蜂窝结构力学超材料弹性及抗冲击性能的研究进展

具有手性蜂窝结构的力学超材料是近年来发展起来的高性能工程材料,它具有轻质、高比刚度、负泊松比、结构参数可调以及力学性能稳定等优点。其不仅可以实现面内变形,面外承载的双重力学作用,还具有出色的隔振、吸声降噪以及控制弹性波的传播等工程应用潜质,在智能结构、车辆船舶、航空航天等领域具有巨大的发展潜力。本文从其弹性和抗冲击两个力学性能方面进行综述。首先介绍并评述了近年来蜂窝结构力学超材料的面内杨氏模量、负泊松比特性以及面外剪切模量等弹性性能的理论分析研究进展。在抗冲击性能方面,从力学模型建立和有限元分析的角度出发,对手性蜂窝结构力学超材料在冲击载荷作用下的整体变形及其抗冲击性能的研究现状分别进行了评述。最后指出针对蜂窝结构力学超材料弹性及冲击性能的研究,可进一步建立内部韧带变形及力的传递力学模型以及深入探索冲击过程吸能机理等,以期为该类力学超材料内部韧带和节点环结构的优化设计提供参考。     

A first-principles study on the structural, elastic, electronic, and optical properties of CdRh2O4

The structural, elastic, electronic, and optical properties of CdRh₂O₄ with cubic $ (Fd\overline{ 3} m) $ and orthorhombic (Pnma) structures have been investigated using a pseudopotential plane wave (PP-PW) method within the local density approximation (LDA). The calculated lattice parameters agree reasonably with the experimental values. The single-crystal elastic stiffness constants C _ij s of the cubic and orthorhombic phases are investigated using the stress–strain method. In addition, the polycrystalline elastic properties including bulk modulus, shear modulus, Young’s modulus, bulk modulus–shear modulus ratio, Poisson’s ratio, and elastic anisotropy ratio are determined based on Voigt–Reuss–Hill approach. The use of the hybrid functional sX-LDA leads to considerably improved electronic properties compared to standard LDA approach. On the other hand, the dielectric function, refraction index, reflectivity, conductivity function, and energy-loss spectra were obtained and analyzed on the basis of electronic band structures and density of states.     

Mechanical properties of potentially-smart carbon/epoxy composites with asymmetrically embedded shape memory wires

Embedding Shape Memory Alloy (SMA) wires in composite structures enables controlling of their mechanical properties. The main aim of this study is to characterize experimentally the mechanical properties of two-layer smart composite structures which are made of one layer of carbon fibers epoxy laminate and one layer of epoxy embedded with SMA wires. A carbon/epoxy layer was first fabricated using vacuum infusion method. Then a SMA/epoxy layer was prepared separately and then laid over the completely cured carbon/epoxy layer using the hand lay-up process. The final structure is smart and has potential of being specifically bent under controlled thermal loading, due to the embedded pre-strained SMA wires. However the temperature was kept constant and there was no thermal excitation of the SMA wires in this experimental study. The configuration of the material constituents through the thickness of the structure renders the cross-section to be unsymmetrical. The specimens were tested in a specially developed unsymmetrical tensile testing machine. From the readings of force from the testing machine and strain gages, the tensile and shear stress–strain relations of the composite materials were obtained. The elastic and shear moduli and also Poisson’s ratio of the composite materials were defined and it was observed that, the effective moduli increased with increasing density of SMA wires in the layer. It is concluded that, due to the asymmetrical material variation, finding the mechanical properties via conventional testing machine is not accurate and a special testing machine is needed.     

Elastic properties and sound velocities of silicane/graphane hybrids

Using ab initio density functional theory, the effect of hydrogen arrangement on the elastic properties of silicene–graphene hybrid is studied. Mechanical stability, elastic constants and sound velocities of pure and five configurations of hydrogenated SiC sheet, namely, chair, table, boat, zigzag and armchair, are explicitly examined. To reveal the anisotropic properties of the six structures, the polar plots of Young’s modulus, Poisson ratio and acoustic waves speed are given. Compared to graphene, it is shown that all the isotropic systems are less stiffer with lower in-plane Young’s modulus and stronger with their larger Poisson ratio, moreover, their compressional and shear waves propagate faster. The analysis of linear elastic behavior shows that the armchair configuration has an auxetic structure. The result of this work could be used for the design of future silicane–graphane based nanodevices with potentially large technological impact in nanomechanics.     

