composite.

Lost circulation is a critical issue in drilling. This work reports the investigations on shape memory epoxy polymers (SMEPs) with high switching temperatures, and good thermal, mechanical and shape memory properties for plugging in deep drilling. The SMEPs are produced by curing diglycidyl ether of bisphenol A (DGEBA) and castor oil glycidyl ether (COGE) with a flexible poly(oxypropylene) diamine (D230) and a rigid 4, 4′ ‐diaminodiphenyl methane (DDM). Increasing D230 contents or using COGE, glass transition temperature (Tg, 93–163 °C, DMA analyses) and crosslink density of SMEPs are reduced, shape recovery rate is increased, while tensile strength, elongation at break and impact strength are improved. Further, shape memory epoxy syntactic foams (SMEFs) with high‐temperature adhesion are developed by introducing polymer microspheres into SMEPs. The SMEFs have compression ratio of 52–58%, and expansion ratio of 100% by adding 2% wt. microspheres. They retain the Tg and thermal stability of SMEPs, and show a delayed, slower recovery than SMEPs. Long fracture plugging experiments exhibit that a pressure‐bearing capability of 8.99–9.81 MPa is achieved with SMEFs in sealing fracture slots at a high temperature. [ABSTRACT FROM AUTHOR]

One of the biggest problems the maritime industries confront is the erosion of composite structures brought on by interaction with sludge and sea debris. In the current study, erosion behaviour of cenosphere reinforced syntactic foam is examined in relation to specimen rotation angle, slurry concentration, and rotational speed. Moulding method was used to create the specimens. Sea sand particles were used as the erodent material in experiments carried out in a slurry pot erosion test rig. The impact of particular parameters on erosion is examined using the Taguchi L9 array. A scanning electron microscope was used to analyse the morphology of the eroded surfaces in order to analyse the erosion behaviour. Maximum erosion was seen when the Cenosphere content is least in the syntactic foams and the erosion process may be classified as semi-ductile in nature. It has been found that Cenosphere content have a major impact on erosion behaviour. The erosion damage in the matrix diminishes as the volume % of cenospheres rises, indicating that cenospheres boost the material's resistance against erosion. [ABSTRACT FROM AUTHOR]

To investigate the compression properties and failure mechanisms of syntactic foams, two different resin matrices and three types of hollow glass microspheres (HGMs) were used to prepare syntactic foams with different properties. Results showed that the compression strength of syntactic foams increases with the increase in the compression strength of HGMs or resin matrix. Low-strength HGMs will be the first to break down as a source of cracks under stress. When the stress value exceeds the compression strength of the matrix, the matrix will deform and break, leading to the overall destruction of syntactic foam. High-strength HGMs not only act as a lightweight filler but also act as a reinforcing phase to improve the strength of the syntactic foam. The matrix located at the interface will experience initial cracking when subjected to stress, becoming the origin of subsequent cracks. Consequently, the weaker part between the HGMs and the matrix will be the initial point of failure, resulting in the formation of a crack source. The failure of the matrix will ultimately result in the complete destruction of the syntactic foam.

Protecting occupants or payloads in crashes and blasts is of utmost importance in both moving and immobile structures. One way of achieving this is by using a sacrificial energy absorber. Composite tubes have been studied as potential energy absorbers due to their ability to fail progressively under axial compression. In this study, the energy absorption capability of these tubes is enhanced by adding hollow glass particles to the matrix. Drop-weight tests are performed on composite tubes, and a digital image correlation (DIC)-based technique is used to capture their load-displacement behaviour. This eliminates the use of electronic data acquisition systems, load cells, and accelerometers. The load-displacement curves of the tubes are obtained from the DIC-based technique and examined to understand their crushing behaviour. Although the mean crush load shows a drop, an increase in crush length is noticed. The specific energy absorbed by the tubes improves with an increase in GMB volume fraction. The addition of 0.1, 0.2, 0.3 and 0.4 vol fractions of GMB results in the specific energy absorption increasing by 6.6%, 14.7%, 24% and 36.6%, respectively, compared to neat glass fibre-epoxy tubes. Visual examination of the tubes and comparison with tubes subject to quasi-static compression is also performed.

