通孔泡沫铝的动态压缩行为

在SHPB装置上对渗流法制备的通孔泡沫铝进行了动态压缩实验,研究了相对密度为0.341~0.419的通孔泡沫铝在10-3~2000 s-1应变率范围内的压缩响应特征和应变率相关性,并用扫描电镜（scanning electron microscope,SEM)分析了泡沫铝的压缩变形特征。实验结果表明,通孔泡沫铝有明显应变率效应,随应变率上升,泡沫铝流动应力提高。SEM观察结果揭示,在动态压缩下,通孔泡沫铝宏观上均匀变形,微观变形机制以泡孔横向伸展坍塌为主。     

泡沫铝合金动态力学性能实验研究

利用分离式霍布金森压杆(SHPB)实验技术和MTS材料实验机对两组不同孔径、不同密度的开孔泡沫铝合金进行了准静态和动态压缩实验研究。实验结果表明:泡沫铝合金的静态和动态变形过程均具有泡沫材料变形的三个阶段特征。开孔泡沫铝合金的变形是均匀变化过程，并不出现局部的变形带。与相对密度对力学性能的影响相比，孔径大小的影响可以忽略不计。在考察的应变率范围内，屈服应力对应变率并不很敏感。     

Dynamic crushing of aluminum foams: Part II – Analysis

Part II of this study uses micromechanically accurate foam models to simulate and study the dynamic crushing of open-cell foams. The model starts as random soap froth generated using the Surface Evolver software to mimic the microstructure of the foams tested. The linear edges of the cellular microstructure are “dressed” with appropriate distributions of solid to match those of ligaments in the actual foams and their relative density. The ligaments are modeled as shear-deformable beams with variable cross sections discretized with beam elements in LS-DYNA, while the Al-alloy is modeled as a finitely deforming elastic–plastic material. The numerical contact algorithm of the code is used to model ligament contact and limit localized cell crushing. The quasi-static and all dynamic crushing experiments in Part I are simulated numerically. The models are shown to reproduce all aspects of the crushing behavior including the formation and evolution of nearly planar shocks, the force acting at the two ends, the shock front velocity, the strain in the crushed material behind the shock, and the energy absorbed.     

The elastic–plastic behaviour of foam under shock loading

An experimental investigation of the elastic–plastic nature of shock wave propagation in foams was undertaken. The study involved experimental blast wave and shock tube loading of three foams, two polyurethane open-cell foams and a low-density polyethylene closed-cell foam. Evidence of precursor waves was observed in all three foam samples under various compressive wave loadings. Experiments with an impermeable membrane are used to determine if the precursor wave in an open-cell foam is a result of gas filtration or an elastic response of the foam. The differences between quasi-static and shock compression of foams is discussed in terms of their compressive strain histories and the implications for the energy absorption capacity of foam in both loading scenarios. Through a comparison of shock tube and blast wave loading techniques, suggestions are made concerning the accurate measurements of the principal shock Hugoniot in foams.     

铝/硅橡胶复合材料动态压缩行为的研究

通过向开孔泡沫铝中填充硅橡胶而制备了铝 /硅橡胶复合材料 ,在Hopkinson压杆实验装置上对这种材料进行了动态压缩实验 ,分析了其动态压缩应力 应变响应特征 ,并与开孔结构泡沫铝的压缩行为进行了比较。结果表明 :铝 /硅橡胶复合材料的压缩应力 应变响应具有两个阶段的特征 ,即弹性和塑性变形阶段 ;这种复合材料具有较强的应变率效应 ,随应变率的提高 ,其屈服强度和流动应力显著上升。     

Permeability of open-cell foams under compressive strain

A model for the behavior of low-density, open-cell foam under compressive strain is proposed. Using this model, a tractable relationship between the normalized permeability and the applied strain is developed. An experimental study of the effect of strain on the permeability of open-cell polyurethane foams is presented. The experiments are performed using a Newtonian fluid in the fully laminar regime, where viscous forces are assumed to dominate. The model is found to describe the experimental data well and be independent of the foam cell size, the direction of flow with respect to the foam rise direction, and the properties of the saturating fluid. In a companion paper, the model for the permeability of open-cell foam is combined with Darcy’s law to give the contribution of viscous fluid flow to the stress–strain response of a reticulated foam under dynamic loading.     

