Study on quasi-static compressive properties of aluminum foam-epoxy resin composite structures

Closed cell aluminum foam (AF) has extensive application prospects due to its extended plateau stress region and high energy absorption capacity. As one of the most important manufacturing routes for aluminum foams, the gas injection method still does not guarantee an excellent energy absorption performance. In order to improve the energy absorption capacity while remaining the plateau region extended, epoxy resin (ER) was infiltrated into the aluminum foams in various composite forms. In this paper, different AF-ER composite structures were designed and their uniaxial quasi-static compressive behaviors were investigated. The experimental results indicate that the plateau stress and energy absorption capability of the AF-ER composite structures increase with increasing amount of epoxy resin. Additionally, both the stress fluctuation and the peak stress in the plateau region become insignificant, which is beneficial for energy absorption applications. The composite form is also confirmed to have great effect on the compressive property of the AF-ER composite structures. At last, the Young's modulus of the composite structure is theoretically deduced while the plateau stress and the energy absorption capacity are fitted with the composite parameters by considering the contribution of aluminum foam, epoxy resin and the reciprocity of these two materials. The present model is found to have good agreement with experimental data.     

Strain-rate effects in Ni/Al composite metal foams from quasi-static to low-velocity impact behaviour

Metal foams are used as absorbers for kinetic energy but predominantly, they have only been investigated under quasi-static load-conditions. Coating of open-cell metal foams improves the mechanical properties by forming of Ni/Al hybrid foam composites. The properties are governed by the microstructure, the strut material and geometry. In this study, the strain-rate effects in open-cell aluminium foams and new Ni/Al composite foams are investigated by quasi-static compression tests and low-velocity impact. For the first time, drop weight tests are reported on open-cell metal foams, especially Ni/Al composite foams. Furthermore, size-effects were evaluated. The microstructural deformation mechanism was analysed using a high-speed camera and digital image correlation. Whereas pure aluminium foams are only strain-rate sensitive in the plastic collapse stress, Ni/Al foams show a general strain-rate sensitivity based on microinertia effects and the rate-sensitive nano-nickel coating. Ni/Al foams are superior to aluminium foams and to artificial aluminium foams with equal density.     

The energy-absorbing characteristics of composite tube-reinforced foam structures

This paper reports the findings of a research study investigating the energy-absorbing characteristics of polymer foams reinforced with small carbon fibre reinforced epoxy tubes. Initial attention focuses on establishing the influence of tube diameter on the specific energy absorption (SEA) characteristics of the chamfered CFRP tubes. Here, it is shown that the SEA of the tubes increases rapidly with decreasing diameter/thickness ratio, with the highest values being close to 93 kJ/kg. Similar tests were conducted at dynamic rates of strain, where it was observed that the measured values of SEA were lower than the corresponding quasi-static data, possibly due to rate-sensitive effects in the delamination resistance of the composite material. In the next stage of the investigation, the composite tubes were embedded in a range of polymer foams in order to establish the influence of both tube arrangement and foam density on the crush behaviour of these lightweight structures. In addition, a limited number of blast tests have been undertaken on structures based on these core materials. Here, extensive crushing of the composite tubes was again observed, suggesting that these structures should be capable of absorbing significant energy when subjected to this severe loading condition. Finally, the results of these tests are compared with previously-published data from studies on a range of different cores materials. Here, it has been shown that the energy-absorbing characteristics of these systems exceed values associated with other core materials, such as aluminium honeycombs, polymer honeycombs and metal foams.     

Aluminium foam–polymer composites: processing and characteristics

Dissemination of closed cell metal foam unique properties (low density, efficient energy absorption, high vibration/sound attenuation) in real life products has often been difficult to realise. With advanced pore morphology (APM) aluminium foam–polymer hybrids a new and simplified process route targeted at application in foam-filled structures (e.g. automotive A-pillar) has been introduced. APM foams are made from spherical, small volume foam elements joined to each other in a separate process step. Joining the aluminium foam elements by adhesive bonding delivers composite foam with approximately 80–95 wt.% aluminium foam and 5–20 wt.% adhesive (polymer). Setting up cellular structures from spherical foam elements allows for automatic part production, good pore morphology control and cost effective aluminium foam application. An automated production line is displayed and discussed. Mechanical properties of APM aluminium foam–polymer hybrids are similar to other closed cell aluminium foams. Integration of APM foams in profiles resulted in significantly improved properties as observed for conventional closed cell aluminium foam fillings. The unique properties of APM composite foams make them an attractive alternative as a cost effective and easily applicable material of construction with targeted uses such as energy absorbing reinforcement of composite structures.     

