Compressive properties of closed-cell aluminum foams with different contents of ceramic microspheres

In this paper, closed-cell aluminum foams with different kinds and contents of ceramic microspheres are obtained using melt-foaming method. The distribution and the effects of the ceramic microspheres on the mechanical properties of aluminum foams are investigated. The results show that both kinds of ceramic microspheres distribute in the foams uniformly with part in the cell wall matrix, some in adhere to the cell wall surface and part embed in the cell wall with portion surface exposed to the pores. Ceramic microspheres have an important effect on the yield strength, mean plateau stress, densification strain and energy absorption capacities of aluminum foams. Meanwhile, the content of ceramic microsphere in aluminum foams should be controlled in order to obtain good combination of compressive strength and energy absorption capacity. The reasons are discussed.     

Compressive properties of closed cell Al alloy–fly ash particle composite foams

Abstract

The closed cell aluminium alloy–fly ash particle composite (Al/FA) foams containing 1·5 wt-% fly ash were manufactured by molten body transitional foaming process. The quasi-static compressive properties of Al/FA have been investigated. Results show the compressive stress–strain curves of Al/FA foams exhibit three regions, i.e. the elastic region, the plastic plateau region and the densification region. A linear relationship between the densification strain and the relative density was obtained. The relation between the plastic collapse stress and the relative density can be described with Gibson and Ashby’s model. The energy absorption capacities of the Al/FA foams gradually increase with increasing strain and relative density.     

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。     

Manufacturing of Al/SiCp composite foams using calcium carbonate as foaming agent

Abstract

The aluminium composite foams reinforced by different volume fractions of SiC particles were manufactured with the direct foaming route of melt using different contents of CaCO₃ foaming agent. The density of produced foams changed from 0·43 to 0·76 g cm^?3. The microstructural features and compressive properties of the Al/SiC_p composite foams were investigated. Compressive stress–strain curve of Al/SiC_p composite foams is not smooth and exhibits some serrations. At the same relative density of composite foams, the plateau stress of the composite foams increases with increasing volume fraction of SiCp and decreasing weight percentage of CaCO₃. The relation between plateau stress, relative density, weight percentage of CaCO₃ and SiCp volume fraction of Al/SiCp composite foams with a given particle size was investigated.     

On the mechanical properties of heat-treated expanded perlite–aluminium syntactic foam

In this paper, a syntactic foam is fabricated by counter-gravity infiltrating packed bed of expanded perlite particles with A356 aluminium alloy. The samples are subjected to a T6 heat treatment. The impact of heat treatment on microstructure characteristics, mechanical properties, deformation behaviour, and cell wall fracture mechanism are investigated. The compression stress–strain curves of the heat treated foams showed the three stages of elasticity, stress plateau and densification. Heat treatment resulted in a significant increase in plateau stress and absorbed energy. It is found that the effect of density on mechanical properties after heat treated conditions is more significant in comparison to untreated conditions. Under compression, the heat treated foams shows more uniform deformation. The improvement in compression characteristics by heat treatment is found to be a result of refined microstructure and higher ductility of the cell walls. Heat treatment reduces the deleterious impact of the columnar dendritic structure of the cell wall and the casting defects on mechanical properties. It limits the crack propagation by increasing the aspect ratio and interparticle distance of the Si particles in the Al matrix.     

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。     

复合泡沫塑料模量和屈服强度的理论预测

基于广义自洽原理，利用四相球模型研究了复合泡沫塑料在拉伸加载下的力学性能，并对其可能发生的破坏进行了分析，发现模型退化后给出的泡沫材料强度预测结果与实验值符合较好。通过分析微珠与基体界面的法向应力集中系数和基体相的von Mises应力分布，可以发现，当微珠壁极薄时，微珠的力学行为与实心柔性粒子相似，随着微珠壁厚的增加，微珠对材料整体力学行为的影响与实心刚性粒子的影响接近相同。通过引入破坏影响因子，对复合泡沫塑料的强度预测进行研究，提出了一种有效的预测方法。     

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.     

Compressive properties of aluminum foam produced by powder-Carbamide spacer route

Effects of cell shape and size, and relative density of aluminum foam on its compressive behavior have been investigated. Aluminum foams were produced via aluminum powder-Carbamide spacer route. The results show that angular cells significantly reduce mechanical properties of the foam. They also indicate that compressive properties of the foams, including plateau stress (σ_pl), densification strain (ε_D), and energy absorption, increase by cell size and relative density of the foams. Experimental results were compared with theoretical predictions; they were fairly corresponded to theoretical conceptions; this arises from near-ideal architecture of the foams with almost spherical cells, in this study. Constant values of C, n and α in theoretical modulus and densification strain equations wear calculated as 1.22, 2.09 and 0.95, respectively. The values indicate compressive behavior approaches to ideal morphology foam via employing spherical space holder.     