Syntactic foams, containing up to 23 wt% of untreated waste tyre-derived rubber particles mixed with hollow glass microspheres (HGMs) and epoxy, were produced via compressed air-assisted injection moulding. This study investigated the effect of rubber particles on the through-the-thickness compression properties and the vibration-damping response of the ternary syntactic foams. The interaction between foam constituents was investigated using SEM, with the objective to establish a microstructure-compression property relationship for rubber-incorporated foam. The effect of rubber particles on the energy absorbed by the foam under compression deformation was also studied. The substitution of epoxy for rubber particles did not alter the compression properties (crush behaviour) of the foam. A laser Doppler vibrometer was used to measure the vibration-damping response and natural resonance frequency. The vibration-damping ratio of the rubber-incorporated foam increased by up to 75% upon the addition of 23 vol % rubber particles.

Abstract Lost circulation is a critical issue in drilling. This work reports the investigations on shape memory epoxy polymers (SMEPs) with high switching temperatures, and good thermal, mechanical and shape memory properties for plugging in deep drilling. The SMEPs are produced by curing diglycidyl ether of bisphenol A (DGEBA) and castor oil glycidyl ether (COGE) with a flexible poly(oxypropylene) diamine (D230) and a rigid 4, 4′ ‐diaminodiphenyl methane (DDM). Increasing D230 contents or using COGE, glass transition temperature (Tg, 93–163 °C, DMA analyses) and crosslink density of SMEPs are reduced, shape recovery rate is increased, while tensile strength, elongation at break and impact strength are improved. Further, shape memory epoxy syntactic foams (SMEFs) with high‐temperature adhesion are developed by introducing polymer microspheres into SMEPs. The SMEFs have compression ratio of 52–58%, and expansion ratio of 100% by adding 2% wt. microspheres. They retain the Tg and thermal stability of SMEPs, and show a delayed, slower recovery than SMEPs. Long fracture plugging experiments exhibit that a pressure‐bearing capability of 8.99–9.81 MPa is achieved with SMEFs in sealing fracture slots at a high temperature.

This paper presents research findings on the influence of tyre-derived rubber particle size on the mechanical properties of rubberised syntactic foam manufactured through stir casting. The study examined how the rubber particle size affected the shear, in-plane compression, and through-thickness compression properties, as well as the flexural properties of sandwich composites with rubberised syntactic foam core. Rubber particles of various sizes (425 µm) were integrated into the syntactic foam at both low (9 wt%) and high (23 wt%) concentrations. Rubber particles measuring less than 150 µm, promoted agglomeration and increased void volume due to elevated viscosity, leading to a reduction in the mechanical properties of the rubberised foam. Conversely, larger rubber particles exceeding 425 µm reduced the mechanical properties of the syntactic foam due to debonding at the matrix/rubber interface. This study identified the optimal rubber particle size for achieving the highest mechanical properties in rubberised foam, which falls within the range of 150–425 µm. This research demonstrates the sustainable development of multifunctional composites from recovered waste tyres.

The present study focuses on the synthesis and characterisation of a lightweight ceramic material with electromagnetic interference (EMI) shielding properties, achieved using mullite containing micrometre-sized hollow spheres (cenospheres) and CoFe2O4 nanoparticles. This research explores compositions with varying CoFe2O4 contents ranging from 0 up to 20 wt.%. Conventional sintering in an air atmosphere is carried out at a temperature between 1100 and 1300 °C. The addition of ferrite nanoparticles was found to enhance the process of sintering cenospheres, resulting in improved material density and mechanical properties. Furthermore, this study reveals a direct correlation between the concentration of ferrite nanoparticles and the electromagnetic properties of the material. By increasing the concentration of ferrite nanoparticles, the electromagnetic shielding effect of the material (saturation magnetisation (Ms) and remanent magnetisation (Mr)) was observed to strengthen. These findings provide valuable insights into designing and developing lightweight ceramic materials with enhanced electromagnetic shielding capabilities. The synthesized ceramic material holds promise for various applications that require effective electromagnetic shielding, such as in the electronics, telecommunications, and aerospace industries. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

By applying the physical vapour deposition method, hollow ceramic microspheres were coated with titanium, and subsequently, they were sintered using the spark plasma sintering technique to create a porous ceramic material that is lightweight and devoid of a matrix. The sintering process was carried out at temperatures ranging from 1050 to 1200 °C, with a holding time of 2 min. The samples were subjected to conventional thermal analyses (differential scanning calorimetry, thermogravimetry, dilatometry), oxidation resistance tests, and thermal diffusivity measurements. Phase analysis of the samples was performed using the XRD and the microstructure of the prepared specimens was examined using electron microscopy. The titanium coating on the microspheres increased the compressive strength and density of the resulting ceramic material as the sintering temperature increased. The morphology of the samples was carefully examined, and phase transitions were also identified during the analysis of the samples. [ABSTRACT FROM AUTHOR]

