负泊松比结构力学设计、抗冲击性能及在车辆工程应用与展望

轻量化多功能负泊松比结构由于具有优异的可设计性、拉胀特性、剪切模量、断裂韧性、抗冲击吸能、减震降噪等特性,在车辆吸能结构设计和多功能优化方面具有巨大的应用潜力.本文详细综述了负泊松比结构的力学设计及其在车辆工程中的典型应用:(1)负泊松比基本概念及其力学特性, 以及近几十年来的快速发展趋势;(2)负泊松比材料与结构构型设计方法的基本分类、负泊松比泡沫材料微结构特征及制备工艺、负泊松比复合材料设计方法的基本发展历程以及前沿人工智能设计方法;(3)针对典型负泊松比结构的力学设计进行详细介绍, 主要包括手性结构、方格旋转结构、双箭头内凹结构、内凹蜂窝结构、拉伸扭转效应负泊松比结构等;(4)负泊松比材料与结构的冲击吸能特性及相关的实验、理论和模拟研究;(5)负泊松比材料与结构在汽车轻量化设计领域的典型应用, 主要包括汽车吸能盒、B柱、发动机罩、安全带、悬架、免充气轮胎等典型吸能结构件;(6)负泊松比结构在汽车工程中的应用前景, 所面临技术挑战和巨大应用潜力.      

In-plane mechanical behaviors of 2D repetitive frameworks with four-coordinate flexible joints and elbowed beam members

The mechanical properties of an open-framework structure constructed of joints and beam members are strongly influenced by both its geometrical configuration and joint flexibility. This paper clarifies the relationship between joint flexibility and Poisson's ratio, which is a mechanical criterion for solid deformation, and discusses two types of in-plane anisotropic structures made up of four-coordinate flexible joints and elbowed beam members. Uniaxial tensile analyses estimate the linear and nonlinear elastic properties of these frameworks by applying straightforward joint modeling with multi-rotational degrees of freedom. The numerical results show that these proposed frameworks produce a variety of deformability dependent on the joint flexibility in auxetic deformation with a negative Poisson's ratio, the folding mechanics under kinematic indeterminacy, and the transition of Poisson's ratio between the positive and negative values. The geometrical and topological aspects of the obtained mechanical behaviors are discussed.     

负泊松比材料与结构的力学性能研究及应用

本文综述了负泊松比材料与结构研究的发展过程,包括负泊松比材料拉伸膨胀理论研究、天然和人工制备负泊松比材料与结构的力学性能研究、负泊松比材料与结构的制备以及其在各个领域的广泛应用,并对未来负泊松比材料与结构的力学性能研究及应用进行了展望.      

闭孔泡沫铝的塑性泊松比

在表征闭孔泡沫铝的力学性能中,塑性泊松比是较为重要的参数之一。本文应用Kelvin十四面体模型构建出不同相对密度的闭孔泡沫铝三维细观模型并采用LS-DYNA对所得细观模型进行单轴准静态压缩计算。数值模拟分析发现,随着轴向应变的增加,泡沫铝泊松比-轴向应变曲线呈倒S形,存在峰值和极小值,曲线变化规律与泡沫铝胞孔的变形有密切关系。根据泊松比-轴向应变曲线与胞孔变形之间的关系,给出了平均塑性泊松比的定义。计算结果显示,随着相对密度的提高,闭孔泡沫铝的平均塑性泊松比增大。当闭孔泡沫铝的相对密度低于0.1时,其平均塑性泊松比接近于零,计算中可以忽略;当闭孔泡沫铝相对密 度大于0.1时,其平均塑性泊松比随相对密度的增加而呈线性从0.17增加到0.5     

Connexions between the moduli for anistropic elastic materials

Summary For homogeneous isotropic elastic materials there are simple interrelations connecting Young's modulus, Poisson's ratio, the rigidity modulus and the modulus of compression. However for anisotropic materials the situation is quite different. Young's modulus is a function of direction and Poisson's ratio and the rigidity modulus are functions of pairs of orthogonal directions. Here some simple universal connexions between the moduli for various directions are simply derived for general anisotropic materials. No particular symmetry is assumed in the material.     

