微振激励下黏弹性阻尼器微观链结构力学模型

减小微振动对高精密仪器至关重要,利用黏弹性阻尼器进行微振动抑制是一个新兴而又具有挑战性的课题.本文采用分子链网络模型方法分析了黏弹性材料的微观分子链结构,综合考虑材料分子链结构中的网络链和自由链对黏弹性材料力学性能的影响,提出一种基于材料微观分子链结构的微振激励下黏弹性阻尼器力学模型.模型分别采用标准线性固体模型和Maxwell模型来描述网络链和自由链中单个链的力学性能,并分别采用8链网络模型和3链网络模型考虑两种类型分子链的综合效应,引入温频等效原理描述温度对微振激励下黏弹性阻尼器力学性能的影响.该模型能够描述温度和频率对黏弹性阻尼器动态力学性能的影响,并能够反映黏弹性材料的微观结构与材料力学性能的关系.为验证所提模型的有效性及考察黏弹性阻尼器在微振激励下的耗能能力和动态力学性能,在微振条件下对黏弹性阻尼器进行了动态力学性能试验.研究结果表明黏弹性阻尼器具有较好的微振耗能能力,其动态力学性能受温度和频率影响较大,所提的力学模型能够精确地描述微振激励下黏弹性阻尼器动态力学性能随温度和频率的变化关系.      

半晶态聚合物拉伸变形的微观机理

应用大规模分子动力学方法,采用粗粒化聚乙烯醇模型,模拟了晶区与非晶区随机交杂的半晶态聚合物模型系统,研究了半晶态聚合物在单轴拉伸变形过程中的应力-应变行为和微观结构演变.应力-应变曲线表现出4个典型变形阶段:弹性变形、屈服、应变软化和应变强化.在拉伸变形过程中,主要存在晶区折叠链之间的滑移、晶区破坏、非晶区的解缠结,以及分子链沿拉伸方向重新取向等4种主要的微结构演变形式.在屈服点附近,晶区分子链之间排列紧密程度减小而发生滑移,之后晶区变化需要的应力变小,从而形成应变软化现象.随着应变的增大,经各分子链段协同作用使非晶区分子链的解缠结和重新取向行为扩展到相对宏观尺度,导致拉伸应力增大而形成应变强化现象.      

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。     

复合材料周期性线弹性微结构的拓扑优化设计

提出复合材料周期性线弹性微结构拓扑优化设计的模型，模型1设计具有极值弹性特性的复合材料，模型2设计工况最刚微结构单胞。通过该模型和均匀化技术可以获得优化的微结构单胞，进而改善或者得到最优宏观特性的复合材料。为了便于制造和应用，用胞体材料而不是多相材料来得到复合材料的极值弹性特性和最大刚度。优化结果表明，该模型与数值方法相结合可以有效地实现微结构的拓扑优化设计。     

微流控通道内细胞及其初级纤毛的力传导行为

细胞处于复杂的生理环境之下,附着在细胞表面的初级纤毛被认为是重要的力学信号传感器,其与细胞的代谢、发育、分裂和增殖等生理活动密切相关.为了研究细胞及其初级纤毛在微流体环境下的力传导行为,本文建立了力-电协同驱动下的矩形微流控通道和含有多孔黏弹性属性的贴壁细胞有限元模型系统.考察了细胞的细胞质和细胞核在振荡层流下的应力、应变、孔隙压力和孔隙流速等力学信号响应,量化研究了初级纤毛作为细胞独特的力学感受器的生物力学行为. 结果表明:细胞在振荡层流下的力学响应表现出和外加力-电驱动载荷相同的震荡规律.渗透率是细胞多孔弹性力学行为的主要影响因素. 初级纤毛是细胞主要的力学感受器,细胞可以通过纤毛长度和直径调节其力学感受敏感性(应力影响区域),随着初级纤毛长度的增大, 其纤毛挠曲刚度减小, 但是敏感性增大.模型的建立为进一步研究微流体剪切作用下的细胞生长、分化等微观机理提供基础,同时也为检测细胞微结构器(纤毛等蛋白链)的力学性能提供了理论技术支持.      

