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

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

Comparison of tensile and compressive characteristics of vinyl ester/glass microballoon syntactic foams

The present study is focused on the synthesis and characterization of vinyl ester/glass microballoon syntactic foams. Tensile and compressive properties of vinyl ester matrix syntactic foams are characterized. Results show that the compressive strength and moduli of several syntactic foam compositions are comparable to those of the neat matrix resin. Due to the lower density of syntactic foams, the specific compressive properties of all compositions are higher than those of the neat resin. Similar trends are observed in the tensile properties. Mechanical properties of vinyl ester matrix syntactic foams are compared to well-documented mechanical properties of epoxy matrix systems. The comparison shows that low cost vinyl ester resins, which are extensively used in marine applications, can result in syntactic foams with comparable performance to epoxy matrix systems. In addition, tensile modulus is found to be 15–30% higher than the compressive modulus for all syntactic foam compositions. This difference is related to the possibility of particle fracture in the stress range where modulus is calculated in the compressive stress–strain curves.     

Characterization of epoxy syntactic foams by dynamic mechanical analysis

Epoxy syntactic foams that are capable of withstanding use-temperatures in the range of 106 to 175°C were fabricated with DGEBA or novolac based epoxy resins and appropriate amine hardener materials. These foams were characterized for dynamic mechanical properties in single cantilever mode. The storage modulus, loss modulus and tan δ values were recorded over a wide temperature range. A typical density value of around 0.45 g/cm³ of the syntactic foams made respectively from a cycloaliphatic amine hardener, aromatic amine hardener-I, aromatic amine hardener-II recorded storage modulus (E′) values of 1354 MPa, 1500 MPa and 1530 MPa respectively and tan δ values of 0.0139, 0.0090, 0.01039 respectively at 30°C. The storage modulus values gradually decreased with increasing temperature while the loss modulus values showed corresponding gradual increase in the same temperature range. There is a steep variation in these values when the material softens in the vicinity of the glass transition temperature (T _g) indicating the upper temperature limits to which they can be put in use. The reduction in the storage modulus values with increasing temperature and the glass transition temperature values are characteristic of the resin/hardener systems as well as the curing/post curing cycles employed.     

空心玻璃微球含量对环氧复合泡沫塑料性能的影响

以空心玻璃微球（HGM）填充环氧树脂制备了密度为0.56~0.91 g/cm3的HGM/环氧复合泡沫塑料。研究了HGM含量对复合泡沫塑料黏度、力学性能、动态力学性能及隔热性能的影响。结果表明:表面偶联处理后增加了HGM的表面亲油性,改善了其与基体树脂间的相容性和界面性能,有利于HGM/环氧复合泡沫塑料性能的提高;体系黏度与HGM含量呈正相关,与温度呈负相关;随着HGM含量的增加,HGM/环氧复合泡沫塑料的压缩强度、弯曲强度和拉伸强度均有一定程度的降低,但是比强度变化不大,材料得到很大程度的轻质化;HGM的引入使得HGM/环氧复合泡沫塑料玻璃化转变温度向低温方向偏移,储能模量呈现先减小后增加的趋势,导热系数由纯环氧树脂的0.203 W/（m·K）减小到HGM含量为40wt%时的0.126 W/（m·K）。HGM/环氧复合泡沫塑料阻尼性能和隔热性能均有所提高。      

Quasi-static uni-axial compression behaviour of hollow glass microspheres/epoxy based syntactic foams

Hollow glass microspheres/epoxy foams of different densities were prepared by stir casting process in order to investigate their mechanical properties. The effect of hollow spheres content and wall thickness of the microspheres on the mechanical response of these foams is studied extensively through a series of quasi-static uni-axial compression tests performed at a constant strain rate of 0.001 s^?1. It is found that strength of these foams decreases linearly from 105 MPa (for the pure resin) to 25 MPa (for foam reinforced with 60 vol.% hollow microspheres) with increase in hollow spheres content. However, foams prepared using hollow spheres with a higher density possess higher strength than those prepared with a lower one. The energy absorption capacity increases till a critical volume fraction (40 vol.% of the hollow microspheres content) and then decreases. Failure and fracture of these materials occur through shear yielding of the matrix followed by axial splitting beyond a critical volume fraction.     

