Failure modes in open-cell foams with hollow struts

Failure modes in open-cell foams with hollow struts are analyzed. As the walls of the struts become thin, the failure mode of the foam can change from plastic collapse of struts loaded in bending to plastic collapse of struts loaded in direct compression. For very thin strut walls an elastic strut wrinkling failure mode is possible. Expressions for the failure modes are derived, and a failure mode map is presented to clarify when the various modes are expected. The analysis suggests that a hollow strut foam can be substantially stronger than a solid strut foam with the same relative density.     

开孔泡沫金属压缩实化特性研究

对开孔泡沫金属微结构实化过程进行分析,理论分析表明开孔泡沫金属的实化应变不但与其相对密度成幂律关系,而且还与泡沫金属的单元结构特征密切相关.由此提出了一种用于描述相对密度较低的开孔泡沫金属压缩实化特性的数学模型,在该模型中泡沫结构和筋条材料对泡沫金属实化特性的影响以材料常数的形式体现,理论分析结果与试验结果完全一致.     

High-frequency tortuosity relaxation in open-cell foams

Propagation of ultrasounds through open-cell polymeric foams is studied using air-coupled ultrasound and Fourier spectral analysis (both phase and magnitude) in the frequency range 0.1 to 6 MHz. A detailed micrographic study is first performed to determine struts dimensions and cell geometry, hence, a unit cell model to describe these foams is proposed. Ultrasound phase velocity and transmission loss were then measured. Variation of these magnitudes with the frequency follows the shape of a sigmoid growth. This behavior of the phase velocity can be explained by introducing an apparent tortuosity with a relaxation-like behavior, which can be explained by considering a probabilistic tortuous walk, as it has recently been performed for other kind of foams. However and unlike in previous studies, the present one shows the whole transition of this sigmoid growth for all studied foams. This is achieved by a precise selection of the foam samples and by the fabrication of new air-coupled transducers that enlarge the experimental working frequency range to lower frequencies. The study of the measured sigmoid growth is used to determine the probability function, required by the probabilistic tortuous walk model, which best describes the variation of the apparent tortuosity.     

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.     

Effect of structure on the creep of open-cell nickel foams

A method of constructing the deformation mechanism maps of open-cell foams is proposed. Starting with the deformation behaviour of the constitutive material and taking account of foam geometry, we are able to determine the dominant deformation mechanisms in the foam as a function of the temperature and the applied-stress. The influences of cell-size, cell-strut shape and scales in the microstructure are studied. The model is applied successfully to the experimental results available for open-cell pure nickel foams.     

微结构对泡沫材料蠕变性能的影响

针对较低密度开孔泡沫的正四面体模型,通过引入支柱结点处的体积修正使新模型能够用于预测较大密度范围内的泡沫材料的蠕变性能,并且基于该修正模型,分析了斜支柱的弯曲变形机制以及剪切变形机制对蠕变应变率的影响。结果表明:当泡沫材料的相对密度较低时,支柱的弯曲变形机制决定了其蠕变速率;而当相对密度较高时,支柱的剪切变形作用机制开始主导其蠕变速率。通过与实验结果的比较验证了本文预测的有效性。      

Effect of Cell Morphology and Heat Treatment on Compressive Properties of Aluminum Foams

In this article, the compressive behavior and anisotropy of both open- and closed-cell aluminum foams under different heat treatments were examined. For the closed-cell A356/SiCp foam, due to the age-hardening effect, the yield strength of the heat-treated specimens was found to be more than 200% of that of the as-cast specimens. The yield strength of the foam in the transverse direction was however only slightly higher than that in the longitudinal direction, which may be related to the relatively spherical cell structure of the foam. For open-cell Al6061 foams, heat treatment results in a significant increase in yield strength and also changes the failure mode from ductile to brittle. The open-cell foam further demonstrates a strong anisotropy. The causes of such phenomena are discussed in the article.     

Open-cell cavity-integrated injection-molded acoustic polypropylene foams

In this study, a commercially available foam injection-molding machine was enhanced with a mold opening technique to produce polypropylene open-cell acoustic foams. Gas counter-pressure was used to improve the cell morphology and uniformity of the injection-molded foams. Their structure and thickness were controlled by applying different degrees of mold opening. The sample structure, the cell morphology, and the acoustic behavior of the foams were characterized. A foamed structure with an open-cell content of 67% and an expansion ratio of 4.6 was obtained when the mold was opened by 4.5 mm. Although further opening of the mold did not significantly increase the open-cell content, it triggered crack creation in the middle of the foams, where the creation of cavities was also facilitated. The injection-molded foams with a cavity and a high open-cell content, presented remarkable acoustic properties: a peak absorption coefficient of 0.95 was observed for foam with a 73% open-cell content and a 9 mm cavity. An automated system was also developed to perforate the acoustic foams, and the acoustic properties of foams both with and without perforation were studied. While perforating the foams widened their absorption coefficient frequency spectrum, it did not improve their transmission loss.     