Mechanical response of a unidirectional graphite fiber/polyimide composite as a function of temperature

In this work, the mechanical response of a unidirectional composite based on T650-35 graphite fibers embedded in a PMR-15 polyimide resin was analytically and numerically predicted as a function of temperature and subsequently compared with the available experimental data. The Eshelby/Mori–Tanaka (E/M–T) method was used to predict the elastic properties of the composite, whereas a finite element unit cell was employed to predict the stress vs. strain curves of the composite under elasto-plastic conditions. It was shown that for the temperature range from 25 to 315 °C the predicted elastic properties of the composite agreed closer with the experiment in the case of the longitudinal and transverse Young’s moduli than in the case of the longitudinal shear modulus. The comparisons for the transverse shear modulus and the longitudinal Poisson’s ratio were uncertain. The agreements between the numerically predicted and experimentally determined stress–strain curves of the composite were found to be dependent on temperature and the type of loading. The experimental and numerical research data and the approaches presented in this work should significantly extend our knowledge of the effect of elevated temperatures on the mechanical behavior of unidirectional high temperature polymer matrix composites.     

三向编织玻璃/环氧复合材料刚度性能

通过实验研究了三向编织玻璃/环氧复合材料的刚度性能 , 并考虑编织角和试件宽度参数的影响 , 探讨了拉伸和压缩刚度性能的差异。实验结果表明 : 在同一纤维体积分数条件下 , 随着编织角的增大 , 试件的纵向弹性模量有所减小 , 泊松比 (在编织角约大于 35° 时) 也有所减小 ; 宽度为两倍和三倍单胞宽度的试件的刚度性能基本相同; 试件的纵向弹性模量和泊松比远大于横向弹性模量和泊松比; 拉伸和压缩时试件的弹性模量和泊松比基本接近 ; 在横向拉伸和压缩时试件的应力2应变曲线具有明显的非线性特征。实验结果为编织复合材料结构设计提供了数据参考。      

Mechanical properties of and critical conditions for crack initiation in electronic glass

In this paper, the mechanical properties of electronic glass are tested using a combination of the Vickers indentation test and a multiple-loading nanoindentation test to obtain the elastic modulus, Poisson’s ratio and hardness values. The basic mechanical property parameters of the electronic glass and its stress–strain curve are found using atomic force microscopy analysis of the indentation morphology. The critical pressure and depth for crack initiation and the corresponding load and depth can be obtained during vertical loading on the electronic glass. When cracks extend to the surface, the results show that the electronic glass is isotropic. Several loading cycles causes a fatigue effect on the surface of the electronic glass, which decreases its elastic–plastic response. While the loadings are increasing, the elastic–plastic response rates are decreasing bur it rends stability finally. These results can provide a reference and guide for micro machining and surface microstructure machining of electronic glass.     

The structural and mechanical properties of CdN compound: A first principles study

First principles calculations are performed to investigate the structural, elastic, and mechanical properties of CdN for various structures: NaCI, CsCl, ZnS, wurtzite, WC, CdTe, NiAs, and CuS. The local density and generalized gradient approximations are used for modeling exchange–correlation effects. Our calculations indicate that CuS (B18) structure is energetically the most stable among the considered structures. The some basic physical properties such as lattice parameters, bulk modulus, and second-order elastic constants are calculated. We have also predicted the shear modulus, Young’s modulus, Poison’s ratio, Debye temperature, and sound velocities. Our structural and some other results are consistent with the available theoretical data.     

Prediction of Young’s modulus and Poisson ratio for foamed metals using octahedron model

Abstract

The present paper deals with the mathematical–physical expression of Young’s modulus and Poisson ratio of foamed metals. As it is known that, Young’s modulus and Poisson ratio are two basic mechanical parameters of engineering materials. Foamed metal is a class of excellent engineering materials with dual attributes of structural and functional characteristics; therefore, these two parameters are investigated for these materials, and the relevant mathematical–physical expressions are derived from the ‘octahedron model’ of porous materials in the present paper. The results show that the apparent Young’s modulus displays a quite complicated mathematical relationship to porosity of the porous body, and the apparent Poisson ratio is just a characteristic of the material constant almost not relative to porosity of the foamed metal.     

聚酰胺非晶相弹性性质预测

聚合物基复合材料宏观有效力学性质的确定需要复合材料中各个组分的基本力学性质。采用基于拓扑的关联指数法对11种典型聚酰胺非结晶相的体积模量、泊松比、杨氏模量、剪切模量等弹性性能进行了预测。结果表明,当温度从1K逐步增加到(Tg-20)K时,体积模量、杨氏模量和剪切模量随温度的增加呈指数规律减小;而泊松比则呈线性增加。当温度在玻璃化温度Tg附近(Tg-20)相似文献   

加热过程中小鼠皮肤热物性及力学参数变化研究

本文以小鼠作为实验对象,研究了离体小鼠皮肤组织加热过程中热物性及力学参数的变化,讨论了温度及水分等因素的影响规律。结果表明:在加热过程(37~47℃)中,皮肤组织应力应变曲线近似为线性,弹性模量与温度相关,且随着温度的升高而逐渐减小,泊松比变化与之相似;加热初期(37~45℃),组织含水率减少不明显,导热系数随温度的升高而缓慢增加;随着温度的升高与时间的推移,组织逐渐失水,含水率对导热系数的影响高于温度对其的影响,45℃时导热系数开始急剧下降,在45~47℃,导热系数值下降了5. 4%,比热容变化与之相似。     

