Formation, microstructure and development of the localized shear deformation in low-carbon steels

An investigation has been made into the effect of microstructural parameters on the propensity for forming shear localization produced during high speed torsional testing by split Hopkinson bar with different average rates of 610, 650 and 1500 s(-1) in low carbon steels. These steels received the quenched, quenched and tempered as well as normalized treatments that provide wide microstructural parameters and mechanical properties. The results indicate that the occurrence of the shear localization is susceptible to the strength of the steels. In other words, the tendency of the quenched steel to form a shear band is higher than that of the other two steels. It is also found that there is a critical strain at which the shear localization occurs in the steels. The critical strain value is strongly dependent on the strength of the steels. Before arriving at this point, the material undergoes a slow work-hardening. After this point, the material suffers work-softening, corresponding to a process during which the deformation is gradually localized and eventually becomes spatially correlated to form a macroscopic shear band. Examinations by SEM reveal that the shear localization within the band involves a series of sequential crystallographic and non-crystallographic events including the change in crystal orientation, misorientation, generation and even perhaps damage in microstructures such as the initiation, growth and coalescence of the microcracks. It is expected that the sharp drop in the load-carrying capacity is associated with the growth and coalescence of the microcracks rather than the occurrence of the shear localization, but the shear localization is seen to accelerate the growth and coalescence of the microcracks. The thin foil observations by TEM reveal that the density of dislocations in the band is extremely high and the tangled arrangement and cell structure of dislocations tends to align along the shear direction. The multiplication and interaction of dislocations seems to be responsible for work-hardening of the steels. The avalanche of the dislocation cells corresponds to the sharp drop in shear stress at which the deformed specimen is broken. Double shear bands and kink bands are also observed in the present study. The principal band develops first and its width is narrower than that of the secondary band.

Electric characteristics of pseudo-spark during breakdown phase

An analysis of the time-dependent resistive voltage and power deposition during the breakdown phase of pseudo-spark is presented. The voltage and current were measured by specially designed low-inductance capacitive voltage divider and current measuring resistor. The measured waveforms of voltage and current are digitized and processed by a computer program to remove the inductive component, so as to obtain resistive voltage and power deposition. The influence of pressure, cathode geometry and charging voltage of storage capacitors on the electrical properties in the breakdown phase are investigated. The results suggest that the breakdown phase of pseudo-spark consists of three stages. The first stage is mainly hollow cathode discharge. In the second stage, field-enhanced thermionic emission takes place, resulting in a fast voltage drop and sharp rise of discharge current. The third stage of discharge depends simply on the parameters of the discharge circuit.

Crack tip behavior and crack-propagation in ductile materials

Dilatational plastic equations, which can include the effects of ductile damage, are derived based on the equivalency in expressions for dissipated plastic work. Void damage developed internally at the large-strain stage is represented by an effective continuum being strain-softened and plastically dilated. Accumulation of this local damage leads to progressive failure in materials. With regard to this microstructural background, the constitutive parameters included for characterizing material behaviour have the sense of internal variables. They are not able to be determined explicitly by macroscopic testing but rather through computer simulation of experimental curves and data. Application of this constitutive model to mode-I cracking examples demonstrates that a huge strain concentration accompanied by a substantial drop of stress does occur near the crack tip. Eventually, crack propagation is simulated by using finite elements in computations. Two numerical examples show good accordance with experimental data. The whole procedure of study serves as a justification of the constitutive formulation proposed in the text.

DMFC内部气液两相流动与电性能的短时落塔实验研究

利用国家微重力实验室落塔提供的短时微重力实验环境，对常重力和微重力条件下直接甲醇燃料电池（DMFC）内部的气液两相流动形态和相应电性能等的影响进行了实验研究，发现在微重力条件下，DMFC阳极流道内CO_2气泡速度很小，气泡尺寸随着时间的推移而不断长大，甚至堵塞流道；流道堵塞现象随电流增大而急剧强化．电性能曲线显示，在浓差极化区存在显著的重力效应，电性能的恶化随浓差极化程度的加强而增大．

