Fracture behaviour and fracture toughness of ductile closed-cell metallic foams

Standard fracture mechanics tests were carried out on two different types of aluminium foam, ALPORAS^® foams and ALULIGHT^® foams, with a variety of densities. Standard fracture toughness tests on compact tension (CT) specimens with widths from 50 mm to 300 mm and in situ tests in the scanning electron microscope were performed. Fracture toughness values in terms of the critical stress intensity factor, K_IC, the critical J-integral, J_IC, and the critical crack-tip opening displacement, COD_5,i, were determined. To identify the fracture process, local deformation measurements were performed on the foam surfaces with a digital image processing system.From the deformation measurements it is evident that the deformation is strongly localised on different length scales. A relatively large fracture process zone, 6–8 cells in height, is developed, where only few of them are heavily deformed. On the cell wall level the deformation is again strongly localised to the thinnest parts of the cell wall, where cracks initiate and propagate. The crack propagates through the foam, building many secondary cracks and crack bridges. The comparison of K vs. Δa (crack extension), J vs. Δa and COD vs. Δa with the current fracture processes at the crack tip and the load–displacement response reveals that COD gives the most reliable measured values to characterise the fracture toughness.     

The effect of cooling rate on the structure and properties of closed-cell aluminium foams

The application of different cooling rates as a strategy to enhance the structure of aluminium foams is studied. The potential to influence the level of morphological defects and cell size non-uniformities is investigated. AlSi6Cu4 alloy was foamed through the powder compact route and then solidified, applying three different cooling rates. Foam development was monitored in situ by means of X-ray radioscopy while foaming inside a closed mould. The macro-structure of the foams was analysed in terms of cell size distribution as determined by X-ray tomography. Compression tests were conducted to assess the mechanical performance of the foams and measured properties were correlated with structural features of the foams. Moreover, possible changes in the ductile–brittle nature of deformation with cooling rate were analysed by studying the initial stages of deformation. We observed improvements in the cell size distributions, reduction in micro-porosity and grain size at higher cooling rates, which in turn led to a notable enhancement in compressive strength.     

闭孔泡沫铝的电磁屏蔽性能

采用粉末冶金发泡法制备闭孔泡沫铝,通过调整发泡剂含量、发泡温度、粘度、保温时间等手段,制得孔隙率可调、孔洞分布均匀的闭孔泡沫铝样品,并测试了不同孔隙率、孔径泡沫铝样品的电磁屏蔽性能.结果表明:在100～1000MHz内,泡沫铝的电磁屏蔽性能在60～90dB之间,且随着孔隙率、孔径的增加,泡沫铝的电磁屏蔽性能下降.     

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.     

Toughness of aluminium alloy foams

The fracture behaviour of closed cell aluminium-based foams (trade-name “Alulight”) is characterized for the compositions Al–Mg1–Si0.6 and Al–Mg1–Si10 (wt%), and for a relative density in the range 0.1–0.4. The toughness testing procedures are critically analysed, and the origins of the observed R-curve behaviour for metal foams are explored. A major contribution to the observed increasing crack growth resistance with crack advance is in the development of a crack bridging zone behind the crack tip. The crack bridging response is quantified in terms of a crack traction vs extra displacement curve by performing independent tests on deep notch specimens. The area under the bridging traction vs extra displacement curve from the deep notch tests is approximately equal to the measured initiation toughness J_IC, in support of the crack bridging concept. A line spring model is then used to interpret the fracture response. The effect of material composition and relative density upon the initiation toughness is measured, and the accuracy of an existing micromechanical model for the fracture toughness of a brittle foam is assessed. Finally, the reduction in tensile and compressive strengths due to the presence of an open hole is determined; it is found that the Alulight foams are notch-insensitive, with the net section strength equal to the unnotched strength.     

闭孔泡沫材料3-D几何建模及力学性能分析

为建立闭孔泡沫金属材料泡孔及其孔壁结构形状数字化模型,提出在Voronoi多面体内填充空心椭球(Hollow Ellipsoid Filled in Voronoi Cell,HEFIVC),并以椭球半轴长度及其方位角为变量、胞体质量最小为目标建立优化模型,迭代模拟闭孔泡沫金属材料中气泡的长大过程,成功构建了不同孔隙率的泡沫铝几何模型。通过拟合确定了HEFIVC模型中最小孔壁厚度与孔隙率间的关系。将具有周期性边界的闭孔泡沫材料HEFIVC几何模型导入MSC.MARC有限元软件,模拟分析了低孔隙率泡沫铝的静态压缩力学性能,通过与泡沫铝压缩实验结果对比,验证了该模型的准确性。     