MECHANOTRANSDUCTION OF THE CELL AND ITS PRIMARY CILIUM IN THE MICROFLUIDIC CHANNEL 1)

细胞处于复杂的生理环境之下,附着在细胞表面的初级纤毛被认为是重要的力学信号传感器,其与细胞的代谢、发育、分裂和增殖等生理活动密切相关.为了研究细胞及其初级纤毛在微流体环境下的力传导行为,本文建立了力-电协同驱动下的矩形微流控通道和含有多孔黏弹性属性的贴壁细胞有限元模型系统.考察了细胞的细胞质和细胞核在振荡层流下的应力、应变、孔隙压力和孔隙流速等力学信号响应,量化研究了初级纤毛作为细胞独特的力学感受器的生物力学行为. 结果表明:细胞在振荡层流下的力学响应表现出和外加力-电驱动载荷相同的震荡规律.渗透率是细胞多孔弹性力学行为的主要影响因素. 初级纤毛是细胞主要的力学感受器,细胞可以通过纤毛长度和直径调节其力学感受敏感性(应力影响区域),随着初级纤毛长度的增大, 其纤毛挠曲刚度减小, 但是敏感性增大.模型的建立为进一步研究微流体剪切作用下的细胞生长、分化等微观机理提供基础,同时也为检测细胞微结构器(纤毛等蛋白链)的力学性能提供了理论技术支持.     

基于弹性力学第一性原理的数据驱动力学建模

提出了一种基于弹性力学第一性原理的数据驱动力学建模方法,其能够从基于弹性力学方程的数值计算结果建立简洁且能准确捕捉变形机制的力学模型。基于有限元计算得到的高精度数据和无监督数据驱动控制方程识别方法Seq-SVF,从梁的载荷和位移数据中自动识别出了Timoshenko梁形式的弯曲控制微分方程,得到了三种不同加载条件下剪切影响系数关于结构尺寸和力学参数的函数表达式。揭示了经典模型适用的加载条件,同时还给出了一种未发现的新模型。通过将基于弹性力学的第一性原理计算与数据驱动范式相结合,克服了传统建模方法的局限性和对人类经验的强依赖性,为建立简洁的力学模型提供了一种新途径。     

多相复合材料微结构布局优化设计

基于对传统双向渐进结构优化(BESO)方法的改进,通过均匀化方法建立微观尺度多相材料布局分布与宏观结构材料弹性张量之间的联系,集成宏观结构所得到的位移场,推导出带有宏观结构力学特性的微观灵敏度。提出以宏观结构最大刚度为目标、对极小尺度周期性排列的材料微结构胞元进行布局优化设计的方法。本文算例结果表明,该方法在微结构多相材料布局优化设计过程中,不仅克服了拓扑优化领域常见的"棋盘格"现象,得到了边界清晰、受力合理的多相材料合理分布布局优化结果,而且整体优化过程稳定,具有很强的工程实际应用价值。     

反应诱导聚合物双层结构弯曲行为的建模与分析

聚合物软材料兼具柔软性和大变形能力,作为一种智能材料在软体机器人等领域应用广泛．化学活性聚合物分子内包含具有较高反应活性的官能团,如环氧、酯键、羟基、羧基等,可以在一定条件下发生化学反应引起材料体积和性质变化．因此研究化学反应如何调控聚合物变形对开发新的功能型聚合物材料有重要指导意义．本文建立化学活性聚合物-弹性基底双层结构的理论模型并分析其在化学反应诱导下的有限弯曲行为．引入化学反应进度作为Helmholtz自由能独立的状态变量,考虑化学反应对聚合物体积变化和模量的影响,以及反应过程独立的能量耗散机制,并基于Neo-Hookean模型建立起各层内的超弹性本构关系．最后利用Newton-Raphson方法对反应完全时的平面应变稳态问题进行数值求解,得到不同几何和反应影响参数下各层内的弯曲变形和应力分布．     

颗粒增强橡胶细观力学性能二维数值模拟

在细观层次上建立了具有随机分布形态的代表性体积单元,通过细观力学有限元方法对炭黑颗粒填充橡胶复合材料的宏观力学性能进行了研究。采用二维平面应变模型进行单轴压缩模拟仿真,通过施加周期边界条件保证了代表性体积单元变形场的协调性。重点研究讨论了颗粒的随机分布形态和粒径大小、刚度、体积分数对复合材料宏观应力-应变关系曲线和有效弹性模量的影响。结果表明:炭黑颗粒的填充显著提升了橡胶材料的刚度,在炭黑含量Vf=0.2513时,复合材料的有效弹性模量值高于橡胶初始模量值的2倍;复合材料的有效弹性模量随颗粒所占体积分数的增加而增大。     

