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Bauschinger and size effects in thin-film plasticity due to defect-energy of geometrical necessary dislocations

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Acta Mechanica Sinica 2011年第2期27卷 266-276页

作者： Zhan-Li Liu · Zhuo Zhuang · Xiao-Ming Liu · Xue-Chuan Zhao · Yuan Gao department of engineering mechanics, School of Aerospace, tsinghua university, 100084 beij.ng, china department of engineering mechanics School of Aerospace Tsinghua University Beijing China

The Bauschinger and size effects in the thinfilm plasticity theory arising from the defect-energy of geometrically necessary dislocations （GNDs） are analytically investigated in this paper. Firstly, this defect-energy is deduced based on the elastic interactions of coupling dislocations （or pile-ups） moving on the closed neighboring slip plane. This energy is a quadratic function of the GNDs density, and includes an elastic interaction coefficient and an energetic length scale L. By incorporating it into the work- conjugate strain gradient plasticity theory of Gurtin, an energetic stress associated with this defect energy is obtained, which just plays the role of back stress in the kinematic hardening model. Then this back-stress hardening model is used to investigate the Bauschinger and size effects in the tension problem of single crystal Al films with passivation layers. The tension stress in the film shows a reverse dependence on the film thickness h. By comparing it with discrete-dislocation simulation results, the length scale L is determined, which is just several slip plane spacing, and accords well with our physical interpretation for the defect- energy. The Bauschinger effect after unloading is analyzed by combining this back-stress hardening model with a friction model. The effects of film thickness and pre-strain on the reversed plastic strain after unloading are quantified and qualitatively compared with experiment results.

关键词： Thin film · Crystal plasticity · Defect energy · Back stress · Size effect

Ultra-Broad-Band LiNbO3 Electro-Optic Modulators with Novel Complex Electrode

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Chinese Journal of Lasers 2000年第2期9卷 128-133页

作者： ZHOU Weiqin WU Boyu PEng Jihu GU Jinghua HU Jinyang (department of Electronic engineering, tsinghua university, beij.ng 100084, china) 清华大学北京 100084

The characteristics of Z- cut LiNbO3optical modulators with novel complex electrode are discussed by using the finite element method （FEM） and studied experimentally in this paper. The travelling wave electrode consists of upper and lower parts. It can easily realize phase velocity matching between microwave and optical wave and drastically reduce microwave electrode loss. An electrooptic modulator with larger than 100 GHz bandwidth and half wave voltage 6 V is designed, fabricated and measured. Vπis 5. 1 V and the attenuation coefficient ac is 0. 3 dB/(cm· GHz1/2). The experimental results show that the modulator has great potentiality for expanding bandwidth.

关键词： electro-optic medulator LiNbO3 modulator travelling wave electrode complex electrode

HIGH TEMPERATURE OXIDATION AND STRUCTURE OF SCALE AND WHISKERS FOR Fe-Ni-Cr SEALIng ALLOYS

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Acta Metallurgica Sinica(English Letters) 1993年第8期6卷 147-150页

作者： DOng Zhili MA Jusheng TAng Xiangyun YOU Liang tsinghua university,beij.ng,china lecturer department of Materials Science and engineering,tsinghua university,beij.ng 100084,china tsinghua Univ Beijing China

The oxidation rate,the growth,morphology and structure of oxide scale and whiskers for Fe-Ni-Cr sealing alloys in H_2-H_2O atmosphere at high temperatures have been studied. The growth rate of scale is controlled by *... 详细信息

关键词： oxidation oxide scale oxide whisker Fe-Ni-Cr sealing alloy

Numerical Simulation of Shrinkage Cavity Formation in Spheroidal Graphite Iron Castings Based on Micro Modeling

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International Journal of Minerals,Metallurgy and Materials 1998年第1期12卷 9-12页

作者： Wenzhen Li Baicheng Liu Shantong Jin (Metallurgy engineering School, university of Science and Technology beij.ng, beij.ng 100083, china)(department of Mechanical engineering, tsinghua university, beij.ng 100084, china) Metallurgy engineering School University of Science and Technology Beijing Beijing 100083 China department of Mechanical engineering Tsinghua University Beijing 100084 China

A micro-modeling method (MM) for the quantitative prediction of the shrinkage cavity formation in SGiron castings is proposed. The mathematical models describing the volume changes during the solidification ofspheroidal graphite cast iron are established based on the models of solidification kinetics. The shrinkage cavityformation of T-shaped SG iron castings is calculated with MM method. The calculated results are compared with theexperimental results. It is shown that the predicted size, shape and distribution of shrinkage cavity by MM methodare in good agreement with the measured results.

关键词： solidification shrinkage numerical simulation micro modeling spheroidal graphite cast iron

TWO-DIMENSIONAL SIMULATION OF FLOWS IN PLANE-WALL DIFFUSERS

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Journal of Hydrodynamics 1992年第4期4卷 43-53页

作者： Yuan Ming-shun department of Hydraulic engineering,tsinghua university,beij.ng 100084,P.R.china

A numerical model has been developed for computing turbulent flow in plane-wall *** model solves the weakly compressible flow equations with the application of the Smagorinsky's subgrid-scale turbulence model and the boundary conditions of partial slip and no slip at the solid *** examples include two-dimensional calculations of unstalled and stalled flows in diffusers of small diverging angle,as well as transitory stall flow in a symmetric diffuser of 16°total *** the unstalled and stalled flows,the calculated pressure recovery and velocity profile are compared with experimental *** the transitory stall flow,the com- putational result shows the unsteady flow features including the vortex shedding and stall washout phenomena that have been experimentally observed.