Stress analysis of syntactic foams

The efficient design of syntactic foam composites demands an accurate knowledge of the stress distribution throughout the constituents. The shape and distribution of the included voids have a marked effect on the load-carrying action of the composite.In this work the finite element method is employed for the stress analysis of such honeycomb composites, and three basic modules are investigated. Regular arrangements of cubic voids are first studied and spherical voids in both a simple cubic and a face centre cubic distribution are then considered. The effects of a structurally stiff liner are finally investigated and it is shown how improvements can be made in the composite behaviour by the choice of optimum liner characteristics.     

Microstructural characterisation of syntactic foams

Three types of hollow microspheres with different average diameters (100–150 μm) and two aluminium alloys as matrix material were used to produce metal matrix syntactic foams (MMSFs) by pressure infiltration. The phases, which formed at matrix-filler interface, were investigated by X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). The investigation showed that in syntactic foams, with the Al99.5 matrix, an exchange reaction took place between the matrix and the amorphous components of ceramic hollow microspheres. The reaction resulted in significant formation of alumina and Si precipitates. Because of this diffusion reaction, the hollow microspheres’ walls were degraded. In the case of the AlSi12 matrix the reaction was suppressed by the considerable Si content of the matrix. Therefore, the wall of the hollow microspheres remained unharmed and no real interface layer was found.     

Effective elastic moduli of syntactic foams

An analytical approach to compute the effective elastic moduli of syntactic foams is presented. Using the concentric spherical model, the effective elastic constants are estimated, and compared with other theories and experimental data. The computed effective elastic moduli are within the lower and upper bounds, and agree with the experimental data.     

Three-phase syntactic foams: structure-property relationships

This study addresses a unique problem that occurs in composite materials containing hollow reinforcements, that is the density-filler content relationship. Unlike a traditional solid reinforcement, a hollow reinforcement can be manufactured with a variety of densities. Subsequently, when fabricating composite materials with a particular density requirement, a large variation in volume percent of that reinforcing phase can occur. Hollow reinforcements under consideration are carbon microballoons (CMBs) of various densities determined by both tap density and pycnometry. Our approach is to study several different densities and volume percents of microballoons while maintaining a constant volume percent (8.5%) of the polymer binder phase. The resulting syntactic foams are three-phase materials consisting of binder, microballoon (MB), and interstitial void phases. The volume of the MB and binder phase is measured by helium pycnometry. The complementary volume of the interstitial void phase will depend on the volume of microballoons in the billet. Mechanical characterization is done by compression and flexure testing and results are discussed to highlight structure-property relationships. Results show that, in addition to bulk density of the foam, the packing arrangement of the CMBs is an important factor in the mechanical behavior of the foam and is shown to be an important design criterion.     

Compressive characteristics of metal matrix syntactic foams

The compressive behaviour of eight different metal matrix syntactic foams (MMSFs) are investigated and presented. The results showed that the engineering factors as chemical compositions of the matrix material, the size of the microballoons, the previously applied heat treatment and the temperature of the compression tests have significant effects on the compressive properties. The smaller microballoons with thinner wall ensured higher compressive strength due to their more flawless microstructure and better mechanical stability. According to the heat treatments, the T6 treatments were less effective than expected; the parameters of the treatment should be further optimised. The elevated temperature tests revealed ∼30% drop in the compressive strength. However, the strength remained high enough for structural applications; therefore MMSFs are good choices for light structural parts working at elevated or room temperature. The chemical composition - microballoon type - heat treatment combinations give good potential for tailoring the compressive characteristics of MMSFs.     

Manufacture and compression properties of syntactic foams

A study has been made of the processability, bulk density and uniaxial compression properties of a syntactic foam system with varying volume fractions of phenolic microballoons. Short-term compression tests showed that the compression yield strength and initial tangent modulus of elasticity were linearly dependent on the bulk density (and the volume fraction of microballoons). The microballoon concentration and resin binder composition was found to be crucial to the ease of manufacturing syntactic foams.     

