Electrical properties of carbon nanofiber reinforced multiscale polymer composites

Carbon nanofiber (CNF) reinforced epoxy matrix nanocomposites and CNF reinforced glass hollow particle filled syntactic foams are studied for electrical properties. The effect of CNF weight fraction, hollow particle volume fraction, and hollow particle wall thickness on impedance and dielectric constant are characterized. The results show that the impedance decreases and the dielectric constant increases with increasing CNF content in the composites. Nanocomposites containing 10 wt.% CNFs showed significantly higher dielectric constant because of the presence of a continuous network of CNFs in the composite. CNF reinforced syntactic foams showed higher dielectric constant than the neat resin. The CNF content had a more prominent effect on the dielectric constant than the glass hollow particle volume fraction and wall thickness. The Maxwell–Garnett and the Jayasundere–Smith models are modified to include the effect of hollow particle wall thickness and obtain predictions of dielectric constants of syntactic foams. The semi-empirical predictions obtained from Maxwell–Garnett models are closer to the experimental values. Lightweight syntactic foams, tailored for electrical properties, can be useful in electronic packaging applications.     

Thermal expansion behavior of hollow glass particle/vinyl ester composites

Ceramic particle-reinforced composites have better dimensional stability than the matrix polymer at high temperatures. In hollow-particle filled composites (syntactic foams), the coefficient of thermal expansion (CTE) can be controlled by two parameters simultaneously: wall thickness and volume fraction of particles, which are explored in this study. The CTE was experimentally measured to be up to 60.4 % lower than the matrix material with the addition of glass microballoons for the twelve compositions of syntactic foams characterized using a thermomechanical analyzer. The CTE values have a stronger dependence on particle volume fraction than the wall thickness within the range of parameters explored. The experimental trends are analyzed by using Turner’s and Kerner’s models modified for syntactic foams. The results from the modified Turner’s model show close correlation with the experimental values with a maximum difference of ±15 %. Parametric studies show that syntactic foams of a wide range of densities can be tailored to obtain the same CTE value. The experimental and theoretical results are helpful in developing syntactic foams with desired properties for thermal applications.     

Poisson's ratio of hollow particle filled composites

The present study is focused on the experimental measurement of the Poisson's ratio of glass hollow particle filled composites called syntactic foams. The effect of hollow particle wall thickness and volume fraction on the Poisson's ratio of the composites is investigated. Results show that the Poisson's ratio of the composites is lower than that of the neat vinyl ester resin used as the matrix material. Despite being a fundamental elastic constant, a reliable value of the Poisson's ratio of the various types of composites is not readily available. These experimental results can be useful in benchmarking available theoretical models and guiding modeling efforts on functionally graded composites.     

A functionally graded syntactic foam material for high energy absorption under compression

A functionally graded structure for hollow particle (microballoon) filled syntactic foams is fabricated that is capable of withstanding compression for 60–75% strain without any significant loss in strength. The new functionally graded structure is based on creating a gradient in microballoon wall thickness. This material has the same volume fraction of microballoons throughout the structure, eliminating the undesirable effects of the present functionally graded composites containing a gradient of particle volume fraction. Three compositions of such material are fabricated and tested in the present study. Results show that the compressive modulus, strength, and total energy absorption of the new syntactic foams can be controlled by using appropriate type and volume fraction of microballoons.     

Thermoanalytical characterization of epoxy matrix-glass microballoon syntactic foams

Syntactic foams are finding new applications where their thermal stability and high temperature response are important. Therefore, the high temperature response of these advanced composites needs to be characterized and correlated with various material parameters. The present study is aimed at evaluating the effect of microballoon (hollow particle) volume fraction (Φ) and wall thickness (w) on thermoanalytical characteristics of epoxy matrix syntactic foams containing glass microballoons. These composites are characterized to determine the glass transition temperature (T _g), the weight loss, and the char yield. It is observed that T _g decreases and the char yield increases due to the presence of microballoons in the resin. The T _g is increased with an increase in Φ but is not significantly affected by w. The thermal stability is increased by increasing w and is relatively less sensitive to Φ. Understanding the relations between thermal properties of syntactic foams, the microballoon wall thickness, and microballoon volume fraction will help in developing syntactic foams optimized for mechanical as well as thermal characteristics. Due to the increased interest in functionally graded syntactic foams containing a gradient in microballoon volume fraction or wall thickness, the results of the present study are helpful in better tailoring these materials for given applications.     

