Surface deformation-dependent mechanical properties of bending nanowires: an ab initio core-shell model

An ab initio core-shell model is proposed to evaluate the surface effect in bending nanowires, in which the elastic modulus depends on the surface relaxation and deformation induced by external loading. By using first-principles calculations based on the density functional theory(DFT), the surface and bulk properties are calculated for Ag, Pb, and Si nanowires. The obtained theoretical predictions of the effective Young’s modulus of nanowires agree well with the experimental data, which shows th...     

SIZE EFFECTS OF ELASTIC MODULUS OF FCC METALS BASED ON THE CAUCHY-BORN RULE AND NANOPLATE MODELS

In the present research, a simple quasi-continuum model, the Cauchy-Born rule model, is used to investigate the size effects of elastic modulus for fcc metals. By considering a nanoplate model and calculating the strain energy for the nano-sized plate under tension and bending, the relationship between the elastic modulus and the plate thickness is found. Size effects of the elastic modulus are displayed by the relative differences of the elastic modulus between the nano-sized plate sample and the bulk sample. By comparing the present results with those of others, the effectiveness of the Cauchy-Born rule model in studying the size effects of material properties are shown.     

砂岩球体法向恢复系数实验研究

法向恢复系数是岩崩块石运动分析的关键参数,其取值直接决定了块石的运动轨迹。本文中采用自行设计的碰撞实验装置和声频采样技术,测定了砂岩球体碰撞的法向恢复系数,研究了粒径、碰撞速度、含水状态和板的弹性特性4个因素对恢复系数的影响。结果表明:砂岩球体法向恢复系数存在复杂的尺寸效应,恢复系数随粒径的增大先增大后减小;碰撞过程中存在的黏弹性耗能机理和弹塑性损伤耗能机理共同作用产生了复杂的尺寸效应;受砂岩非均质特性的作用,粒径较小时,恢复系数的速度效应较明显(随速度增大而增大),粒径较大时速度对恢复系数的影响消失;砂岩饱和使黏弹性耗能和弹塑性损伤耗能增加,使恢复系数比风干时低;等效弹性模量对恢复系数的影响较大,等效弹性模量越大,法向恢复系数越小。     

Sample boundary effect in nanoindentation of nano and microscale surface structures

Nanoindentation is a widely used technique to characterize mechanical properties of materials in small volumes. When the sample size is comparable to the indent size, the indentation-induced plastic zone can be affected by the sample boundary which may cause inaccurate interpretation of the mechanical properties. In this study, the sample boundary effect is investigated by performing experiments and atomistic simulations of nanoindentation into nano- and micro-scale Au pillars and bulk Au (0 0 1) surfaces. In experiments, a more compliant deformation is observed in pillar indentations compared to bulk Au. The elastic modulus decreases with increasing indent size over sample size ratio. Atomistic simulations are performed to gain insights on the mechanisms of pillar deformation and pillar boundary effect. The reduced modulus has a similar trend of decrease with increasing indent size over sample size ratio. Significantly different dislocation activities and dislocation interactions with the pillar boundary contribute to the lower value of the reduced modulus in the pillar indentation. The presence of the free surface would allow the dislocations to annihilate, causing a higher plastic recovery during the pillar unloading process.     

Elastic properties of chiral,anti-chiral,and hierarchical honeycombs:A simple energy-based approach

The effects of two geometric refinement strategies widespread in natural structures, chirality and self-similar hierarchy, on the in-plane elastic response of two-dimensional honeycombs were studied systematically. Simple closed-form expressions were derived for the elastic moduli of several chiral, antichiral, and hierarchical honeycombs with hexagon and square based networks. Finite element analysis was employed to validate the analytical estimates of the elastic moduli. The results were also compared with the numerical and experimental data available in the literature. We found that introducing a hierarchical refinement increases the Young's modulus of hexagon based honeycombs while decreases their shear modulus. For square based honeycombs, hierarchy increases the shear modulus while decreasing their Young's modulus. Introducing chirality was shown to always decrease the Young's modulus and Poisson's ratio of the structure. However, chirality remains the only route to auxeticity. In particular, we found that anti-tetra-chiral structures were capable of simultaneously exhibiting anisotropy, auxeticity,and remarkably low shear modulus as the magnitude of the chirality of the unit cell increases.     