Dynamic crushing behavior of 3D closed-cell foams based on Voronoi random model

Dynamic crushing responses of three-dimensional cellular foams are investigated using the Voronoi tessellation technique and the finite element (FE) method. FE models are constructed for such closed-cell foam structures based on Voronoi diagrams. The plateau stress and the densification strain energy are determined using the FE models. The effects of the cell shape irregularity, impact loading, relative density and strain hardening on the deformation mode and the plateau stress are studied. The results indicate that both the plateau stress and the densification strain energy can be improved by increasing the degree of cell shape irregularity. It is also found that the plastic deformation bands appear firstly in the middle of the model based on tetrakaidecahedron at low impact velocities. However, the crushing bands are seen to be randomly distributed in the model based on Voronoi tessellation. At high impact velocities, the “I” shaped deformation mode is clearly observed in all foam structures. Finally, the capacity of foams absorbing energy can be improved by increasing appropriately the degree of cell shape irregularity.     

Si元素对碳纳米管增强铝基复合泡沫组织与性能的影响

针对金属基复合材料,添加合金元素是提升其综合性能的有效途径.本文通过高能球磨和填加造孔剂法,制备了添加Si元素的碳纳米管(CNTs)增强铝基(CNTs/Al-Si)复合泡沫,通过准静态压缩实验测试其压缩性能和吸能性能,进一步研究烧结温度和不同Si元素含量对CNTs/Al-Si复合泡沫微观组织、压缩性能和吸能性能的影响,...     

高应变率加载下复合泡沫塑料的吸能特性及失效机理研究

通过SHPB冲击实验装置对空心玻璃微球填充聚氨酯复合泡沫塑料进行了动态压缩实验,获得了不同密度复合泡沫塑料在高应变率加载条件下的应力-应变曲线,研究了材料的动态力学性能。基于所获得的应力-应变曲线,进一步分析和讨论了复合泡沫塑料的能量吸收特性,发现材料最佳吸能点的包络线是同一直线。此外,通过动态变形试件的扫描电镜分析,还研究了这类新材料的动态失效问题。     

FE modeling of the compressive behavior of porous copper-matrix nanocomposites

Compressive mechanical test and numerical simulation via finite element modeling have been employed on closed-cell copper-matrix nanocomposite foams reinforced by alumina particles. The FE analysis' purpose was to model the foam deformation behavior under compressive loading and to investigate the correlation between material characteristics and the compressive mechanical behavior. Exploring this, several foam samples with different conditions were manufactured and compression test was carried out on the samples. Scanning electron microscopy and image analysis have been performed on the foam samples to obtain the required data for the numerical simulation. The stress–strain curves exhibited plateau stress between 18 and 112.5 MPa and energy absorption in the range of 20.03–51.20 MJ/m³ for the foams with different relative densities. The foams exhibited enhanced mechanical properties to an optimum value, as a consequence of increasing the reinforcing nanoparticles, through both experimental tests and numerical simulation data. Also, the validated model of copper-matrix nanocomposite foams has been used to probe stress distribution in the foams. In addition, the results obtained by numerical simulation via ABAQUS CAE finite element modeling provided support for experimental test results. This confirmed that FEM is a favorable technique for predicting mechanical properties of nanocomposite copper foams.     

Effects of specimen aspect ratio on the compressive properties of Mg alloy foam

Closed-cell AZ31 Mg alloy foams were successfully prepared by melt-foaming method. The effects of specimen aspect ratio (the thickness/width ratio, AR) on the compressive properties of closed-cell Mg alloy foams were investigated systematically. The results showed that the length of stress–strain plateau stage extended and ideality energy absorption efficiency improved with the specimen AR increasing and the yield strength decreased. Specimens with the AR = 1.00 possess good combination of yield strength, plateau stage length and compressive stability when compressed under the experiment conditions.     

Effect of hollow sphere size and size distribution on the quasi-static and high strain rate compressive properties of Al-A380–Al2O3 syntactic foams

Metal matrix syntactic foams are promising materials for energy absorption; however, few studies have examined the effects of hollow sphere dimensions and foam microstructure on the quasi-static and high strain rate properties of the resulting foam. Aluminum alloy A380 syntactic foams containing Al₂O₃ hollow spheres sorted by size and size range were synthesized by a sub-atmospheric pressure infiltration technique. The resulting samples were tested in compression at strain rates ranging from 10^?3 s^?1 using a conventional load frame to 1720 s^?1 using a Split Hopkinson Pressure-bar test apparatus. It is shown that the quasi-static compressive stress–strain curves exhibit distinct deformation events corresponding to initial failure of the foam at the critical resolved shear stress and subsequent failures and densification events until the foam is deformed to full density. The peak strength, plateau strength, and toughness of the foam increases with increasing hollow sphere wall thickness to diameter (t/D) ratio. Since t/D was found to increase with decreasing hollow sphere diameter, the foams produced with smaller spheres showed improved performance. The compressive properties did not show measurable strain rate dependence.     