The influence of dewetting in filled elastomers on the changes of their mechanical properties

Summary The changes of mechanical properties of filled polymers and their dependence on deformation history are the subject of this paper. For most of filled polymers in practical use, the theory of linear viscoelasticity cannot be applied, even at small deformations. In this work, samples of glass bead filled polybutadiene rubber with different filler levels were investigated at small strains (<10%). The evolution of the relaxation modulus and Poisson's ratio was observed in cyclic experiments, which were also applied in inducing a defined deformation history for the succeeding relaxation experiments. In these experiments, the relaxation modulus and Poisson's ratio were measured as functions of time, with strain, strain rate, filler level and the preceeding deformation history as parameters. The results indicate dewetting as the main reason for the changes of the mechanical properties of the filled materials.The financial support of this work by the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.     

原子力显微镜在二维材料力学性能测试中的应用综述

以石墨稀为代表, 二维材料有着诸多优异的性质, 在下一代电子器件等领域拥有广阔的应用前景. 目前绝大多数关于二维材料的研究都集中在其电子学和光学的性质和应用, 对于其力学性质的研究则相对欠缺, 而力学性质在二维材料的研究和应用中都有着至关重要的意义. 原子力显微镜是低维材料力学性质表征的主要手段, 例如基于原子力显微镜的纳米压痕技术. 本文首先简要介绍了二维材料的基本背景以及原子力显微镜的工作原理. 进一步展示了纳米压痕技术的工作原理和理论背景, 并回顾了利用纳米压痕技术研究二维材料面内力学性质的相关实验和理论工作, 同时探讨了原子力显微镜在表征二维材料力学性能中存在的测量误差及来源. 由于二维材料展现出强烈的各向异性, 纳米压痕技术在能够很好地测量二维材料面内力学性质的同时, 对于二维材料层间力学性质表征等方面存在明显的局限性. 第三部分介绍了一种全新的基于原子力显微镜的埃(?)压痕技术, 该技术能够将形变尺度控制在0.1 nm以内, 从而精确地表征和调控二维材料的层间范德华作用力, 即层间力学性质. 作者在第三部分介绍了通过埃压痕技术表征和调控的石墨烯、氧化石墨烯等二维材料的层间力学性质. 最后简要介绍了范德华异质结材料的基本性质, 探讨了埃压痕技术在该材料力学性质研究中的潜在应用.      

Bedding plane-emb e dde d augmente d virtual internal bonds for fracture propagation simulation in shale

To effectively simulate the fracture propagation in shale, the bedding plane(BP) effect is incorporated into the augmented virtual internal bond(AVIB) constitutive relation through BP tensor. Comparing the BP-embedded AVIB with the theory of transverse isotropy, it is found the approach can represent the anisotropic properties induced by parallel BPs. Through the simulation example, it is found that this method can simulate the stiffness anisotropy of shale and can represent the effect of BPs on hydraulic fracture propagation direction. Compared with the BP-embedded virtual internal bond(VIB), this method can account for the various Poisson's ratio. It provides a feasible approach to simulate the fracture propagation in shale.     

基于Oda裂隙结构张量的流纹岩各向异性试验研究

岩石内天然存在长度、倾角和形态不同的裂隙,造成岩石的各向异性特征。为揭示岩石内天然随机裂隙发育特征对岩石物理力学特性的影响规律,以泥巴山隧址区采集裂隙性流纹岩为研究对象,首先对试样裂隙进行素描统计分析;然后基于Oda裂隙结构张量,获得天然随机分布裂隙的几何统计参数;最后对裂隙性流纹岩试样分别进行单轴和常规三轴压缩试验,得到不同应力路径下流纹岩的应力-应变曲线及物理力学参数。分析Oda裂隙结构张量定义的各向异性参数与试验获得的力学参数之间的规律,研究结果表明:(1)Oda裂隙结构张量适用于天然随机分布裂隙的几何统计分析,各向异性参数A(F)越大,裂隙优势方向越明显;(2)单轴压缩下,随着各向异性参数I1和A(F)的增大,流纹岩各向异性程度增大,弹性模量减小,泊松比增大;(3)常规三轴压缩下,流纹岩弹性模量和泊松比随各向异性参数改变的规律较不明显,Oda裂隙结构张量不再适用。     

