泡沫铝率相关性能的有限元模拟

构建了三维随机分布球形泡孔模型,模拟开、闭孔混合结构泡沫铝材料的微细观结构,并通过有限元方法计算了10~104 s-1应变率范围内、孔隙率35%~65%泡沫铝材料的率相关性以及应变率和相对密度变化对泡沫铝动态压缩力学性能的影响。研究表明:中、低应变率下,泡沫铝材料率相关性能主要取决于基体材料的应变率敏感性;高应变率下,泡沫铝材料率相关性能受基体材料的应变率敏感性以及微结构惯性联合作用,且相对密度较低泡沫铝材料的微结构惯性效应更显著。     

泡沫杆撞击刚性壁的动态压溃模型

针对泡沫杆撞击刚性壁的情形建立了2类动态压溃模型:一维冲击波模型和三维细观有限元模 型。以连续介质框架下的应力波理论为基础,并假定了刚性-非线性塑性硬化的加载和刚性卸载的本构关系, 建立了一维冲击波模型,给出了冲击波波后应变与冲击时间的隐式表达式。利用随机Voronoi技术构建了闭 孔泡沫金属结构的三维细观有限元模型,使用ABAQUS/Explicit有限元软件模拟了泡沫材料的动态压溃过 程,并基于最小二乘法计算局部变形梯度和局部应变得到了三维泡沫结构的应变场。通过理论解和数值解的 比较,发现该理论模型能够较好地预测泡沫金属杆撞击刚性壁的力学行为,得到了较为精确的结果。     

基于等效阻尼理论的金属橡胶弹性迟滞力学模型及实验研究

针对弹性多孔金属橡胶非线性迟滞特性力学行为,将迟滞恢复力-位移曲线分解为非线性单值曲线和椭圆,并将等效阻尼理论用于动态力学性能参数识别,从而建立了一种新型的适用于黏弹性阻尼材料的宏观唯象力学模型。采用不同相对密度的环形金属橡胶进行动态实验测试,以验证理论模型的准确性,结果表明该模型可将具有非线性特性的金属橡胶系统进行降阶处理,提高金属橡胶力学模型的预测效率,并能很好地描述金属橡胶的迟滞力学行为。另外,研究了在不同激励频率条件下金属橡胶的阻尼耗能特性。实验结果表明:在高频加载的条件下,黏性阻尼系数对动态加载频率不敏感,阻尼耗能与加载幅值之间呈线性正相关。基于等效阻尼理论的弹性迟滞力学模型具有一定的普适性,可进一步推广应用于类似弹性多孔材料的力学性能表征,为其工程应用提供理论基础。     

随机孔隙缺陷对3D编织复合材料力学性能的影响研究

3D编织复合材料经RTM工艺成型将难以避免产生孔隙缺陷,孔隙缺陷主要包括纤维束内干斑和富树脂区孔隙。经分析RTM工艺孔隙缺陷的产生机理,提出了随机孔隙缺陷分布模型,建立了基于细观胞元的含孔隙缺陷有限元分析模型,并应用概率统计方法,分析了缺陷的随机分布和孔隙率对材料等效弹性性能的影响,可为3D编织复合材料力学性能的设计提供依据。     

滞后细观模型在岩石力学中的应用

对以砂岩为代表的所谓``NME材料'的力学行为研究方面的一些新的概念和模型进行了评介.首先介绍了一种基于所谓``滞后单元'的描述滞后现象的物理模型------Preisach-Mayergoyz(P-M)模型,然后详细阐述了P-M模型应用于模拟岩石的非线性滞后应力应变关系的过程和结果.这种唯象模型很好地描述了宏观上的滞后表现和``离散记忆'效应.接着本文对应变能耗散的力学机制进行了简单分析. 最后,介绍了一种描述弹性波在``NME材料'中传播规律的数学方法, 该方法从一般的弹性波传播规律出发,分析了``NME材料'特殊的力学性质给弹性波传播带来的影响,揭示了产生特殊的弹性波传播规律的原因.      

球形孔洞泡沫铝力学行为的SPH分析

分别基于体心立方元胞模型和随机孔洞模型建立了泡沫铝SPH模型.采用SPH方法分析了准静态加载条件下泡沫铝的单轴压缩力学性能.最大应变达70%.SPH计算所得应力-应变曲线表现出明显的弹性、平台和压实三段.分析了泡沫铝的微观变形机理,结果表明规则模型和随机模型均呈现出明显的局部塑性变形带,其中随机泡孔模型的孔壁变形以弯曲为主,局部塑性变形带表现更为明显.本文研究结果同时还说明了采用SPH方法研究泡沫材料力学行为的可行性与适应性.     