Protecting occupants or payloads in crashes and blasts is of utmost importance in both moving and immobile structures. One way of achieving this is by using a sacrificial energy absorber. Composite tubes have been studied as potential energy absorbers due to their ability to fail progressively under axial compression. In this study, the energy absorption capability of these tubes is enhanced by adding hollow glass particles to the matrix. Drop-weight tests are performed on composite tubes, and a digital image correlation (DIC)-based technique is used to capture their load-displacement behaviour. This eliminates the use of electronic data acquisition systems, load cells, and accelerometers. The load-displacement curves of the tubes are obtained from the DIC-based technique and examined to understand their crushing behaviour. Although the mean crush load shows a drop, an increase in crush length is noticed. The specific energy absorbed by the tubes improves with an increase in GMB volume fraction. The addition of 0.1, 0.2, 0.3 and 0.4 vol fractions of GMB results in the specific energy absorption increasing by 6.6%, 14.7%, 24% and 36.6%, respectively, compared to neat glass fibre-epoxy tubes. Visual examination of the tubes and comparison with tubes subject to quasi-static compression is also performed. [ABSTRACT FROM AUTHOR]

Protecting occupants or payloads in crashes and blasts is of utmost importance in both moving and immobile structures. One way of achieving this is by using a sacrificial energy absorber. Composite tubes have been studied as potential energy absorbers due to their ability to fail progressively under axial compression. In this study, the energy absorption capability of these tubes is enhanced by adding hollow glass particles to the matrix. Drop-weight tests are performed on composite tubes, and a digital image correlation (DIC)-based technique is used to capture their load-displacement behaviour. This eliminates the use of electronic data acquisition systems, load cells, and accelerometers. The load-displacement curves of the tubes are obtained from the DIC-based technique and examined to understand their crushing behaviour. Although the mean crush load shows a drop, an increase in crush length is noticed. The specific energy absorbed by the tubes improves with an increase in GMB volume fraction. The addition of 0.1, 0.2, 0.3 and 0.4 vol fractions of GMB results in the specific energy absorption increasing by 6.6%, 14.7%, 24% and 36.6%, respectively, compared to neat glass fibre-epoxy tubes. Visual examination of the tubes and comparison with tubes subject to quasi-static compression is also performed. [ABSTRACT FROM AUTHOR]

Hollow glass microsphere composites, or syntactic foams, are widely used in marine environments due to their buoyancy. However, in service, loss of buoyancy occurs along time. This behavior is attributed to water absorption that leads to a mass increase of the material. This study investigates the water absorption of three types of syntactic foams under hydrostatic pressure with density ranging from 0.59 to 0.74 g. cm−3. Water absorption measurements are performed for a maximal duration of 3 years and under hydrostatic pressure up to 700 bars. Results show that water absorption is much higher than expected and attributed to the filling of microspheres that collapse over time. Based on results presented here, prediction of water absorption syntactic foams is proposed including the impact of applied hydrostatic pressure. This prediction allows estimating the buoyancy loss in hollow glass microsphere composite as a function of time, depth and sample size. Highlights: Water absorption in syntactic foams is not only due to water diffusion in polymers.Glass bubbles collapse due to immersion in syntactic foam.Long‐term water absorption can be predicted with data obtained from small samples. [ABSTRACT FROM AUTHOR]

Fiber accumulation due to printing ink inconsistency makes additive manufacturing (AM) of reinforced thermoset syntactic foam composites difficult. This study predicts and analyzes the mechanical properties of AM-made carbon fiber-reinforced syntactic thermoset composites to overcome experimental limitations. Thus, an adaptive neuro-fuzzy inference system (ANFIS)-based model creates an accurate mechanical behavior prediction under a variety of conditions without experimental inquiry. Compression and flexure tests assessed the ANFIS model’s validation. The model’s predictions were very close to reality, validating the approach taken to improve the technical assessment of the created composites, which are perfect for weight reduction, mechanical improvement, and product complexity.