Influence of moisture absorption on flexural properties of syntactic foams

This work studies the influence of moisture absorption on the flexural properties of vinyl ester matrix–glass particle syntactic foams. The extent and the effect of moisture absorption are related to the wall thickness and volume fraction of the particles present in the composite. Four compositions of vinyl ester–glass systems are exposed to deionized and sea water conditions. Experimental findings are compared with results on virgin specimens. In general, the exposure of syntactic foams to a water environment yields a deterioration of Young’s modulus. This phenomenon is more prominent with deionized water as compared to sea water and increases with the particle volume fraction. In addition, results from water absorption tests show that syntactic foams have a lower diffusivity as compared to the neat resin. Experimental data are interpreted by using available modeling tools that allow for predicting the composite behavior from the properties of its constituents.     

Effect of volume fraction and wall thickness on the elastic properties of hollow particle filled composites

Hollow particle filled composites, called syntactic foams, are widely used in applications requiring high damage tolerance and low density. The understanding of the mechanics of these materials is largely based on experimental studies. Predictive models that are capable of estimating the elastic properties of these materials over wide variation of particle wall thickness, size, and volume fraction are not yet fully developed. The present study is focused on developing a modeling scheme to estimate the elastic constants for such materials. The elastic properties of an infinitely dilute dispersion of microballoons in a matrix material are first computed by solving a dilatation and a shear problem. A differential scheme is then used to extrapolate the elastic properties of composites with high volume fractions of microballoons. The results show that the model is successful in predicting the Young’s modulus for syntactic foams containing microballoons of a wide range of wall thickness and volume fraction.     

Tensile, thermal and dynamic mechanical properties of hollow polymer particle-filled epoxy syntactic foam

Four types of hollow polymer particles have been employed as fillers in UV-heat-cured epoxy resin. The effects of filler volume fraction and types on tensile properties of syntactic foams are investigated. All hollow particle types have exhibited approximately the same size, but with different groups on particle surface. Volume fraction of hollow particles for the syntactic foams varies up to 0.25. Thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and tensile tests have been performed. According to TGA and DMA test results, the addition of polymer particle leads to stronger interfacial particles-matrix interaction and high loss factor. Tensile tests have also shown that tensile strength and specific properties of all foams decrease with the increasing of particle content. In terms of tensile properties, the particles-matrix compatibility is more significant than that of the surface functional groups of particles.     

Effects of particle clustering on the tensile properties and failure mechanisms of hollow spheres filled syntactic foams: A numerical investigation by microstructure based modeling

Particle clustering originated from manufacturing process is thought to be one of the critical factors to the mechanical performance of hollow spheres filled syntactic foams. Although experimental evidence provides a qualitative understanding of the effects of particle clustering on the mechanical properties of syntactic foams, a quantitative assessment cannot be made in the absence of an appropriate micromechanical modeling strategy. In this study, three-dimensional microstructures of syntactic foams with different degrees of particle clustering were reconstructed based on random sequential adsorption (RSA) method. Three-phase finite element models considering the progressive damage behavior of the microsphere–matrix interface were accordingly developed by means of representative volume element (RVE) to quantitatively investigate the effects of particle clustering on the tensile properties and failure mechanisms of syntactic foams. The simulation results indicate that the elastic behavior of syntactic foams is insensitive to the degree of particle clustering, but the strength properties as well as the failure mechanisms are significantly influenced by the degree of particle clustering. From the micromechanical viewpoint, the clustered regions containing higher concentration of microspheres than the average volume fraction would serve as crack initiation sites due to stress concentration, and consequently lead to a negative effect on tensile strength, fracture strain, and interfacial damage of syntactic foams.     

Viscoelastic properties of hollow glass particle filled vinyl ester matrix syntactic foams: effect of temperature and loading frequency

Viscoelastic properties of hollow particle-reinforced composites called syntactic foams are studied using a dynamic mechanical analyzer. Glass hollow particles of three different wall thicknesses are incorporated in the volume fraction range of 0.3–0.6 in vinyl ester resin matrix to fabricate twelve compositions of syntactic foams. Storage modulus, loss modulus, and glass transition temperature are measured and related to the microstructural parameters of syntactic foams. In the first step, a temperature sweep from ?75 to 195 °C is applied at a fixed loading frequency of 1 Hz to obtain temperature dependent properties of syntactic foams. In the next step, selected four compositions of syntactic foams are studied for combined effect of temperature and loading frequency. A frequency sweep is applied in the range 1–100 Hz and the temperature is varied in the range 30–140 °C. Time–temperature superposition (TTS) principle is used to generate master curves for storage modulus over a wide frequency range. The room temperature loss modulus and maximum damping parameter, Tanδ, are found to have a linear relationship with the syntactic foam density. Increasing volume fraction of particles helps in improving the retention of storage modulus at high temperature in syntactic foams. Cole–Cole plot and William–Landel–Ferry equation are used to interpret the trends obtained from TTS. The correlations developed between the viscoelastic properties and material parameters help in tailoring the properties of syntactic foams as per requirements of an application.     

