Some extensions of elementary plasticity theory

A detailed investigation is presented of yield or loading criteria, for work hardening materials, which lead to constant strain ratios under increasing stress when stress ratios are maintained constant. The experimenter is given a choice of incremental stress-strain theories with which results of simple or complicated combined loading tests may be correlated. These theories are dealt with in order of increasing complexity and, at the same time, increasing capacity to represent experimentally established phenomena.The objective is the use of the mathematically simplest theory which will provide the desired flexibility in the analysis of experimental data.Loading functions of isotropic as well as anisotropic types are examined, among them those depending explicitly on stress alone and those depending on both stress and plastic strain. In the latter cases only those functions are investigated in which plastic strain appears to the first order. The predicted results of some basic tests are examined for several loading criteria.     

关于自主创新和科研时间保障的几点思考

一中科院如何能更好地自主创新自主创新可有如下几种类型:人对自然界的探索本性。王竹溪先生在其《热力学》引言中曾说,热力学是由于人对热的本质的追求。爱因斯坦的相对论是他从事科学活动中对自然规律的总结和发现。人的思维是自由的,没有任何功利目的的科学研究,是自主创新的最高境界。大环境的压力。新中国成立以后,帝国主义的封锁以及中苏关系的破裂,迫使中国只能自力更生、独立自主地发展科学技术。中国的“两弹一星”就是自主创新的最好的例子。在当时的条件下,作为个人来说,他的聪明智慧不能随自己的意志,而要服从一个大的目标。还…     

Graph-theoretic considerations on the invariance of return difference

Contrary to the common assumption, it is shown that the general return difference and the general null return difference are variant under the general transformations between a system of basis circuits and a system of basis cutsets. Necessary and sufficient conditions are presented for the invariance of these functions. Formulation of these functions in terms of the primary systems of equations and illustrative examples are also given.     

Log-periodic quantum magneto-oscillations and discrete-scale invariance in topological material HfTe5

Discrete-scale invariance (DSI) is a phenomenon featuring intriguing log-periodicity that can be rarely observed in quantum systems. Here, we report the log-periodic quantum oscillations in the longitudinal magnetoresistivity (ρ_xx) and the Hall traces (ρ_yx) of HfTe₅ crystals, which reveal the DSI in the transport-coefficients matrix. The oscillations in ρ_xx and ρ_yx show the consistent logB-periodicity with a phase shift. The finding of the logB oscillations in the Hall resistance supports the physical mechanism as a general quantum effect originating from the resonant scattering. Combined with theoretical simulations, we further clarify the origin of the log-periodic oscillations and the DSI in the topological materials. This work evidences the universality of the DSI in the Dirac materials and provides indispensable information for a full understanding of this novel phenomenon.     

Project begins on plasticity in brain structure

Studies on plasticity in brain structure and function, a flagship project of China's basic science research during the 1lth Five-year Plan period (2006-2010), has kicked off recently in Shanghai.     

Trajectory tracking of an autonomous vehicle using immersion and invariance control

An Immersion and Invariance [I & I] controller is designed to control the nonlinear lateral vehicle’s motion, using the steering angle as the only input. Similar to most of the lateral vehicle’s dynamics control law, the cornering stiffness parameters are involved in our proposed controller. Because of the tight relation between tire/road properties and the cornering stiffness parameters, they are not available from the outputs of the sensors and therefore, should be estimated for utilizing in the control law. An online data-driven identification is employed for estimating the cornering stiffness parameters. In addition, a robust model-based fault detection and approximation method in the presence of uncertainties via neural networks is presented. The performance of the obtained control law is investigated via simulation tests in different situations and in the presence of the disturbance. Moreover, some validation tests are performed using the CarSim software to show the effectiveness of our algorithm.     

Swimming direction reversal of flagella through ciliary motion of mastigonemes

Bio-inspired designs can provide an answer to engineering problems such as swimming strategies at the micron or nano-scale. Scientists are now designing artificial micro-swimmers that can mimic flagella-powered swimming of micro-organisms. In an application such as lab-on-a-chip in which micro-object manipulation in small flow geometries could be achieved by micro-swimmers, control of the swimming direction becomes an important aspect for retrieval and control of the micro-swimmer. A bio-inspired approach for swimming direction reversal (a flagellum bearing mastigonemes) can be used to design such a system and is being explored in the present work. We analyze the system using a computational framework in which the equations of solid mechanics and fluid dynamics are solved simultaneously. The fluid dynamics of Stokes flow is represented by a 2D Stokeslets approach while the solid mechanics behavior is realized using Euler-Bernoulli beam elements. The working principle of a flagellum bearing mastigonemes can be broken up into two parts: (1) the contribution of the base flagellum and (2) the contribution of mastigonemes, which act like cilia. These contributions are counteractive, and the net motion (velocity and direction) is a superposition of the two. In the present work, we also perform a dimensional analysis to understand the underlying physics associated with the system parameters such as the height of the mastigonemes, the number of mastigonemes, the flagellar wave length and amplitude, the flagellum length, and mastigonemes rigidity. Our results provide fundamental physical insight on the swimming of a flagellum with mastigonemes, and it provides guidelines for the design of artificial flagellar systems.     

Investigation of microflow reversal by ac electrokinetics in orthogonal electrodes for micropump design

Orthogonal electrodes have been reported to produce high velocity microflows when excited by ac signals, showing potential for micropumping applications. This paper investigates the microflow reversal phenomena in such orthogonal electrode micropumps. Three types of microflow fields were observed by changing the applied electric signals. Three ac electrokinetic processes, capacitive electrode polarization, Faradaic polarization, and the ac electrothermal effect, are proposed to explain the different flow patterns, respectively. The hypotheses were corroborated by impedance analysis, numerical simulations, and velocity measurements. The investigation of microflow reversal can improve the understanding of ac electrokinetics and hence effectively manipulate fluids.