橡胶材料弹性的一种新的螺旋管模型

建立统计力学模型正确描述材料微观结构与宏观力学特性之间的关系是软物质类材料的最大挑战之一,已有的橡胶材料统计模型尚存在一些不足.文章根据橡胶类材料宏观各向同性、连续均匀和不可压缩特性,结合分子链的非高斯统计模型,提出一种橡胶材料网络结构的力学特性模型.该模型将代表体元上对应点之间的传力路径用一个类螺旋管区域约束的分子链子网络来描述,螺旋管的表面随材料的宏观变形做仿射变形,分子链子网络由方向和长度随机的分子链或链段首尾链接而成,在此基础上由分子链的熵推导出描述材料宏观力学特性的本构关系.通过大量的材料测试数据对本构模型进行拟合验证,拟合结果表明该模型具有非常好的精度,并且在采用两个参数时模型具有非常高的可靠性,仅用单轴拉伸实验数据拟合模型就能准确预测全部3类实验数据.该模型使用了仿射的弯曲管假设,能从微观结构尺度上说明材料的不可压缩特性,避免了直管模型的近似性,为微观尺度的随机性和宏观的均匀性的联系提出一个新的模型.     

Elastic–plastic behavior of non-woven fibrous mats

Electrospinning is a novel method for creating non-woven polymer mats that have high surface area and high porosity. These attributes make them ideal candidates for multifunctional composites. Understanding the mechanical properties as a function of fiber properties and mat microstructure can aid in designing these composites. Further, a constitutive model which captures the membrane stress–strain behavior as a function of fiber properties and the geometry of the fibrous network would be a powerful design tool. Here, mats electrospun from amorphous polyamide are used as a model system. The elastic–plastic behavior of single fibers are obtained in tensile tests. Uniaxial monotonic and cyclic tensile tests are conducted on non-woven mats. The mat exhibits elastic–plastic stress–strain behavior. The transverse strain behavior provides important complementary data, showing a negligible initial Poisson's ratio followed by a transverse:axial strain ratio greater than ?1:1 after an axial strain of 0.02. A triangulated framework has been developed to emulate the fibrous network structure of the mat. The micromechanically based model incorporates the elastic–plastic behavior of single fibers into a macroscopic membrane model of the mat. This representative volume element based model is shown to capture the uniaxial elastic–plastic response of the mat under monotonic and cyclic loading. The initial modulus and yield stress of the mat are governed by the fiber properties, the network geometry, and the network density. The transverse strain behavior is linked to discrete deformation mechanisms of the fibrous mat structure including fiber alignment, fiber bending, and network consolidation. The model is further validated in comparison to experiments under different constrained axial loading conditions and found to capture the constraint effect on stiffness, yield, post-yield hardening, and post-yield transverse strain behavior. Due to the direct connection between microstructure and macroscopic behavior, this model should be extendable to other electrospun systems and other two dimensional random fibrous networks.     

Reprint of “Size dependence and orientation dependence of elastic properties of ZnO nanofilms” [In. J. Solids Struct. 45 (2008) 1730–1753]

The elastic properties of ZnO nanofilms with different film thickness, surface orientations and loading directions are investigated by using molecular mechanics (MM) method. The size dependence of elastic properties is relevant to both the film surface crystallographic orientation and loading direction. Both atomic structure analysis and energy calculation are employed to identify the mechanisms of the size-dependent elastic properties, under different loading directions and surface orientations. Upon small axial deformation, the relationship between intralayer and interlayer bond length variation and film elastic stiffness is established; it is found that the atomic layers with larger bond length variation have higher elastic stiffness. The strain energies of atomic layers of ZnO nanofilm and bulk are decoupled, from which the stiffness of film surface, intralayers, and interlayers are derived and compared with their bulk counterparts. The surface stiffness is found to be much lower than that of the interior layers and bulk counterpart, and with the decrease of film thickness, the residual tension-stiffened interior atomic layers are the main contributions of the increased elastic modulus of ZnO nanofilms.     

Size dependence and orientation dependence of elastic properties of ZnO nanofilms

The elastic properties of ZnO nanofilms with different film thickness, surface orientations and loading directions are investigated by using molecular mechanics (MM) method. The size dependence of elastic properties is relevant to both the film surface crystallographic orientation and loading direction. Both atomic structure analysis and energy calculation are employed to identify the mechanisms of the size-dependent elastic properties, under different loading directions and surface orientations. Upon small axial deformation, the relationship between intralayer and interlayer bond length variation and film elastic stiffness is established; it is found that the atomic layers with larger bond length variation have higher elastic stiffness. The strain energies of atomic layers of ZnO nanofilm and bulk are decoupled, from which the stiffness of film surface, intralayers, and interlayers are derived and compared with their bulk counterparts. The surface stiffness is found to be much lower than that of the interior layers and bulk counterpart, and with the decrease of film thickness, the residual tension-stiffened interior atomic layers are the main contributions of the increased elastic modulus of ZnO nanofilms.     