空心微球/环氧树脂复合泡沫塑料的抗压性能与破坏机制

以环氧树脂为基体, 不同粒径空心玻璃微球为填充体, 制备了轻质高强复合泡沫塑料。通过单轴准静态压缩试验研究了空心微球的粒径大小对复合泡沫塑料的抗压性能的影响, 并采用SEM对复合泡沫塑料的微观结构进行观测。通过随机空间分布法建立了空心玻璃微球/环氧树脂复合泡沫塑料的实体模型, 并且使用有限元分析软件对复合泡沫塑料在1 kPa载荷下的应力分布进行了分析。结果表明, 在相同体积含量下, 当空心微球的粒径从30 μm增大到120 μm时, 复合泡沫塑料的抗压强度无明显变化。有限元分析的结果表明, 在复合泡沫塑料中主要承载部分为空心微球, 空心微球上的应力大于树脂基体上的应力。最大应力分布在空心微球的内壁, 结合SEM图像可推测, 空心微球在破裂之前受到充分的挤压, 并且从内壁产生裂纹。     

Enhancement of Energy Absorption in Syntactic Foams by Nanoclay Incorporation for Sandwich Core Applications

Syntactic foams are closed pore foams fabricated by the mechanical mixing of hollow glass particles in a matrix resin. The present study deals with change in compressive properties of syntactic foams due to the incorporation of nano-sized clay (nanoclay) particles. A surface modified clay, Nanomer I.30E, has been used in the fabrication of specimens. Six different types of syntactic foams are fabricated and tested for compressive properties. Three types of hollow particles (microballoons) of glass having different densities are used for fabrication. Each type of microballoon is combined with 0.02 and 0.05 volume fraction of nanoclay, respectively. The combined volume fraction of microballoons and nanocparticles is 0.65 in all kinds of foams. Compressive properties of these samples are compared with those of syntactic foams without nanoclay particles. It is observed that partial intercalation of nanoclay has taken place in the specimens and remaining nanoclay particles are present in small clusters. Such microstructure leads to nearly the same strength with considerable enhancement in fracture strain. Hence, the toughness of the material, measured as the area under stress–strain curve, is found to increase by 80–200% for various kinds of foams tested in the study. Fracture features of syntactic foams with and without nanoclay are compared.     

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.     

Compressive and ultrasonic properties of polyester/fly ash composites

The addition of hollow fillers having appropriate mechanical properties can decrease the density of the resulting composite, called syntactic foams, while concurrently improving its mechanical properties. In this study, hollow fly ash particles, called cenospheres, are used as fillers in polyester matrix material. Cenospheres are a waste by-product of coal combustion and, as such, are available at very low cost. In this study, the composites were synthesized by settling cenospheres in a glass tube filled with liquid polyester resin and subsequently curing the resin. This process resulted in a functionally graded structure containing a gradient in the cenosphere volume fraction along the sample height. Uniform radial sections were cut from each composite and were characterized to observe the relationship between cenosphere volume fraction and compressive properties of the composite. The composite was also tested using ultrasonic non-destructive evaluation method. Results show that the modulus of the composites increases with increasing cenosphere volume fraction. The modulus of composites containing more than 4.9 vol% cenosphere was found to be higher than the matrix resin. In general, the modulus of composites increased from 1.33 to 2.1 GPa for composites containing from 4.9–29.5 vol% cenospheres. The specific strength of the composite was found to be as high as 2.03 MPa/(kg/m³) compared to 0.96 MPa/(kg/m³) for the neat resin. Numerous defects present in fly ash particles caused a reduction in the strength of the composite. However, the reduction in the strength was found to be only up to 22%. Increase of over 110% in the specific modulus and only a slight decrease in the strength indicates the possibility of significant saving of weight in the structures using polyester/fly ash syntactic foams.     

