Kinematostatic modeling of compliant parallel mechanisms

The benefits of compliant mechanisms in terms of precision are not easy to exploit because of the limitations of the existing kinematic models used to analyze them. In practice, compliant mechanisms are more sensitive to external wrenches than conventional mechanisms. In this paper, based on the kinematic constraints and the static equilibrium between the joint coordinates and the external wrenches, a general kinematostatic model of compliant parallel mechanisms is presented. Then, this model is differentiated to provide a quasi-static model that makes it possible to calculate the variation of the pose as a linear function of the motion of the actuators and the variation of the external loads through two new matrices: the quasi-static Jacobian matrix and the Cartesian compliance matrix that give a simple and meaningful formulation of the model of the mechanism.     

The role of rebound spikes in the maintenance of self-sustained neural spiking activity

Nonlinear Dynamics - In general, the mechanisms that maintain the activity of neural systems after a triggering stimulus has been removed are not well understood. Different mechanisms involving at...     

生物网络中的随机变异机制与度负关联性的关系

在生物进化中,复制和变异是两个重要机制。在本文中,我们主要研究变异对生物网络度负关联性的影响。我们讨论了多种随机变异机制,如随机节点删除变异机制、随机删边变异机制、随机加边变异机制和随机重组边变异机制。我们的结论是:随机节点删除变异机制和随机删边变异机制都不会强烈地影响生物网络的度负关联性,随机加边变异机制和随机重组边变异机制都会抑制生物网络的度负关联性。      

The Relationship between Random Divergence Mechanisms and Disassortative Mixing Property in Biological Networks

在生物进化中,复制和变异是两个重要机制。在本文中,我们主要研究变异对生物网络度负关联性的影响。我们讨论了多种随机变异机制,如随机节点删除变异机制、随机删边变异机制、随机加边变异机制和随机重组边变异机制。我们的结论是：随机节点删除变异机制和随机删边变异机制都不会强烈地影响生物网络的度负关联性,随机加边变异机制和随机重组边变异机制都会抑制生物网络的度负关联性。     

TZP陶瓷在水润滑下的磨损机制转变图

在ＳＲＶ磨损试验机上对ＴＺＰ陶瓷在水润滑条件下的摩擦学行为进行了试验研究。用磨损机制转变图概括了ＴＺＰ陶瓷与５２１００钢在水润滑时于不同运行条件下的磨损机制。     

Mechanisms of prestressed reticulate systems with unilateral stiffened components

The stability of space reticulate systems is dependent on the existence of mechanisms. The methods that have been developed to determine them are mainly based on the calculation of a basis of the mechanisms' vectorial subspace by computing the kernel of the transpose equilibrium matrix of the structure. However, they can only consider a bilateral stiffness of the members, which applies to the case for systems composed of bars with traction and compression stiffness. Nevertheless, some classes of reticulate systems, like tensegrity systems, use unilateral rigidity components such as cables. The objective of this paper is to develop a method for calculating the mechanisms which can take into consideration the presence of components with unilateral rigidity. In this case, specific mechanisms associated with these elements may appear; these are referred to as “unilateral mechanisms”. An approach is therefore proposed in order to write a basis of their vectorial subspace. It is included in a methodology devoted to the analysis of space structures with initial stresses. The process is based on the identification of the possible prestress states and of the bilateral mechanisms and, next, to the characterization of the unilateral mechanisms.     

On the Failure and Dynamic Performance of Materials

Waves induced by impact initiate deformation mechanisms within a material that precede later flow. An impulse excites a cascade of deformation mechanisms starting with ultrafast and concluding with slower ones. In metals, brittle glasses and polycrystalline ceramics there are a combination of mechanisms with differing relaxation times that condition a loaded target. In the case of ballistic impact, once failure has occurred, long rod penetration can occur and the depth achieved within each target can be scaled with the deformation strengths recorded during the initial high pressure impulse. A review of material shock response and target preconditioning shows a correlation with the ballistic penetration of the target after loading. This indicates that the effect of an initial loading impulse upon material behaviour is a strong feature of the effects observed in many dynamic phenomena.     

Studies of the mechanisms for buckling and waving in aluminum extrusion by use of a Lagrangian FEM software

The background for this paper is the increasing demands from the extrusion market on tighter geometric tolerances on extruded sections. This calls for an improved understanding of the underlying mechanisms causing shape variations. The focus in this paper has been on buckling or waving of extruded flat sections. In order to study these basic mechanisms a Lagrangian FEM software, MSC SuperForm, has been utilized. The flow conditions have been altered by only varying the feeder geometry. Extrusion experiments have been performed in order to verify both the simulations and the mechanisms observed in the FEM simulations.     

Six-bar motion I. The Watt mechanism

Algebraic, geometric and kinematic properties are derived for the curves generated by points on the floating links of plane, six-link mechanisms with turning pairs. In part I the motion obtained from the Watt kinematic chain has been analyzed; parts II and III are concerned with the motions obtained from the Stephenson chain and extensions involving eight-bar and 2n-bar mechanisms.     

Fragmentation of liquid and liquid-plastic media under unsteady strains

Based on a qualitative analysis of results of experimental studies, the main mechanisms of fragmentation of polar liquids and liquid-plastic media under dynamic loading are determined. In the case of low-viscosity liquids, such mechanisms are thermodynamic instability of foam, hydrodynamic instability of initial disturbances of the free surface, and the action of capillary forces. In the case of a polar high-viscosity liquid, the main mechanisms are shear instability of the structure, responsible for stratification of the medium along the lines of local failure of structural viscosity, and the action of capillary forces. The main mechanisms acting in liquid-plastic structured media (gels) are “spalling” in the zone of tensile stresses if the time of their formation is smaller than the time needed for the gel to transform to the sol state, as well as thermodynamic instability of foam and the action of capillary forces after the medium transition to the sol state. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 4, pp. 62–68, July–August, 2007.     

