Fracture Mechanisms And Size Effects Of Brittle Metallic Foams: In Situ Compression Tests Inside Sem

In situ compressive tests on specially designed small samples made from brittle metallic foams were accomplished in a loading device equipped in the scanning electron microscopy (SEM). Each of the small samples comprises only several cells in the effective test zone (ETZ), with one major cell in the middle. In such a system one can not only obtain sequential collapse-process images of a single cell and its cell walls with high resolution, but also correlate the detailed failure behaviour of the cell walls with the stress-strain response, therefore reveal the mechanisms of energy absorption in the mesoscopic scale. Meanwhile, the stress-strain behaviour is quite different from that of bulk foams in dimensions of enough large, indicating a strong size effect. According to the in situ observations, four failure modes in the cell-wall level were summarized, and these modes account for the mesoscopic mechanisms of energy absorption. Paralleled compression tests on bulk samples were also carried out, and it is found that both fracturing of a single cell and developing of fracture bands are defect-directed or weakness-directed processes. The mechanical properties of the brittle aluminum foams obtained from the present tests agree well with the size effect model for ductile cellular solids proposed by Onck et al. (C) 2008 Elsevier Ltd. All rights reserved.

Mechanics of metallic foams: A fractal approach

A fractal approach was proposed to investigate the meso structures and size effect of metallic foams: For a series At foams of different relative densities, the information dimension method was applied to measure meso structures. The generalized sierpinski carpet was introduced to map the meso structures of the foam according to specific dimension. The results show that the fractal-based model can not only reveal the variation of yield strength with specimen size, but also bridge the meso structures and mechanical proper-ties of Al foams directly. Key words: metallic foams; fractal; size effect; meso structures

Optimum hot electron production with low-density foams for laser fusion by fast ignition

We propose a foam cone-in-shell target design aiming at optimum hot electron production for the fast ignition. A thin low-density foam is proposed to cover the inner tip of a gold cone inserted in a fuel shell. An intense laser is then focused on the foam to generate hot electrons for the fast ignition. Element experiments demonstrate increased laser energy coupling efficiency into hot electrons without increasing the electron temperature and beam divergence with foam coated targets in comparison with solid targets. This may enhance the laser energy deposition in the compressed fuel plasma.

SBS as template to synthesize 3D siliceous mesostructured cellular foams

A mesostructured cellular foam (MCF) with three-dimensional (313) disordered strutlike structure is prepared by using triblock copolymer (poly(styrene-b-butadiene-b-styrene), SBS, M-W = 140K) as template under strong acid conditions. It is the first report to use triblock copolymer with both hydrophobic head and tail groups instead of hydrophilic head and hydrophobic tail copolymers to synthesize siliceous mesostructured cellular foams. The resulted materials have high pore volume (0.92 cm(3)/g) and relatively narrow pore size distributions with a large pore size of 7.9 nm, which will allow for the fixation of large active complexes, reduce diffusional restriction of reactants and enable reactions involving bulky molecules to take place, especially.

The Interaction Between Shock Waves and Foam in a Shock Tube

An experimental study and a numerical simulation were conducted to investigate the mechanical and thermodynamic processes involved in the interaction between shock waves and low density foam. The experiment was done in a stainless shock tube (80mm in inner diameter, 10mm in wall thickness and 5360mm in length). The velocities of the incident and reflected compression waves in the foam were measured by using piezo-ceramic pressure sensors. The end-wall peak pressure behind the reflected wave in the foam was measured by using a crystal piezoelectric sensor. It is suggested that the high end-wall pressure may be caused by a rapid contact between the foam and the end-wall surface. Both open-cell and closed-cell foams with different length and density were tested. Through comparing the numerical and experimental end-wall pressure, the permeability coefficients a and 0 are quantitatively determined.

A porous bulk metallic glass with unidirectional opening pores

Porous Zr-based bulk metallic glass (PMG) with unidirectional opening pores is prepared by electrochemical etching of tungsten wires of the W/bulk metallic glass (BMG) composites. The porosity and pore size can be controlled by adjusting the tungsten wires. The PMG showed no measurable loss in thermal stability as compared to the monolithic Zr-based BMG by water quenching and is more ductile and softer than the pore-free counterpart. The specific surface area of the PMGs is calculated to be 0.65, 3.96, and 10.54 m(2)/kg for 20, 60, and 80 vol % porosity, respectively. (c) 2007 The Electrochemical Society.