空心玻璃微珠填充环氧复合泡沫材料的制备及性能表征

通过优化试验，以大量空心玻璃微珠填充环氧树脂体系制备出了密度低、强度高的复合泡沫材料，并对其密度和水下声学性能进行了表征。结果表明，所研制的轻质高强复合泡沫材料密度在0．3～0．5g／cm^3之间，且在高静水压下具有良好声学性能。     

Analysis of particle-to-particle elastic interactions in syntactic foams

This work analyzes the elastic interaction between a pair of hollow particles embedded in a dissimilar medium subjected to remote uniaxial tensile loading. The Boussinesq-Papkovich stress function approach is combined with a multi-pole series expansion to derive a semi-analytical solution of the Navier-Cauchy equation and determine stress and displacement fields in the inclusions and the surrounding medium. Stress profiles, stress intensification factors, circumferential stress resultants in the particles, and strain energy storage are presented for a broad range of particle wall thicknesses, sizes, and relative inter-particle distances. Results specialized to glass-vinyl ester systems demonstrate that particle wall thicknesses and size distributions can be used to modulate the effects of particle-to-particle interaction on the overall mechanical behavior of syntactic foams.     

Mechanical properties of graphene platelets reinforced syntactic foams

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

Quasi-static compressive behaviour of aluminium cenosphere syntactic foams

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?kg?m^?3. Compression tests at strain rate 0.001/s are performed on foam samples to characterise their mechanical properties which are then used in numerical analysis on commercial finite element analysis software ABAQUS/CAE with isotropic elastic-plastic material model. Experimental and numerical results show good conformity in deformation behaviour with elastic and plateau zones showing average deviations less than 5% and 20%, respectively. Foams showed high yield stress and energy absorption capabilities that can be useful in making blast and impact resistant structures.     

Microstructural failure modes in three-phase glass syntactic foams

Samples of syntactic foam containing hollow glass microspheres of 0.108 and 0.253 g/cm³ tap densities, some with a silane surface treatment, were subjected to different stress states and examined for failure modes. All foams contained the same volume fraction of APO-BMI, a bismaleimide resin binder. The samples were tested in compression and in three-point bend, and mechanical properties were compared between the various foams. Microsphere strength had a strong effect on overall uniaxial compressive strength with interface strength playing a secondary, yet significant role. In three-point bending, the role of the interface was much more critical. Cross sections of the compression test samples were examined by optical microscopy, and fracture surfaces were investigated by scanning electron microscopy.     

On the compressive strength of glass microballoons-based syntactic foams

This work is concerned with particulate composites filled with hollow spherical inclusions, i.e., syntactic foams. We aim at the micromechanical evaluation of the effective uniaxial compressive strength for the most relevant case of glass inclusions of wall thickness of few micrometers (microballoons) filling a thermoset matrix. We develop a three-dimensional Finite Element (FE) modelling which extends and improves that recently proposed by our group. Different microstructures are described by cubic unit cells containing fifty hollow spheres accounting for different filler polydispersions and filler volume fraction f up to 60%. Each microballoon is assumed to undergo brittle failure according to a structural criterion. Here, we account for the matrix nonlinear behaviour and, in a phenomenological way, for the detriment of its mechanical properties, proportional to its defectiveness, which increases with the filler content and becomes extremely relevant at f larger than 50%. Our findings agree with experimental observations from the literature and reveal room for improvement in the effective mechanical properties by acting on the manufacturing process.     

Characterization of reinforced syntactic foams using ultrasonic imaging technique

Non-destructive evaluation of defects like voids in syntactic foam reinforced with epoxy compatible chopped strand glass fibres, employing ultrasonic C-scan immersion through transmission method, was under-taken. The results showed that in four of the five similarly processed foam samples, the voids were uniformly spread while in the fifth, which was processed by a different route, a large spread of low dense area was noticed emphasizing the influence that processing technique has on the amount of voids present in the composites.     

Hygrothermal studies on syntactic foams and compressive strength determination

Syntactic foams are commonly used as core materials in composite sandwich structures for weight sensitive applications such as aircraft and spacecraft structures and boat hulls. Moisture absorption is highly undesirable in these applications. The present study evaluates the hygrothermal properties of two types of syntactic foams. Distribution of outer diameter of cenospheres (hollow particles) incorporated in both types of syntactic foams is the same but there is variation in the internal diameter causing difference in the density of syntactic foams. Epoxy resin is used as matrix material and the volume fractions of matrix and cenospheres are kept at 0.35 and 0.65 by volume, respectively. Moisture absorption experiments are conducted at two different temperatures, 25 and 70 °C and in deionized and salt waters. Non-destructive ultrasonic imaging technique is used to find the extent of moisture penetration and damage to the specimens. Syntactic foam samples are tested for compressive strength after moisture absorption and the results are compared with the compression test results of dry syntactic foam samples.