液滴热毛细迁移问题的研究进展

液滴或气泡的迁移现象无论是在流体力学的基础研究中,还是在材料加工,化学工程等实际应用中都是一个很重要的课题.在微重力环境中,如果液滴或气泡所在的母液中外加一个温度场,则液滴或气泡就会由于表面张力分布的不均匀而发生迁移运动.这种运动被称为Marangoni迁移或热毛细迁移运动.综述了液滴或气泡的热毛细迁移问题历史研究中理论分析,数值模拟以及实验方面的主要结果,阐述了该问题的研究发展过程.目前液滴迁移问题的研究,理论分析解还只限于线性及弱非线性的定常问题,数值模拟工作已经得到了在热对流作用较小时液滴的非定常迁移过程,对于热对流影响很大的情况(Marangoni数大于100)则尚未得到与实验中观测到的相一致的理论结果.在总结前人研究的基础上,给出了热对流作用较大时液滴热毛细迁移非定常问题的最新的数值模拟结果,并对该问题在此情况下产生的新变化给予了分析.最后,分析了当前研究中所存在的问题并进一步展望液滴热毛细迁移问题未来的发展方向.

应用PIVT技术测量单液滴热毛细迁移时的速度场和温度场

在地面实验中,使用粒子图像测速测温(Particle Image Velocimetry and Thermometry, PIVT)技术测量了单滴热毛细迁移时液滴周围同时刻的速度场和温度场.选用密度相近的豆油和硅油作为实验系统的母液和液滴.实验结果表明单滴周围的温度场扰动不同于线性理论预测的随到液滴距离的平方成反比,而是距离一次方成反比,这是由于重力引起的浮力效应造成的,地面实验中重力作用不能忽略.

The Characteristics of Semi-Cokes --The Solid Residues from Coal Partial Gasification

In this paper the proximate analysis and ultimate analysis of sulfur in different semi-cokes generated from Rizhao bituminous coal and Beijing anthracite under different temperatures is done. Also the tendency of the contents of volatile, ash, fixed carbon and sulfur in different semi-cokes along with the different preparation temperatures is studied. Then the combustion experiment of semi-cokes in the drop-tube furnace system was carried out, and the kinetic parameters of different semi-cokes ware calculated.

Numerical Research on Liquid Spray in Supersonic Cross Flow

Numerical simulation was conducted to characterize the kerosene spray injecting into supersonic cross flow, especially focusing on the aerodynamic secondary breakup effect of the supersonic cross flow on the initial droplets. It was revealed that the initial parent drops were broken up into small drops whose diameter is about O(10) micrometers soon after they entered into the supersonic cross flow. During the appropriate range of initial drop size, the parent droplets would be broken up into small drops with the same magnitude diameter no matter how large the initial drops SMD was.

Subcooled Pool Boiling at Microgravity: Preliminary Ground-Based Experimental Results

A temperature-controlled pool boiling (TCPB) device was developed to perform pool boiling heat transfer studies at both normal gravity on Earth and microgravity in the drop tower Beijing and aboard a Chinese recovery satellite. Two platinum wires of 60 ?m in diameter were simultaneously used as heaters and thermometers. The lengths were 30 mm and 40 mm, respectively. The ends of wires were soldered with copper poles to provide low resistance paths for the electric current. The heater resistance, and thus the heater temperature, was kept constant by a feedback circuit similar to that used in constant-temperature hot-wire anemometry. The fluid was R113 at 0.1 Mpa and subcooled by 30 ?C nominally for all cases. The results of the experiments at normal gravity were presented. Four modes, namely single-phase convection, nucleate boiling, transition two-mode boiling, and film boiling were observed. A few data obtained from several preliminary experiments at microgravity in the drop tower Beijing were also presented. A slight increase of the heat flux was obtained.

Numerical Study on Kerosene Spray in Supersonic Flow

Numerical simulation was conducted to study the kerosene spray characteristics injecting into supersonic cross flow. The verification of the simulation was carried out by experimental Schlieren image, and the agreement was obtained by compared the spray plume pictures. Furthermore, the aerodynamic secondary breakup effect of the supersonic cross flow on the initial droplets was investigated. It was revealed that the initial parent drops were broken up into small drops whose diameter is about O(10) micrometers soon after they entered into the supersonic cross flow. During the appropriate range of initial drop size, the parent droplets would be broken up into small drops with the same magnitude diameter no matter how large the initial drops SMD was.