Surface effects on mechanical behavior of elastic nanoporous materials under high strain

This paper studies surface effects on the mechanical behavior of nanoporous materials under high strains with an improved anisotropic Kelvin model. The stress-strain relations are derived by the theories of Euler-Bernoulli beam and surface elasticity. Meanwhile, the influence of strut (or ligament) size on the mechanical properties of nanoporous materials is discussed, which becomes a key factor with consideration of the residual surface stress and the surface elasticity. The results show that the decrease in the strut diameter and the increase in the residual surface stress or the surface elasticity can both lead to an increase in the carrying capacity of nanoporous materials. Furthermore, mechanical behaviors of anisotropic nanoporous materials in different directions (the rise direction and the transverse direction) are investigated. The results indicate that the surface effects in the transverse direction are more obvious than those in the rise direction for anisotropic nanoporous materials. In addition, the present results can be reduced to the cases of conventional foams as the strut size increases to micron-scale, which confirms validity of the model to a certain extent.     

砂岩岩石力学参数各向异性研究

对胜利油田不同深度的四口井的砂岩岩芯进行了三轴压缩试验和动态岩石力学参数试验。根据获得的垂直地层方向和平行地层方向的试验数据,计算了各岩石力学参数的各向异性系数。通过分析围压以及深度对各向异性系数的影响,给出了砂岩峰值强度各项异性系数、静泊松比各向异性系数、纵波波速各向异性系数、纵横波速比和动泊松比各向异性系数与深度的拟合公式。发现峰值强度、纵波波速和纵横波速比的各向异性系数都随着深度的增加而减小,而泊松比各向异性系数随着深度的增加而增大。动、静弹性模量各向异性系数与深度相关性都很差,动弹性模量各向异性较弱,静弹性模量相对较强。     

基于BP神经网络与小冲杆试验确定在役管道钢弹塑性性能方法研究

为了能够在不停输油气工况下获得在役管道材料的弹塑性力学性能, 提出了一种人工智能BP (back-propagation)神经网络、小冲杆试验与有限元模拟相结合,通过确定材料真应力-应变曲线从而获得材料弹塑性力学性能的方法. 首先,通过系统改变Hollomon公式中的参数$K$, $n$值,获得457组具有不同弹塑性力学性能的假想材料本构关系, 其次,将得到的本构关系代入经试验验证的含有Gurson-Tvergaard-Needleman(GTN)损伤参数的小冲杆试验二维轴对称有限元模型,通过有限元计算得到了与真应力-应变曲线一一对应的457条不同假想材料的载荷-位移曲线,最终将两组数据作为数据库输入BP神经网络进行训练,建立了同种材料小冲杆试验载荷-位移曲线与真应力-应变曲线之间的关联关系.通过此关联关系,可利用试验得到的小冲杆载荷-位移曲线获取在役管道钢的真应力-应变曲线,从而确定其弹塑性力学性能.通过对比BP神经网络得到的X80管道钢真应力-应变曲线与单轴拉伸试验的结果以及引用现有文献中不同材料的试验数据对此关系进行验证,证明了该方法的准确性与广泛适用性.      

SHPB实验中端面摩擦效应研究

选取硅橡胶、聚氨酯泡沫、Comp.B炸药、PBX-HMX(97%)炸药以及6061-T6铝合金五种材料试样,对分离式霍普金森压杆（SHPB）实验技术中的端面摩擦效应进行了研究。实验结果表明,SHPB实验技术中的端面摩擦效应与材料的性质相关。另外,基于各向同性弹性理论,运用能量守恒法对SHPB实验中的端面摩擦效应进行了机理分析,研究表明,材料泊松比、端面摩擦系数、试样长径比、轴向应变是SHPB实验中影响端面摩擦效应的四个因素。     

基于拓扑描述函数的特定性能材料设计方法

研究了拓扑描述函数在材料设计中的应用,给出了一种基于拓扑描述函数的特定性能材料设计问题的提法和求解方法.将拓扑描述函数表示成含参数的基函数之和,将材料微结构拓扑优化问题转化为设计基函数描述参数的尺寸优化问题,使问题求解更方便.基于拓扑描述函数的方法可以准确确定设计域上任意点的材料分布,避免了变密度法常出现的棋盘效应、设计变量和有限元单元相关的缺点;与传统的水平集方法相比,其优化模型可以利用现有的优化方法求解,避免了差分法求偏微分方程.具有正泊松比和负泊松比的特定弹性性能材料的设计算例,说明了基于拓扑描述函数的材料设计方法的有效性.     