Elastic Properties of Compacted Clay Soils by Laser Ultrasonics

To evaluate the effect of the excitation frequency on the dynamic properties of soils, the elastic modulus $E$ , shear modulus $G$ , and Poisson’s ratio $\nu $ for three Mexican compacted clayey soils were determined using two techniques: laser ultrasonic and resonant column (RC) tests. For the first, the parameters were determined by measurements of the P- and S-waves at ultrasonic frequencies and variations of the height of the cylindrical soil specimens and for the second one, a harmonic excitation between 5 Hz and 7 Hz was applied. Large variations in the elastic parameters through an ultrasonic axial scanning of the soil specimens were observed; this reveals the heterogeneity of these materials, while a decrease of the sample aspect ratio mainly affects the determination of Poisson’s ratio. The ultrasonic data were integrated with those from RC data to obtain a shear modulus profile covering both high and low frequencies. The interpolation on whether the data are either linear or not is an indication of the viscoelastic behavior of the compacted clayey soils. The specimens were: (a) clay from Texcoco Valley, (b) clay from Mexico Valley, and (c) granular soils from the Parota. Experimental determination of the mechanical properties of soils is very important because soil constitutive models are traditionally calibrated from global boundary measurements taken from laboratory soil specimens. The most difficult parameter to obtain is the Poisson’s ratio, as well as the shear modulus, which is a fundamental parameter for establishing the soil response under low amplitude vibrations and it is extremely important to foundation design.     

Nb–Al–N thin films: Structural transition from nanocrystalline solid solution nc-(Nb,Al)N into nanocomposite nc-(Nb,Al)N/a–AlN

Structures and mechanical properties of thin films of the Nb–Al–N system produced by magnetron sputtering of targets from niobium and aluminum in the Ar–N₂ atmosphere have been studied. It has been shown that as the aluminum concentration increases, the structure of a thin film transforms from the nanocrystalline into the nanocomposite one, which consists of nanocrystallites of solid solutions in a matrix of amorphous aluminum nitride. Hardness, elastic modulus, and yield strength of Nb–Al–N thin films have been studied by nanoindentation in the mode of continuous control of the contact stiffness. It has been found that the transition of the structures of Nb–Al–N thin films from the nanocrystalline to the nanocomposite structures results in an increase of hardness and decrease of elastic modulus due to the formation of a thin amorphous interlayer between grains of nanocrystallites. A high hardness to elastic modulus ratio of Nb–Al–N nanocomposite thin films indicates that the films are a promising material for wear-resistant coatings.     

砷化镓晶体力学特性的各向异性计算

砷化镓因其良好的光电特性被广泛应用于电子与半导体领域, 为推动砷化镓解理加工技术, 对砷化镓材料力学特性的各向异性进行计算并分析。本研究对砷化镓各个晶面之间的夹角、面间距、原子的密度等结构参数进行计算, 基于广义胡克定律结合压痕实验, 分析砷化镓材料表层弹性模量、泊松比、剪切模量、硬度、断裂韧性等力学特性在{100}晶面沿不同晶向力学性能的变化规律。结果表明: 砷化镓不同晶面间结构参数的不同是导致砷化镓力学特性呈现各向异性的主要原因; 砷化镓在{100}晶面上弹性模量、泊松比、剪切模量的各向异性均呈现出周期性变化, 且{100}晶面的剪切模量为恒值59.4 GPa; 砷化镓{100}晶面硬度的各向异性变化幅度较小, 断裂韧性变化幅度较大, 最小值为0.304 MPa·m^1/2, 位于<110>晶向, 确定<110>晶向是裂纹最容易扩展的晶向。     

Mechanics of hollow concrete block masonry prisms under compression: Review and prospects

The aim of this work is to critically assess the mechanical properties of hollow concrete masonry using experimental results from prisms constructed with blocks of two different strengths and four types of mortar. A key conclusion is that mortar is mostly responsible for the non-linear behavior of masonry. Moreover, a strongly non-linear relationship between masonry elasticity modulus and compressive strength is found, which contradicts the simple linear relation proposed by Eurocode 6 [CEN. Eurocode 6: Design of masonry structures – Part 1 – Common rules for reinforced and unreinforced masonry structures. EN-1996-1-1; 2005.]. The porosity of mortar and the state of stress that mortar undergoes in the process of compressive loading can be responsible for changes in the mechanical properties, such as elasticity modulus and Poisson’s ratio. Finally, different types of mortars induce different failure modes in the masonry prisms and there is clear evidence that the failure of hollow concrete masonry starts after onset of mortar crushing. In order to better reproduce the observed experimental behavior, a tentative model for the mortar Poisson’s ratio variation upon loading is also presented.     

