The Temperature Effect on the Compressive Behavior of Closed-Cell Aluminum-Alloy Foams

In this research, the mechanical behavior of closed-cell aluminum (Al)-alloy foams was investigated at different temperatures in the range of 25-450 °C. The main mechanical properties of porous Al-alloy foams are affected by the testing temperature, and they decrease with the increase in the temperature during uniaxial compression. From both the constant/serrated character of stress–strain curves and macro/microstructural morphology of deformed cellular structure, it was found that Al foams present a transition temperature from brittle to ductile behavior around 192 °C. Due to the softening of the cellular structure at higher temperatures, linear correlations of the stress amplitude and that of the absorbed energy with the temperature were proposed. Also, it was observed that the presence of inherent defects like micropores in the foam cell walls induced further local stress concentration which weakens the cellular structure’s strength and crack propagation and cell-wall plastic deformation are the dominant collapse mechanisms. Finally, an energy absorption study was performed and an optimum temperature was proposed.     

胞状AlCu5Mn合金泡沫的压缩性能和能量吸收特性

用熔体发泡法制备孔隙率为51.5%～90.5%、孔结构均匀的胞状铝合金(AlCu5Mn),研究其孔结构、压缩性能、能量吸收能力、能量吸收效率和吸能性能.结果表明:胞状铝合金孔结构由高孔隙率(88.8%)时的大孔径、多边形孔向低孔隙率(62.5%)时的小孔径、球形孔孔结构过渡,其压缩应力(σ)-应变(ε)曲线具有线性变形阶段、屈服平台阶段和致密化阶段三个部分,由线性变形阶段进入屈服平台阶段所对应的ε_s值介于2%～9%之间;屈服强度σ_s~*随着孔隙率的增大而下降,在孔隙率相同的条件下,胞状铝合金的力学性能优于胞状铝和多孔铝合金,其比刚度高于钢;当应变为定值时,胞状铝合金单位体积和单位质量的压缩吸能能力(C和C_m)都随着孔隙率的升高而降低,但是孔隙率在73.5%～82.1%范围内时,其C_m与ε的关系几乎不随孔隙率的改变而改变;对于孔隙率为51.5%～90.5%的胞状铝合金,它们的吸能效率的峰值都大于80%.胞状铝合金的C-σ和C_m-σ关系可以表征其吸能性能,从而可以根据实际工况选择作为减振吸能材料的胞状铝合金的最佳孔结构.     

Study of thermal–mechanical properties of polyurethane foam and the three-dimensional shape of molded bra cups

Molded bras nowadays are dominating the overall bra market place, while the bra cups are mostly made of polyurethane foam sheets. The smooth and seamless inner surface of a molded bra cup gives three-dimensional nice fit to the wearer and provides unlimited designs for different softness and thickness. However, it has been a difficult question to determine the optimum molding conditions for various types of foam and there was no reliable method to measure the cup shape conformity. In this study, the properties of five polyurethane foams were investigated by thermal–mechanical analysis (TMA) and their 3D shapes formed in various molding conditions were measured by a Steinbichler Comet scanner and a new parameterization-based remesh algorithm method. The results revealed that the optimal temperature and dwell time for molding bra cups were greatly affected by the thermal–mechanical properties of polyurethane foams. The softening temperature and deformation properties of the foams tested by TMA can facilitate the determination of optimal molding temperature for the desirable cup shape and thickness. This study provides an effective and quantitative approach to eliminate the time-consuming “trial-and-error” in the molding tests traditionally being used in the industry.     

Assessment of Multiaxial Mechanical Response of Rigid Polyurethane Foams

Multiaxial deformation behavior and failure surface of rigid polyurethane foams were determined using standard experimental facilities. Two commercial foams of different densities were assayed under uniaxial, biaxial, and triaxial stress states. These different stress states were reached in a uniaxial universal testing machine using suitable testing configurations which imply the use of special grips and lateral restricted samples. Actual strains were monitored with a video extensometer. Polyurethane foams exhibited typical isotropic brittle behavior, except under compressive loads where the response turned out to be ductile. A general failure surface in the stress space which accounts for density effects could be successfully generated. All of failure data, determined at the loss of linear elasticity point, collapsed in a single locus defined as the combination of a brittle crushing of closed-cell cellular materials criterion capped by an elastic buckling criterion.     

