利用三维随机模型分析沥青混合料的蠕变行为

将沥青混合料看作由粗骨料和沥青砂组成的两相复合材料,根据给定的级配生成凸多面体骨料,然后利用随机投放算法建立沥青混合料试样的三维随机模型.采用广义Maxwell模型刻画沥青砂的本构行为,其参数通过单轴蠕变实验获得.在对三维随机模型的有效性进行验证之后,采用参数化建模方法建立包含不同骨料分布、含量和级配的沥青混合料有限元...     

Size effect on the static behavior of electrostatically actuated microbeams

We present a new analytical model for electrostatically actuated microbeams to explore the size effect by using the modified couple stress theory and the minimum total potential energy principle. A material length scale parameter is introduced to represent the size-dependent characteristics of microbeams. This model also accounts for the nonlinearities associated with the mid-plane stretching force and the electrostatical force. Numerical analysis for microbeams with clamped-clamped and cantilevered conditions has been performed. It is found that the intensity of size effect is closely associated with the thickness of the microbeam,and smaller beam thickness displays stronger size effect and hence yields smaller deffection and larger pull-in voltage. When the beam thickness is comparable to the material length scale parameter,the size effect is significant and the present theoretical model including the material length scale parameter is adequate for predicting the static behavior of microbeam-based MEMS.     

Micromechanical analysis of the behavior of stiff clay

Cementations formed in geological timescale are observed in various stiff clays.A micromechanical stress strain model is developed for modeling the effect of cementation on the deformation behavior of stiff clay.The proposed approach considers explicitly cementations at intercluster contacts,which is different from conventional model.The concept of inter-cluster bonding is introduced to account for an additional cohesion in shear sliding and a higher yield stress in normal compression.A damage law for inter-cluster bonding is proposed at cluster contacts for the debonding process during mechanical loading.The model is used to simulate numerous stress-path tests on Vallericca stiff clay.The applicability of the present model is evaluated through comparisons between the predicted and the measured results.In order to explain the stress-induced anisotropy arising from externally applied load,the evolution of local stresses and local strains at inter-cluster planes are discussed.     

Nonlinear study of the dynamic behavior of a string-beam coupled system under combined excitations

In this paper,the nonlinear dynamic behavior of a string-beam coupled system subjected to external,parametric and tuned excitations is presented.The governing equations of motion are obtained for the nonlinear transverse vibrations of the string-beam coupled system which are described by a set of ordinary differential equations with two degrees of freedom.The case of 1:1 internal resonance between the modes of the beam and string,and the primary and combined resonance for the beam is considered.The method of multiple scales is utilized to analyze the nonlinear responses of the string-beam coupled system and obtain approximate solutions up to and including the second-order approximations.All resonance cases are extracted and investigated.Stability of the system is studied using frequency response equations and the phase-plane method.Numerical solutions are carried out and the results are presented graphically and discussed.The effects of the different parameters on both response and stability of the system are investigated.The reported results are compared to the available published work.     

Adhesion behavior of endothelial progenitor cells to endothelial cells in simple shear flow

The adhesion of endothelial progenitor cells(EPCs) on endothelial cells(ECs) is one of the critical physiological processes for the regenesis of vascular vessels and the prevention of serious cardiovascular diseases.Here,the rolling and adhesion behavior of EPCs on ECs was studied numerically.A two-dimensional numerical model was developed based on the immersed boundary method for simulating the rolling and adhesion of cells in a channel flow.The binding force arising from the catch bond of a receptor and ligand pair was modeled with stochastic Monte Carlo method and Hookean spring model.The effect of tumor necrosis factor alpha(TNF-α) on the expression of the number of adhesion molecules in ECs was analyzed experimentally.A flow chamber system with CCD camera was set up to observe the top view of the rolling of EPCs on the substrate cultivated with ECs.Numerical results prove that the adhesion of EPC on ECs is closely related to membrane stiff-ness of the cell and shear rate of the flow.It also suggests that the adhesion force between EPC and EC by P-selectin glycoprotein ligand-1 only is not strong enough to bond the cell onto vessel walls unless contributions of other catch bond are considered.Experimental results demonstrate that TNF-α enhanced the expressions of VCAM,ICAM,P-selectin and E-selectin in ECs,which supports the numerical results that the rolling velocity of EPC on TNF-α treated EC substrate decreases obviously compared with its velocity on the untreated one.It is found that because the adhesion is affected by both the rolling velocity and the deformability of the cell,an optimal stiffness of EPC may exist at a given shear rate of flow for achieving maximum adhesion rates.     

利用三维随机模型分析沥青混合料的蠕变行为

将沥青混合料看作由粗骨料和沥青砂组成的两相复合材料,根据给定的级配生成凸多面体骨料,然后利用随机投放算法建立沥青混合料试样的三维随机模型.采用广义Maxwell模型刻画沥青砂的本构行为,其参数通过单轴蠕变实验获得.在对三维随机模型的有效性进行验证之后,采用参数化建模方法建立包含不同骨料分布、含量和级配的沥青混合料有限元模型,通过数值模拟研究骨料分布、含量和级配对沥青混合料蠕变行为的影响.结果表明:骨料分布对沥青混合料力学性质的影响较小;沥青混合料瞬时弹性模量随骨料含量的增加呈近似线性增大,而且弹性模量与骨料含量之间的关系曲线处于Paul上下边界内;在计算考虑的粒径范围内,骨料平均粒径越小,沥青混合料抵抗变形的能力越强.     

偏压及测试环境对离子束DLC膜摩擦学行为的影响

采用线性阳极层离子束技术制备了类金刚石薄膜(DLC膜),研究了不同衬底负偏压和测试环境对DLC薄膜摩擦学性能的影响.结果表明:在-50V偏压下,薄膜硬度和弹性模量最大,这主要与薄膜中高的sp³含量相关;衬底负偏压对薄膜在室温大气条件下的摩擦学性能影响不显著,薄膜总体呈现较低的摩擦系数和磨损率,显示出优异的抗磨损性能;线性离子束制备的含氢DLC薄膜的摩擦学行为受湿度及环境气氛影响较大,归因于环境中的氧气和水分造成的摩擦化学反应.     

金属-碳管复合结构的计算力学研究

系统地研究了金属-碳纳米管复合结构的力学行为,考察了单轴压缩载荷作用下填充管的临界屈曲应变对管内金属原子数目的依赖性,分析了管的几何特征,包括管径、管长及手性,对填充管变形与力学行为的影响,并与连续体力学模型的预测进+行了对比分析.本文的研究结果对金属-碳管复合结构的理论研究和工程应用都具有较好的指导意义.     

决定行人行为和拥挤灾难的简单规律

随着聚众事件的规模和发生次数的增加,对拥挤灾难的研究及人群流动的模拟成为重要的研究领域.但是,即使那些成功的模型方法(如由牛顿力模型发展起来的方法),仍不能符合观察的结果.由法国、瑞士和英国的科学家组成的研究组提出一种认知科学的方法,这种方法是建立在行为探索法的基础上的.他们认为行人根据视觉获得的信息,也就是根据行人与眼     

低振幅中心对称光折变暗和灰空间孤子的时间特性

为了研究中心对称光折变晶体中低振幅暗和灰空间孤子的时间特性,推导出了含时间参量的空间电荷场和光波的动态演化方程.采用数值方法,得到了中心对称光折变晶体中低振幅空间暗和灰孤子强度包络和强度半峰全宽的时间演化特性.结果表明,初始阶段形成的宽度较宽的孤子,其宽度随时间单调递减到一个最小值直至稳态孤子的形成;在相同的演化时间内...