Aluminium smelter waste (ASW) is a big contributor to landfills, and its recycling has been of great interest. This study investigates the tribological properties of aluminium matrix syntactic foams manufactured using an Al 6082 alloy and ASW. Ball-on-disc tests were conducted under both dry and lubricated conditions. Under dry sliding conditions, the coefficient of friction (COF) had an initial sharp increase, followed by a gradual decrease and finally a steady state as the sliding distance increased. The wear surfaces showed the presence of adhesive, abrasive and oxidative wear, with some presence of delamination. Syntactic foams containing small ASW particles led to a decrease in surface roughness, decrease in the average COF and decrease in specific wear. Heating large ASW particles before manufacturing the syntactic foams enhanced overall wear properties because the particles are hardened due to a compositional change. The T6 treatment of the syntactic foams enhanced the wear properties due to the hardening of the Al matrix. The average COF of the ASW syntactic foams was higher than that of the E-sphere syntactic foam, which was predominantly abrasive wear. The specific wear of the ASW syntactic foams can be higher or lower than the E-sphere syntactic foam, depending on the ASW particle size. Under lubricated sliding test conditions, the wear was reduced significantly, and the type changed from predominantly adhesive to predominantly abrasive. The porous ASW particles acted as lubricant reservoirs and provided a constant supply of lubricant, further improving the lubrication effect.

The application of polyvinyl alcohol (PVA) films is still restricted due to their poor hydrophilicity, mechanical strength, and thermal stability. To improve these properties, a teardrop-shaped hollow polystyrene (HPS) nanoparticle was prepared in this work and further applied as the reinforcing nanomaterial. Herein, a teardrop-shaped silica nanoparticle was first used as the hard template; polystyrene was in situ-synthesized on the surface of teardrop-shaped silica, followed by the etching of the silica template. By tuning the synthesis parameters (e.g., styrene concentration and etching time), the optimal structure of HPS was obtained; subsequently, HPS was mixed with PVA by a solution-mixing method, which in turn produced the PVA-based synthetic foam film. Using optimal synthesis parameters of HPS and its loadings, the obtained syntactic foam films exhibited high transparency, excellent mechanical strength and toughness, and good thermal stability. Compared to pure PVA film, the tensile strength and toughness of PVA-based syntactic foam films containing 1.0 wt % HPS were improved by about 55 and 77%, respectively; the glass transition temperature and onset decomposition temperature of PVA-based syntactic foam films increased to about 32 and 66 °C, respectively. The research exemplifies the great promise of HPS toward practical application in polymer syntactic foams. [ABSTRACT FROM AUTHOR]

This paper introduces the conceptual design of a submersible seaplane that merges the maturity of the wing-in-ground (WIG or ekranoplan) crafts and seaplanes with covert hybrid underwater insertion, travel, and recovery. WIG crafts have a higher lift-to-drag ratio and thus improved endurance, while hybrid crafts have recently become feasible due to advances in materials, electric propulsion, and multi-medium computational fluid dynamics. The reconnaissance design can insert, loiter, and extract from underwater, surfaces if necessary; it can fly in or out of ground effect, keep watch on the sea surface while recharging, and travel underwater. This design minimizes Doppler and infrared signatures to evade the surface wave, backscatter radar systems, and cube satellite arrays typical in contested maritime areas. Five critical enabling technologies are overviewed, showing how they enable a conceptual design. This project was conducted in collaboration with two industrial partners, namely Ron Allum and Thales Australia. The conceptual design has been socialised and confirmed at technical conferences from each core discipline and partly confirmed by a recent Chinese design and testing of a similar hybrid uncrewed aerial vehicle (HUAV). Recommendations are made for improving the conceptual design before proof-of-concept prototype testing. Given the seminal nature of HUAV design and research and some of the unique innovations proposed, the lessons learned from this iteration will likely be significant to other designers and researchers. [ABSTRACT FROM AUTHOR]

The effect of CO2 laser drilling on glass microballoon/epoxy syntactic foams are investigated in this study to optimize machining parameters to achieve a clean hole for various industrial applications. The epoxy matrix is reinforced with glass microballoons in concentrations of 0, 20 and 40 vol%. Cutting speed, laser power and additive percentage are input parameters for optimization. Kerf taper angle, surface roughness and ovality percentage are used as output responses to evaluate hole quality. For the optimization study, hybrid multi-criteria decision-making methods such as grey relational analysis and multi-objective optimization with ratio assessment methods are used, with equal weightage given to each output response. According to the study, low power and high speed produce better machining results such as a smaller kerf taper angle, lower surface roughness and a lower ovality percentage. Furthermore, a higher additive percentage is not appropriate for laser in epoxy/glass microballoon composite because it burns the area near the laser and increases surface roughness.