Shear stability of metallic glasses

The effect of sample size on the shear deformation and compressive plasticity of different metallic glasses were investigated. The experimental results showed that the deformation and fracture behaviors of samples prepared from chemically identical Zr-, Ti-, Fe-, or Mg-based metallic glass ingots were strongly dependent on the sample size and machine stiffness, and a super-high compressive plasticity was achieved in the Zr-based metallic glasses with sample size of 1.0 mm in width. It is also found that the sample size can significantly influence the density of elastic energy dissipated in the shear band: with sample size decreasing and machine stiffness increasing, the density of the elastic energy dissipated in the shear band of metallic glasses is prominently decreased, thus the shear deformation turns to be more stable, resulting in the improvement of plasticity in ductile metallic glasses and the transition from fragmentation fracture to shear fracture in brittle metallic glasses. This finding is important for the potential applications of the present metallic glasses and for designing new metallic glasses with better mechanical properties.     

纳米双晶铜单向拉伸弹性性能的应变率效应和尺寸效应

利用分子动力学方法研究了双晶铜在单向拉伸载荷作用下弹性性能的应变率效应和尺寸效应.结果表明:随着加载应变率的改变,纳米双晶铜的初始弹性模量在低应变率区和高应变率区表现出不同的规律.在低应变率区,弹性模量基本不随应变率的变化而变化;但在高应变率区,弹性模量随应变率的增大而增大,弹性模量和加载应变率之间满足近似的对数线性关系.因此,存在一个控制弹性模量应变率效应的应变率阈值.进一步的研究发现,双晶纳米铜的应变率阈值随着截面尺寸的增大而减小,具有明显的尺寸依赖性.     

Simulation of flow around a thin,flexible obstruction by means of a deforming grid overlapping a fixed grid

This paper presents a numerical method for simulation of coupled flows, in which the fluid interacts with a thin deformable solid, such as flows in cardiovascular valves. The proposed method employs an arbitrary Lagrangian–Eulerian (ALE) method for flow near the solid, embodied in the outflow domain in which the mesh is fixed. The method was tested by modelling a two‐dimensional channel flow with a neo‐Hookean obstacle, an idealization of the coupled flow near a cardiovascular valve. The effects of the Reynolds number and the dimensionless elastic modulus of the material on the wall shear stress, the size of the downstream reverse flows, and the velocity and pressure profiles were investigated. The deformation of the obstacle, the pressure drop across the obstacle, and the size of the top reverse flow increased as the Reynolds number increased. Conversely, increasing the elastic modulus of the obstacle decreased the deformation of the obstacle and the size of the top reverse flows, but did not affect the pressure drop across the obstacle over the range studied. Copyright © 2007 John Wiley & Sons, Ltd.     

An expanding cavity model incorporating strain-hardening and indentation size effects

An expanding cavity model (ECM) for determining indentation hardness of elastic strain-hardening plastic materials is developed. The derivation is based on a strain gradient plasticity solution for an internally pressurized thick-walled spherical shell of an elastic power-law hardening material. Closed-form formulas are provided for both conical and spherical indentations. The indentation radius enters these formulas with its own dimensional identity, unlike that in classical plasticity based ECMs where indentation geometrical parameters appear only in non-dimensional forms. As a result, the newly developed ECM can capture the indentation size effect. The formulas explicitly show that indentation hardness depends on Young’s modulus, yield stress, strain-hardening exponent and strain gradient coefficient of the indented material as well as on the geometry of the indenter. The new model reduces to existing classical plasticity based ECMs (including Johnson’s ECM for elastic–perfectly plastic materials) when the strain gradient effect is not considered. The numerical results obtained using the newly developed model reveal that the hardness is indeed indentation size dependent when the indentation radius is very small: the smaller the indentation, the larger the hardness. Also, the indentation hardness is seen to increase with the Young’s modulus and strain-hardening level of the indented material for both conical and spherical indentations. The strain-hardening effect on the hardness is observed to be significant for materials having strong strain-hardening characteristics. In addition, it is found that the indentation hardness increases with decreasing cone angle of the conical indenter or decreasing radius of the spherical indenter. These trends agree with existing experimental observations and model predictions.     

沥青混凝土级配曲线的数值设计方法与对有效弹性模量影响的研究

沥青混凝土有效性能的预测和评估是其在道路工程与大坝防渗等应用中的关键问题.论文利用细观随机建模技术和有限元方法研究了级配曲线对沥青混凝土有效弹性模量的影响.首先,基于改进的蒙特卡洛法分别实现了服从工程筛分级配曲线或给定函数级配曲线的沥青混凝土细观随机模型的高效生成;然后,利用建立的模型分析了骨料的含量、弹性模量和试样尺...     