泡沫金属的微惯性效应和动态塑性泊松比

采用三维Voronoi技术和显式有限元方法来研究闭孔和开孔两种泡沫金属的动态塑性泊松比问题和微惯性效应。细观数值模拟的结果表明:塑性泊松比随着轴向应变的增加而下降,塑性泊松比的峰值随着冲击速度的增加而下降;相对密度增加时,泡沫金属塑性泊松比增加;微惯性对平台应力的影响不大。该数值模拟结果能够解释侧向约束情况下闭孔泡沫金属的压溃应力随着加载速率的提高而下降的实验现象。     

Finite element micromechanics model of impact compression of closed-cell polymer foams

Finite element analysis, of regular Kelvin foam models with all the material in uniform-thickness faces, was used to predict the compressive impact response of low-density closed-cell polyethylene and polystyrene foams. Cell air compression was analysed, treating cells as surface-based fluid cavities. For a typical 1 mm cell size and 50 s^?1 impact strain rate, the elastic buckling of cell faces, and pop-in shape inversion of some buckled square faces, caused a non-linear stress strain response before yield. Pairs of plastic hinges formed across hexagonal faces, then yield occurred when trios of faces concertinaed. The predicted compressive yield stresses were close to experimental data, for a range of foam densities. Air compression was the hardening mechanism for engineering strains <0.6, with face-to-face contact also contributing for strains >0.7. Predictions of lateral expansion and residual strains after impact were reasonable. There were no significant changes in the predicted behavior at a compressive strain rate of 500 s^?1.     

EFFECT OF SPECIMEN ASPECT RATIO ON FATIGUE LIFE OF CLOSED CELL Al-Si-Ca ALLOY FOAM

Quasi-static and compressive fatigue tests on the closed cell Al-Si-Ca alloy foam specimens with three different aspect ratios were performed. It turned out that the onset of cyclic shortening of foam with a lower aspect ratio took place earlier and the fatigue strength was lower compared with the specimen with a higher aspect ratio, although all the dimensions of specimen satisfied the seven times the cell size criterion, while the quasi-static stress-strain curves were almost same having same Young＇s modulus, yield stress and plateau stress. Therefore, the seven times the cell size criterion for the quasi-static compression behavior was not applicable to the fatigue analysis of Al-Si-Ca alloy foam.     

高速冲击载荷下梯度金属泡沫夹芯梁的动态响应与失效

采用泡沫弹冲击加载实验对梯度金属泡沫夹芯梁结构开展了不同冲击强度下的动态响应和失效研究，分析了由三种不同密度泡沫铝组成的等面密度的五种不同梯度的夹芯结构在夹支边界条件下的抗高速冲击性能，结合三点弯曲实验，研究梯度效应对夹芯结构抗冲击性能的影响。研究表明：密度梯度对结构的失效过程和失效模式有着明显的影响，且夹芯梁结构的初始失效模式对结构整体响应和主要的能量吸收机制起着主导作用；当冲击条件不足以使得均质芯材发生压缩时，均质及负梯度夹芯结构初始失效模式为整体弯曲变形，低强度芯层位于前两层的梯度结构随着冲击强度的变化出现不同程度的局部芯层压缩；当冲击强度较低时，梯度结构通过丰富的局部失效表现出明显优于均质结构的抗冲击变形能力；当冲击强度大于临界值时，均质结构具有更好的抗冲击变形能力。通过合理地设计密度梯度实现逐层压缩吸能，能够有效的提升防护结构的抗冲击性能。     