Characterisation of aluminium matrix syntactic foams under drop weight impact

It is a challenging task to develop a lightweight, and at the same time, strong material with high energy absorption for applications in military vehicles, which are able to withstand impact and blast with minimum injury to occupants. This paper presents a study on aluminium matrix syntactic foams as a possible core material for a protection system on military vehicles. Experimental work was first carried out which covers sample preparation through pressure infiltration and impact tests on aluminium matrix syntactic foams manufactured. Numerical models were then developed using commercial finite element code ABAQUS/Explicit to simulate the dynamic behaviour of the foam. The effect of strain rate on their compressive behaviour was investigated as these properties are vital in terms of the applications of these materials. Characterisation of the foam behaviour under low velocity impact loading and an identification of the underlying failure mechanisms were also carried out to evaluate the effective mechanical performance. It was found that samples subjected to drop weight impact offered a 20–30% higher plateau stresses than those of the samples subjected to quasi-static compression loading. The degree of correlation between the numerical simulations and the experimental results has been shown to be reasonably good.     

漂珠聚氨酯复合泡沫力学特性及本构模型研究

以粉煤灰漂珠为主要组分的复合泡沫具有较高的比强度和比吸能,在轻质抗冲击结构设计和缓冲防护领域极具应用潜力。然而,漂珠尺寸和增强相等因素对材料力学性能和行为的影响机制尚不清楚,且当前研究尚未构建该类复合泡沫的力学模型,不利于开展结构设计中材料选型和数值仿真等工作。为此,该研究针对漂珠尺寸和蜂窝铝增强相对复合泡沫的力学性能和变形行为的影响规律进行系列准静态压缩实验研究,在此基础上采用Avalle理论构建该复合泡沫的力学模型。结果表明:①当相对密度小于0.29时,漂珠尺寸对复合泡沫的力学性能几乎没有影响;当相对密度大于0.29时,漂珠尺寸对复合泡沫力学性能的影响随密度的增大而增大;②对于含增强相的复合泡沫,含小尺寸漂珠的复合泡沫力学性能有明显提高,铝蜂窝的额外增强效果对包含小尺寸漂珠的复合泡沫更为明显,该增强机制主要是将材料的初始失效模式由剪切转变为轴向压溃;③使用Avalle理论构建的本构模型,其应力平台阶段和能量耗散特性的拟合与实验结果一致,可较为准确地预测该材料的基本力学性能。该研究可为粉煤灰的综合利用及其复合泡沫在轻质抗冲击结构设计中的应用提供理论参考和基本预测模型。     

Aluminium foams for transport industry

Foamed materials are widespread in transportation industry applications. While polymeric foams have been applied for many years foamed metals are now beginning to move into the focus of interest. A powder metallurgical method which allows the production of aluminium foams with porosity levels up to 90% is described. The foams typically have closed pores and densities ranging from 0.4 to 1 g cm⁻³, so that this foamed metals float on water. The unique mechanical properties of metal foams are described. The density dependence of metal foam properties is shown with the Young's modulus, flexural strength and compression strength as examples. A non-linear dependency of these properties on the density is found and discussed. The discussion then focuses on the energy absorption properties of aluminium foams and tools to select appropriate foams for a given energy absorption task.     

Deformation characteristics of metal foams

The deformation behaviour of a series of aluminium and zinc foams was investigated by uniaxial testing. Because the deformation behaviour of metal foams is expected to be anisotropic owing to the existence of a closed outer skin and with respect to the foaming direction, a series of measurements was carried out where the orientation of the outer skin and the foaming direction were varied. Stress–strain diagrams and corresponding compression strengths were determined for aluminium- and zinc-based foams. The influence of an age-hardening heat treatment was investigated. Finally, the axial deformation behaviour of aluminium tubes filled with aluminium foam was tested under uniaxial loading conditions. The results of the measurements are discussed in the context of possible applications of metal foams as energy absorbers. © 1998 Chapman & Hall     

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.     