Laser forming of open-cell aluminium foams

Open-cell aluminium foams cannot be formed by conventional mechanical processes because of the occurrence of failure during deformation. In this study, open cell-aluminium foams with different densities were laser bent by means of a diode laser. Laser bending tests were performed with different values of laser power and scan velocity, and were compared with mechanical bending tests. High bending angles were reached for proper combinations of process parameters: 90° bending was obtained by sharing out the total angle among many parallel bending lines. The process efficiency was measured in terms of bending increase per single laser pass: similar efficiencies were observed for the foams with different density at low number of scans, but higher efficiencies were measured for the low density foam at high number of scans because of the lower pre-heating during the process.     

Compressive characteristics of closed-cell aluminum foams with different percentages of Er element

In the present study, closed-cell aluminum foams with different percentages of erbium(Er) element were successfully prepared. The distribution and existence form of erbium(Er) element and its effect on the compressive properties of the foams were investigated. Results show that Er uniformly distributes in the cell walls in the forms of Al3 Er intermetallic compound and Al-Er solid solutions. Compared with commercially pure aluminum foam, Er-containing foams possess higher micro-hardness, compressive strength and energy absorption capacity due to solid solution strengthening and second phase strengthening effects. Additionally, the amount of Er element should be controlled in the range of 0.10 wt.%-0.50 wt.% in order to obtain a good combination of compressive strength and energy absorption properties.     

基于不同材料模型的闭孔泡沫铝疲劳分析

使用真实多孔模型和三种均质材料模型(可压缩泡沫模型、等向强化模型和随动强化模型)对AlSi7闭孔泡沫铝进行疲劳分析.所用三种均质材料模型的数值分析是基于先前在应力比R=0.1的振荡拉伸载荷下进行疲劳测试所得实验结果.计算结果表明,等向强化模型和随动强化模型均适用于分析闭孔泡沫铝的疲劳行为.此外,与等向强化模型相比,随动...     

闭孔泡沫Zn-22Al在单轴准静态加载下的压缩行为(英文)

通过使用氢化物发泡剂,采用熔体发泡法制备闭孔泡沫Zn-Al合金,在准静态条件下研究其压缩性能。在压缩试样过程中,分析发泡材料的结构,并研究其形态和压缩性能之间的关系。结果表明,应力-应变行为具有典型的闭孔泡沫金属和脆性泡沫金属的特征;在平稳阶段的控制变形机制是脆性破碎;泡沫Zn-22Al合金的抗压强度得到了显著提高,其压缩性能符合Gibson-Ashby模型     

Novel strengthening method of closed-cell aluminum foams through surface treatment by resin

Aluminum foams are focused on as a lightweight structural material because of their excellent energy absorbing capacity. However, compressive strength of aluminum foams is much lower than that of dense aluminum. This is due to local buckling of the inhomogeneous cell structure. The authors carried out infiltration of open surface pores with polyester resin because buckling starts at the open surface pores. Compressive tests using commercial aluminum foams show significant increases in compressive strength and absorbed energy. Since the density of resin is not high, the specific compressive strength and specific absorbed energy are also increased.     

Structure and deformation correlation of closed-cell aluminium foam subject to uniaxial compression

We report the results of an experimental and numerical study conducted on a closed-cell aluminium foam that was subjected to uniaxial compression with lateral constraint. X-ray computed tomography was utilized to gain access into the three-dimensional (3-D) structure of the foam and some aspects of the deformation mechanisms. A series of advanced 3-D image analyses are conducted on the 3-D images aimed at characterizing the strain localization regions. We identify the morphological/geometrical features that are responsible for the collapse of the cells and the strain localization. A novel mathematical approach based on a Minkowski tensor analysis along with the mean intercept length technique were utilized to search for signatures of anisotropy across the foam sample and its evolution as a function of loading. Our results show that regions with higher degrees of anisotropy in the undeformed foam have a tendency to initiate the onset of cell collapse. Furthermore, we show that strain hardening occurs predominantly in regions with large cells and high anisotropy. We combine the finite element method with the tomographic images to simulate the mechanical response of the foam. We predict further deformation in regions where the foam is already deformed.     