Compressive Fatigue Behavior of Bovine Cancellous Bone and Bone Analogous Materials under Multi‐Step Loading Conditions

In this study the compressive cyclic behavior of bovine cancellous bone and three open‐cell metallic foams including AlSi7Mg foams (30 and 45 ppi) and CuSn12Ni2 foam (30 ppi) has been investigated. Multi‐step fatigue tests are carried out to study the deformation behavior under increasing compressive cyclic stresses. Short multi‐step tests, with steps of 300–500 cycles, are used to identify the cyclic yield stress (σ_cy) and the stress at failure (σ_fail). The residual strain accumulation, or cyclic creep, is observed during these tests. Long multi‐step tests, with 5000 cycles at selected stress ranges (0.4σ_cy, 0.6σ_cy, 0.8σ_cy, and σ_cy), are also carried out to study further the compressive fatigue behavior of the materials. Scanning electron microscopy (SEM) has been used to characterize the microstructure of the foams and the bone prior to and post mechanical testing. Particular attention is paid to the role of cyclic creep and buckling in the failure processes. The results show that residual strain accumulation seems to be the predominant driving force leading to failure of foams and bones. Although foams and bone fail by the same mechanism of cyclic creep, the deformation behavior at the transient region of each step is different for both materials. The maximum strain ε_max of foams decrease suddenly during the change of each step. This behavior may be explained by the rapidly developing microdamage in the cell struts that occur at the transient region of each step. Bones show more gradual decrease of ε_max, where microdamage may be accumulated progressively during the fatigue test.     

Closed-cell Al alloy composite foams: Production and characterization

Foamy Al alloy SiCp composites of different densities ranging from 0.4 to 0.7 g/cm³ were manufactured by melt-foaming process, which consisted of direct CaCO₃ addition into the molten A356 aluminum bath. Mechanical properties and morphological observations indicated that the three-stage deformation mechanism of typical cellular foams is dominant in the produced A356 aluminum foams. Middle-stage stress plateau shrinkage plus compressive strength and bending stress enhancements were observed in denser foams. With the same Al/SiCp ratio, energy absorption ability and plastic collapse strength of the closed-cell foams were increased with the foam density. Doubling cell-face bending effects resulted in larger compressive than bending strengths in the closed-cell foams; while stiffness lowering was due to the cell-face stretching conditions.     

Innovative processing and mechanical properties of high temperature syntactic foams based on a thermoplastic/thermoset matrix

An innovative processing of syntactic foams based on high-glass transition temperature thermoplastic is reported. The aim is to propose an insulating material able to withstand both continuous effluent temperature up to 150°C and hydrostatic pressure up to 300 bar. Uniaxial compression and tension tests have been performed. Two factors seems to govern the mechanical properties: the wall thickness/radius ratio (e/r) and the volume fraction of microspheres. At room temperature, the study of the strain recovery at the yield stress shows that the plastic deformation is negligible compared to the elastic and anelastic part. The study of the yielding behavior at room temperature has shown a large influence of the type of microsphere compared to the influence of the volume fraction. The syntactic foams exhibit good performance at 150°C as expected from the matrix selection. Comparison with a few classical models have been done.     

Effect of Porosity and Cell Size on the Dynamic Compressive Properties of Aluminum Alloy Foams

The dynamic mechanical properties of open-cell aluminum alloy foams with different relative densities and cell sizes have been investigated by compressive tests.The strain rates varied from 700 s^-1 to 2600 s^-1.The experimental results showed that the dynamic compressive stress-strain curves exhibited a typical three-stage behavior:elastic,plateau and densification.The dynamic compressive strength of foams is affected not only by the relative density but also by the strain rate and cell size.Aluminum alloy foams with higher relative density or smaller cell size are more sensitive to the strain rate than foams with lower relative density or larger cell size.     

Modified Method for Dynamic Stress‐Strain Curve Determination of Closed‐Cell Foams

The paper presents a modified method for the non‐linear dynamic stress‐strain curve determination with the use of cushion curves provided by the manufacturers of closed‐cell foams. This method consists of two main stages: stage 1 – the cushion curves transformation to the coordinate system of dynamic stress‐energy density, combined with the cubic b‐spline curve approximation, and stage 2 – development of the dynamic stress‐strain curve considering that the area under the curve is equal to the energy density of a collision. The stress‐strain curves are one of the basic parameters of the foam material models used to describe the collision process, in the modelling environments of finite element method (FEM), e.g. LS‐DYNA. Performed verification consisting in the reconstruction of cushion curves using LS‐DYNA showed that the proposed method of the dynamic stress‐strain curves determination is efficient and effective. Taking into account that access to the stress‐strain curves is hindered (cushioning material manufacturers do not publish them), and the access to charts of cushion curves is easier, the proposed method for determining the stress‐strain curves can relevantly shorten the process of preparing the input data, which are required during modelling of spatial structures using FEM. Copyright © 2016 John Wiley & Sons, Ltd.