低密度开孔泡沫材料力学模型的理论研究进展

开孔泡沫材料主要用于隔音、减振和填充方面，对其力学行为进行理论描述，探讨力学性能与密度及复杂微结构的关系具有十分重要的学术价值和工程意义．为了促进国内泡沫材料力学的发展和交流，文中对低密度开孔泡沫材料力学模型的研究历史进行了简要回顾，重点介绍了能较好地反映开孔泡沫材料真实胞体结构特点的十四面体胞体模型和随机胞体模型，并报道了近年来基于十四面体胞体模型和随机胞体模型研究低密度开孔泡沫材料力学行为的一些理论工作、同时，也对国内的一些相关研究情况进行了简要评述，指出了该领域今后的一些研究方向．     

闭孔泡沫铝材料十四面体模型的改进

基于对闭孔泡沫铝发泡过程更为合理的假设,提出了描述胞体结构的改进的十四面体模型,使之可以反映密度增大时质量集中于支柱和顶点的情况。采用有限元方法及耦合边界条件,研究了闭孔泡沫铝的相对弹性模量、泊松比等弹性特征与胞体参数的关系,给出了拟合的弹性模量的计算公式,并对模型在弹性压缩变形下应力分布进行了分析。通过与已有模型的比较表明,改进模型可以较好地模拟闭孔泡沫铝材料的弹性性能。     

Kelvin结构开孔泡沫材料的弹性性能研究

采用一修正的十四面体结构模型(Kelvin结构模型)对开孔泡沫金属的弹性性能进行研究,对低密度开孔泡沫材料表现出不可压的特性进行了分析。该模型考虑作用在泡沫筋条上的弯矩、剪力和轴向力,以及轴向力的平衡。修正模型的数值计算结果与实验结果及其他模型的结果进行了对比,结果表明修正模型计算的杨氏模量比原有模型的略有提高,筋条截面为星形的修正模型计算的结果与实验比较符合。在密度等同的条件下,筋条截面惯性矩越大的开孔泡沫材料,其弹性模量也越大,而泊松比则越小。Kelvin结构的开孔泡沫材料的泊松比随相对密度的减小而趋于0.5。     

基于表面效应三维纳米多孔金属力学性能的有限元分析

纳米多孔金属是一类包含大量纳米尺度孔洞的金属材料,孔洞突出的表面效应,使得其具有比传统多孔金属更为优异的力学性能.相对于理论和分子动力学仿真,有限元方法更适用于复杂结构模型,但受限于理论难度,以往研究仍将纳米多孔金属模型简化为较为简单的二维结构,因此无法真实刻画纳米多孔金属的力学性能.为此,基于Gurtin-Murdoch表面理论,成功构建计入纳米表面效应的有限元表面单元,并考虑微观结构非均匀性,发展面向一般三维纳米多孔金属力学行为的有限元计算模型,将计算得到的纳米孔附近应力分布与参考文献进行对比分析,验证了所构建有限元模型的有效性.通过对包含单球孔和随机多球孔的纳米多孔金属进行单轴拉伸和单轴压缩模拟,揭示了孔隙率、孔洞数量和表面参数对纳米多孔金属杨氏模量、压缩屈服强度和吸能性的影响规律.结果表明:所构建的有限元模型可准确捕捉纳米孔附近应力分布,相对于表面拉梅常数,纳米多孔金属的杨氏模量显著依赖于孔洞表面残余应力和加载方向.所构建的有限元模型为纳米多孔金属力学性能预测提供科学依据.     

On the microstructure of open-cell foams and its effect on elastic properties

Synthetic open-cell foams have a complex microstructure consisting of an interconnected network of cells resulting from the foaming process. The cells are irregular polyhedra with anywhere from 9 to 17 faces in nearly monodisperse foams. The material is concentrated in the nearly straight ligaments and in the nodes where they intersect. The mechanical properties of such foams are governed by their microstructure and by the properties of the base material. In this study micro-computed X-ray tomography is used to develop 3D images of the morphology of polyester urethane and Duocel aluminum foams with different average cell sizes. The images are used to establish statistically the cell size and ligament length distributions, material distributions along the ligaments, the geometry of the nodes and cell anisotropy. The measurements are then used to build finite element foam models of increasing complexity that are used to estimate the elastic moduli. In the most idealized model the microstructure is represented as a regular Kelvin cell. The most realistic models are based on Surface Evolver simulations of random soap froth with N³ cells in spatially periodic domains. In all models the cells are elongated in one direction, the ligaments are straight but have a nonuniform cross sectional area distribution and are modeled as shear deformable beams. With this input both the Kelvin cell models and the larger random foam models are shown to predict the elastic moduli with good accuracy but the random foams are 5–10% stiffer.     

