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The influence of nano-particle tracers on the slip length measurements by microPTV

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Acta Mechanica Sinica 2013年第3期29卷 411-419页

作者： Xu Zheng Gao-Pan Kong Zhan-Hua Silber-Li state key lab.ratory of nonlinear mechanics Institute of MechanicsCAS

Direct measurement of slip length is based on the measured fluid velocity near solid boundary. However, previous micro particle image velocimetry/particle tracking velocimetry （microPIV/PTV） measurements have reported surprisingly large measured near-wall velocities of pressure-driven flow in apparent contradiction with the no-slip hy-pothesis and experimental results from other techniques. To better interpret the measured results of the microPIV/PTV, we performed velocity profile measurements near a hy-drophilic wall （z = 0.25-1.5 μm） with two sizes of tracer particles （φ 50 nm and φ200 nm）. The experimental results indicate that, at less than 1 μm from the wall, the deviations between the measured velocities and no-slip theoretical values obviously decrease from 93% of φ200 nm particles to 48% of φ50 nm particles. The Boltzmann-like exponential measured particle concentrations near wall were found. Based on the non linear Boltzmann distribution of particle concentration and the effective focus plane thickness, we illustrated the reason of the apparent velocity increase near wall and proposed a method to correct the measured velocity profile. By this method, the deviations between the corrected measured velocities and the no-slip theoretical velocity decrease from 45.8% to 10%, and the measured slip length on hy-drophilic glass is revised from 75 nm to 16 nm. These results indicated that the particle size and the biased particle concentration distribution can significantly affect near wall velocity measurement via microPIV/PTV, and result in larger measured velocity and slip length close to wall.

关键词： MicroPIV/PTV · Slip length · Nano-particle tracer · Particle concentration distribution · Boltzmann distribution

STRAIN REGULARITY IN REINFORCERS OF SHORT-FIBER/ WHISKER REINFORCED COMPOSITE AND ITS APPLICATION

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Chinese Journal of Aeronautics 2000年第4期13卷 204-210页

作者：王迺鹏刘秋云刘晓宇 Stt. key lab. nonlinear mechanics Institute of Mechanics CAS Beijing 100080 China

Based on the study of strain distribution in short-fiber/whisker reinforced metal matrix composites, a deformation characteristic parameter λ is defined as the ratio of the root-mean-square strain of reinforcers to the macro-linear strain along the same direction. Quantitative relation between λ and microstructure parameters of the composite is obtained. As an example of applying and verifying λ, the stress-strain curve of ［AlBO］w/Al composite under tensile loading is predicted and favorably compared with experiments. By using λ, the stiffness modulus of the composite with arbitrary reinforcer orientation under any loading condition is predicted from the microstructure parameters of material.

关键词： short-fiber/whisker reinforced composite strain distribution stiffness prediction

SIZE EFFECTS OF ELASTIC MODULUS OF FCC METALS BASED ON THE CAUCHY-BORN RULE AND NANOPLATE MODELS

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Acta Mechanica Solida Sinica 2014年第2期27卷 111-121页

作者： Jianyun Liu Jingru Song Yueguang Wei state key lab.ratory of nonlinear mechanics Institute of Mechanics Chinese Academy of Sciences

In the present research, a simple quasi-continuum model, the Cauchy-Born rule model, is used to investigate the size effects of elastic modulus for fcc metals. By considering a nanoplate model and calculating the strain energy for the nano-sized plate under tension and bending, the relationship between the elastic modulus and the plate thickness is found. Size effects of the elastic modulus are displayed by the relative differences of the elastic modulus between the nano-sized plate sample and the bulk sample. By comparing the present results with those of others, the effectiveness of the Cauchy-Born rule model in studying the size effects of material properties are shown.

关键词： quasi-continuum method Cauchy-Born rule size effect elastic modulus fcc-metal

Onset and Direction of Shear Banding Instability in Metallic Glasses

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Journal of Materials Science & Technology 2014年第6期30卷 616-621页

作者： Yan Chen Lanhong Dai state key lab.ratory of nonlinear mechanics Institute of Mechanics Chinese Academy of Sciences

The shear banding instability occurs as the homogenous deformation in metallic glasses （MGs） develops to a critical point, at which the discontinuity in deformation rate is incipient across nano-scale shear bands. When and where the shear instability takes place is an important issue for understanding the shear band origin. However, such condition and direction of shear localization concerning the unique properties of MGs is still lacking for general stress state. In this paper, a new constitutive is introduced for MGs accounting for the pressure sensitivity, dilatancy and structural evolution; the shear banding is regarded as the appearance of instability in the constitutive description of inelastic deformation. Tying the bifurcation theory to the new constitutive, the general condition of deformation localization is derived. The shear band orientation corresponding to the easiest direction of shear instability is then obtained in dependence on pressure sensitivity, dilatancy and Poisson＇s ratio for MGs. The range of the predicted shear band angles is consistent with the experimental observations.

