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New discrete element models for elastoplastic problems

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Acta Mechanica sinica 2009年第5期25卷 629-637页

作者： Ming Cheng Weifu Liu Kaixin Liu LTCs and Department of Mechanics and aerospace engineering,college of engineering, Peking University, 100871 Beijing, China LTCs and Department of Mechanics and aerospace engineering College of Engineering Peking University 100871 Beijing China

The discrete element method （DEM） has attractive features for problems with severe damages, but lack of theoretical basis for continua behavior especially for nonlinear behavior has seriously restricted its application. The present study proposes a new approach to developing the DEM as a general and robust technique for modeling the elastoplastic behavior of solid materials. New types of connective links between elements are proposed, the interelement parameters are theoretically determined based on the principle of energy equivalence and a yield criterion and a flow rule for DEM are given for describing nonlinear behavior of materials. Moreover, a numerical scheme, which can be applied to modeling the behavior of a continuum as well as the transformation from a continuum to a discontinuum, is obtained by introducing a fracture criterion and a contact model into the DEM. The elastoplastic stress wave propagations and the tensile failure process of a steel plate are simulated, and the numerical results agree well with those obtained from the finite element method （FEM） and corresponding experiment, and thus the accuracy and efficiency of the DEM scheme are demonstrated.

关键词： DEM Continuum Plasticity Fracture

Fracture analysis of a functionally graded interfacial zone between two dissimilar homogeneous materials

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science China(Physics,Mechanics & Astronomy) 2006年第5期49卷 540-552页

作者： CHENG Zhanqi1,2 & ZHONG Zheng1 1. school of aerospace engineering and Applied mechanics, Tongji University, shanghai 200092, China 2. college of Civil engineering, Zhengzhou University, Zhengzhou 450002, China 1. school of aerospace engineering and Applied mechanics Tongji University Shanghai 200092 China 2. college of Civil engineering Zhengzhou University Zhengzhou 450002 China

In this paper the plane elasticity problem for a functionally graded interfacial zone containing a crack between two dissimilar homogeneous materials has been considered. It is assumed that in the interfacial zone the reciprocal of the shear modulus is a linear function of the coordinate, while Possion’s ratio keeps constant. By utilizing the Fourier transformation technique and the transfer matrix method, the mixed boundary problem is reduced to a system of singular integral equations that are solved numerically. The influences of the geometric parameters and the graded parameter on the stress intensity factors are investigated. The numerical results show that the graded parameters, the thickness of interfacial zone, the crack size and location have significant effects on the stress intensity factors.

关键词： functionally graded material interfacial zone fracture stress intensity factor

Crank conditions and rotatability of 3-RRR planar parallel mechanisms

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science China(Technological sciences) 2009年第12期52卷 3601-3612页

作者： LI RuiQin 1 &DAI Jian s 2 1 college of Mechanical engineering and Automation,North University of China,Taiyuan 030051,China 2 Department of Mechanical engineering,King’s college London,University of London,WC2R 2.s,UK college of Mechanical engineering and Automation North University of China Taiyuan 030051 China Department of Mechanical engineering King’s College London University of London WC2R 2LS London UK

This paper investigates the rotatability of planar parallel mechanisms by revealing the crank conditions that relate to link *** paper extends the theorem of assemblability and the rotatability criterion and applies these criteria to parallel mechanisms in investigating the crank conditions of a parallel mechanism,leading to classification of three types of crank existing conditions including three-crank’s,six-crank’s and nine-crank’s of 3-RRR planar parallel *** results are summarized as constraint conditions in relation with link parameters with a concluding table and case studies.

关键词： crank rotatability planar parallel mechanism

Research on Cooling Effect with Coolant Groove structure Parameters in Platelet Transpiration Cooled Nosetip

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Journal of Thermal science 2010年第5期19卷 438-444页

作者： RONG Yisheng and LIU Weiqiang college of aerospace and Material engineering,National Univ.of Defense Technology,Changsha 410073,China college of aerospace and Material engineering National Univ. of Defense Technology Changsha China