Modeling the mode I fracture toughness of anisotropic low-density rigid PUR and PIR foams

Low-density foams have to possess a sufficient resistance to cracking in order to ensure the mechanical integrity of foam materials in service, even when not intended for load-bearing applications. In this study, mode I fracture toughness in the foam rise direction has been experimentally characterized for anisotropic rigid commercial polyurethane foams as well as for polyisocyanurate foams produced using polyols derived from rapeseed oil and filled with a montmorillonite nanoclay. Rectangular parallelepiped unit-cell based scaling relations expressing foam toughness via its relative density, cell dimensions, geometrical anisotropy, and the ultimate tensile stress of the base polymer have been employed for prediction of foam toughness. Assuming a brittle fracture of foam struts, a conservative estimate of toughness is obtained. It is demonstrated that considering the yielding of foam struts at the crack front as the criterion of crack extension provides a closer estimate of foam toughness.     

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.     

Statistical properties of microcracking in polyurethane foams under tensile test, influence of temperature and density

We report tensile failure experiments on polyurethane (PU) foams. Experiments have been performed by imposing a constant strain rate. We work on heterogeneous materials for whom the failure does not occur suddenly and can develop as a multistep process through a succession of microcracks that end at pores. The acoustic energy and the waiting times between acoustic events follow power-law distributions. This remains true while the foam density is varied. However, experiments at low temperatures (PU foams more brittle) have not yielded power-laws for the waiting times. The cumulative acoustic energy has no power-law divergence at the proximity of the failure point which is qualitatively in agreement with other experiments done at imposed strain. We notice a plateau in cumulative acoustic energy that seems to occur when a single crack starts to propagate.     

Single steel strut mechanical testing: challenges and future research directions

Open-cell stainless steel foams, composed of hollow struts, are excellent candidates for energy absorption and thermo-mechanical applications. The basic mechanical element responsible for the function of these foams is the single strut. However, testing and characterisation of single foam struts to predict the foam strength have stirred up a new debate about approaches to micro-tensile testing of such elements. In this paper, we present a protocol for in-situ micro-tensile testing of hollow steel struts using a custom-made grip system. The adapted grips make it possible to perform analysis of the deformation of multiple sintered struts. Here we present and discuss challenges encountered during such micro-tensile testing of hollow steel struts.     

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     

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.     

The wet Kelvin model for air flow through open-cell polyurethane foams

Computational Fluid Dynamics (CFD) was used to compute the air-flow permeability K for laminar flow, for wet Kelvin foam models, as a function of cell size and cell face hole size. The predictions were compared with experimental data for a range of open-cell polyurethane (PU) foams. This suggests that the foam permeability is a function of the area of largest hole in the cells. The predictions are almost the same as those for dry Kelvin foams, showing that the face hole size and cell size are the main factors that determines foam permeability.     

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.     

Preparation of silicon carbide foams using polymeric precursor solutions

A simple method was developed to produce silicon carbide foams using polysilane polymeric precursors. Polyurethane foams were immersed in polysilane precursor solutions to prepare pre-foams. Subsequently, these were heated in nitrogen at different temperatures in the range of 900°C to 1300°C. The silicon carbide foams produced in this manner showed well-defined open-cell structures and the struts in the foams were free of voids. The shrinkage which accompanies pyrolysis of the pre-foams was reduced with increasing the concentration of the polymeric precursor solutions.     

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.     

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。     

Pore connectivity of aluminium foams: effect of production parameters

This work studies the effect of some production parameters on the pore connectivity grade (i.e., the open-cell content associated cracks and missing cell walls) of aluminium foams produced via powder metallurgy route. Two types of precursors, extruded and hot uniaxially compressed, were used to create a varied group of Al–Si and Al–Si-Mg alloy-based foams in a wide porosity range. The cellular structure and defects were characterized by gas pycnometry and X-ray tomography. The analysis performed points to a high pore connectivity in all foam specimens, despite these materials are classified as closed celled due to their appearance, and a significant dependence on all the parameters varied. These dependences and the related mechanisms are discussed in the paper in terms of (i) the dissimilar foam evolution at initial stages (effect of precursor processing technique), (ii) the solidification shrinkage of each alloy (effect of composition) and (iii) the cell wall thinning (effect of foam porosity and local drainage). In addition, it has been observed that the interconnections are preferably located in the central parts of the samples, thus suggesting the possible effect of the cooling conditions on defect generation.