相对密度对泡沫铝力学性能和能量吸收性能的影响

对不同相对密度的两种胞孔结构--开孔和闭孔泡沫铝进行了单轴压缩试验,研究了相对密度对泡沫铝力学性能和能量吸收性能的影响.结果表明:随着相对密度的增大,泡沫铝的屈服强度与流动应力也相应增加,通过对本实验结果进行拟合,得出泡沫铝的屈服强度与相对密度的关系式.泡沫铝材料吸收的能量随着应变量的增大而增加,在相同应变量下,高密度开孔泡沫铝的吸收能比低密度闭孔材料多.吸能效率反映材料本身的一种属性,高的理想吸能效率表明泡沫铝是一种优良的吸能材料.     

Polymer foams for personal protection: cushions, shoes and helmets

Three areas, where polymer foam products are used in personal protection, are reviewed to contrast the foam micromechanisms and the use of Finite Element Analysis (FEA) for engineering design. For flexible open-cell foams used for seating cushions, the main deformation mechanism is cell edge bending; regular cell models can predict much of the compressive response. Hyperelastic FEA models can then predict the forces for foam indentation. For flexible closed-cell foams used in shoe midsoles, cell air compression dominates the response; diffusive air loss leads to foam deterioration with use. Hyperelastic FEA models can predict the interaction between the foam and the heelpad. Finally, for rigid closed-cell foams used in helmets, the permanent stretching and wrinkling of cell faces dominates the response. Crushable foam FEA models, which consider the yield surface and hardening, predict different responses for impacts on the road and on a kerbstone.     

纳米纤维增强闭孔泡沫材料的力学性能

为研究纳米纤维增强闭孔泡沫材料的力学性能,采用Voronoi随机泡沫模型对闭孔泡沫材料的细观几何结构进行模拟,并将纳米纤维随机分布在泡沫材料的胞壁中,利用改进的自动搜索耦合（ASC）技术将纤维单元与基体单元进行耦合,建立了能够反映纳米纤维增强闭孔泡沫材料细观结构的数值模型。在此基础上,进一步研究了泡沫模型随机度、相对密度以及纳米纤维长径比和质量分数对纳米纤维增强闭孔泡沫材料弹性模量与屈服强度的影响规律。结果表明:由所建立的数值模型得到的纳米纤维增强闭孔泡沫材料的弹性模量和屈服强度与实验值吻合较好;提高泡沫模型的随机度会使复合泡沫材料的弹性模量和屈服强度增加,而当随机度达到0.450以后,材料的弹性模量和屈服强度几乎不再发生变化;当相对密度在0.05~0.30范围内变化时,复合泡沫材料的弹性模量与屈服强度几乎随相对密度的增加呈线性增长;提高纳米纤维长径比和质量分数也会使材料的弹性模量和屈服强度得到提高,但当纤维长径比达到500以后,纤维长径比的增强作用逐渐减弱。所得结论对纳米纤维增强闭孔泡沫材料的制备具有重要意义。      

Effect of cell shape anisotropy on the compressive behavior of closed-cell aluminum foams

The closed-cell Al–Si foams have been prepared by molten body transitional foaming process using TiH₂ foaming agent. The cell shape anisotropy ratio of specimens with various relative densities was measured. The quasi-static compressive behavior of Al–Si foams in both longitudinal and transverse directions were investigated. The results show that Al–Si foam loaded in the transverse direction exhibits a lower stress drop ratio. The relationship between plastic collapse stress ratio and cell shape anisotropy is in accordance with Gibson and Ashby model. The plastic collapse stress and the energy absorption property of Al–Si foams increase following power law relationship with relative density. Moreover, Al–Si foams exhibit higher plastic collapse stress and the energy absorption property in the longitudinal direction than that in the transverse direction.     