A platelet transpiration cooled nosetip is considered as thermal protection system （TPs） to prevent hypersonic ve- hicle from the serious aerodynamic heating. Based on the one dimensional flow model, a distribution model of coolant is proposed for the temperature calculation. When si = sj （i, j=1,… 24）, the first cooling effect parameter Pmax is proposed and its relationship with total mass flux and sc0/si is investigated. The result shows that Pmax increases while the total mass flux increases, and when the mass flux is fixed, Pmax increases rapidly at the be- ginning and then turns to a nearly stable value while sc0/si increases. Then under the precondition of cooling ef- fect, we fix sc0/si to insure there is enough space for the pipe. Numerical investigation shows the design of the nosetip makes the transpiration cooling extremely effective. In order to reduce the temperature difference on the nosetip, the second cooling effect parameter Pdiff is proposed and different Pdiff with different 0gi （i=1,..., 23） are analyzed. According to the cases we design, Pdiff decreases while the upstream 0gi decreases or the down- stream 0gi increases. The best result among cases shows Pdiff is reduced by 15.1%.

关键词： platelet transpiration cooling nosetip thermal protection system aerodynamic heating

Three vortex motion in the slightly viscous flow

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science China(Physics,Mechanics & Astronomy) 2010年第4期53卷 735-744页

作者： ZHU Hao & CHENG KeMing college of aerospace engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China 1. college of aerospace engineering Nanjing University of Aeronautics and Astronautics Nanjing 210016 China

The dynamics of three vortices moving in an ideal fluid in a plane can be expressed in Hamiltonian form. However, when viscosity can not be ignored, the system of point vortices has not this delicious structure. This investigation focuses on the viscosity effect on the motion of three vortices. since the viscous diffusion of vortices is simulated by Brownian motion, the equations of intervortical distances are obtained by Ito formula. These equations show that there exist two viscosity effects on intervortical distances: stable expansion and random impulse. Furthermore, via employing the asymptotic solution to these equations, these two viscous effects have been shown to destroy the self-similar process of collapse and make it into a new configuration, which is similar to the near collapse in the ideal case.

关键词： point vortex slightly viscous flow Fokker-Planck equation collapse Ito formula

A new navigation approach of terrain contour matching based on 3-D terrain reconstruction from onboard image sequence

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science China(Technological sciences) 2010年第5期53卷 1176-1183页

作者： LI LiChun1,2,YU QiFeng2.sHANG Yang2,YUAN Yun2,LU HongWei3 & LIU XiaoLin4 1 Beijing aerospace Control Center,Beijing 100094,China 2 college of aerospace and Material engineering,National University of Defense Technology,Changsha 410073,China 3 Equipment Research Institute of PLA’s second Artillery,Beijing 100085,China 4 college of Mechatronic engineering and Automation,National University of Defense Technology,Changsha 410073,China 1. Beijing aerospace Control Center Beijing 100094 China2. College of Aerospace and Material Engineering National University of Defense Technology Changsha 410073 China3. Equipment Research Institute of PLA’s Second Artillery Beijing 100085 China4. College of Mechatronic Engineering and Automation National University of Defense Technology Changsha 410073 China

This article presents a passive navigation method of terrain contour matching by reconstructing the 3-D terrain from the image sequence(acquired by the onboard camera).To achieve automation and simultaneity of the image sequence processing for navigation,a correspondence registration method based on control points tracking is proposed which tracks the sparse control points through the whole image sequence and uses them as correspondence in the relation geometry ***,a key frame selection method based on the images overlapping ratio and intersecting angles is explored,thereafter the requirement for the camera system configuration is *** proposed method also includes an optimal local homography estimating algorithm according to the control points,which helps correctly predict points to be matched and their speed ***,the real-time 3-D terrain of the trajectory thus reconstructed is matched with the referenced terrain map,and the result of which provides navigating *** digital simulation experiment and the real image based experiment have verified the proposed method.

