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Effect of Spark Plasma Sintering Temperature on Electrochemical Properties of La_(0. 82)Mg_(0. 18)Ni_(3. 50)Co_(0. 15) Alloy

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Journal of Iron and Steel Research International 2016年第5期23卷 459-465页

作者： Xiao-ping DONG Yan-rong PANG Li-ying YANG Qing WANG Zhi-yuan LI department of mechanics Hebei University

Electrochemical properties of La_（0. 82）Mg_（0. 18）Ni_（3. 50）Co_（0. 15） alloys synthesized by spark plasma sintering（ SPS） were investigated based on the electrochemical measurements,physical parameters and microstructure observation. The sintering behavior of La_（0. 82）Mg_（0. 18）Ni_（3. 50）Co_（0. 15） alloys at the temperatures of 900,950 and 1 000 ℃ is characterized by four stages,i. e.,initial slight shrinkage,expansion,abrupt shrinkage and slight expansion. The maximum shrinkage displacement increases with increasing sintering temperature. All of the alloys consist of（ La,Mg）_2（ Ni,Co）_7 phase; additionally,temperatures of 900 and 950 ℃ are beneficial to the formation of（ La,Mg）（ Ni,Co）_3 phase,whereas the LaNi_5 phase is easy to form in the alloy synthesized by SPS at 1 000 ℃. The electrochemical measurements indicate an evident change of the electrochemical performance of the alloys associated with increasing the sintering temperature. The discharge capacity of the alloys first increases and then decreases as sintering temperature rises,whereas their cycle stability clearly grows all the ***,the charging-discharging potential difference and discharging efficiency both demonstrate that the electrochemical properties of the alloy electrodes first augment and then decline with increasing sintering temperature.

关键词： spark plasma sintering La-Mg-Ni system microstructure electrochemical property

Calculation of thermal physical parameters of dissociated air by the dissociation degree method

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Applied Mathematics and mechanics(English Edition) 2018年第7期39卷 1045-1056页

作者： Yaopeng ZHAO Wei CAO department of mechanics Tianjin University

The high temperature gas occurs behind shock or near the wall surface of vehicle in the hypersonic flight. As the temperature exceeds 2 000 K, 4 000 K, respectively, O2 and N2 molecules are successively dissociated. Because of variable components at dif- ferent temperatures and pressures, the dissociated air is no longer a perfect gas, In this paper, a new method is developed to calculate accurate thermal physical parameters with the dissociation degree providing the thermochemical equilibrium procedure. Based on the dissociation degree, it is concluded that few numbers of equations and the solutions are easily obtained. In addition, a set of formulas relating the parameter to the dissociation degree are set up four-species, O2 molecule The thermodynamic properties of dissociated air containing and N2 molecule, O atom and N atom, are studied with the new method, and the results are consistent with those with the traditional equilibrium constant method. It is shown that this method is reliable for solving thermal physical parameters easily and directly.

关键词： dissociated air thermal physical parameter equilibrium constant dissociation degree

Flat-plate hypersonic boundary-layer ?ow instability and transition prediction considering air dissociation

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Applied Mathematics and mechanics(English Edition) 2019年第5期40卷 719-736页

作者： Yufeng HAN Wei CAO department of mechanics Tianjin University

The effects of air dissociation on ?at-plate hypersonic boundary-layer ?ow instability and transition prediction are studied. The air dissociation reactions are assumed to be in the chemical equilibrium. Based on the ?at-plate boundary layer, the ?ow stability is analyzed for the Mach numbers from 8 to 15. The results reveal that the consideration of air dissociation leads to a decrease in the unstable region of the ?rst-mode wave and an increase in the maximum growth rate of the second mode. High frequencies appear earlier in the third mode than in the perfect gas model, and the unstable region moves to a lower frequency region. When the Mach number increases, the second-mode wave dominates the transition process, and the third-mode wave has little effect on the transition. Moreover, when the Mach number increases from 8 to 12, the N-factor envelope becomes higher, and the transition is promoted. However, when the Mach number exceeds 12, the N-factor envelope becomes lower, and the transition is delayed. The N-factor envelope decreases gradually with the increase in the altitude or Mach number.