关键词： Metallic glasses Shear banding instability Bifurcation

Total energy equation leading to exchange-correlation functional

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Science China(Physics,mechanics & Astronomy) 2015年第5期58卷 93-98页

作者： LIU Feng WANG TzuChiang state key lab.ratory of nonlinear mechanics Institute of Mechanics Chinese Academy of Sciences

By solving the total energy equation, we obtain the formula of exchange-correlation functional for the first time. This functional is usually determined by fitting experimental data or the numerical results of models. In the uniform electron gas limit, our exchangecorrelation functional can exactly reproduce the results of Perdew-Zunger parameterization from the jellium model. By making use of a particular solution, our exchange-correlation functional could take into accotmt the case of non-uniform electron density, and its validity can be confirmed through comparisons of the band structure, equilibrium lattice constant, and bulk modulus of aluminum and silicon. The absence of mechanical prescriptions for the systematic improvement of exchange-correlation functional hinders further development of density-functional theory （DFT）, and the formula of exchange-correlation functional given in this study might provide a new perspective to help DFT out of this awkward situation.

关键词： Density-functional theory exchange-correlation functional bulk modulus band structure aluminum silicon

A new thermo-elasto-plasticity constitutive equation for crystals

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Science China(Physics,mechanics & Astronomy) 2015年第5期58卷 99-108页

作者： CHEN Cen TANG QiHeng WANG TzuChiang state key lab.ratory of nonlinear mechanics Institute of Mechanics Chinese Academy of Sciences

Based on the crystal plasticity theory and interatomic potential, in this paper a new thermo-elasto-plasticity constitutive model is proposed to study the behavior of metal crystals at finite temperature. By applying the present constitutive model, the stress-strain curves under uniaxial tension at different temperatures are calculated for the typical crystal A1, and the calculated results are compared with the experimental results. From the comparisons, it can be seen that the present theory has the capability to describe the thermo-elasto-plastic behavior of metal crystals at finite temperature through a concise and explicit calculation process.

关键词： thermo-elasto.plasticity theory constitutive equation interatomic potential critical resolved shear stress hardening

A two-parameter model to predict fatigue life of high-strength steels in a very high cycle fatigue regime

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Acta Mechanica Sinica 2015年第3期31卷 383-391页

作者： Chengqi Sun Xiaolong Liu Youshi Hong The state key lab.ratory of nonlinear mechanics Institute of Mechanics Chinese Academy of Sciences

In this paper, ultrasonic （20 kHz） fatigue tests were performed on specimens of a high-strength steel in very high cycle fatigue （VHCF） regime. Experimental results showed that for most tested specimens failed in a VHCF regime, a fatigue crack originated from the interior of specimen with a fish-eye pattern, which contained a fine granular area （FGA） centered by an inclusion as the crack origin. Then, a two-parameter model is proposed to predict the fatigue life of high-strength steels with fish-eye mode failure in a VHCF regime, which takes into account the inclusion size and the FGA size. The model was verified by the data of present experiments and those in the literature. Furthermore, an analytic formula was obtained for estimating the equivalent crack growth rate within the FGA. The results also indicated that the stress intensity factor range at the front of the FGA varies within a small range, which is irrespective of stress amplitude and fatigue life.

关键词： Very high-cycle fatigue - High-strength steels Fatigue life Inclusion size Crack growth rate

A new thermo-elasto-plasticity constitutive theory for polycrystalline metals

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Acta Mechanica Sinica 2015年第3期31卷 338-348页

作者： Cen Chen Qiheng Tang Tzuchiang Wang state key lab.ratory of nonlinear mechanics Institute of Mechanics Chinese Academy of Sciences

In this study, the behavior of polycrystalline metals at different temperatures is investigated by a new thermo-elasto-plasticity constitutive theory. Based on solid mechanical and interatomic potential, the constitutive equa- tion is established using a new decomposition of the deformation gradient. For polycrystalline copper and magnesium, the stress-strain curves from 77 to 764 K （copper）, and 77 to 870 K （magnesium） under quasi-static uniaxial loading are calculated, and then the calculated results are compared with the experiment results. Also, it is determined that the present model has the capacity to describe the decrease of the elastic modulus and yield stress with the increasing temperature, as well as the change of hardening behaviors of the polycrystalline metals. The calculation process is simple and explicit, which makes it easy to implement into the applications.

关键词： Thermo-elasto-plasticity constitutive theory Yield stress Hardening behaviors Finite temperature

The effect of agglomerate on micro-structural evolution in solid-state sintering

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Acta Mechanica Sinica 2012年第5期28卷 1323-1330页

作者： Chao Wang Shao-Hua Chen state key lab.ratory of nonlinear mechanics Institute of MechanicsChinese Academy of Sciences

Discrete element method （DEM） is used in the present paper to simulate the microstructural evolution of a planar layer of copper particles during sintering. Formation of agglomerates and the effect of their rearrangement on densification are mainly focused on. Comparing to the existing experimental observations, we find that agglomerate can form spontaneously in sintering and its rearrangement could accelerate the densification of compacts. Snapshots of numerical simulations agree qualitatively well with experimental observations. The method could be readily extended to investigate the effect of agglomerate on sintering in a three- dimensional model, which should be very useful for understanding the evolution of microstructure of sintering systems.

关键词： Solid-state sintering Discrete element method Agglomerate Densification Micro-structural evolution