Compressive properties of aluminum foams by gas injection method

The compressive properties of aluminum foams by gas injection method are investigated under both quasi-static and dynamic compressive loads in this paper.The experimental results indicate that the defo...     

The partition of elastic strain energy in solid foams and lattice structures

Rapid advances in additive manufacturing techniques promise that, in the near future, the fabrication of functional cellular structures will be achieved with the desired cellular microstructures tailored to specific applications. It is therefore essential to develop a detailed understanding of the relationship between macroscopic mechanical response and cellular microstructure. The present study reports on the results of a series of computational experiments that explore the effect of topology and microstructural irregularity (or non-periodicity) on deformation modes of cellular structures under both uniaxial and biaxial stress states. A simple quantitative technique based on the partition of elastic strain energy into bending and stretch components is used to identify the distribution of deformation modes at a microstructural level. The relationship between nodal connectivity, morphological regularity and deformation modes is then explored through their influence on biaxial yield surfaces as obtained from finite element analyses.     

Creep-buckling of cellular solids

Honeycombs and foams, loaded at low temperatures (T<0.3 T_m) deform by elastic deformation, elastic buckling and plastic collapse of their cell wall. At more elevated temperatures, creep contributes to the deformation, which becomes time dependant. Deformation by the creep-bending of cell walls has been analysed in some detail, but deformation by creep-buckling has not. In this paper we analyse for the first time the collapse of cellular structures by creep-buckling, deriving the critical load and time required for the buckling event to commence.     

Plasticity and damage in cellular amorphous metals

Compressive mechanical properties of low-density, open-cell Zr-based bulk metallic glass foams processed by the salt replication method are investigated as a function of relative density and pore size in the ranges 14–28% and 150–355 μm, respectively. Scaling behaviors for strength and stiffness are discussed in the context of models developed for conventional metal foams, and appropriate modifications presented where necessary. Deformation and damage features not addressed by such models are then discussed in terms of the unique conditions allowing ductility in amorphous metal foams. It is shown that despite a small number of brittle uniaxial strut failures, ductile deformation by strut bending predominates in the foams, with the result that all but the densest foam could be compressed to a nominal strain in the vicinity of 80% without macroscopic fracture.     

Effects of strut geometry and pore fraction on creep properties of cellular materials

A set of analytical models based on engineering beam analysis is developed to predict creep behavior of cellular materials over a broad range of relative density. Model predictions, which take into account the presence of mass at strut nodes and consider different possible deformation mechanisms and foam architectures, are compared to experimental creep results for a replicated nickel-base foam and a reticulated aluminum foam. As porosity decreases, the controlling creep mechanism in the foams changes from strut bending, to strut shearing, and ultimately to strut compression.     

石膏型渗流制备泡沫铝填充圆管压缩行为研究

采用石膏型渗流制备开孔泡沫铝并填充到薄壁圆管,制成泡沫铝夹心管。通过准静态压缩试验研究了泡沫铝夹心管的压缩行为。结果表明：采用石膏型渗流法制备的泡沫铝孔隙率在85%左右,其压缩变形阶段可分为弹性段、塑性平台段和致密化段;空心圆管的压缩行为与其本身的结构参数有关;泡沫铝夹心管的力学性能与吸能能力比空心圆管和泡沫铝有了一定的提高,且石膏型渗流法所制泡沫铝夹心管质量较轻。     

Effect of Ce content on mechanical properties of Ce/Cr coated open-cell Ni–Cr–Fe alloy foams

The Ce/Cr coating was homogenously deposited onto the reticulated open-cell Ni–Cr–Fe alloy foam by the pack cementation process. The mechanical properties of the Ce/Cr coated alloy foams were investigated by the quasi-static compression test. Simultaneously, the deformation and failure mechanisms of Ce/Cr coated alloy foams were discussed. The results show that the adding amount of CeO₂ powders influences the mechanical properties of the Ce/Cr coated alloy foams. Despite an increase in density as compared to the uncoated foams, the Ce/Cr coated foams exhibit improvement in both yield strength and energy-absorption performance. Especially, the energy-absorption performance of 2% Ce/Cr (mass fraction) coated alloy foam is averagely 1.9 times as high as that of the bare Ni–Cr–Fe alloy foam. In addition, the mechanical properties of the Ce/Cr coated alloy foams increase with the increase of strain rate. The distortion and cracking are mainly the deformation behavior of the Ce/Cr coated alloy foam, confirmed by SEM images.     