关键词： air dissociation transition prediction boundary layer

Effect of wall-cooling on Mack-mode instability in high speed flat-plate boundary layers

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Applied Mathematics and mechanics(English Edition) 2016年第9期37卷 1219-1230页

作者： Shaolong ZHANG Jianxin LIU Jisheng LUO department of mechanics Tianjin University

The instability of the Mack mode is destabilized by wall-cooling in a high speed boundary layer. The aim of this paper is to study the mechanism of the wall cooling effect on the Mack mode instability by numerical methods. It is shown that the wall-cooling can destabilize the Mack mode instability, similar to the previous conclusions with the exception that the Mack mode instability can be stabilized by wall-cooling if the wall temperature is extremely low. The reversed wall temperature is related to a freestream condition. If the Mach number increases to a large enough value, e.g., about 7, the reversed wall temperature will tend to be zero. It seems that the Mack mode instability is determined by the region between the boundary layer edge and the critical layer. When the wall temperature decreases, this region becomes wider, and the boundary layer becomes more unstable. Additionally, a relative supersonic unstable mode can be observed when the velocity of the critical layer is less than 1 - liMa or is cancelled by the wall-cooling effect. These results provide a deeper understanding on the wall-cooling effect in high speed boundary layers.

关键词： stability wall-cooling high speed boundary layer Mack mode instability

Stability analysis method considering non-parallelism:EPSE method and its application

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Applied Mathematics and mechanics(English Edition) 2016年第1期37卷 27-36页

作者： Gaotong YU Jun GAO Jisheng LUO department of mechanics Tianjin University

The e-N method is widely used in transition prediction. The amplitude growth rate used in the e-N method is usually provided by the linear stability theory （LST） based on the local parallel hypothesis. Considering the non-parallelism effect, the parabolized stability equation （PSE） method lacks local characteristic of stability analysis. In this paper, a local stability analysis method considering non-parallelism is proposed, termed as EPSE since it may be considered as an expansion of the PSE method. The EPSE considers variation of the shape function in the streamwise direction. Its local characteristic is convenient for stability analysis. This paper uses the EPSE in a strong non-parallel flow and mode exchange problem. The results agree well with the PSE and the direct numerical simulation （DNS）. In addition, it is found that the growth rate is related to the normalized method in the non-parallel flow. Different results can be obtained using different normalized methods. Therefore, the normalized method must be consistent.

关键词： parabolized stability equation (PSE) expansion of PSE (EPSE) linear stability theory (LST) normalized method non-parallel

Global instability of Stokes layer for whole wave numbers

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Applied Mathematics and mechanics(English Edition) 2016年第8期37卷 999-1012页

作者： Wei KONG Jisheng LUO department of mechanics Tianjin University

The study on the global instability of a Stokes layer, which is a typical unsteady flow, is usually a paradigm for understanding the instability and transition of unsteady flows. Previous studies suggest that the neutral curve of the global instability obtained by the Floquet theory is only mapped out in a limited range of wave numbers （0.2 ≤ a ≤ 0.5）. In this paper, the global instability is investigated with numerical simulations for all wave numbers. It is revealed that the peak of the disturbances displays irregularity rather than the periodic evolution while the wave number is beyond the above range. A ＂neutral point＂ is redefined, and a neutral curve of the global instability is presented for the whole wave numbers with this new definition. This work provides a deeper understanding of the global instability of unsteady flows.