The paper presents the application of the casting method for the production of porous composites, called syntactic foams, of the casting alloy - solid particles type. This method was used to produce composites based on Al alloys reinforced with particles of clinoptilolite, a natural mineral from the zeolite group. Before the casting process, tests were carried out on the morphology, physicochemical properties and chemical composition of the zeolite, which was obtained from a rock called zeolite tuff, mined in a quarry in Kucin, (VSK PRO-ZEO s.r.o., Slovakia). Observations of the microstructure of the produced composites were also carried out using a scanning electron microscope. Diffractometric tests of zeolite rock as delivered for research and of the produced samples reinforced with zeolite particles were also carried out. Initial studies of the density and porosity of the produced composites were performed. The usefulness of the presented method of composite production was assessed on the basis of the conducted structural tests, with particular emphasis on the particle distribution in the alloy matrix.

In this paper, the effect of temperature on the creep-recovery behavior of a polypropylene matrix syntactic foam material under low stresses is analyzed. Previous dynamic mechanical analyses have shown that the mechanical response of the composite material is strongly time dependent with the polymeric nature of its matrix despite a high volume fraction of hollow glass microspheres. The permanent deformations are more pronounced at the higher temperatures. With the low level of the applied stress, the results lead to the assumption that the microcracks can be generated in the matrix of the composite material. Under the effect of temperature gradients in the offshore environment, the response of the material could evolve from a linear viscoelastic behavior to a behavior of which one part could be associated with the viscoelasticity of the matrix and a second with its viscoplasticity. We propose to use hooke, spectral triangular, and Zapas-Crissmann models to predict the overall creep response of a polypropylene matrix syntactic foam at the different temperatures. The results showed that the creep deformation at the higher temperatures conforms well to the global model including a power law that takes into account the permanent deformations of the polypropylene matrix composite of syntactic foam material type. [ABSTRACT FROM AUTHOR]

The research on lightweight materials for advanced engineering applications attracted the development of metal matrix syntactic foams. The automobile sector has started using Al-based alloys in structural components such as crash-box, underride-guard, fenders, dampers, A, B, and C Pillars. The present study explores the energy absorption behavior; microstructural characterization such as SEM, EDS, and XRD analysis of hollow glass microsphere (HGM) filled aluminum matrix syntactic foam. A380 aluminum alloy reinforced with different volume fractions 10%, 20%, 30%, and 35% of hollow glass microspheres were used in the fabrication of syntactic foam using the stir casting technique. The quasi-static compression test conducted, evaluated the plateau strength, which improved from 284.14 to 341.69 MPa, and energy absorption capacity was observed in the range 139.25–187.92 MJ/m3. The plateau strength and energy absorption capacity were improved by 16.82% and 25.89% for the 35 vol.% HGM sample as compared with 10 vol.% HGM filled aluminum matrix syntactic foam. The addition of a calcium thickening agent in the casting process improved the bonding between aluminum and HGM particle and also the homogeneous distribution of HGM. The XRD analysis revealed the chemical reaction that occurred between aluminum and SiO2 that produced the AlSiO2 and Al2SiO5 interfacial compounds. This reaction tends to collapse the HGM cell wall and fills it with matrix material. [ABSTRACT FROM AUTHOR]

This study aims to develop a novel technique in manufacturing nanocomposite bimodal foams containing expandable polymeric microballoons. Low density polyethylene syntactic foams were prepared via injection molding process, afterwards, a batch refoaming method was utilized to create bimodal structure. The effects of microballoon and nanoclay content and foaming time and temperature on microstructure and physical properties of foams were investigated. The results revealed that refoaming leads to a considerable decrease in density due to nucleation of microcells along with re-expansion of microballoons, as well as CO2 diffusion in voids between the matrix and microballoon surfaces. Microballoon content has no significant effect on cell size of bimodal foams, while a great growth in cell density was observed as its content increased. Results also indicated that at low and high foaming temperature and time, melt strength and gas loss are the overcoming phenomena, respectively leading to an optimal processing temperature and time. [ABSTRACT FROM AUTHOR]

