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.     

Effect of carbon nanofibers on tensile and compressive characteristics of hollow particle filled composites

The effect of presence of carbon nanofibers on the tensile and compressive properties of hollow particle filled composites is studied. Such composites, called syntactic foams, are known to have high specific modulus and low moisture absorption capabilities and are finding applications as core materials in aerospace and marine sandwich structures. The results of this study show that addition of 0.25 wt.% carbon nanofibers results in improvement in tensile modulus and strength compared to similar syntactic foam compositions that did not contain nanofibers. Compressive modulus decreased and strength remained largely unchanged for most compositions. Tensile and compressive failure features are analyzed using scanning electron microscopy.     

Effects of density and strain rate on properties of syntactic foams

Syntactic foams are characterized for high strain rate compressive properties using Split-Hopkinson Pressure Bar (SHPB) technique in this study. The results at high strain rates are compared to quasi-static strain rate compressive properties of the same material. Four different types of syntactic foams are fabricated with the same matrix resin system but different size microballoons for testing purpose. The microballoons have the same outer radius. However, their internal radius is different leading to a difference in their density and strength. The volume fraction of the microballoons in syntactic foams is maintained at 0.65. Such an approach is helpful in isolating and identifying the contribution of matrix and microballoons to the dynamic compressive properties of syntactic foams. Results demonstrate considerable increase in peak strength of syntactic foams for higher strain rates and increasing density. It is also observed that the elastic modulus increases with increasing strain rate and density. Scanning electron microscopy is carried out to understand the fracture modes of these materials and the density effect on high strain rate properties of syntactic foam.     

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.     

Studies on compressive failure features in syntactic foam material

Syntactic foam made by mechanical mixing of glass hollow spheres in epoxy resin matrix is characterized for compressive properties in the present study. Volume fraction of hollow spheres in the syntactic foam under investigation is kept at 67.8%. Effect of specimen aspect ratio on failure behavior and stress-strain curve of the material is highlighted. Considerable differences are noted in the macroscopic fracture features of the specimen and the stress-strain curve with the variation in specimen aspect ratio, although compressive yield strength values were within a narrow range. Post compression test scanning electron microscopic observations coupled with the macroscopic observations taken during the test helped in explaining the deviation in specimen behavior and in gathering support for the proposed arguments.     

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.     

Response of sandwich composites with nanophased cores under flexural loading

It is well known that in a sandwich structure, the core plays an important role in enhancing the flexural rigidity and by controlling the failure mechanisms. If the core is made from foam, the strength of the core material and the debond strength at the core–skin interface almost entirely dictate the performance of structural sandwich composites especially under flexure. In this investigation attempts have been made to improve the performance of the sandwich by strengthening the core but partially sacrificing the debond fracture toughness of the sandwich construction. Strengthening of the core has been accomplished by infusing nanoparticles into the parent polymer of the core material when it was in the liquid stage. The core material is polyurethane foam made from polymeric isocyanate (Part A) and reacting with polyol (Part B). Spherical nanoparticles such as TiO₂ of about 29 nm in diameters were dispersed in Part A of liquid polyurethane through an ultrasonic cavitation process. The amount of nanoparticles infused into liquid foam varied from 1 to 3% by weight. Once Part A was doped with nanoparticles, it was mixed with Part B, and was cast in a rectangular mold to produce the nanophased polyurethane foam. The nanophased foam was then used with regular S-2 Glass fiber preforms and SC-15 epoxy to manufacture sandwich composites in a VARTM set up. Test coupons were then extracted from foam as well as from sandwich panels to conduct flexural and various other chemical tests. A parallel set of control panels were also made with neat polyurethane core materials. Thermogravimetric and SEM analyses have indicated that the decomposition temperature of the nanophased foams increases by about 27 °C and the cell size almost doubles with nanoparticle infusion. A significant improvement in flexural strength and stiffness has also been observed with 3% loading of TiO₂ nanoparticles. Debond fracture toughness parameters (G_c) were also determined for both categories of sandwich constructions, and it was seen that nanoparticle infusion reduces the value of G_c by almost a factor of three. Despite this reduction, strength of nanophased sandwich increased by about 53% over the neat system. Details of manufacturing and analyses of test results are included in the paper.     

热固性环氧泡沫材料的研究进展

热固性环氧泡沫材料因具有优异的性能而在夹芯材料领域具有广泛应用,并成为新的研究热点.总结了近年来的新型制备技术、新型结构泡沫材料以及空心微球填充环氧合成泡沫的研究概况,详细概述了环氧泡沫材料的改性研究新进展,包括在泡沫基体中引入新基团/新组分、短切纤维增强、纳米材料改性、橡胶粒子改性等.     