基体性能对泡沫铝力学行为的影响

用渗流法制备了不同基体的开孔泡沫铝,利用MTS810和SHPB研究了其准静态和动态力学性能。实验结果表明,泡沫铝基体的性能对泡沫铝材料的力学行为有显著的影响。准静态压缩时脆性泡沫有非常长而平缓的屈服平台区,韧性泡沫的屈服段的应力随着应变的增加而缓慢增加。脆性泡沫的吸能效果总体优于韧性泡沫。     

The multiaxial yield behaviour of an aluminium alloy foam

The multi-axial yield behaviour of the aluminium alloy foam Alulight has been measured. Triaxial tests have been performed on a range of relative densities in order to compare the hydrostatic stress versus strain response with the uniaxial compressive response, and to probe the yield surface after prior hydrostatic compression. It is found that the degree of strain hardening in hydrostatic compression exceeds that for uniaxial compression, and the yield surface remains almost self-similar in shape after hydrostatic compaction. The measured yield surface provides support for the phenomenological yield model of Deshpande and Fleck (V. S. Deshpande, N. A. Fleck, Journal of Mechanics and Physics of Solids, 48, (2000), 1253). Upon reviewing the available experimental evidence from this and previous studies it is found that a broad correlation emerges between the relative density and the shape of the yield surface for metallic foams.     

A study of fabricating and compressive properties of cellular Al–Si (355.0) foam using TiH2

Al–Si (355.0) alloy foam has been produced by Alporas method (in which foam alloy melts, and titanium hydride is used as a blowing agent). Mechanical behavior such as quasi-static compression (strain–stress curves, energy absorption capacity), also the effects of thermal properties on the macroscopic structure of the produced foam were investigated. In addition, the effect of energy absorption capacity on percentage porosity has also been studied. The research shows that the produced foam with an average cell size and proper distribution has a more mechanical stability compared to the foams with no such characteristics. It was found that yield strength tends to increase from 12.51 MPa for porosity 74.0% to 22.32 MPa for porosity 54.0%. This foam has also been compared with other foams such as Al-pure foam and Mg foam. It can be stated that Al–Si (355.0) foam has a higher yield strength in comparison to Al-pure foam and Mg foam.     

Piezoresistive and compression resistance relaxation behavior of water blown carbon nanotube/polyurethane composite foam

The in-situ bulk polycondensation process in combination with a ball milling dispersion process was used to prepare the water blown multiwall carbon nanotubes (CNT)/polyurethane (PU) composite foam. The mechanical properties, piezoresistive properties, strain sensitivity, stress and resistance relaxation behaviors of the composite foams were investigated. The results show that the CNT/PU composite foam has a better compression strength than the unfilled polyurethane foams and a negative pressure coefficient behavior under uniaxial compression. The resistance response of CNT/PU nanocomposites foam under cyclic compressive loading was quite stable. The nanocomposite foam containing a weight fraction of carbon nanotubes close to the percolation threshold presents the largest strain sensitivity for the resistance. The characteristic of resistance relaxation of CNT/PU composite foam is different from the stress relaxation due to the different relaxation mechanism. During compressive stress relaxation, the CNT/PU foam composites have excellent resistance recoverability while poor stress recoverability.     

泡沫铝硅合金动态压缩力学性能及吸能特性研究

利用分离式霍普金森压杆研究了泡沫铝硅合金的动态压缩力学性能,得到了应变率为1400s-1~2500s-1的动态应力-应变曲线,且与准静态压缩实验结果进行了对比,分析了应变率对泡沫铝硅材料压缩强度和吸能特性的影响。动态压缩实验过程中,针对泡沫铝硅合金的低阻抗特点,采用LC4铝压杆和半导体应变片改进了测试装置和方法,保证了实验结果的可靠性。结果表明:应变率对泡沫铝硅合金的流动应力有着明显的影响,其流动应力随着应变率的增大而增大;由于惯性效应和胞孔的坍塌,在弹性极限处应力出现波动,且波动应力随应变率的增大而增大。该文还讨论了泡沫铝硅合金在不同应变率下的吸能效率。     

Characterization of close-celled cellular aluminum alloys

The deformation behaviour of two different types of aluminium alloy foam are studied under tension, compression, shear and hydrostatic pressure. Foams having closed cells are processed via batch casting, whereas foams with semi-open cells are processed by negative pressure infiltration. The influence of relative foam density, cell structure and cell orientation on the stiffness and strength of foams is studied; the deformation mechanisms are analysed by using video imaging and SEM (scanning electronic microscope). The measured dependence of stiffness and strength upon relative foam density are compared with analytical predictions. The measured stress versus strain curves along different loading paths are compared with predictions from a phenomenological constitutive model. It is found that the deformations of both types of foams are dominated by cell wall bending, attributed to various process induced imperfections in the cellualr structure. The closed cell foam is found to be isotropic, whereas the semi-open cell foam shows strong anisotropy.     