关键词： Stokes layer instantaneous instability Floquet theory neutral curve

3D analysis for pit evolution and pit-to-crack transition during corrosion fatigue

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Journal of Zhejiang University-Science A(Applied Physics & Engineering) 2013年第4期14卷 292-299页

作者： Xiao-guang HUANG Jin-quan XU department of Engineering mechanics China University of Petroleum department of Engineering mechanics Shanghai Jiao Tong University

This paper presents a deterministic model to predict the pit evolving morphology and crack initiation life of corrosion *** on the semi-ellipsoidal pit assumption,the thermodynamic potential including elastic energy,surface energy and electrochemical energy of the cyclically stressed solid with an evolving pit is established,from which specific parameters that control the pit evolution are introduced and their influence on the pit evolution are *** critical pit size for crack nucleation is obtained from stress intensity factor criterion and the crack nucleation life is evaluated by Faraday's ***,this paper presents a numerical example to verify the proposed model and investigate the influence of cyclic load on the corrosion fatigue crack nucleation *** corrosion pit appears approximately as a hemisphere in its early formation,and it gradually transits from semicircle to *** strain energy accelerates the morphology evolution of the pit,while the surface energy decelerates *** higher the stress amplitude is,the smaller the critical pit size is and the shorter the crack initiation life is.

关键词： Pit Evolving morphology Thermodynamic potential Critical pit size Crack nucleation

Critical transition Reynolds number for plane channel flow

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Applied Mathematics and mechanics(English Edition) 2017年第10期38卷 1415-1424页

作者： Yongming ZHANG department of mechanics Tianjin University Tianjin Key Laboratory of Modern Engineering mechanics

The determination of the critical transition Reynolds number is of practical importance for some engineering problems. However, it is not available with the current theoretical method, and has to rely on experiments. For supersonic/hypersonic boundary layer flows, the experimental method for determination is not feasible either. Therefore, in this paper, a numerical method for the determination of the critical transition Reynolds number for an incompressible plane channel flow is proposed. It is basically aimed to test the feasibility of the method. The proposed method is extended to determine the critical Reynolds number of the supersonic/hypersonic boundary layer flow in the subsequent papers.

关键词： critical transition Reynolds number plane channel flow boundary layerflow

Free vibration of functionally graded material beams with surface-bonded piezoelectric layers in thermal environment

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Applied Mathematics and mechanics(English Edition) 2009年第8期30卷 969-982页

作者：李世荣苏厚德程昌钧 department of Engineering mechanics Lanzhou University of Technology department of mechanics Shanghai University

Free vibration of statically thermal postbuckled functionally graded material （FGM） beams with surface-bonded piezoelectric layers subject to both temperature rise and voltage is studied. By accurately considering the axial extension and based on the Euler-Bernoulli beam theory, geometrically nonlinear dynamic governing equations for FGM beams with surface-bonded piezoelectric layers subject to thermo-electro- mechanical loadings are formulated. It is assumed that the material properties of the middle FGM layer vary continuously as a power law function of the thickness coordinate, and the piezoelectric layers are isotropic and homogenous. By assuming that the amplitude of the beam vibration is small and its response is harmonic, the above mentioned non-linear partial differential equations are reduced to two sets of coupled ordinary differential equations. One is for the postbuckling, and the other is for the linear vibration of the beam superimposed upon the postbuckled configuration. Using a shooting method to solve the two sets of ordinary differential equations with fixed-fixed boundary conditions numerically, the response of postbuckling and free vibration in the vicinity of the postbuckled configuration of the beam with fixed-fixed ends and subject to transversely nonuniform heating and uniform electric field is obtained. Thermo-electric postbuckling equilibrium paths and characteristic curves of the first three natural frequencies versus the temperature, the electricity, and the material gradient parameters are plotted. It is found that the three lowest frequencies of the prebuckled beam decrease with the increase of the temperature, but those of a buckled beam increase monotonically with the temperature rise. The results also show that the tensional force produced in the piezoelectric layers by the voltage can efficiently increase the critical buckling temperature and the natural frequency.

关键词： functionally graded material laminated beams with piezoelectric layers thermal buckling free vibration natural frequency