Syntactic foams with low density as well as low thermal conduction and proper mechanical performance are vitally important for aerospace, marine, and automotive industries. Here, phenolic-based syntactic foams were fabricated by combining the hollow glass microsphere (GMs) with phenolic resin of in situ synthesis. Benefited from the stirring and hot-pressing treatment, microspheres dispersed homogeneously in the resin matrix and it greatly reduced the density of the composites. Stretching and compression tests were performed to investigate the mechanical behavior of the foams. It is found that both the compressive and tensile strength decreased as the filler loadings increasing. While the elasticity modulus was improved. On the other hand, thermal properties tests revealed superior thermal stability and thermal insulate performance of the composites. The final residue content of the synthetic foam with 40 wt% filler was improved by ∼31.5% than that of the neat one at 700°C. And samples with 20 wt% microspheres reached a minimum thermal conductivity value of approximately 0.129 W (m·K)−1 which is ∼46.7% lower than that of neat resin [0.298 W (m·K)−1]. This work provides a feasible strategy to construct syntactic foams with low density and ideal thermal properties.

Hollow polymer microspheres with superior elastic properties, high thermal stability, and energy absorbance capabilities are essential in many applications where shock and vibration need to be mitigated, such as in civil, medical, and defense industries. In this paper, the synthesis, fabrication, and characterization of hollow thermoset microspheres for syntactic polymer foam were studied. The hollow polymer microspheres (HPMs) were made by developing core--shell composites and thermally removing the polystyrene core to yield a polysiloxane shell. The HPMs were embedded into a polydimethylsiloxane (PDMS) matrix to form a polymer syntactic foam. The mechanical energy absorption characteristic of polymer syntactic foams was measured by cyclic uniaxial compression testing following ASTM 575. The engineered compression response was demonstrated by fabricating and testing syntactic foams with different porosities, ranging from a 50 vol% to 70 vol% of HPMs. Through scanning electron microscopy (SEM), we observed that the HPM contributes to the energy absorption of the syntactic foam. Moreover, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) determined the necessity of a profound study to understand the effects of varying HPM synthesis parameters, as well as the syntactic foam fabrication methods. It was shown that the compressive modulus and toughness can be increased by 20% using a 70 vol% of porosity with synthesized HPM syntactic foams over bulk PDMS. We also found that the energy absorbed increased by 540% when using a 50 vol% of porosity with fabricated HPM-PDMS syntactic foams. [ABSTRACT FROM AUTHOR]

In this work, mechanical performance on sandwich composites consisting of syntactic foam filled with hollow glass microsphere in an epoxy resin matrix as the core material and hybrid kenaf/glass fibers face-sheet based composites panel is studied. The main purpose of this work is to design a suitable lightweight composite panel material for structural application. Four different sequences of face-sheet reinforcement (e.g., kenaf-kenaf; glass-glass; glass-kenaf, and kenaf-glass) were adopted. In this study, the mechanical properties and failure modes are carefully investigated. The sandwich composites were characterized by compressive, tensile, and flexural properties. The results showed that compressive strength was highest at the "kenaf-glass" face-sheets, tensile strength was highest at "glass-glass" face-sheets, and flexural at "glass-glass" face-sheets with 52.1%, 67.6%, and 74.4% increase to "kenaf-kenaf" face-sheets, respectively. The tensile strength and modulus values with the specific strengths and modulus values showed that the mechanical properties of the sandwich composites can be influenced by hybridizing the face-sheet laminates of the composite. This shows that kenaf fiber has limited strength when used as reinforcement alone but gave an excellent performance when combined with glass fiber. [ABSTRACT FROM AUTHOR]

The addition of hollow aluminium oxide bubbles to the 7075 aluminium matrix results in a lightweight syntactic foam with a reduced density and an increased peak compression strength. The presence of ceramic bubbles also aids in a reduced coefficient of thermal expansion and thermal conductivity in comparison to aluminium alloys. In spite of their enhanced material properties, the inclusion of hollow aluminium oxide bubbles presents the challenge of poor machinability. In order to elucidate the problem of poor surface machinability, an attempt has been made to develop a thermo-mechanical finite element machining model using AdvantEdgeTM software with which surface quality and machined syntactic foam material can be analyzed. If the novel model developed is combined with virtual reality technology, CNC technicians can observe the machining results to evaluate and optimize the machining program. The main novelty behind this software is that the material foam is assumed as a homogeneous material model for simplifying the material model as a complex heterogeneous material system. The input parameters used in this study are cutting speed, feed, average size and volume fraction of hollow aluminium oxide bubbles, and coolant. For the output parameters, the numerical analysis showed a 6.24% increase in peak tensile machining induced stress as well as a 51.49% increase in peak cutting temperature as cutting speed (25 m/min to 100 m/min) and uncut chip thickness (0.07 mm to 0.2 mm) were increased. The average size and volume fraction of hollow aluminium oxide bubbles showed a significant impact on the magnitude of cutting forces and the depth of tensile induced stress distribution. It was observed on the machined surface that, as the average size of hollow aluminium oxide bubbles became coarser, the peak machining induced tensile stress on the cut surface reduced by 4.47%. It was also noted that an increase in the volume fraction of hollow aluminium oxide bubbles led to an increase in both the peak machining induced tensile stress and the peak cutting temperature by 29.36% and 20.11%, respectively. This study also showed the influence of the ceramic hollow bubbles on plastic deformation behavior in 7075 aluminium matrix; the machining conditions for obtaining a favorable stress distribution in the machined surface and sub-surface of 7075 closed cell syntactic foam are also presented. [ABSTRACT FROM AUTHOR]