Experimental analysis of the flexural properties of sandwich panels with cellular core materials

This paper addresses the flexural properties of sandwich structures with cellular core materials. Experimental three point bending tests are conducted in order to determine the flexural stiffness and the load‐carrying capacity of these advanced composites. In addition, the significant failure modes after exceeding the load‐carrying capacity are identified. The results of these analyses are compared for sandwich structures containing various core materials. These core materials comprise two aluminium foams, namely M‐Pore® and Alporas®, honeycomb structures and novel metallic hollow sphere structures (MHSS).     

Effect of strain rate and density on dynamic behaviour of syntactic foam

The final objective of this study is to improve the mechanical behaviour of composite sandwich structures under dynamic loading (impact or crash). Cellular materials are often used as core in sandwich structures and their behaviour has a significant influence on the response of the sandwich under impact. Syntactic foams are widely used in many impact-absorbing applications and can be employed as sandwich core. To optimize their mechanical performance requires the characterisation of the foam behaviour at high strain rates and identification of the underlying mechanisms.Mechanical tests were conducted on syntactic foams under quasi-static and high strain rate compression loading. The material behaviour has been determined as a function of two parameters, density and strain rate. These tests were complemented by experiments on a new device installed on a flywheel. This device was designed in order to achieve compression tests on foam at intermediate strain rates. With these test machines, the dynamic compressive behaviour has been evaluated in the strain rate range up [6.7 · 10⁻⁴ s⁻¹, 100 s⁻¹].Impact tests were conducted on syntactic foam plates with varying volume fractions of microspheres and impact conditions. A Design of Experiment tool was employed to identify the influence of the three parameters (microsphere volume fraction, projectile mass and height of fall) on the energy response. Microtomography was employed to visualize in 3D the deformation of the structure of hollow spheres to obtain a better understanding of the micromechanisms involved in energy absorption.     

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.     

Enhancing mechanical and fracture properties of sandwich composites using nanoparticle reinforcement

Polyurethane (PU) foam is reinforced with SiC nanoparticles to develop core materials for sandwich composites. Isocyanate component (Part A) of PU foam was dispersed with SiC nanoparticles, and then mixed with polyol (Part B) to manufacture nanophased core materials. Nanoparticle reinforcement varied from 0.1 to 2.0 wt% of the total polymer. Both pristine and silane functionalized SiC nanoparticles were used in the investigation. Nanophased foams were tested in compression and flexure to determine the mechanical properties. Fracture toughnesses K _IC and G _IC were determined using the SENB test. Sandwich panels were fabricated and tested for face-core debond fracture toughness using the tilted sandwich debond test. The study has revealed that reinforcement of the foam by pristine nanoparticles substantially enhances mechanical properties but degrades fracture toughness. This loss in fracture toughness, however, may be recovered with the use of functionalized nanoparticles. Small concentrations (0.1–0.2 wt%) of functionalized nanoparticles provided large improvement in debond fracture toughness of sandwich specimens.     

Syntactic foams from copolymers based on epoxidized soybean oil

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

Radius ratio effect on high-strain rate properties of syntactic foam composites

The high-strain rate compressive properties of syntactic foams are characterized in this study. This study is performed using a pulse-shaped Split-Hopkinson Pressure Bar technique. Nine different types of syntactic foams are fabricated with the same matrix resin system but three different size microballoons and three different microballoon volume fractions. The microballoons have the same outer radius of 40 μm, but different internal radii leading to a difference in their densities. The volume fractions of the microballoons in the syntactic foams are maintained at 0.1, 0.3, and 0.6. Analysis is carried out on the effect of the microballoon radius ratio at each volume fraction on the high-strain rate properties. This approach is helpful in separating and categorizing the contribution of matrix and microballoons to 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 little or no significant change in both compressive strength and modulus of syntactic foams at all radius ratios when tested at strain rates of 400–500/s compared to quasi-static rates. However, higher dynamic strength and stiffness values are obtained consistently at all radius ratios when tested at 800–1000/s compared to quasi-static values. It is observed that the radius ratio does not affect the syntactic foam properties significantly when tested at the same high-strain rate and volume fraction. Scanning electron microscopy is carried out to understand the fracture modes of the syntactic foams.     

Sea-water effects on foam-cored composite sandwich lay-ups

This article concerns the effects of sea-water on foam cored composite sandwich structures under long-term exposure. Special attention is focused on sea-water induced damage in foam materials, weight gains and expansional strains, as well as on possible degradation in the properties of foam materials due to such extended exposure. In addition, sea-water effects on the fracture behavior of foam materials and on face/core interfacial debonding fracture are investigated experimentally and interpreted by means of computational fracture mechanics.     

