A meso elastoplastic constitutive model for polycrystalline metals based on equivalent slip systems with latent hardening

A meso material model for polycrystalline metals is proposed, in which the tiny slip systems distributing randomly between crystal slices in micro-grains or on grain boundaries are replaced by macro equivalent slip systems determined by the work-conjugate principle. The elastoplastic constitutive equation of this model is formulated for the active hardening, latent hardening and Bauschinger effect to predict macro elastoplastic stress-strain responses of polycrystalline metals under complex loading conditions. The influence of the material property parameters on size and shape of the subsequent yield surfaces is numerically investigated to demonstrate the fundamental features of the proposed material model. The derived constitutive equation is proved accurate and efficient in numerical analysis. Compared with the self-consistent theories with crystal grains as their basic components, the present theory is much simpler in mathematical treatment.

浮动磁头气动力静态特性

本文给出了最新浮动磁头的气动力静态特性。文中采用了数值方法求解滑流条件下滑块轴承的修正雷诺方程,描述了运用积分离散、Newton—Raphson线性化方法的有限差分法,给出了运用Booy方法的基本思想所形成的线性化有限差分方程组的数值计算方法,计算出在压缩数很大范围内(A可达5000以上)浮动磁头的气动参数。

具有粘滞阻尼与非线性结构阻尼的四边固支板的振动响应

本文以正则化的完全满足四边固支板边条件的广义傅立叶坐标基f_i(x,y)为基函数,将各阶振型展成广义傅立叶级数,求解了四边固支板在无阻尼情况下的自由振动。以后,以应力应变滞后一位相v的非线性结构阻尼模型,建立了粘滞阻尼和结构阻尼共同影响下的振动微分方程,求得了它的自由振动解,正弦响应和随机响应。已编制程序,对长短比为a/b=2的板进行了计算。结果表明:低阶频率和振型收敛较快,较高阶频率则要取足够的项数,方能有较为令人满意的结果。

交流电弧的外迴路对弧柱特性的影响

本文研究了交流电弧的弧柱与外迴路之间的耦合作用,以及电弧边界随时间的波动对电弧特性的影响,用数值方法给出了电弧的特性曲线。结果表明,当ωθ≤5时外迴路与弧柱间的耦合作用显著。弧电流在过零点后发生畸变,畸变大小与外迴路阻抗及ωθ值有关。电弧边界和热流势随时间波动较大,相对波动量可达50％以上。当ωθ≤1时,本文从理论上证实存在熄弧峰值电压,与实验结果一致。

The mechanism of energy storage by shearing the solar magnetic field

In this paper, a complete set of MHD equations have been solved by numerical calculations in an attempt to study the dynamical evolutionary processes of the initial equilibrium configuration and to discuss the energy storage mechanism of the solar atmosphere by shearing the magnetic field. The initial equilibrium configuration with an arch bipolar potential field obtained from the numerical solution is similar to the configuration in the vicinity of typical solar flare before its eruption. From the magnetic induction equation in the set of MHD equations and dealing with the non-linear coupling effects between the flow field and magnetic field, the quantitative relationship has been derived for their dynamical evolution. Results show that plasma shear motion at the bottom of the solar atmosphere causes the magnetic field to shear; meanwhile the magnetic field energy is stored in local regions. With the increase of time the local magnetic energy increases and it may reach an order of 4×10^25 J during a day. Thus the local storage of magnetic energy is large enough to trigger a big solar flare and can be considered as the energy source of solar flares. The energy storage mechanism by shearing the magnetic field can well explain the slow changes in solar active regions.

Experimental Study on Flow Induced Vibration of a Cylinder with Two Degrees of Freedom Near a Rigid Wall

The flow-induced vibration of a cylinder with two degrees of freedom near a rigid wall under the action of steady flow is investigated experimentally. The vibration amplitude and frequency of the cylinder and the vortex shedding frequency at the wake flow region of the cylinder are measured. The influence of gap-to-diameter ratio upon the amplitude response is analyzed. The experimental results indicate that when the reduced velocity (Vr) is in the range of 1.2 < Vr < 2.6, only streamwise vibration with small amplitude occurs, whose frequency is quite close to its natural frequency in the still water. When the reduced velocity Vr > 3.4, both the streamwise and transverse vibrations of the cylinder occur. In this range, the amplitudes of transverse vibration are much larger than those of streamwise vibrations, and the amplitudes of the streamwise vibration also get larger than those at the range of 1.2 < Vr < 2.6. At the range of Vr > 3.4, the frequency of streamwise vibration undergoes a jump at certain values of Vr, at which the streamwise vibrating frequency is twice as much as the transverse one. However, when the streamwise vibration does not experience a jump, its frequency is the same as that of the transverse vibration. The maximum values of second streamwise and transverse vibration amplitudes increase with increasing gap-to-diameter ratios.