空心颗粒填充复合材料的建模与弹塑性仿真计算

空心玻璃微珠（hollow glass microballon,简称HGM）填充复合材料称为复合泡沫材料,近来已被广泛应用在工业中。利用RSA(Random Sequential Adsorption)方法生成了含不同体积比的HGMs填充代表体元模型,然后用有限元方法计算得出了材料的应力-应变关系,将材料属性简化为双线性随动强化模型,分析了HGM填充比、壁厚对材料的有效弹性常数、屈服极限的影响,并分析了材料内部细观应力场和塑性应变分布情况。结果发现,HGM壁厚比对材料有效弹性模量和屈服极限的增减起着决定性作用,而对于任意填充模式来说,其比模量和比强度总是大于纯树脂,这一点体现了该材料轻质的优良特性。材料基体的应力集中部位分布以及塑性应变区域的分布也取决于HGM的壁厚比。     

Three-dimensional modeling of the mechanical property of linearly elastic open cell foams

Three-dimensional Voronoi models are developed to investigate the mechanical behavior of linearly elastic open cell foams. Dependence of the Young’s modulus, Poisson’s ratio and bulk modulus of the foams on the relative density is evaluated through finite element analysis. Obtained results show that in the low density regime the Young’s modulus and bulk modulus of random Voronoi foams can be well represented by those of Kelvin foams, and are sensitive to the geometric imperfections inherent in the microstructure of foams. In contrast, the compressive plateau stress of the foams is less sensitive to the imperfections. Failure surface of the foams subject to multi-axial compression is determined and is found to comply with the maximum compressive principal stress criterion, consistent with available experimental observations on polymer foams. Numerical results also show that elastic buckling of cell edges at microscopic level is the dominant mechanism responsible for the compressive failure of elastic open cell foams.     

宽γ辐照剂量范围内硅泡沫本构模型研究

基于实验和理论建模研究了白炭黑增强硅泡沫材料在γ辐照剂量范围为0~1000kGy作用后的单轴压缩力学行为。实验结果表明辐照导致硅泡沫出现明显硬化现象,初始杨氏模量和固定应变下应力幅值均随γ辐照剂量近似线性增加。辐照后硅泡沫泡孔结构完整,硅橡胶基体中高分子交联反应占主导,且交联密度随辐照剂量线性增大。基于实验分析结果,实现了Ogden Hyperfoam超弹本构模型参数与辐照剂量的关联。结果表明初始剪切模量参数与辐照剂量成线性关系,硬化指数和泊松比参数与辐照剂量无关。基于应力应变实验数据拟合得到模型参数,并与未参与拟合的实验数据对比,验证了模型的准确性,表明该模型能够表征宽辐照剂量范围内硅泡沫的压缩力学行为。     

宽γ辐照剂量范围内硅泡沫本构模型研究

基于实验和理论建模研究了白炭黑增强硅泡沫材料在γ辐照剂量范围为0~1000kGy作用后的单轴压缩力学行为。实验结果表明辐照导致硅泡沫出现明显硬化现象，初始杨氏模量和固定应变下应力幅值均随γ辐照剂量近似线性增加。辐照后硅泡沫泡孔结构完整，硅橡胶基体中高分子交联反应占主导，且交联密度随辐照剂量线性增大。基于实验分析结果，实现了Ogden Hyperfoam超弹本构模型参数与辐照剂量的关联。结果表明初始剪切模量参数与辐照剂量成线性关系，硬化指数和泊松比参数与辐照剂量无关。基于应力应变实验数据拟合得到模型参数，并与未参与拟合的实验数据对比，验证了模型的准确性，表明该模型能够表征宽辐照剂量范围内硅泡沫的压缩力学行为。     

Modelling open cell-foams based on the Weaire–Phelan unit cell with a minimal surface energy approach

The complex architecture of open cell foams has most often been described by Kelvin cell models. It has been shown that the accuracy to predict the elastic properties of open cell foams increases with an increasing level of detail and resemblance to real foam microstructures. However, the Kelvin cell does not possess pentagonal faces which are the most abundant within real open cell foams. Therefore this study focuses on the use of the Weaire–Phelan unit cell to model the elastic properties of an open cell polyurethane foam. Optical and scanning electron microscopy were used to characterise the architecture of the open cell foam. Surface Evolver software was used to minimize the surface energy and introduce the typical architectural characteristics of the open cell foam to the FE-model. The E-modulus and Poisson coefficient of the Kelvin and Weaire–Phelan cell show a similar behaviour as a function of density. The Weaire–Phelan cell predicts however a higher dependency of the shear modulus on the density. When the influence of the elongation of the cells in the rise direction of the foam and the uncertainty of the solid material properties of the polyurethane is taken into account, a good accuracy of the Kelvin cell and Weaire–Phelan structure based FE-models versus experimental compression tests is found.     