Aluminium foams as a filler for leading edges: Improvements in the mechanical behaviour under bird strike impact tests

The use of aluminium foams as filler materials in aeronautical leading edges is investigated. Particularly, the improvement of the mechanical behaviour of the filled structure respect to the hollow one is analysed by means of standard bird strike impact tests. For this purpose, a collection of AlSi₁₀ foams were fabricated using the powder metallurgical route (PM), and introduced into leading edges profiles, maintaining or reducing the total weight of the composite structure (leading edge + aluminium foam) in comparison with the original one (hollow structure). Bird strike impact tests were carried out in both types of structures, comparing the global deformation and total load transferred in the tests. The results showed that the composite structure, a 13% lighter than the original one, showed four times better behaviour in terms of global deformation and an improvement of two times in the transmitted load.     

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.     

Adaptation of density distributions for optimising aluminium foam structures

Abstract

Metallic foams show some potential for being produced with controlled spatial variations in their density. This suggests employing them as graded materials in space filling lightweight structures designed in analogy to cortical bone, a natural cellular material, that displays increased density in regions of high loading. In the present study the influence of the mechanical properties of aluminium foams on the results of an optimisation of the foam density distribution with regard to structural strength and stiffness was examined. Regression formulae for the relationships between stiffness and strength of metallic foams on one hand and effective density on the other hand can be fitted to the results of uniaxial compression tests of a certain brand of metallic foam. These results and additional assumptions such as overall isotropy and a yield surface suitable for cellular materials can be implemented into a finite element program adapted for performing stiffness or strength optimisation on the basis of a density adaptation similar to the remodelling of bone. Some applications are presented that show how foams with gradients in the apparent density may be employed to obtain optimal structural behaviour for classical design problems.     

泡沫铝相对密度对泡沫铝-聚氨酯复合材料吸能性能的影响

目的通过准静态压缩试验研究泡沫铝-聚氨酯复合材料(AF-PU)的压缩性能,并利用试验数据探究泡沫铝(AF)相对密度对整体材料吸能性能的影响规律。方法首先使用相关仪器和材料制备AF-PU复合材料,其次用相关仪器对其进行准静态压缩试验。结果 AF-PU复合材料通过相关仪器进行准静态压缩试验,计算得到相对应的应力-应变曲线,同时通过计算得到相对应的吸能-应变曲线。当泡沫铝相对密度从5.6%增加到6.7%时,整体复合材料的屈服强度提升了22.18%。在压缩过程中,当复合材料的压缩应变为0.8时,整体复合材料的总吸能增加了70.08%。结论 AF-PU复合材料的吸能性能随着AF相对密度的增加而增强。     

负泊松比三明治结构填充泡沫混凝土的面内压缩性能

为改善负泊松比三明治结构的受压破坏模式且提高其缓冲吸能能力,提出一种填充泡沫混凝土的新型复合三明治结构。在负泊松比结构中填充不同密度(409 kg/m3、575 kg/m3、848 kg/m3、1 014 kg/m3)的泡沫混凝土得到负泊松比填充结构,并对无填充负泊松比结构、负泊松比填充结构和泡沫混凝土对照试块在准静态压缩下的破坏模式和吸能特性进行比较。根据荷载-位移关系和破坏模式得到以下结论:当填充物密度较小时,负泊松比填充结构能够将填充物的泊松比限制在较小的数值,胞元表现出内凹的变形模式,结构发生逐渐被压实的压缩破坏;当填充物密度较大时,结构发生“X”型剪切破坏,塑性铰区域和剪切带附近的胞壁发生断裂破坏;泡沫混凝土填充物的密度越大,填充结构的压实应变越小,吸收的能量越多,但当填充物密度超过一定值后,填充物密度的增加对负泊松比填充结构能量吸收能力的提升作用不再明显,结构的比吸能降低。