Prediction of the effect of artificial aging heat treatment on the yield strength of an open-cell aluminum foam

The effect of artificial aging on the compression yield strength of an open-cell AA6101 foam is studied using both experimental and modeling approaches. Isothermal calorimetry is used to analyze the precipitation kinetics of the foam. The modeling work combines the established approaches for predicting the yield strength of open-cell metallic foams as a function of the relative density and normalized strength, as well as the age hardening behavior of AA6101 alloy. The foam yield strength is related to the evolution of precipitate content during aging and is modeled for artificial aging at 180 and 220 °C. It is shown that the model predictions match very well with the experimentally determined yield strength values. The overall results suggest that the presented analytical and modeling approaches can effectively be used to predict the precipitation hardening behavior and/or optimize processing and properties of AA6101 foams.     

开孔泡沫金属热传输性能研究进展

介绍了开孔泡沫金属的结构特点,分别从紧凑型换热器、相变储能系统、热管以及催化反应器四个方面的应用分析了国内外开孔泡沫金属热传输性能应用研究现状,指出了泡沫金属在这些领域应用的前景和存在的问题,提出了今后开孔泡沫金属热传输性能的研究及应用方向。     

Investigation on the influence of cell shape anisotropy on the mechanical performance of closed cell aluminium foams using micro-computed tomography

The mechanical behaviour of closed-cell aluminium foams made by both powder metallurgy (LKR) and liquid state (Hydro) processes is investigated. Hydro foams exhibit a significant anisotropy in their mechanical behaviour. The transverse direction stands out as the most favourable one in terms of strength. In contrast, LKR foams show an almost isotropic compressive behaviour. Both foams perform at a level far below the theoretical predictions. The reduced values are a result of imperfections and defects in the cellular microstructure. X-ray microfocus computed tomography (μCT) is therefore used for internal investigation of the foam cell structure. 2D and 3D quantitative image analyses have been performed on μCT images to characterise the morphometric parameters of the foams. The main parameters of interest are cell size, cell size distribution and cell features information. A preferred cell orientation in Hydro foams is observed along the normal and the transverse directions of the specimen. This cell shape anisotropy is quantified using the dimensions of the three axes of the equivalent ellipsoids. The orientation of the cells is well characterised by pole figures of the three axes of equivalent ellipsoids. The influence of this geometrical anisotropy on the mechanical behaviour of the foam is discussed.     

Acoustic properties of closed-cell aluminum foams with different macrostructures

As structural materials, closed-cell aluminum foams possess obvious advantages in product dimension, strength and process economics compared with open cell aluminum foams. However, as a kind of structure-function integration materials, the application of closed-cell aluminum foams has been restricted greatly in acoustic fields due to the difficulty of sound wave penetration. It was reported that closed-cell foams with macrostructures have important effect on the propagation of sound waves. To date, the relationship between macrostructures and acoustic properties of commercially pure closedcell aluminum foams is ambiguous. In this work, different perforation and air gap types were designed for changing the macrostructures of the foam. Meanwhile, the effect of macrostructures on the sound absorption coefficient and sound reduction index were investigated. The results showed that the foams with half-hole exhibited excellent sound absorption and sound insulation behaviors in high frequency range(2500 Hz). In addition, specimens with air gaps showed good sound absorption properties in low frequency compared with the foams without air gaps. Based on the experiment results, propagation structural models of sound waves in commercially pure closed-cell aluminum foams with different macrostructures were built and the influence of macrostructures on acoustic properties was discussed.     

Effects of nanoparticle impregnation of polyurethane foam core on the performance of sandwich beams

Polyurethane foam core has been considered for sandwich design due to its high stiffness and toughness. One of the methods of potential improvement of the foam is impregnating it with stiff nanoparticles, increasing its stiffness and collapse strength. In this paper we use the Mori–Tanaka and the self-consistent methods to estimate an increase in the stiffness of closed-cell and open-cell foams impregnated by stiff nanoparticles that do not affect foam geometry. Subsequently, the effect of the impregnation of the foam core on the response of a sandwich beam is demonstrated. The response characteristics considered in the paper include bending deflections in a three-point test, global buckling and wrinkling loads, and the effect of the foam impregnation on its compressive collapse stress associated with local instability of the elements of the foam. Recognizing that the introduction of nanoparticles in the foam material creates local stress concentrations around the inclusions we also examine the local strength of the impregnated foam using the classical Goodier solution for dilute particles and the Mori–Tanaka based stress analysis for a finite particle concentration. In addition, we demonstrate the approach to the evaluation of the resilience of impregnated foam utilizing the dilute approach. While numerical examples concentrate on polyurethane foam, the methodologies and conclusions can be extrapolated to any polymeric foam material reinforced by nanoparticles.     