High-temperature elastic moduli of thermoelectric SnTe1±x – y SiC nanoparticulate composites

In waste heat recovery applications, thermoelectric (TE) generators are subjected to thermal gradients and thermal transients, creating mechanical stresses in the TE legs. Such stresses are functions of the elastic moduli of the TE material. For SnTe_1±x matrices (where x = 0.0 or 0.016) composite specimens with 0–4 vol% SiC nanoparticle (SiC_NP) additions, the elastic moduli (Young’s modulus, shear modulus, and Poisson’s ratio) were measured by resonant ultrasound spectroscopy from room temperature (RT) to 663 K. The effects of matrix composition and the SiC_NP additions on the RT intercepts and the slopes of the elastic modulus as a function of temperature are also discussed.     

Numerical investigations on the mechanical properties of metallic hollow spheres structure with perforations under compression

This paper numerically investigates the compressive mechanical properties of the perforated hollow spheres structures with different geometrical and physical properties. In these structures, the metallic hollow spheres are perforated regularly with several holes, which open the inner sphere volume and surface and are bonded in simple cubic, body-centered cubic and face-centred cubic patterns. The 5×5 cells finite element models under uniaxial compression are established by ABAQUS 6.14 software for simulation. The influence of the spheres’ spatial pattern, as well as base materials for the spheres and bonding necks on the structural mechanical properties are evaluated. By changing the wall thickness, hole diameter and bonding radius in the body-centered cubic packing finite element model, the elastic modulus, Poisson's ratio and initial yield stress are calculated and discussed as functions of these geometrical parameters and their average densities.     

Structural,elastic, and lattice dynamical properties of YB2 compound

In this work, density functional theory calculations on the structural, mechanical, lattice dynamical, and thermodynamical properties of YB₂ in AlB₂-type and monoclinic (C2/m) structures are reported. The local density approximation has been used for modeling exchange–correlation effects. We have predicted the lattice constants, bulk modulus, elastic constants, shear modulus, Young’s modulus, Poison’s ratio, Debye temperature, and sound velocities. Furthermore, the phonon dispersion curves, corresponding phonon density of states, some thermodynamical quantities such as internal energy, entropy, heat capacity, and their temperature-dependent behaviors are presented. Our structural and some other results are in agreement with the available experimental and theoretical data.     

Brazilian disk test and digital image correlation: a methodology for the mechanical characterization of brittle materials

In this paper, an optimized and reliable approach for the evaluation of the mechanical properties of brittle materials is proposed and applied to the characterization of geopolymer mortars. In particular, the Young’s modulus, the Poisson’s ratio and the tensile strength are obtained by means of a Brazilian disk test combined with the digital image correlation (DIC) technique. The mechanical elastic properties are evaluated by a special routine, based on an over-deterministic method and the least square regression, that allows to fit the displacement fields experienced by the samples during the experiment. Error sources, like center of the disk location and rigid-body motion components, were analyzed and estimated automatically with the proposed procedure in order to perform an accurate evaluation of the elastic constants. The strain field measured by DIC and the computed elastic properties were then used to perform a local stresses analysis. This latter was exploited to investigate the failure mechanisms and to evaluate the tensile strength of the investigated material and the obtained data were compared with those predicted by the ASTM and ISMR standards. Three different loading platens (flat, rod and curved) were adopted for the Brazilian test in order to evaluate their effect on the elastic properties calculation, on the failure mechanisms and tensile strength evaluation. Results reveal that the curve platens are the most suitable for the tensile strength calculation, whereas the elastic properties did not show any influence from the loading configuration. Furthermore, the proposed procedure, of easy implementation, allows to accurately calculate Young’s modulus, Poisson’s ratio and the tensile strength of brittle materials in a single experiment.     

Compressive mechanical properties of closed-cell aluminum foam–polymer composites

The compressive mechanical properties of two kinds of closed-cell aluminum foam–polymer composites (aluminum–epoxy, aluminum–polyurethane) were studied. The nonhomogeneous deformation features of the composites are presented based on the deformation distributions measured by the digital image correlation (DIC) method. The strain fluctuations rapidly grow with an increase in the compressive load. The uneven level of the deformation for the aluminum–polyurethane composite is lower than that for the aluminum–epoxy composite. The region of the preferentially fractured aluminum cell wall can be predicted by the strain distributions in two directions. The mechanical properties of the composites are investigated and compared to those of the aluminum foams. The enhancement effect of the epoxy resin on the Young’s modulus, the Poisson’s ratio and the compressive strength of the aluminum foams is greater than that of the polyurethane resin.