二维材料纳米尺度摩擦行为及其机制

二维材料是指厚度仅有单层或数层原子的晶体或非晶材料,其优异的物理、力学和化学性能给纳米尺度超薄固体润滑材料的设计和发展带来了新的契机。同时,二维材料独特而简单的拓扑结构也为深入了解摩擦的微观机制提供了一个理想的对象。本文综述了以石墨烯为主的二维材料纳米尺度摩擦和磨损研究的进展。根据相对运动形式的不同,我们分别介绍了二维材料的层间滑动和表面摩擦行为,并详细阐述了这些独特行为背后的微观物理机制;同时我们还重点介绍了若干种影响和调控二维材料表面摩擦性能的典型方法和策略,以及二维材料纳米尺度的磨损行为及其失效模式。最后,我们还对纳米尺度二维材料摩擦研究进行了小结,并展望了该领域尚待探索的若干研究方向。     

Higher-order constraints for higher kinematic pairs and their application to mobility and shakiness analysis of mechanisms

The mobility analysis of mechanisms rests on an adequate formulation of the constraints defining its configuration space (c-space). Whereas there is no general method for a global analysis, the higher-order mobility analysis, which locally approximates the c-space, is applicable to general mechanisms. It requires an efficient method for the evaluation of higher-order constraints, i.e. constraints on velocity, acceleration, jerk, etc. Such a method is known for linkages comprising lower pair joints only. In this paper a method for the efficient evaluation of higher-order constraints for mechanisms comprising higher pair joints is proposed. The method builds on the results for the lower pair linkages. It leads to a computationally simply recursive algorithm. This is applied to the mobility analysis that allows to determine the local finite mobility, to detect singularities, and to identify shaky mechanisms.     

Atomistic insights into dislocation-based mechanisms of void growth and coalescence

One of the low-temperature failure mechanisms in ductile metallic alloys is the growth of voids and their coalescence. In the present work we attempt to obtain atomistic insights into the mechanisms underpinning cavitation in a representative metal, namely Aluminum. Often the pre-existing voids in metallic alloys such as Al have complex shapes (e.g. corrosion pits) and the defromation/damage mechanisms exhibit a rich size-dependent behavior across various material length scales. We focus on these two issues in this paper through large-scale calculations on specimens of sizes ranging from 18 thousand to 1.08 million atoms. In addition to the elucidation of the dislocation propagation based void growth mechanism we highlight the observed length scale effect reflected in the effective stress-strain response, stress triaxiality and void fraction evolution. Furthermore, as expected, the conventionally used Gurson's model fails to capture the observed size-effects calling for a mechanistic modification that incorporates the mechanisms observed in our (and other researchers') simulation. Finally, in our multi-void simulations, we find that, the splitting of a big void into a distribution of small ones increases the load-carrying capacity of specimens. However, no obvious dependence of the void fraction evolution on void coalescence is observed.     

发动机高温自润滑配气阀座材料的技术攻关分析

高温自润滑配气阀座是某工程发动机的关键部件。文章对3种配气阀座的研制全过程进行了技术分析,并就三者的物理机械性能、马力试验考核、材料研制和测试条件及综合性能对比情况作了论述,最后还从与美国同类产品相比的角度提出了作者的看法。     

Fundamental Study of Pore Scale Mechanisms in Microbial Improved Oil Recovery Processes

A fundamental study of microscopic mechanisms and pore-level phenomena in the Microbial Improved Oil Recovery method has been investigated. Understanding active mechanisms to increase oil recovery is the key to predict and plan MIOR projects successfully. This article presents the results of visualization experiments carried out in a transparent pore network model. In order to study the pore scale behavior of bacteria, dodecane and an alkane oxidizing bacterium, Rhodococcus sp. 094, suspended in brine, are examined for evaluating the performance of bacterial flooding in the glass micromodel. The observations show the effects of bacteria on remaining oil saturation, allowing us to get better insight on the mechanisms. Bacterial mass composed of bacteria and bioproducts growth in the fluid interfaces and pore walls have been recorded and are presented. No gas is observed throughout any of the experiments. The biomass blocks some pores and pore-throats, and thereby changing the flow pattern. As a consequent, the flow pattern change together with the previously proposed mechanisms, including the interfacial tension reduction and wettability changes are recognized as active mechanisms in the MIOR process.     

Ellipticity loss analysis for tangent moduli deduced from a large strain elastic–plastic self-consistent model

In order to investigate the impact of microstructures and deformation mechanisms on the ductility of materials, the criterion first proposed by Rice is applied to elastic–plastic tangent moduli derived from a large strain micromechanical model combined with a self-consistent scale-transition technique. This approach takes into account several microstructural aspects for polycrystalline aggregates: initial and induced textures, dislocation densities as well as softening mechanisms such that the behavior during complex loading paths can be accurately described.     

神经系统疾病与认知动力学(II): 神经振荡与认知动力学

大脑神经系统具有从慢到快多种不同的振荡节律, 这些节律振荡被认为参与了大脑多种功能的实现, 其中高频的伽马同步振荡被认为与大脑的认知功能最为相关. 本文阐述了生物学实验方面关于伽马振荡及其功能的研究进展, 并针对实验中伽马振荡的频率敏感依赖于外部刺激特征的现象, 综述了基于神经网络模型进行变频伽马振荡及其认知功能的动力学建模研究工作, 解释了视觉刺激调控的变频率伽马振荡动力学产生机理, 提出了基于同步抑制增强全局放电率对比度的神经认知机制. 研究成果有助于理解神经系统同步振荡的产生机理及其认知作用, 为大脑认知原理以及类脑智能的研究奠定基础.