软质聚氨酯泡沫的动态压缩力学性能和本构模型

采用Instron 9350落锤试验机研究了中低应变率下软质聚氨酯泡沫的动态压缩力学性能,分析了其应力-应变响应特征和应变率敏感性,讨论了应变率对材料应变率敏感性指数和能量吸收特性的影响,并基于实验结果建立了可准确描述其压缩力学响应的率相关本构模型。结果表明,软质聚氨酯泡沫的静动态压缩应力-应变响应具有典型的三阶段特征,且呈现出明显的应变率强化效应。准静态加载下,材料具有较高的吸能效率但能量吸收值较小,应变率对最大吸能效率和比吸能的影响较小;动态加载下,随着应变率的增加,最大吸能效率显著减小而比吸能明显增大。考虑应变率影响的修正Sherwood-Frost模型和修正Avalle模型都能够很好地表征软质聚氨酯泡沫的静动态压缩应力-应变响应,但修正Avalle模型的参数较少,更便于工程应用。研究结果可为软质聚氨酯泡沫抗冲击结构的设计和优化提供指导。     

Multiscale mass-spring models of carbon nanotube foams

This article is concerned with the mechanical properties of dense, vertically aligned CNT foams subject to one-dimensional compressive loading. We develop a discrete model directly inspired by the micromechanical response reported experimentally for CNT foams, where infinitesimal portions of the tubes are represented by collections of uniform bi-stable springs. Under cyclic loading, the given model predicts an initial elastic deformation, a non-homogeneous buckling regime, and a densification response, accompanied by a hysteretic unloading path. We compute the dynamic dissipation of such a model through an analytic approach. The continuum limit of the microscopic spring chain defines a mesoscopic dissipative element (micro-meso transition) which represents a finite portion of the foam thickness. An upper-scale model formed by a chain of non-uniform mesoscopic springs is employed to describe the entire CNT foam. A numerical approximation illustrates the main features of the proposed multiscale approach. Available experimental results on the compressive response of CNT foams are fitted with excellent agreement.     

Hydrodynamic Characterization of Nickel Metal Foam,Part 1: Single-Phase Permeability

This article presents results of the investigation of the fluid dynamic behavior in CVD processed nickel metal foams. An experimental facility was developed to measure the single-phase permeability in nickel metal foams in Darcian flow regime. Data on permeability values of seven different nickel foam samples was obtained. The pore sizes of the foam were obtained with scanning electron microscope. By defining friction factor and Reynolds number based on the permeability length scale a correlation was obtained for the foam permeability in Darcian flow regime. The result from this study was compared with the correlations reported by other researchers, and was found to be in good agreement.     

负梯度闭孔泡沫金属的力学性能分析

运用三维Voronoi技术生成闭孔梯度泡沫模型,结合有限元分析方法模拟负梯度闭孔泡沫金属在不同冲击速度下的力学行为。结果表明,随着冲击速度的提高,得到了与均匀泡沫一样的三种变形模式：准静态模式,过渡模式和冲击模式。通过对名义应力应变曲线和变形模式的研究,提出了一种新的定义局部密实化应变的方法,并研究了相对密度和密度梯度对它的影响。分别建立了相对密度和密度梯度与冲击速度的变形模式图。通过引入密实化因子,确定了三种变形模式对应的临界冲击速度。最后讨论了不同冲击速度下,密度梯度大小对泡沫材料能量吸收能力的影响。结果表明,在高速冲击的变形初期,密度梯度的绝对值越大,泡沫材料的能量吸收能力越强。     

Modeling the dynamic response of visco-elastic open-cell foams

This article introduces a mesoscopic formulation for modeling the dynamic response of visco-elastic, open-cell solid foams. The effective material response is obtained by enforcing on a representative 3D unit cell the principle of minimum action for dissipative systems. The resulting model accounts explicitly for the foam topology, the elastic and viscous properties of the cell wall, and the inertial effects arising from non-affine motion within the cells. The microinertial effects become significant in retarding the foam collapse during exceedingly high strain-rate loading. As an application example, a heterogenous case of compressive deformation at high strain rate is simulated utilizing the present model as a constitutive update in a non-linear finite element analysis code. This FEM simulation shows the ability of the model to capture the progressive foam collapse during the dynamic compression as observed in experimental studies. Using the microscopic model, the inertial and viscous strain-rate effects are investigated through the foam density, viscosity, and relative density. Based on the physics incorporated into the local cell model, we provide insight into the physical mechanisms responsible for the experimentally observed strain-rate effects on the behavior of dynamically loaded foam materials.     