Joining stainless steel metal foams

Abstract

Metallic foams produced from stainless steel are one of the most recently developed ultralightweight materials. These foams have very low densities and high energy absorption capacities and are therefore expected to have widespread applications in the manufacture of ultralightweight structural components. The fabrication of load bearing structural components such as implants, and high temperature air or fluid filters, are potential application areas depending on the form of the cell structure of the foam. Closed cell metal foams are typically suitable for structural uses whereas open cell foams tend to be preferred for functional applications. Development of adequate joining technologies for these materials is an essential step for their widespread industrial utilisation. The present paper describes a brazing method that is capable of providing excellent joints between 316 stainless steel foams and a conventional 316 stainless steel bulk alloy. Having optimised the bonding conditions and using a Cu–Ti alloy as the filler metal, bonds between a foam and a bulk alloy were produced. No apparent plastic deformation of the metal foam occurred in the course of the 10 min length brazing process, and the resulting bonds had tensile strengths higher than that of the stainless steel foam.     

Thermal transport and fire retardance properties of cellular aluminium alloys

Closed-cell aluminium alloy foams exhibit exceptional resistance to fire. It is unclear why this happens, although the protection imparted by oxide Al₂O₃ layers has been suggested. This work attempts to uncover the thermal transport processes in metallic foams. The apparent thermal conductivities of two-dimensional foams having a variety of cellular microstructures are first calculated. These include regular honeycombs, Voronoi structures and Johnson–Mehl models. The effects of several types of geometric imperfection—Plateau borders, cell-edge misalignments, fractured cell edges, missing cells, inclusions and cell size variations—are studied by using analytical as well as finite element methods. The focus is on metallic foams where the transport of heat is dominated by solid conduction and thermal radiation; contributions from gaseous conduction and convection are neglected. The coupling of solid conduction with thermal radiation is dealt with by using the method of finite elements. These results are then applied to solve the transient temperature field of a cellular metal plate subjected to a sudden introduction of a high-temperature source of heat such as fire. The factors which dictate the thermal and structural fire retardance of cellular metallic foams are identified.     

泡沫铝的制备方法及应用进展

概述了泡沫铝的各种制备方法研究进展。根据制备过程中铝的状态可以将制备方法分为三类：固相法、液相法、电沉积法。液态铝能够通过直接注入气体、加入发泡剂或生成过饱和固一气共晶体的方法制得泡沫铝,间接方法包括熔模铸造法和渗流铸造法。如果往铝粉末压块中加入发泡剂,通过加热使发泡剂分解同样能得到泡沫铝。类似的方法还包括粉浆烧结法、散粉烧结法等。最后描述了泡沫铝的结构和优良性能,并对泡沫铝在各领域的应用进行了概括和展望。     

Toughening Effects through Optimizing Cell Structure and Deformation Behaviors of Al-Mg Foams

As widely used protective materials, the application of Al foams is still limited by their low intrinsic mechanical properties caused by the brittleness of struts. The introduction of Mg is recently demonstrated effective to improve the mechanical performance of Al foams; however, the mechanism of Mg modification is still not clear. In this work, Al-Mg foams are developed through a powder metallurgy process with excellent compression performance and high energy absorption capacity. The effects of Mg modification on the cell structure, toughness, and deformation behavior are investigated systematically. As a result, the small cell size of ~ 1.8 mm and the high sphericity of 0.92 are achieved with 5% of Mg addition, delivering high compression stress of 8.5 ± 0.43 MPa and energy absorption capacity of 6.9 ± 0.36 MJ/m³, simultaneously. The synergistic mechanism for the improved mechanical performance is also demonstrated to be the combination of stress transfer and plastic deformation behavior of cells. The results provide a new strategy to develop high-performance foam materials by improving toughness and further promote the practical application.     