颗粒介质弹性参数测量方法及装置研究

本文发展了一种实施简便的颗粒介质弹性参数测量方法,设计并加工了基于该方法的实验装置。利用加载设备对该实验装置在一个方向上加卸载,在加载方向和与之垂直的另一方向上测量载荷和位移后即可获得颗粒介质的等效弹性模量和泊松比。该实验装置的容器还可方便地调节尺寸,适用于测量不同粒径颗粒堆积在不同大小容器时的弹性参数。本文还研究了尺寸效应的影响,分析了容器尺寸和颗粒粒径分别对介质弹性参数的影响规律。     

纳米金属丝拉伸破坏及其应变率效应

建立了适于研究纳米金属快速变形破坏过程的分子动力学模型,并对不同应变率工况下不同截面尺寸单晶镍纳米丝的零温单向拉伸破坏过程进行了分子动力学模拟.模拟得到各种纳米镍丝的应力-应变曲线、屈服应变、屈服强度、断裂强度和初始弹性模量,提出了纳米金属丝快速变形力学性能的应变率效应预测公式并加以验证.计算表明金属纳米丝的屈服应变与尺寸和应变率无关,屈服强度、断裂强度和弹性模量与应变率呈对数关系.     

光斑尺寸对K9玻璃近红外激光损伤阈值的影响

基于热弹性模型 ,用有限元法对不同光斑近红外激光辐照下K9玻璃样品中的温度和压力分布进行了计算。对厚 2mm、半径 10mm的圆片样品的计算结果表明 ,K9玻璃的损伤受表面环向拉伸应力的控制 ,光斑大小对损伤阈值有较大的影响 ,对光束总入射功率为 2 8kW的激光辐照 ,当光斑半径从 0 .4mm增加到 3mm时 ,损伤所需时间从 14ms增加到 1s。在一定光斑尺寸范围内 ,样品损伤 (表面拉伸断裂 )所需的激光功率密度在同一量级内变化 ,在更小光斑时 ,损伤时的最高温升趋于 5 6 5K。     

单纤维微拉伸力学性能测试与分析

针对微尺度材料力学性能测试与尺度效应实验研究的需要,自行研制了一台FMT-I型高精度纤维材料微拉伸力学性能实验装置,并基于LabVIEW软件平台开发了相应的数据采集与控制系统,实现了测试过程的全自动化。该装置的测力传感器由非接触式激光位移传感器和两端固定的薄梁组成,可同时测量试样的拉伸力和上夹持端的位移量,帮助精确地获取试样的载荷-变形曲线。采用该装置对微米级直径的多晶铜丝、316L不锈钢纤维和T300碳纤维进行了拉伸测试。实验结果表明,直径为18～105μm多晶铜丝的拉伸力学行为并无明显的尺度效应;多晶铜丝和316L不锈钢纤维的弹性模量分别在43.9～60.0GPa和102.9～111.5GPa之间,均低于宏观尺度下材料的弹性模量;316L不锈钢纤维的抗拉强度和延伸率随着丝径的减小而降低;T300碳纤维的弹性模量为235.4±12.4GPa,抗拉强度为3238.2±280.8MPa,断裂应变约为1.5%。另外,相同的细铜丝材料的测试结果与Instron5848型商用拉伸试验机的测试结果进行了对比,吻合良好。通过系统的实验分析表明,该装置具有较高的精度和稳定性,适用于各种纤维材料的拉伸力学性能测试。     

实心砖石古塔动力特性与结构损伤分析

为研究“丝绸之路”起点世界文化遗产建筑兴教寺测师塔的动力性能及结构损伤,采用超低频动态测试系统进行了原位动力测试试验.在环境随机激励下,采集了该塔各楼层顶部水平振动的速度响应信号,经滤波后通过积分变换进行自谱及互谱分析,得到了结构沿水平方向的前2阶自振频率与振型.并依据结构测绘结果,建立测师塔数值模型,计算了弹性模量逐渐降低时的振动特性,并与测试结果进行对比,依据结构损伤参数识别的改进形法,进行了测师塔结构损伤分析.结果表明,该塔沿东西与南北两个水平方向的前2阶频率值相近,第1阶振型呈弯曲型,第2阶振型呈弯剪型;与无损砌体弹性模量的取值比较,测师塔结构的等效弹性模量降低较多,其结构整体损伤较为严重.因此,可通过确定无损砌体的弹性模量,采用动力测试及数值计算依据等效弹性模量进行残损砖石古塔结构损伤识别.     