Crushability maps for structural polymeric foams in uniaxial loading under rigid confinement

A family of epoxy-based polymeric foams with various initial porosity levels was subjected to quasi-static uniaxial loading in rigid confinement (uniaxial strain) to investigate their crushability characteristics. Two issues were investigated. The first issue was the uniformity of deformation in a specimen as a function of porosity level by creating a grid of equally spaced thin stripes on the surface and by monitoring their pattern during the experiment. It was found that the higher the porosity of foam, the more non-uniform the deformation in the specimen. However, the localized non-uniform deformation did not affect the global stress-strain response, especially at large deformations. The second issue was the development of a new analysis tool, called “crushability map”. The purpose of the tool is to depict the evolution of porosity, bulk density and energy absorption as functions of applied strain, stress, and porosity. These maps can assist in characterizing the residual crushability or energy absorption capability of foams as a function of residual porosity. The maps can be used as a design tool for selection of suitable foams for a given application in conjunction with various design criteria.     

Deformation rate effects on failure modes of open-cell Al foams and textile cellular materials

The compressive behavior of open-cell aluminum alloy foam and stainless steel woven textile core materials have been investigated at three different deformation rate regimes. Quasi-static compressive tests were performed using a miniature loading frame, intermediate rates were achieved using a stored energy Kolsky bar, and high strain rate tests were performed using a light gas gun.In agreement with previous studies on foam materials, the strain rate was not found to have a significant effect on the plateau stress of metallic foams. For all the tests, real time imaging of the specimen combined with digital image correlation analysis allowed the determination of local deformation fields and failure modes. For the Kolsky bar tests, the deformations in the foam specimen were found to be more distributed than for the quasi-static test, which is attributed to moderate inertia effects. The differences in failure mode are more dramatic for the gas gun experiments, where a full compaction shock wave is generated at the impact surface. The stresses in front and behind the shock wave front were determined by means of direct and reverse gas gun impact tests, i.e., stationary and launched specimen, respectively. A one-dimensional shock wave model based on an idealized foam behavior is employed to gain insight into the stress history measurements. We show that the predictions of the model are consistent with the experimental observations. Woven textile materials exhibited moderate dependence of strength on the deformation rate in comparison with open-cell foam materials.     

基于三维细观力学模型的开孔泡沫弹性性能预测

泡沫材料的宏观力学性能主要取决于基体材料的力学特性及其微细观结构特征,基于细观力学模型的分析方法是泡沫材料力学性能研究的重要途径。文中基于Matlab语言和Abaqus软件构建了描述中等孔隙率开孔弹性泡沫材料微结构特征的三维随机分布球形泡孔模型,并采用有限元方法对弹性泡沫压缩变形进行了模拟,并计算给出了不同孔隙率弹性泡沫材料弹性模量、剪切模量、体积模量以及泊松比的分布,建立了相应的唯象表达式。与理论模型及测试结果的比较表明,本文基于三维随机泡孔模型模拟结果构建的唯象表达式能够对弹性泡沫材料的弹性力学性能给出很好的预测。     

填充硅橡胶的泡沫铝复合材料的力学性能

用渗流法向开孔泡沫铝-硅合金和泡沫纯铝中充填硅橡胶获得含硅橡胶的泡沫材料, 在材料试验机和SHPB上对含硅橡胶的复合材料进行动态与准静态压缩实验。实验结果表明:含硅橡胶的泡沫复合材料只有弹性段和塑性段两个阶段,具有更高的应变率敏感性,其应力-应变曲线抖动幅度比较大。     

泡沫材料的应变率效应

对泡沫材料的应变率敏感性进行了系统深入的讨论,认定这种材料是应变率敏感材料,这种敏感性主要是由于泡孔的变形特性产生的。泡沫材料变形的局部化、微观惯性和致密性导致压垮应力明显提高,基体的应变率效应及泡孔的形状大小并不能对泡沫材料应变率敏感性起主导作用。