MSC折纸结构设计及其双稳态解析与实验

折纸是一门古老艺术,其本质是将平面材料沿着事先设计好的折痕进行折叠,进而形成一个复杂的三维结构．柔性折纸结构是实现三维结构轻量化的重要途径．因此,解析折纸结构几何特性和力学性质十分必要．本文以MSC(Magic Spiral Cube)为研究对象,通过实际折痕和虚拟折痕的方法,建立了该结构的几何模型,确定了实现完全展开和完全折叠对刚性面和可变形面的设计条件,在虚拟折痕上引入了扭转刚度,证明了该扭转刚度与柔性面变形的等效性,从而得到了MSC 折纸结构的弹性势能,得到了使结构变形的力与位移本构．通过力学特性分析,发现了MSC折纸结构具有双稳态特性,这种特性是由面内变形诱发的,与虚拟折痕刚度与弹性折痕刚度的比值有直接的关系．最后,我们对MSC折纸结构进行设计和制备,通过实验,验证了理论 模型的准确性．本文的研究结果不仅进一步加深了我们对于MSC折纸结构力学特性的认识,同时也为其工程应用提供理论基础．     

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.      

Virtual improvement of ice cream properties by computational homogenization of microstructures

Optimal shape design of microstructured materials has recently attracted a great deal of attention in materials science. The shape and the topology of the microstructure have a significant impact on the macroscopic properties. This paper presents different computational models of random microstructures, to virtually improve the physical properties of ice cream. Several sensory properties of this heterogeneous material issued from food industry are directly controlled by the elastic and thermal conducting ones. The material effective elastic and thermal conducting properties are obtained through direct large scale numerical simulations. The different formulations address the problem of finding the shape of the representative microstructural element for random heterogeneous media that increase the elastic moduli and thermal conductivity compared to existing products. The computational models are established using finite element method and images of virtual microstructures. In this paper we propose a new model of microstructures. This model is constructed with hexagonal prismatic rods and plates with volume fractions around 0.7 for the hard phase represented by hexagons of ice. A comparison between three two-phase elastic heterogeneous microstructures models is drawn. This illustrates the concept of design of microstructures using computational homogenization tools.     

Computing elastic moduli on 3-D X-ray computed tomography image stacks

A numerical task of current interest is to compute the effective elastic properties of a random composite material by operating on a 3D digital image of its microstructure obtained via X-ray computed tomography (CT). The 3-D image is usually sub-sampled since an X-ray CT image is typically of order 1000³ voxels or larger, which is considered to be a very large finite element problem. Two main questions for the validity of any such study are then: can the sub-sample size be made sufficiently large to capture enough of the important details of the random microstructure so that the computed moduli can be thought of as accurate, and what boundary conditions should be chosen for these sub-samples? This paper contributes to the answer of both questions by studying a simulated X-ray CT cylindrical microstructure with three phases, cut from a random model system with known elastic properties. A new hybrid numerical method is introduced, which makes use of finite element solutions coupled with exact solutions for elastic moduli of square arrays of parallel cylindrical fibers. The new method allows, in principle, all of the microstructural data to be used when the X-ray CT image is in the form of a cylinder, which is often the case. Appendix A describes a similar algorithm for spherical sub-samples, which may be of use when examining the mechanical properties of particles. Cubic sub-samples are also taken from this simulated X-ray CT structure to investigate the effect of two different kinds of boundary conditions: forced periodic and fixed displacements. It is found that using forced periodic displacements on the non-geometrically periodic cubic sub-samples always gave more accurate results than using fixed displacements, although with about the same precision. The larger the cubic sub-sample, the more accurate and precise was the elastic computation, and using the complete cylindrical sample with the new method gave still more accurate and precise results. Fortran 90 programs for the analytical solutions are made available on-line, along with the parallel finite element codes used.     

Elastic behaviour of a regular lattice for meso-scale modelling of solids

A modelling strategy is proposed to link the meso-scale mechanical response of a solid material to the macroscopic material behaviour. The model is based on a regular lattice of truncated octahedral cells, with sites at the cell centres linked by two sets of bonds. The relationship between the macroscopic elastic behaviour of the model and the elastic properties of the bonds is studied numerically. The results demonstrate that, in contrast to previously proposed lattice arrangements, any elastic properties of metallic or cementitious materials can be obtained by appropriate selection of the axial and the shear stiffness of the bonds. Discussion of the modelling approach includes the potential of the site-bond model to simulate the evolution of damage driven not only by mechanical deformation but also by processes that involve the interaction of different mechanisms.