关键词： diffuser turbulent flow separation transitory stall Large-eddy simulation

On the dynamics in a transitional boundary layer

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Communications in Nonlinear Science and Numerical Simulation 2001年第3期6卷 111-171页

作者： C. B. LEE and R. LEE department of mechanics and engineering Science/State Key Laboratory for Studies of Turbulence and Complex Systems, Peking university, beij.ng 100871, china department of engineering mechanics, tsinghua university, beij.ng 100084, Chi department of mechanics and engineering Science / State Key Laboratory for Studies of Turbulence and Complex Systems Peking University Beijing 100871 China department of engineering mechanics Tsinghua University Beijing 100084 China

A model is presented to describe the dynamic physical processes that occur in a transitional boundary layer flow. The CS-solitons, the closed vortex, the secondary closed vortex and the chain of ring-like vortices are postulated to be the basic flow structures of the transitional boundary layer as well as the turbulent boundary layer. It is argued that the central features of transitional and developed turbulent boundary layer flows can be explained in terms of how the series vortices interact with each other, and with the CS-solitons. The physical process that leads to the regeneration of the new closed vortex along the border of the CS-soliton, as well as the processes of evolution of the vortices to high frequency vortices, is described. The model is supported by the important developments in a number of "kernel" experiments, which illustrate both transitional and developed turbulent boundary layer. An important aspect of the model is that it has been formulated to be consistent with accepted rational mechanics concepts that are known to provide a proper physical description of other flows.

关键词： boundary layer transition dynamic processes onset of turbulence

Evaluation of Surface Cracks Using Magnetic Flux LeakageTesting

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Journal of Materials Science & Technology 2002年第4期18卷 319-321页

作者： Luming LI, Songling HUAng, Peng ZHEng and Keren SHINUT Center, Mechanical engineering department, tsinghua university, beij.ng 100084, china NDT Cent. Mech. Eng. Dept. Tsinghua Univ. Beijing 100084 China

The magnetic field distribution characteristics of surface cracks with various widths are discussed based on finite element (FEM) results. The crack depth was 0.20 mm, the width range was from 0.02 to 1.00 mm. The results showed that crack width and lift-off (the distance between surface and sensor) will influence signals. Discussed in this paper is the influence of various lift-off parameters on the peak to peak values of the normal component in magnetic flux leakage testing. The effects can be applied to evaluate surface breaking cracks of different widths and depths. An idea is presented to smooth narrow, sharp crack tips using alternating current (AC) field magnetization.

关键词： Magnetic flux leakage testing (MFL) Slot width Finite element method (FKM)

APPLICATION OF MOLECULAR ORBITAL THEORY TO ANALYSIS OF PHASE STABILITY FOR ALLOYS

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Acta Metallurgica Sinica(English Letters) 1993年第3期6卷 145-152页

作者： YIN Hong TAO Kun PAN Jinsheng LI Hengde tsinghua university,beij.ng,china professor.department of Materials Science and engineering,tsinghua university,beij.ng 100084,china tsinghua Univ Beijing China

The critical _d vahues ( _ ) of the γ/(γ+σ).γ/(γ+μ) and γ/(γ+γ') phase boundaries in some ternar alloy phase diagrams at various temperatures are calculated by averaging the _d values of sererai selected characteristic points at the phase *** equations for the temperature dependence of the critical _d of γ/(γ+σ).γ/(γ+μ) and γ/(γ+γ') phase boundaries are *** accuracy of the analysis is discussed in detail. It is found for the first time that the average value of the bond order _ at the phase boundaries ts also approximatelr a constant and therefore a critical average bond order _ like - can be introduced for the analysts of phase stability

关键词： molecular orbital theory allov phase stabilitv

Application of the wavelet transforms on axial strain calculation in ultrasound elastography

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Progress in Natural Science:Materials International 2006年第9期16卷 942-947页

作者： LUO Jianwen, BAI Jing and SHAO Jinhua (department of Biomedical engineering, School of Medicine, tsinghua university, beij.ng 100084, china) [a]department of Biomedical engineering School of Medicine Tsinghua University Beijing China

In ultrasound elastography, the axial strain distribution within biological tissues is calculated as the numerical derivative (differentiation) of the estimated axial displacement field. Unfortunately, the numerical derivative is unstable because it can greatly amplify the noises, especially at high frequencies. This work focuses on the axial strain calculation from the estimated axial displacements using wavelet transforms (WTs), including continuous wavelet transforms (CWTs) and discrete wavelet transforms (DWTs). The feasibility of the WT-based method using the quadratic spline function is verified by computer simulations and some phantom data. Results indicate that the WT-based method can effectively reduce the noise amplification in axial strain calculation.

关键词： ultrasound elastography axial strain differentiation wavelet transforms.