Effect of homogenizing heat treatment on the compressive properties of closed-cell Mg alloy foams

Closed-cell AZ31 Mg alloy foams were successfully prepared by melt-foaming method. Homogenizing heat treatment was applied on the foams and the effects of heat treatment on compressive properties of closed-cell Mg alloy foams were investigated systematically. The results showed that homogenizing heat treatment enhanced the compressive properties in terms of yield strength, mean plateau strength, available energy absorption capacity and ideality energy absorption efficiency of the foams. In addition, homogenizing heat treatment greatly reduced the stress drop rates of the foams. Specimens homogenized at the temperature of 753 K for 24 h possessed good combination of yield strength, compressive stability, available energy absorption capacity and ideality energy absorption efficiency under the present experiment conditions. And the reasons were discussed.     

Conductive properties of foam materials with open or closed cells

The work is devoted to computer simulations of the effective conductive properties of open and closed-cell foam materials. The conductivity of the solid phase of the foam is assumed to be much larger than the one of the filler. For the calculation of the effective conductivity, a complex cell element of the foam that consists of a typical cell and its nearest neighbors is introduced. This element is embedded in the medium with the conductivity of the solid phase. For the calculation of the field and field flux in the complex cell, 3D-integral equations for the fields in heterogenous media are used. The effective conductivity is the coefficient that relates the average field and the field flux in the central part of the complex cell. The method is applied to the calculation of the effective conductivity of open cell foams with various shapes of ligaments. Transition from open to closed-cell foams is considered. Predictions of the method are compared with the experimental data available in the literature.     

开孔与闭孔泡沫铝的压缩力学行为

研究了开孔与闭孔两种胞孔结构不同、制备工艺不同的泡沫铝在准静态压缩载荷下的压缩响应曲线.结果表明:开孔与闭孔泡沫铝压缩应力-应变曲线均具有多孔泡沫材料明显的三阶段特征,即线弹性段、塑性屈服平台段及致密段;相对密度对泡沫材料的力学性能(如杨氏模量、屈服强度)有很大影响;在准静态下,开孔泡沫铝表现出明显的应变率效应,而闭孔泡沫不如开孔敏感;泡沫铝材料表现为弱的各向异性;胞孔结构影响两种泡沫材料的压缩响应曲线.     

Mechanical properties of open-cell foam synthetic thoracic vertebrae

This study presents comprehensive morphological and mechanical properties (static, dynamic) of open-cell rigid foams (Pacific Research Laboratories Inc. Vashon, WA) and a synthetic vertebral body derived from each of the foams. Synthetic vertebrae were comprised of a cylindrical open-cell foam core enclosed by a fiberglass resin cortex. The open-cell rigid foam was shown to have similar morphology and porosity as human vertebral cancellous bone, and exhibited a crush or fracture consolidation band typical of open-celled materials and cancellous bone. However, the foam material density was 40% lower than natural cancellous bone resulting in a lower compressive apparent strength and apparent modulus in comparison to human bone. During cyclic, mean compression fatigue tests, the synthetic vertebrae exhibited an initial apparent modulus, progressive modulus reduction, strain accumulation and S-N curve behaviour similar to human and animal vertebral cancellous bone. Synthetic open-cell foam vertebrae offer researchers an alternative to human vertebral bone for static and dynamic biomechanical experiments, including studies examining the effects of cement injection. Presented, in part, at the XXth Congress of the International Society of Biomechanics and 29th Annual Meeting of the American Society of Biomechanics, Cleveland, OH, July 31-August 5, 2005     

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.     