基于原位观测的泡沫金属细观与宏观压缩实验研究

采用原位观测的方法研究了脆性泡沫铝材料在压缩载荷下细观与宏观断裂破坏规律和吸能机理。针对多孔泡沫金属材料提出一种细观原位加载实验方法，采用特别设计与制备的试件，在S570扫描电镜下研究了特定胞孔在压缩过程中孔壁的失效顺序和破坏规律，并揭示了能量吸收的细观机理。对块体材料的宏观压缩实验表明，脆性泡沫铝是以多个断裂带的形式破坏。研究发现，孔壁缺陷和胞孔形态缺陷是诱发断裂带形成与发展的重要因素。依据尺寸效应对细观与宏观实验下泡沫铝的性能进行了比较。

基于分形的泡沫金属细观结构与尺寸效应研究

提出采用分形理论对泡沫金属的细现结构及尺寸效应进行研究的方法.针对一系列具有不同相对密度和细观结构的泡沫铝,证明了其细观结构在一定尺度内符合分形特征,比较了小岛分维、计盒分维和信息分维等算法对泡沫金属分形表征的适用性,分析了细观结构特征对分维的影响.结合推广的sierpinski垫片模型研究了泡沫铝的屈服强度与分维的联系,建立了泡沫铝屈服强度的尺寸效应模型.研究结果表明,由于引入了表征细现结构特征的分形维数,该模型除能表征屈服强度随试样尺寸的变化规律外,还在一定程度上直接反映了泡沫金属细观结构特征对力学性能的影响.

Pool boiling heat transfer of ultra-light copper foam with open cells

High speed visualizations and thermal performance studies of pool boiling heat transfer on copper foam covers were performed at atmospheric pressure, with the heating surface area of 12.0 mm by 12.0 mm, using acetone as the working fluid. The foam covers have ppi (pores per inch) from 30 to 90, cover thickness from 2.0 to 5.0 mm, and porosity of 0.88 and 0.95. The surface superheats are from -20 to 190 K, and the heat fluxes reach 140 W/cm(2). The 30 and 60 ppi foam covers show the periodic single bubble generation and departure pattern at low surface superheats. With continuous increases in surface superheats, they show the periodic bubble coalescence and/or re-coalescence pattern. Cage bubbles were observed to be those with liquid filled inside and vented to the pool liquid. For the 90 ppi foam covers, the bubble coalescence takes place at low surface superheats. At moderate or large surface superheats, vapor fragments continuously escape to the pool liquid. Boiling curves of copper foams show three distinct regions. Region I and II are those of natural convection heat transfer, and nucleate boiling heat transfer for all the foam covers. Region III is that of either a resistance to vapor release for the 30 and 60 ppi foam covers, or a capillary-assist liquid flow towards foam cells for the 90 ppi foam covers. The value of ppi has an important effect on the thermal performance. Boiling curves are crossed between the high and low ppi foam covers. Low ppi foams have better thermal performance at low surface superheats, but high ppi foams have better one at moderate or large surface superheats and extend the operation range of surface superheats. The effects of other factors such as pool liquid temperature, foam cover thickness on the thermal performance are also discussed.

聚（ε-己内酯）泡沫塑料制备与性能研究

随着人们环保意识的普遍提高，可生物降解材料已经成为了世界范围的研究热点。开展可生物降解高分子泡沫塑料的结构与性能的研究，对以可生物降解的高分子泡沫塑料替代不能生物降解的泡沫塑料，遏制白色污染，保护生态环境，具有重要的科学价值和社会意义。本工作选用聚（ε-己内醋）（PCL）为原料，通过辐射方法引发PCL交联，提高其熔体粘度和强度；以AC为发泡剂，制备PCL泡沫材料：添加无机纳米粒子-碳酸钙，进一步提高PCL的发泡倍率。研究辐射剂量、凝胶含量、分子量、AC发泡剂含量、碳酸钙含量等因素对制备泡沫塑料的影响，优化组合，获得PCL泡沫材料。主要研究结果如下：1．采用辐照交联的方法实现了PCL的交联，凝胶含量最高可达到41％；2．交联后的PCL热稳定性略有下降，结晶性能基本没有变化。但是，抗张性能变化很大，弹性模量提高（260MPa-430MPa），断裂伸长率下降（1500％-30％）。由此推测，提高了PCL的熔体强度；3．制备出了PCL泡沫塑料，发泡倍率可控制在2-12范围内。其中，发泡倍率与发泡剂含量、PCL分子量关系密切。当辐照计量大于5Mrad后，辐照计量对发泡倍率影响不大，但是，对泡孔的结构影响很大，即辐照剂量大，得到的泡孔直径小；4．体系中添加CoCO_3使得PCL的发泡倍率进一步提高，最高可达到15倍，并且使泡孔数量增加，体积减小。添加30wt％碳酸钙，PCL泡沫塑料的孔径为147μm，单位体积泡孔数为29.6*10~8cell/cm~3，壁厚4.48μm。