Quantitative characterisation of low-density,high performance polymeric foams using high resolution X-ray computed tomography and laser confocal microscopy

Advanced polymeric foams are used in a wide range of industrial and R&D applications. Their properties strongly depend upon the cell structure. Traditionally, their microstructure has been studied using optical or electron microscopy, limiting the investigations to sample’s surface only. To overcome this shortcoming, the use of X-ray micro-tomography imaging is here adopted to allow for a complete 3D, non-destructive analysis of advanced polymeric foams. This work brings to fruition high resolution and low-contrast imaging techniques to perform quantitative analyses onto polymeric foams, where the low contrast of reconstructed structural features might hamper accurate quantitative analyses. Local structural features of individual cells are statistically evaluated, showing the key role of the achieved resolution.     

Compressive and energy absorption properties of closed-cell magnesium foams

The quasi-static compressive mechanical behavior and deformation mechanism of closed-cell magnesium foams were studied, and the effects of the density of magnesium foams on the compressive and energy absorption properties were also discussed. The results show that the compressive process of closed-cell magnesium foams is characterized by three deformation stages: linear elastic stage, collapsing stage and densification stage. At the linear elastic stage, the peak compressive strength (σ ₀) and Young’s modulus (E ₀) increase as the density increases. Magnesium foams can absorb energy at the collapsing stage. In a certain strain range, the energy absorption capacity also increases as the density of magnesium foams increases.     

The effect of cell wall microstructure on the deformation and fracture of aluminium-based foams

This study primarily concerns the role of cell wall microstructure in influencing the mechanical behaviour of metallic foams. Three closed-cell foams have been examined, having rather similar relative densities and cell structures but significant differences in cell wall microstructure. It is concluded that these differences can substantially affect the micro-mechanisms of deformation and failure under different types of loading and can also have an influence on the macroscopic mechanical response. Cell wall ductility and toughness are impaired by high volume fractions of coarse eutectic, fine oxide films and large brittle particles, all of which were present in one or more of the foams studied. This impairment can lead to extensive brittle fracture of cell walls, with little energy absorption, even under nominally compressive loading conditions. The influence of cell wall ductility tends to become more significant when the loading state is such that local tensile stresses are generated.     

脆性材料磨粒加工的纳米尺度去除机理

以共价键或离子键结合的脆性单晶、多晶和光学玻璃是能源、通信、交通和医疗领域新兴微电子和光电器件的核心材料。为满足高性能器件的制造需求,脆性材料通常需要经过磨削、研磨、抛光等超精密磨粒加工,获得具有原子级光滑的表面、近无损伤的亚表面和微米甚至纳米级的加工精度。优化磨粒加工工艺不仅可以有效地提高加工效率,降低制造成本,还能够延长脆性材料元器件的服役寿命,但开发高效率、低损伤超精密磨粒加工技术需深入理解脆性材料纳米尺度的去除机理。本文基于划擦力学原理,揭示脆性材料纳米尺度磨粒加工去除的本质,阐明磨粒加工过程中脆性材料脆性–塑性转变去除的基本原理,概述单磨粒纳米划擦脆性材料的形变和去除机制,以及磨粒加工过程中脆性材料的去除机理及材料微观结构对其局部变形及后续去除的影响规律,提出实现脆性材料高效延性加工的控制策略,有助于推动脆性材料超精密磨粒加工技术的进一步发展。      

Deformation behaviour of closed-cell aluminium foams in tension

The mechanical behaviour of commercially available ALPORAS aluminium foam with two different densities was studied under tension loading. In addition to the common stress–strain measurements, local deformation, notch-opening displacement and damage evolution were determined. The deformation characteristics deviated from those observed in aluminium foams under compression. No deformation bands or plastic instabilities could be observed in tension, which are very frequent in compression of metallic foams. Four regimes were evident in the stress–strain curves and deformation maps: the linear elastic regime, the plastic regime with no significant crack initiation and propagation, the regime of formation of a fracture process zone and, finally, the regime of fracture, where a main crack propagates through the specimen and leads to failure. The fracture strain was only a few per cent, with the higher-density foam showing a larger fracture strain, and the plastic Poisson's ratio was about 0.35. The notched specimens showed increasing fracture strengths in terms of the net section stress with increasing notch depth. It is suggested that a change in stress state, caused by a non-vanishing Poisson's ratio, in front of the notch tip creates an increase of the fracture strength similar to the behaviour in ductile bulk metals.