Buoyant materials, as an important counterweight material of the deep-sea devices, play an important role in providing the equipment with enough buoyance as much as possible. Solid buoyant materials (SBMs) have received extensively attention in deep-water surveying and development and petroleum exploration fields in recent years due to their low density, high strength and low water absorption characteristics.In this paper, the classification and the application of SBMs and their recent developments both at home and broad were firstly discussed. Typically, the SBMs can be mainly divided into the chemical and the composite syntactic foams according to their chemical composition and the composite syntactic foam was especially elaborated in this study. Secondly, four basic types of the composite syntactic foams, namely metal-, polymer-, and ceramic- matrix and other types of the syntactic foams, based on their chemical composition of the matrix and the reinforcement. The influence factors such as the essential composition, the surface microstructure, and the loading rate on the physical, mechanical, and the failure mode of the syntactic foams were summarized. The dynamical behavior and failure mechanism under different loading rates can be analyzed and revealed by the computed X-ray tomography technique combined with the finite element method. The prospects of improving overall mechanical performance and advanced experimental characterization methods of syntactic foams are summarized as follows: the overall mechanical performance can be improved by modifying functional groups of filler and resin matrix or adding a second reinforcement phase; the microstructure can be characterized and the failure mechanism revealed by means of μ-CT and scanning electron microscope.

Abstract The most common fillers for syntactic foam development are glass microballoons or fly ash cenospheres. However, the need for improved properties and reduced cost requires exploring other possibilities. In this work, syntactic foams of vinyl ester resin reinforced with expanded glass particles (EG) were fabricated and compressive properties of the composites were studied. The foam EGS was reinforced with smaller particles of size 0.04–0.125 mm in 15, 30, and 45 vol.% and EGL had larger particles of size 0.25–0.5 mm in 15 and 30 vol.%. Compression testing reveals that 45 vol.% smaller particles enhance the modulus of the EGS foam by up to 48% with a 3.4% reduction in yield strength compared to neat resin. Strain rate sensitivity was observed in both the modulus and yield strength values for all composites. Composite EGS with 15 and 30 vol.% filler showed better strength and modulus than the neat resin at 0.01 s−1 strain rate, which is an overall improvement in the material. When compared with the literature data, the foams made in the present work have the highest compressive strength and modulus for the same density.

[Display omitted] • Successful manufacturing of CuSn-Fe hollow spheres syntactic foams by a novel infiltration method. • Improvement of the compressive behavior for the syntactic foams due to the transformation of ε into β phase. • Excellent matrix-reinforcement bonding due to CuSn intergranular infiltration in the sphere walls. • The absence of interfacial products during the infiltration process. This work analyzes the manufacturing process, phase transformations and compression behavior of un-treated and heat-treated syntactic foams consisting of a Cu20Sn alloy matrix reinforced with Fe-hollow spheres. Manufacturing process included the infiltration of the molten alloy into preforms of spheres sintered at 1000 °C during 1 h, obtaining syntactic foams with porosities near 52 %. These foams were heat-treated at 750 °C for 1 h, followed by quenching in water, which led to phase transformations from ε to β. This fact improved the compressive properties of the foams, increasing compression strengths from 195 to 243 MPa. Interfaces were not found between matrix and the Fe-hollow spheres. [ABSTRACT FROM AUTHOR]