静气动弹性计算方法研究

对基于结构网格的Euler方程及N-S方程求解器和基于非结构网格的Euler方程求解器,采用结构模态分析方法和柔度矩阵方法,对无人机大展弦比机翼在Ma=0.6,α=2°,飞行高度20 km的巡航状态下的静气动弹性特性进行了数值模拟.验证了两种求解器对静气动弹性模拟的准确性.同时,对模态分析方法和柔度矩阵方法进行了对比研究,发现柔度矩阵方法更适用于静气动弹性数值模拟.另外,对应用物面法向偏转方法替代网格变形技术模拟静气动弹性进行了研究,计算表明物面法向偏转方法可以大大提高静气动弹性计算效率和克服机翼结构变形过大时动网格技术无法处理的不足.

Vertical Bearing Capacity of a Partially-Embedded Pipeline on Tresca Soils

Slip-line field solutions are presented for the ultimate load of submarine pipelines on a purely cohesive soil obeying Tresca yield criterion, taking into account of pipe embedment and pipe-soil contact friction. The derived bearing capacity factors for a smooth pipeline degenerate into those for the traditional strip-line footing when the embedment approaches zero. Parametric studies demonstrate that the bearing capacity factors for pipeline foundations are significantly influenced by the pipeline embedment and the pipe-soil frictional coefficient. With the increase of pipeline embedment, the bearing capacity factor Nc decreases gradually, and finally reaches the minimum value (4.0) when the embedment equals to pipeline radius. As such, if the pipeline is directly treated as a traditional strip footing, the bearing capacity factor Nc would be over evaluated. The ultimate bearing loads increase with increasing pipeline embedment and pipe-soil frictional coefficient.

5.5W CWYb3+ : Y2O3 ceramic laser pumped with 970 nm laser diode

CW laser output has been demonstrated for polycrystalline transparent 10 at.% Yb3+-doped Y2O3 ceramics. End-pumped with 970 nm laser diode, a maximum output power of 5.5 W has been obtained with absorbed pump power of 31.1 W. The slope efficiency is 25% while the threshold pump power is 5.6 W. Saturation is not observed in our experiments, indicating higher laser output can be expected with higher pump power. (C) 2004 Elsevier B.V. All rights reserved.

Low temperature growth of c-axis oriented AlN films on gamma-LiAlO2 by radio frequency magnetron sputtering

Aluminum nitride (AlN) films were prepared on gamma-LiAlO2 substrates by radio frequency (rf) magnetron sputtering. The influence of substrate temperature (T-s) and nitrogen (N-2) concentration on film growth was investigated. The X-ray diffraction (XRD) results reveal that highly c-axis oriented AlN films can be obtained in the temperature range from room temperature (RT) to 300 degrees C. A smoother surface and a crystalline quality decrease with increasing N-2 concentration have been observed by XRD and atomic force microscopy (AFM) for the films deposited at lower substrate temperature. On the contrary, the degradation of the surface smoothness and the higher crystalline quality can be observed for the films deposited at a higher substrate temperature with N-2-rich ambient. The growth mechanism which leads to different crystalline quality of the films is discussed. (C) 2008 Elsevier B.V. All rights reserved.

Luminescence behavior of Yb(3+)heavy-doped yttrium lanthanum oxide transparent ceramics

Yb3+ heavy-doped yttrium lanthanum oxide transparent ceramics were fabricated and their spectroscopic properties were investigated. The absorption bands of (YbxY0.9-xLa0.1)(2)O-3 (x = 0.05-0.15) ceramics are broad at wavelength of 900-1000 nm. The absorption cross-sections centered at 974 nm and the emission cross-sections at 1031 nm of Yb3+ ion are 0.89-1.12 x 10(-20) cm(2) and 1.05 x 10(-20) cm(2) respectively. The up-conversion luminescence intensity of Yb3+-doped yttrium lanthanum oxide ceramics increased firstly, then decreased with the increase of Yb3+ ion content. (C) 2008 Elsevier B.V. All rights reserved.