Experimental Study on Subcooled Pool Boiling in Microgravity Utilizing

Experiments about Simple Liquid Flows in Microtubes

The behavior of micro-scale flow is significant for the performance of Micro-Electro-Mechanical- Systems (MEMS) devices. Some experiments about liquid flow through microtubes with diameters about 3similar to20mum are presented here. The liquids used in our experiments include some simple liquids with small molecules, such as non-ion water and several kinds of organic liquids (CCL4, C6H5C2H5 and Isopropanol etc.). The flow rate and the normalized friction cocfficients were measured in micro-flow experimental apparatus. The results show that when the driven pressure varies from 0 to 1Mpa, the flow behaviors in 20mum microtube for both polar and non-polar liquids are in agreement with Hagen-Poiseuille law of the classical theory. It means that N-S equation based on continuous medium still acts well in this case. For higher pressure drop from 1 to 30Mpa, in the microtubes with diameter of 3similar to10mum, the normalized friction coefficients of organic liquids can't keep constant with pressure increases. However the non-ion water reveals different trends.

Pool Boiling in Microgravity: Recent Results and Perspectives for the Project DEPA-SJ10

Two research projects on pool boiling in microgravity have been conducted aboard the Chinese recoverable satellites. Ground-based experiments have also been performed both in normal gravity and in short-term microgravity in the Drop Tower Beijing. Steady boiling of R113 on thin platinum wires was studied with a temperature-controlled heating method, while quasi-steady boiling of FC-72 on a plane plate was investigated with an exponentially increasing heating voltage. In the first case, slight enhancement of heat transfer is observed in microgravity, while diminution is evident for high heat flux in the second one. Lateral motions of bubbles on the heaters are observed before their departure in microgravity. The surface oscillation of the merged bubbles due to lateral coalescence between adjacent bubbles drives it to detach from the heaters. The Marangoni effect on the bubble behavior is also discussed. The perspectives for a new project DEPA-SJ10, which has been planned to be flown aboard the Chinese recoverable satellite SJ-10 in the future, are also presented.

Two-phase Flow in Fuel Cells in Short-term Microgravity Condition

A visual observation of liquid-gas two-phase flow in anode channels of a direct methanol proton exchange membrane fuel cells in microgravity has been carried out in a drop tower. The anode flow bed consisted of 2 manifolds and 11 parallel straight channels. The length, width and depth of single channel with rectangular cross section was 48.0 mm, 2.5 mm and 2.0 mm, respectively. The experimental results indicated that the size of bubbles in microgravity condition is bigger than that in normal gravity. The longer the time, the bigger the bubbles. The velocity of bubbles rising is slower than that in normal gravity because buoyancy lift is very weak in microgravity. The flow pattern in anode channels could change from bubbly flow in normal gravity to slug flow in microgravity. The gas slugs blocked supply of reactants from channels to anode catalyst layer through gas diffusion layer. When the weakened mass transfer causes concentration polarization, the output performance of fuel cells declines.

Occurrence of spanning of a submarine pipeline with initial embedment

For better understanding the mechanism of the occurrence of pipeline span for a pipeline with initial embedment, physical and numerical methods are adopted in this study. Experimental observations show that there often exist three characteristic phases in the process of the partially embedded pipeline being suspended: (a) local scour around pipe; (b) onset of soil erosion beneath pipe; and (c) complete suspension of pipe. The effects of local scour on the onset of soil erosion beneath the pipe are much less than those of soil seepage failure induced by the pressure drop. Based on the above observations and analyses, the mechanism of the occurrence of pipeline spanning is analyzed numerically in view of soil seepage failure. In the numerical analyses, the current-induced pressure along the soil surface in the vicinity of the pipe (i.e. the pressure drop) is firstly obtained by solving the N-S equations, thereafter the seepage flow in the soil is calculated with the obtained pressure drop as the boundary conditions along the soil surface. Numerical results indicate that the seepage failure (or piping) may occur at the exit of the seepage path when the pressure gradient gets larger than the critical value. The numerical treatment provides a practical tool for evaluating the potentials for the occurrence of pipe span due to the soil seepage failure.