On the compressive strength of open-cell metal foams with Kelvin and random cell structures

Two families of finite element models of anisotropic, aluminum alloy, open-cell foams are developed and their predictions of elastic properties and compressive strength are evaluated by direct comparison to experimental results. In the first family of models, the foams are idealized as anisotropic Kelvin cells loaded in the <100> direction and in the second family more realistic models, based on Surface Evolver simulations of random soap froth with N³ cells are constructed. In both cases the ligaments are straight but have nonuniform cross sectional area distributions that resemble those of the foams tested. The ligaments are modeled as shear deformable beams with elasto-plastic material behavior. The calculated compressive response starts with a linearly elastic regime. At higher stress levels, inelastic action causes a gradual reduction of the stiffness that eventually leads to a stress maximum, which represents the strength of the material. The periodicity of the Kelvin cell enables calculation of the compressive response up to the limit stress with just a single fully periodic characteristic cell. Beyond the limit stress, deformation localizes along the principal diagonals of the microstructure. Consequently beyond the limit stress the response is evaluated using finite size 3-D domains that allow the localization to develop. The random models consist of 3-D domains of 216, 512 or 1000 cells with periodicity conditions on the compressed ends but free on the sides. The compressive response is also characterized by a limit load instability but now the localization is disorganized resembling that observed in experiments. The foam elastic moduli and strengths obtained from both families of models are generally in very good agreement with the corresponding measurements. The random foam models yield 5–10% stiffer elastic moduli and slightly higher strengths than the Kelvin cell models. Necessary requirements for this high performance of the models are accurate representation of the material distribution in the ligaments and correct modeling of the nonlinear stress–strain response of the aluminum base material.     

Modeling and prediction of bulk properties of open-cell carbon foam

The emerging ultralightweight material, carbon foam, was modeled with three-dimensional microstructures to develop a basic understanding in correlating microstructural configuration with bulk performance of open-cell foam materials. Because of the randomness and complexity of the microstructure of the carbon foam, representative cell ligaments were first characterized in detail at the microstructural level. The salient microstructural characteristics (or properties) were then correlated with the bulk properties through the present model. In order to implement the varying anisotropic nature of material properties in the foam ligaments, we made an attempt to use a finite element method to implement such variation along the ligaments as well as at a nodal point where the ligaments meet. The model was expected to provide a basis for establishing a process-property relationship and optimizing foam properties.The present model yielded a fairly reasonable prediction of the effective bulk properties of the foams. We observed that the effective elastic properties of the foams were dominated by the bending mode associated with shear deformation. The effective Young's modulus of the foam was strongly influenced by the ligament moduli, but was not influenced by the ligament Poisson's ratio. The effective Poisson's ratio of the foam was practically independent of the ligament Young's modulus, but dependent on the ligament Poisson's ratio. The effective Young's modulus of the carbon foam was dependent more on the transverse Young's modulus and the shear moduli of the foam ligaments, but less significantly on the ligament longitudinal Young's modulus. A parametric study indicated that the effective Young's modulus was significantly improved by increasing the solid modulus in the middle of the foam ligaments, but nearly invariant with that at the nodal point where the ligaments meet. Therefore, appropriate processing schemes toward improving the transverse and shear properties of the foam ligaments in the middle section of the ligaments rather than at the nodal points are highly desirable for enhancing the bulk moduli of the carbon foam.     

Compressive response of open-cell foams. Part I: Morphology and elastic properties

This study is concerned with the understanding and modeling of the compressive response of open cell foams. The response starts with a nearly linear elastic regime which terminates into a limit load followed by an extensive load plateau. The plateau, which is responsible for the excellent energy absorption capacity of foams, is followed by a second stiff branch. Results from polyester urethane open cell foams with relative densities of about 0.025 are used to illustrate this behavior using experiments coupled with several levels of modeling. The experiments include characterization of the microstructure and the properties of the base material and measurement of the compressive response of the foams of various cell sizes.A sequence of models for predicting the complete response of such foam is developed. The foam is idealized to be periodic using the space-filling Kelvin cell assigned the major geometric characteristics found in the foams tested. The cells are elongated in the rise direction, the ligaments are assumed to be straight, to have Plateau border cross-sections and nonuniform cross-sectional area distribution. The ligaments are modeled as shear-deformable extensional beams and the base material is assumed to be linearly elastic. Prediction of the initial elastic moduli are addressed in Part I. Closed form expressions for the material constants are presented as well as results using a FE model of the characteristic cell. Comparison between measurements and predictions is very favorable. The paper finishes with results from a limited parametric study of the elastic moduli. The results demonstrate that inclusion of the geometric complexities mentioned above is essential for successful prediction of the moduli of such foams. The nonlinear parts of the response including the foam crushing behavior are addressed in Part II.