关键词： terrain contour matching vision-based navigation 3-D reconstruction control points key frame registration optimal local homography

Orientation angle rotatability of planar serial n-link manipulators

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science China(Technological sciences) 2010年第6期53卷 1620-1637页

作者： LI RuiQin1,XIAO DengHong1 & DAI Jian s2 1 college of Mechanical engineering and Automation,North University of China,Taiyuan 030051,China 2 Department of Mechanical engineering,King’s college London,University of London,London WC2R 2.s,UK 1. college of Mechanical engineering and Automation North University of China Taiyuan 030051 China2. Department of Mechanical Engineering King’s College London University of London London WC2R 2LS UK

This paper presents a new method of analyzing the orientation angle rotatability of a planar serial n-link manipulator by examining the rotatability of its virtual equivalent *** orientation boundary region decomposition is proposed based on the polynomial discriminant derived from a serial three-link *** angle workspace indicates a set of angle ranges by which the end-effector can reach with certain orientation for every point in the reachable position *** introducing a virtual equivalent mechanism,the open-chain kinematic problem is converted into a close-chain mechanism,and the Grashof-type condition is used to examine the rotatability of the end-effector *** reachable position workspace can be divided into several Grashof intervals and non-Grashof intervals based on the extreme positions and singular configurations of a *** derived equations in this paper can be used to calculate the variation rule of the orientation angle workspace of any given planar serial n-link manipulator on the basis of its link *** orientation angle rotatability of a seven-link serial manipulator is solved by the proposed method and graphical representations of the orientation angle workspaces are mapped onto a three-dimensional coordinate *** theoretical approach has practical significance for the design of a highly dexterous serial n-link manipulator with proposed link relations.

关键词： Grashof criterion planar serial manipulator orientation angle workspace virtual equivalent mechanism

An exploration for the macroscopic physical meaning of entropy

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science China(Technological sciences) 2010年第7期53卷 1809-1816页

作者： WU Jing1 & GUO ZengYuan2 1 college of Energy and Power engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China 2 Key Laboratory for Thermal science and Power engineering of Ministry of Education, school of aerospace, Tsinghua University, Beijing 100084, China 1. college of Energy and Power engineering Nanjing University of Aeronautics and Astronautics Nanjing 210016 China2. Key Laboratory for Thermal Science and Power Engineering of Ministry of Education School of Aerospace Tsinghua University Beijing 100084 China

The macroscopic physical meaning of entropy is analyzed based on the exergy (availability) of a combined system (a closed system and its environment), which is the maximum amount of useful work obtainable from the system and the environment as the system is brought into equilibrium with the environment. The process the system experiences can be divided in two sequent sub-processes, the process at constant volume, which represents the heat interaction of the system with the environment, and the adiabatic process, which represents the work interaction of the system with the environment. It is shown that the macroscopic physical meaning of entropy is a measure of the unavailable energy of a closed system for doing useful work through heat interaction. This statement is more precise than those reported in prior literature. The unavailability function of a closed system can be defined as T0s and p0V in volume constant process and adiabatic process, respectively. Their changes, that is, AiTgs) and A (p0V) represent the unusable parts of the internal energy of a closed system for doing useful work in corresponding processes. Finally, the relation between Clausius entropy and Boltzmann entropy is discussed based on the comparison of their expressions for absolute entropy.

关键词： entropy macroscopic physical meaning exergy heat interaction

sURFACE-ENHANCED CANTILEVER sENsORs WITH NANO-POROUs FILMs

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Acta Mechanica solida sinica 2010年第1期23卷 1-12页

作者： Huiling Duan(state Key Laboratory for Turbulence and Complex system,CAPT and Department of Mechanics and aerospace engineering,college of engineering,Peking University,Beijing 100871,China) [a] state Key Laboratory for Turbulence Complex system CAPT Department of Mechanics aerospace engineering College of Engineering Peking University Beijing 100871 China

Developing surface-enhanced microcantilevers with improved sensitivities is of longstanding interest. In this paper, the design of surface-enhanced cantilever sensors using nano- （micro-） porous films as surface layers is proposed. The static deformation and resonance frequencies of these surface-enhanced sensors with the simultaneous effects of the eigenstrain, the surface stress and the adsorption mass are analyzed. It is shown that the sensitivities of these novel cantilever sensors for the static deformation and resonance frequencies can be tuned by the porosity, the size of the pores and the structure of the porous films. For the three kinds of cantilever consisting of solid films, films with aligned cylindrical micro-scale pores, and those with nano-scale pores, the nano-porous one has the highest static and dynamic sensitivities, whereas the solid one has the lowest.

关键词： cantilever sensors nano- (micro-) porous films surface stress curvature resonance frequency