环氧树脂复合泡沫材料的压缩力学性能

对空心玻璃微珠填充环氧树脂复合泡沫材料进行了准静态压缩实验, 研究了材料的宏观压缩力学性能, 并提出了弹性模量和屈服强度的预测公式。此外, 对压缩试件的断口进行了宏、细观观察, 研究了材料的压缩破坏机理。结果表明, 复合泡沫材料在压缩过程中, 具有普通泡沫材料的应力-应变曲线的典型特征, 在应变为2 %左右时材料发生屈服, 在应变大于30 %后发生破坏。此外, 材料的杨氏模量和强度均随密度的减小而下降, 预测公式给出的结果与实验值基本一致。压缩试件断口的宏、细观观察表明, 复合泡沫材料主要的破坏形式为剪切引起的弹塑性破坏。      

Damage prediction in composite sandwich panels subjected to low-velocity impact

The paper illustrates the application of a finite element tool for simulating the structural and damage response of foam-based sandwich composites subjected to low-velocity impact. Onset and growth of typical damage modes occurring in the composite skins, such as fibre fracture, matrix cracking and delaminations, were simulated by the use of three-dimensional damage models (for intralaminar damage) and interfacial cohesive laws (for interlaminar damage). The nonlinear behaviour of the foam core was simulated by a crushable foam plasticity model. The FE results were compared with experimental data acquired by impact testing on sandwich panels consisting of carbon/epoxy facesheets bonded to a PVC foam. Good agreement was obtained between predictions and experiments in terms of force histories, force–displacement curves and dissipated energy. The proposed model was also capable of simulating correctly nature and size of impact damage, and of capturing the key features of individual delaminations at different depth locations.     

Modelling of aluminium foam core sandwich panels under impact perforation

Aluminium foam core sandwich panels are good energy absorbers for impact protection applications, such as light-weight structural panels, packing materials and energy absorbing devices. In this study, the high-velocity impact perforation of aluminium foam core sandwich structures was analysed. Sandwich panels with 1100 aluminium face-sheets and closed-cell A356 aluminium alloy foam core were modelled by three-dimensional finite element models. The models were validated with experimental tests by comparing numerical and experimental damage modes, output velocity, ballistic limit and absorbed energy. By this model the influence of foam core and face-sheet thicknesses on the behaviour of the sandwich panel under impact perforation was evaluated.     

The elliptic paraboloid failure criterion for cellular solids and brittle foams

Summary Cellular solids and brittle foams are increasingly finding application in constructions mainly as core materials for loaded sandwich structures where the loading of the structure generates multiaxial stress states on them. It has been established that the principal mechanism of deformation is based on the cell-wall bending and closed-cell as well as open-cell foams present similar stiffnesses. Therefore simple relations are found for their tensile, compressive and shear strengths and their elastic properties.It has been established in this paper that the modes of failure of such materials can be satisfactorily described by the elliptic paraboloid failure criterion for the general orthotropic body. Then, as soon as the yield or failure stresses in simple tension and compression are measured along the three principal stress directions of the material the failure locus is unequivocally defined and all the properties of the material under any complicated load can be accurately established. However, since these materials fail in the compression-compression-compression octant of the principal stress space by elastic buckling, the EPFS-criterion is cut-off by an ellipsoid surface, with intercepts along the principal axes the respective compressive failure stresses.The criterion thus established yields satisfactory results. It has been tested among others in a reticulated vitreous carbon foam as well as in an aluminium foam. The experimental results for these foams existing in the literature are fitting better with this universal criterion than any other considered, thus indicating the validity of the elliptic paraboloid failure criterion also for this type of materials.     

空心微珠/金属基复合泡沫制备方法与吸能性能的研究进展

金属基复合泡沫是由空心微珠和金属基体复合而成的一种新型结构功能多孔复合材料。它具有许多优异的性能,如轻质、高比强度、高比刚度、高吸能能力、隔热、吸声隔音及电磁屏蔽等,高吸能能力是金属基复合泡沫的突出特点,在防撞、减振、缓冲及防爆抗振的汽车、航空航天、军事装备及船舶等领域具有广阔的应用前景。本文对金属基复合泡沫的基体材料、空心微珠填充材料、影响金属基复合泡沫压缩吸能性能的因素及压缩吸能机制进行了概述,重点报道了金属基复合泡沫常用的制备工艺及近年来铝基、镁基、锌基及钢基复合泡沫吸能性能的研究进展,分析了当前研究中存在的一些问题,并对金属基复合泡沫的应用现状作了阐述,最后展望了金属基复合泡沫的研究发展趋势。