缝纫复合材料层合板面内弹性模量分析

基于对缝纫孔附近局部细观结构的分析,提出了一种预测缝纫复合材料层合板面内力学性能的理论模型．从分析缝孔单胞的纤维弯曲几何特征入手,最终得到单向板及层合板的弹性常数．通过有限元分析研究了缝纫参数对复合材料层合板面内等效模量的影响．研究结果表明,缝纫造成单向板及层合板面内材料性质的不均匀,随着缝纫密度和缝纫线直径的增加,层合板的等效模量逐渐降低．     

2-D Normal compliances for elastic and visco-elastic binder contact with finite particle size effect

The close-form 2-D normal force–displacement compliance relation (binder contact law) is derived for a system of two elastic cylindrical particles bound by an elastic or visco-elastic binder based on the approach developed by Zhu; Zhu and Zhu. A new result of finite particle size effect on the compliance is also obtained. One important application of this binder compliance is in the area of the homogenization analysis of fibrous composites, and computation of the binder compliance based effective transverse bulk modulus is conducted in this article with its comparison to the corresponding upper and lower bounds.     

基于高速3D-DIC技术的砂岩动力特性粒径效应研究

利用分离式霍普金森压杆（SHPB）,对粗砂岩、中等粒径砂岩和细砂岩进行了应变率为69～83 s–1的动态单轴抗压实验,研究了粒径尺寸效应对砂岩动力特性的影响。通过三维数字图像相关（3D-DIC）技术分析高速摄像图像,获得了砂岩的实时应变场,据此分析了动态荷载下3种粒径砂岩的动力变形特性和裂纹开展行为。结果表明,砂岩弹性应变储能可逆释放的临界应变率随着粒径的减小而升高,动态压缩强度随着粒径减小而增大,动态强度应变率敏感度则与强度规律相反。相较于静态条件下,中等粒径砂岩和细砂岩的动态弹性模量增长了2～3倍,粗砂岩的动态弹性模量增长达5倍以上。细砂岩的动态泊松比相较于静态提高了约25%,中等粒径砂岩的动态泊松比约为静态时的70%。动态裂纹首先出现于试件内部,然后传播至表面,呈现出应变局部化,动态荷载下岩石裂纹的孕育和扩展相比静态条件下均有所提前,其中细砂岩在动态荷载条件下的归一化裂纹起裂阈值仅为峰值强度的10%。微观分析表明,矿物粒径大小和黏土矿物含量分别在砂岩的动力力学性质和裂纹开展行为方面发挥主要作用。     

辽西风积土动力特性实验研究

通过一系列动态三轴实验,研究了原状饱和辽西风积土在不同固结压力条件下的动态强度、动态弹性模量和阻尼比。结果表明,辽西风积土的破坏动态强度随着固结围压和固结应力比的增大而增大,而动态剪切强度明显区别于静态剪强度;其动态弹模量随着固结应力比和固结围压的增加而增加,而阻尼比随着固结应力比和固结围压的增大而减小。同时,分别拟合得到最大动态弹模量和最大阻尼比与有效固结应力的关系式,并且对动态弹模量和阻尼比进行了归一化处理。     

Representative elementary volume analysis of polydisperse granular packings using discrete element method

The representative elementary volume (REV) for three-dimensional polydisperse granular packings was determined using discrete element method simulations. Granular mixtures of various sizes and particle size distributions were poured into a cuboid chamber and subjected to uniaxial compression. Findings showed that the minimum REV for porosity was larger compared with the REV for parameters such as coordination number, effective elastic modulus, and pressure ratio. The minimum REV for porosity and other parameters was found to equal 15, 10, and 5 times the average grain diameter, respectively. A study of the influence of sample size on energy dissipation in random packing of spheres has also confirmed that the REV size is about 15 times the average grain diameter. The heterogeneity of systems was found to have no effect on the REV for the parameters of interest for the narrow range of coefficient of uniformity analyzed in this paper. As the REV approach is commonly applied in both experimental and numerical studies, determining minimum REV size for polydisperse granular packings remains a crucial issue.