Finite element modelling of the mechanism of deformation and failure in metallic thin-walled hollow spheres under dynamic compression

Recent interest in lightweight metallic hollow sphere foams for aerospace applications requires a better physical understanding of dynamic properties of single spheres. Finite element modelling supported by high rate experiments was developed to investigate the underlying deformation and failure mechanisms of electrodeposited nickel thin-walled hollow spheres. Parametric simulation was performed to further explore the effect of sphere geometry (wall thickness to diameter ratio) and loading rate. It was found that decreasing the ratio of wall thickness to diameter tends to transit the side wall failure mode from bending to buckling. For a thin-walled sphere (the thickness to diameter ratio less than a critical value), the macroscopic dynamic behaviour is primarily dominated by the two deformation and failure mechanisms: (1) buckling failures of wall materials and (2) self-contacts of wall surfaces and wall-anvil contacts. At higher impact velocity (greater than a critical velocity), inertia effect due to dynamic localisation of wall crushing arises and significantly influences the deformation/failure mode of the sphere, resulting in an increased initial crushing strength and asymmetric deformation. Finally, the behaviour of hollow spheres was correlated to explore the power law behaviour of bulk foams with respect to the relative density; it was found that metallic thin-walled hollow sphere foams can be better approximated as open-cell rather than closed-cell foams.     

高温处理后闭孔泡沫铝压缩性能及变形机理

目的 探究温度和孔隙率对闭孔泡沫铝材料压缩力学性能和变形机理的影响。方法 将孔隙率为84.3%～87.3%的泡沫铝试件在温度25～700 ℃内进行加热处理,对处理后的试样开展准静态压缩实验。结果 在准静态压缩条件下,闭孔泡沫铝材料在不同温度加热处理后的压缩应力–应变曲线均经历了3个阶段:弹性阶段、塑性平台阶段和密实阶段。孔隙率从87.3%减小到84.3%时,其弹性模量增大了44.4 MPa,屈服强度增大了0.39 MPa,平台应力增大了0.94 MPa。孔隙率为84.3%的泡沫铝,在25 ℃时,其弹性模量为141.4 MPa、屈服强度为4.25 MPa、平台应力为4.75 MPa;当加热温度为500 ℃时,弹性模量减小到了128.0 MPa、屈服强度减小到了4.22 MPa、平台应力减小到了4.51 MPa。结论 泡沫铝的弹性模量、抗压屈服强度和平台应力均随孔隙率的增加而减小;加热温度低于500 ℃以下时,泡沫铝材料力学性能变化很小,但屈服强度和弹性模量均小幅度降低;在压缩载荷下,泡沫铝的变形破坏模式呈现出先从试件铝基体较薄弱部分产生孔壁塑性变形、孔洞坍塌,并逐渐出现断裂压缩带,直至泡沫铝孔洞完全坍塌密实。     

防锈闭孔泡沫Al-Mg-Re基合金的制备与性能

在熔体发泡法制备工艺基础上,引入合金化阻燃技术,制备了Al-Mg-Re基防锈闭孔泡沫铝合金.实验结果表明,熔体发泡前同时加入Mg、Ca和稀土制备的防锈闭孔泡沫Al-Mg-Re基合金孔结构均匀,由于Mg和稀土元素的双重作用,Al-Mg-Re基防锈闭孔泡沫铝合金具有优异的耐腐蚀性能.     

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.     

Experimental Investigation on Steel Foams Fabricated by Sintering-Dissolution Process

This article describes a new process to manufacture open-cell steel foams. Calcium chloride anhydrous is used as a space holder. By changing the values of the main manufacturing parameters such as volume percentage, and the size and shape of the space holder, we produce different steel foam samples which cover a wide range of solid fraction, pore size, and shape. The effects of space-holder content and sintering condition such as temperature and time on the porosity of steel foam samples are discussed. The microstructure and composition of steel foam samples are observed and analyzed by scanning electron microscope and X-ray diffraction. The compressive curves of steel foams are measured by a universal testing machine. The experiment results show the compressive strength of steel foam samples with porosities between 65% and 85% is in the range of 66.4 ～ 12.9 MPa. The compressive strength depends mainly on the porosity and pore shape. The absorbed energy per unit volume (W) of steel foams with porosities between 85% and 65% is in range of 6.8 ～ 31.2 MJ/m³. Under the condition of identical porosity, the absorbed energy per unit volume (W) of steel foam is about three times of aluminum foam. In compression, steel foam specimens show heterogeneous macroscopic deformation.     

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。