Volume fraction effect on high strain rate properties of syntactic foam composites

The volume fraction effect on the high strain rate compressive properties of syntactic foams is characterized using a pulse-shaped Split-Hopkinson Pressure Bar (SHPB) technique. Eighteen different types of syntactic foams are fabricated with the same matrix resin system but six different microballoon volume fractions and three different size microballoons. The volume fractions of the microballoons in the syntactic foams are maintained at 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6. The microballoons have the same mean outer radius of 40 μm, but different internal radii leading to a difference in their density. Analysis is carried out on the effect of microballoon volume fractions on the high strain rate properties for each type of syntactic foam. This approach is helpful in understanding the effect of microballoon reinforcement at different volume fractions on the dynamic compressive properties of syntactic foams. The results at high strain rates are compared to quasi-static strain rate compressive properties of the same material. The results show that there is a decrease in both compressive strength and modulus as the microballoon volume fraction increases for the same type of syntactic foam at all strain rates. However, at strain rates of quasi-static and 450/s, the decrease tends to be gradual across all volume fractions, while for strain rates of 800/s, there is a dramatic decrease from 10 to 20% followed by a gradual decline for most specimens. The fracture mode plays a major role in the dynamic behavior of syntactic foams.     

Determination of Dynamic Modulus of Syntactic and Particulate Foams by a Non‐Destructive Approach

Abstract: Developments in aviation posed requirement of lightweight, high strength and highly damage‐tolerant materials. Sandwich‐structured composites fulfilling these requirements have become the first choice for many aerospace applications as well as structural components for ground transport and marine vessels. Sandwich composites are a special class of composite materials which are widely used because of their high specific strength and high bending stiffness. Syntactic foams, which are hollow particle‐filled core materials used in sandwich composites, have recently emerged as attractive material for applications requiring low weight, low moisture absorption and high insulation properties. Quasi‐static and dynamic properties of these syntactic foams are commonly determined though various destructive techniques such as quasi‐static compression and split Hopkinson pressure bar testing. However, there is a need for characterising these materials non‐destructively in the field. The present study focuses on the prediction of dynamic Young's modulus using ultrasonic testing in various types of hollow particle‐reinforced syntactic foam and solid particulate composites. Hollow particle‐filled syntactic foams and solid particulate composites are fabricated with three different volume fractions of 10%, 30% and 60%. Longitudinal and shear wave velocities are used for calculating the dynamic modulus. Effect of longitudinal attenuation behaviour along with longitudinal and shear wave velocities on the varying density and volume fraction of syntactic foams is also discussed.     

短纤维增强复合泡沫弹性性能数值模拟

采用二元模型对短纤维增强复合泡沫(SFRSF)材料进行了简化模拟,考虑了纤维在空间中分布的随机性,并分别采用不同单元类型在不考虑网格匹配的情况下对纤维和基体单独进行网格划分。之后,采用改进的单元嵌入技术(EET)耦合纤维与基体的自由度,并引入杆单元模拟界面相,描述了材料内部纤维与基体的传载机制,从而建立了能反映材料细观结构的有限元数值模型。在此基础上,研究了碳纤维含量和长度以及空心微珠含量和壁厚对SFRSF杨氏模量的影响规律。结果表明,该数值模型对SFRSF杨氏模量的预测与实验值吻合较好。增加碳纤维的含量和纤维长度能够有效提高SFRSF材料的杨氏模量,适当增加空心微珠壁厚一定程度上可以增加其杨氏模量。     

考虑界面效应的复合泡沫塑料弹性性能数值仿真预测与试验研究

通过对不同空心陶瓷微珠含量的环氧基复合泡沫塑料进行准静态拉伸实验, 研究了填充微珠的体积分数对复合泡沫塑料弹性模量和泊松比的影响。基于其细观结构特征, 利用三维立方单胞有限元模型模拟了细观应力/应变场; 将内聚力单元引入细观有限元模型, 以此来模拟空心微珠与基体材料之间界面相的力学行为。将有限元预测结果以及两种传统的细观解析法与实验数据对比, 发现基于界面理想粘接假设的有限元模型和传统细观解析法均过高估计了复合泡沫塑料的弹性模量和泊松比; 复合泡沫塑料的弹性性能强烈地依赖于界面相的力学性质, 只有考虑界面效应的细观有限元模型才能给出较为精确的预测, 从而验证了文中细观建模方法的合理性。     

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.     

Melt flow and solidification during infiltration in making steel matrix syntactic foams

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 alumina hollow spheres with an average size of around 3.97?mm to make the pores and enhance the properties of the foams. During the process, melt flow along with heat transfer during the infiltration is investigated. The study shows that melt flow and velocity distribution of molten steel are significantly unstable; the critical solid fraction of the steel is 0.67; and the alumina ceramic mould in which the foams are prepared must be preheated to at least 1000°C to prevent incomplete infiltration. Finally, the microstructure of syntactic foams is studied.