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.     

On Behaviors of Functionally Graded SMAs under Thermo-Mechanical Coupling

An analytical solution is obtained for the Functionally Graded Shape Memory Alloy(FG-SMA) composites subjected to thermo-mechanical coupling. Young's modulus and thermal expansion coefficient of the material are assumed to vary in different forms of power function through the thickness, with the Poisson's ratio being constant. An SMA constitutive model is combined with the averaging techniques of composite to determine the mechanical properties of the FG-SMA composites. Different phase transformation steps and the corresponding stress distributions through the thickness direction are given. The results show that the average stresses decrease as the transformations proceed.     

Structure-property relationships in polymer composites with micrometer and submicrometer graphite platelets

The objectives of this work were (a) to investigate the influence of micrometer and submicrometer scale graphite platelets of different aspect ratios and volume fractions on the effective and local quasi-static and dynamic properties of composites with micrometer and submicrometer scale reinforcement, and (b) to compare and evaluate mechanical property measurements of inhomogeneous materials via local (microscale) and bulk (macroscale) experimental methods. Small platelet volume fractions (0.5%) provided proportionally larger increase of the elastic and storage moduli compared to large volume fractions (3.0%). Randomly distributed 15 μm platelets provided marginally higher composite stiffness compared to 1 μm platelets while small volume fractions (0.5%) of 15 μm platelets had a pronounced effect on the effective Poisson's ratio. It was found that local property measurements of inhomogeneous materials conducted by nanoindentation are not representative of the bulk behavior even when the characteristic length of the inhomogeneity is an order of magnitude smaller than the indentation contact area. In this case, statistical averaging of data from a large number of indentations does not result in agreement with bulk measurements. On the other hand, for small aspect ratio platelets with dimensions two orders of magnitude smaller than the nanoindentation contact area, the nanoindenter-obtained properties agreed well with the effective material behavior. It was found that platelets residing at the specimen surface contribute the most to nanoindentation data, which implies that this technique is only valid for well-distributed nanoparticulate and microparticulate systems, and that nanoindentation cannot be used for depth profiling of microstructured composites.     

Interpretation of experimental data for Poisson's ratio of highly nonlinear materials

The Poisson's ratio of a material is strictly defined only for small strain linear elastic behavior. In practice, engineering strains are often used to calculate Poisson's ratio in place of the mathematically correct true strains with only very small differences resulting in the case of many engineering amterials. The engineering strain definition is often used even in the inelastic region, for example, in metals during plastic yielding. However, for highly nonlinear elastic materials, such as many biomaterials, smart materials and microstructured materials, this convenient extension may be misleading, and it becomes advantageous to define a strainvarying Poisson's function. This is analogous to the use of a tangent modulus for stiffness. An important recent application of such a Poisson's function is that of auxetic materials that demonstrate a negative Poisson's ratio and are often highly strain dependent. In this paper, the importance of the use of a Poisson's function in appropriate circumstances is demonstrated. Interpretation methods for coping with error-sensitive data or small strains are also described.     

Wave Propagation Characteristics in Thick Conventional and Auxetic Cellular Plates

Based on Mindlin plate models and Kirchhoff plate models,this study was concerned with the wave propagation characteristics in thick conventional and auxetic cellular structures,with the objective to clarify the effects of negative Poisson's ratio,shear factor and orthotropic mechanical properties on the dynamic behaviors of thick plates.Numerical results revealed that the predictions using variable shear factor in Mindlin plate models resulted in high wave frequencies,which were more significant for plates with negative values of Poisson's ratio.The present study can be useful for the design of critical applications by varying the values of Poisson's ratio.