The current work focuses on predicting the buckling and free vibration frequencies (fn) of cenosphere reinforced epoxy based syntactic foam beam under varying loads. Critical buckling loads (Ncr) and fn are predicted using the differential quadrature method (DQM). Ncr and fn have been calculated for beams of varying cenosphere volume fractions subjected to axial load under clamped-clamped (CC), clamped-simply (CS), simply-simply (SS), and clamped-free (CF) boundary conditions (BC′s). Upon increasing the cenosphere volume fraction, Ncr and fn of syntactic foam composites increases. These numerical outcomes are compared with the theoretical values evaluated through the Euler-Bernoulli hypothesis and further compared with experimental outcomes. Results are observed to be in precise agreement. The results of the DQM numerical analysis are given out for the different BC′s, aspect ratios, cenosphere volume fractions, and varying loads. It is perceived that depending on the BC′s, the type of axial varying loads and aspect ratios has a substantial effect on the Ncr and fn behaviour of the syntactic foam beams. A comparative study of the obtained results showed that the beam subjected to parabolic load under CC boundary conditions exhibited a higher buckling load.

Syntactic foam made from hollow glass microspheres (HGM) in an epoxy matrix has proven to be a good material with a strong structural strength. Understanding filler particle size variation is important in composite material formation, especially in syntactic foam, because of its numerous applications such as aerospace, marine, and structural purposes. In this present work, the effects of particle variation in different sizes (20–24 µm, 25–44 µm, 45–49 µm, and 50–60 µm) on the mechanical properties of the syntactic foam composites with a focus on flexural strength, modulus, and fracture surfaces are investigated. The particle sizes are varied into five volume fractions (5, 10, 15, 20, and 25 vol%). The results show that the highest flexural strength is 89 MPa at a 5 vol% fraction of 50–60 µm particle size variation with a 69% increase over the neat epoxy. This implies that the incorporation of HGM filler volume fraction and size variation has a strong effect on the flexural strength and bending modulus of syntactic foam. The highest particle size distribution is 31.02 at 25–44 µm. The storage modulus E' increased at 30 °C, 50 °C, and 60 °C by 3.2%, 47%, and 96%, respectively. The effects of wall thickness and aspect ratio on the size of the microstructure, the fracture surfaces, and the viscoelastic properties are determined and reported accordingly. [ABSTRACT FROM AUTHOR]

One of the remarkable properties of special materials is the piezoelectric phenomenon. The piezoelectric effect is the ability of materials and crystals to convert mechanical energy into electrical energy and vice versa. One way to increase the piezoelectric effect of materials is foaming. The presence of cavities in a foamed material causes the accumulation of electric charge and improves the piezoelectric property of the material. In this paper, a new method is carried out to produce nanocomposite bimodal foams. Piezoelectric low density polyethylene syntactic foams containing expandable polymeric microballoons (EPM) were manufactured using injection molding, then the refoaming process was conducted via batch method. In order to investigate the piezoelectric properties of samples, the effect of microballoon weight% and nanoclay content were studied. The results showed that refoaming process significantly enhanced the piezoelectric properties of both composite and nanocomposite foam specimens. The data of peak-to-peak voltage revealed that the highest piezoelectric response was achieved for the bimodal sample containing the lowest weight% of microballoon (2.5 wt%) as well as highest level of nanoparticle (2 wt%). [ABSTRACT FROM AUTHOR]

The paper presents the application of the casting method for the production of porous composites, called syntactic foams, of the casting alloy - solid particles type. This method was used to produce composites based on Al alloys reinforced with particles of clinoptilolite, a natural mineral from the zeolite group. Before the casting process, tests were carried out on the morphology, physicochemical properties and chemical composition of the zeolite, which was obtained from a rock called zeolite tuff, mined in a quarry in Kucin, (VSK PRO-ZEO s.r.o., Slovakia). Observations of the microstructure of the produced composites were also carried out using a scanning electron microscope. Diffractometric tests of zeolite rock as delivered for research and of the produced samples reinforced with zeolite particles were also carried out. Initial studies of the density and porosity of the produced composites were performed. The usefulness of the presented method of composite production was assessed on the basis of the conducted structural tests, with particular emphasis on the particle distribution in the alloy matrix. [ABSTRACT FROM AUTHOR]

Epoxy composites with hollow glass microspheres are considered. A method is proposed for predicting their local elastic characteristics and limiting strength characteristics in uniaxial compression. The method takes account of the composition of the material, the content of the components in the syntactic foam, and the characteristic size of the microspheres (the ratio of the shell thickness to the cavity radius). The variation in these parameters is taken into account in model calculations. [ABSTRACT FROM AUTHOR]