SiO2包覆CaCO3在无保护气氛下制备闭孔泡沫镁（英文）

在熔体发泡法工艺中,发泡剂的分解速度和浸润性直接影响泡沫金属的孔结构和孔隙率。为减缓泡沫镁发泡剂CaCO3的发泡速度并提高与镁熔体的浸润性,采用非均匀形核法,以硅酸钠为原料,盐酸为酸化剂,在CaCO3表面包覆SiO2钝化膜。采用TGA-DTA、XRD、SEM等方法对包覆后CaCO3的热稳定性和包覆层的微观结构进行分析。结果表明:包覆后的CaCO3分解温度提高;包覆层中的SiO2为无定形态;在CaCO3颗粒表面形成网络状结构。对比实验表明:包覆后的CaCO3发泡速度平稳。同时,采用合金化阻燃工艺在无气体保护条件下制备出较大尺寸的泡沫镁试样,并且试样孔径细小,孔结构均匀,孔隙率在60%-70%。     

Oxidation behaviour of metallic glass foams

In this study, the effects of porosity on the oxidation behaviour of bulk-metallic glasses were investigated. Porous Pd- and Fe-based bulk-metallic glass (BMG) foams and Metglas ribbons were studied. Oxidizing experiments were conducted at 70 °C, and around 80 °C below glass-transition temperatures, (T_gs). Scanning-electron microscopy/energy-dispersive spectroscopy (SEM/EDS) studies revealed little evidence of oxidation at 70 °C. Specimens exhibited greater oxidation at T_g - 80 °C. Oxides were copper-based for Pd-based foams, Fe-, Cr-, and Mo-based for Fe-based foams, and Co-based with borosilicates likely for the Metglas. Pd-based foams demonstrated the best oxidation resistance, followed by Metglas ribbons, followed by Fe-based foams.     

Multi-axial yield behaviour of polymer foams

The yield behaviour of two densities of a ductile PVC foam manufactured by Divinycell have been investigated for a range of axisymmetric compressive and tensile stress states. The yield surface is found to be described adequately by the inner envelope of a quadratic function of mean stress and effective stress and a maximum compressive principal stress criterion. Under tensile loadings the deformation of these foams is governed by cell wall bending, and the uniaxial and hydrostatic tensile strengths have comparable magnitudes. Under compressive loadings the foams deform by elastic buckling of the cell walls, and the uniaxial and hydrostatic compressive strengths are again nearly equal.     

Yielding and post-yield behaviour of closed-cell cellular materials under multiaxial dynamic loading

The paper focuses on characterisation of yielding and post-yield behaviour of metals with closed-cell cellular structure when subjected to multiaxial dynamic loading, considering the influence of the relative density, base material, strain rate and pore gas pressure. Research was conducted by extensive parametric fully-coupled computational simulations using the finite element code LS-DYNA. Results have shown that the macroscopic yield stress of cellular material rises with increase of the relative density, while its dependence on the hydrostatic stress decreases. The yield limit also rises with increase of the strain rate, while the hydrostatic stress influence remains more or less the same at different strain-rates. The macroscopic yield limit of the cellular material is also strongly influenced by the choice of base material since the base materials with higher yield limit contribute also to higher macroscopic yield limit of the cellular material. By increasing the pore gas filler pressure the dependence on hydrostatic stress increases while at the same time the yield surface shifts along the hydrostatic axis in the negative direction. This means that yielding at compression is delayed due to influence of the initial pore pressure and occurs at higher compressive loading, while the opposite is true for tensile loading.     

浸渍法制备泡沫铝的显微组织和力学性能(英文)

利用聚合物泡沫采用压力浸渗铸造工艺制备开孔泡沫铝。所制备的泡沫铝能够很好地复制聚合物泡沫的几何尺寸。开孔泡沫铝的强度比闭孔泡沫铝的低很多,从而得到更多的应用。添加陶瓷颗粒可以改善泡沫铝的力学性能。本研究中,向 AC3A 铝合金中添加 SiC 颗粒得到复合材料泡沫。在复合材料泡沫中,SiC 颗粒嵌入在合金基体中及孔筋表面。高体积分数的陶瓷颗粒使合金泡沫铝的压缩强度、能量吸收、显微硬度增大。这些性能的改善归结为于泡沫铝的结构改变以及 SiC 颗粒存在于结点和孔筋处而引起的强度增加。     

Weld structure of joined aluminium foams with concentrated solar energy

Concentrated solar energy was applied to welded foam plates in non-protective atmosphere. The filler was a pore-generating aluminium–silicon alloy placed between two commercial aluminium foam plates. The heating device provided enough energy to melt and foam the filler. The heat affected surfaces on foam plates and welding mechanisms were correlated with heating conditions. Test plate thickness controlled filler foaming, and two runs were necessary to complete foaming. Weld characterization through tensile tests and microstructural study was performed. The role of the oxide layer on the weld was analyzed and the main welding mechanisms identified: a mechanical form-fit and a metallurgical connection.