Physical parameters affecting sonoluminescence: A self-consistent hydrodynamic study

We studied the dependence of thermodynamic variables in a sonoluminescing ~SL! bubble on various physical factors, which include viscosity, thermal conductivity, surface tension, the equation of state of the gas inside the bubble, as well as the compressibility of the surrounding liquid. The numerical solutions show that the existence of shock waves in the SL parameter regime is very sensitive to these factors. Furthermore, we show that even without shock waves, the reflection of continuous compressional waves at the bubble center can produce the high temperature and picosecond time scale light pulse of the SL bubble, which implies that SL may not necessarily be due to shock waves.

How important are shock waves to single-bubble sonoluminescence?

By solving numerically the full set of hydrodynamic equations governing the pulsation of a bubble,we show that shock waves are often absent in a stable sonoluminescing bubble. Nevertheless, for a wide range of physical parameters, a continuous compressional wave emerges and heats up the bubble, and the resulting black-body radiations have pulse heights and widths that agree with experimental data. Shock waves, being much less robust, are not essential for stable single-bubble sonoluminescence.

Microstructure Study Of Laser Welding Cast Nickel-Based Superalloy K418

Experiments of laser welding cast nickel-based superalloy K418 were conducted. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness. The corresponding mechanisms were discussed in detail. Results show that the laser welded seam have non-equilibrium solidified microstructures consisting of Cr-Ni-Fe-C austenite solid solution dendrites as the dominant and some fine and dispersed Ni-3(Al,Ti) gamma' phase as well as little amount of MC needle carbides and particles enriched in Nb, Ti and Mo distributed in the interdendritic regions, cracks originated from the liquation of the low melting points eutectics in the HAZ grain boundary are observed, the average microhardness of the welded seam and HAZ is higher than that of the base metal due to alloy elements' redistribution of the strengthening phase gamma'. (C) 2008 Elsevier B.V. All rights reserved.

Novel Mechanical Behavior Of Zno Nanorods

A novel stress-strain relation with two stages of linear elastic deformation is observed in [0 0 0 1]-oriented ZnO nanorods under uniaxial tensile loading. This phenomenon results from a phase transformation from wurtzite (WZ, P6(3)mc space group) to a body-centered tetragonal structure with four-atom rings (denoted as BCT-4) belonging to the P4(2)/mnm space group. The analysis here focuses on the effects of nanorod size and temperature on the phase transformation and the associated mechanical behavior. It is found that as size is increased from 19.5 to 45.5 angstrom, the critical stress for nucleation of the transformation decreases by 25% from 21.90 to 16.50 GPa and the elastic moduli of the WZ- and BCT-4-structured nanorods decrease by 24% (from 299.49 to 227.51 GPa) and 38% (from 269.29 to 166.86 GPa), respectively. A significant temperature effect is also observed, with the critical stress for transformation initiation decreasing 87.8% from 17.89 to 2.19 GPa as temperature increases from 300 to 1500 K. (c) 2007 Elsevier B.V. All rights reserved.

Transmission Electron Microscopy Characterization of Laser Welding Cast Ni-Based Superalloy K418 Turbo Disk And Alloy Steel 42Crmo Shaft

Microstructure characterization is important for controlling the quality of laser welding. In the present work, a detailed microstructure characterization by transmission electron microscopy was carried out on the laser welding cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft and an unambiguous identification of phases in the weldment was accomplished. It was found that there are gamma-FeCrNiC austenite solid solution dendrites as the matrix, (Nb, Ti) C type MC carbides, fine and dispersed Ni-3 Al gamma' phase as well as Laves particles in the interdendritic region of the seam zone. A brief discussion was given for their existence based on both kinetic and thermodynamic principles. (c) 2007 Elsevier B.V. All rights reserved.

Research on laser welding of cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft

Exploratory experiments of laser welding cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft were conducted. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. The corresponding mechanisms were discussed in detail. Results showed that the laser-welded seam had non-equilibrium solidified microstructures consisting of FeCr0.29Ni0.16C0.06 austenite solid solution dendrites as the dominant and some fine and dispersed Ni3Al gamma' phase and Laves particles as well as little amount of MC short stick or particle-like carbides distributed in the interdendritic regions. The average microhardness of the welded seam was relatively uniform and lower than that of the base metal due to partial dissolution and suppression of the strengthening phase gamma' to some extent. About 88.5% tensile strength of the base metal was achieved in the welded joint because of a non-full penetration welding and the fracture mechanism was a mixture of ductility and brittleness. The existence of some Laves particles in the welded seam also facilitated the initiation and propagation of the microcracks and microvoids and hence, the detrimental effects of the tensile strength of the welded joint. The present results stimulate further investigation on this field. (c) 2006 Elsevier B.V. All rights reserved.

Dissimilar autogenous full penetration welding of superalloy K418 and 42CrMo steel by a high power CWNd : YAG laser

Experiments of autogenous laser full penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 3.5 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser welding velocity, flow rate of side-blow shielding gas, defocusing distance were investigated. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. Results show that high quality full penetration laser-welded joint can be obtained by optimizing the welding velocity, flow rate of shielding gas and defocusing distance. The laser-welded seam have non-equilibrium solidified microstructures consisting of gamma-FeCr0.29Ni0.16C0.06 austenite solid solution dendrites as the dominant and very small amount of super-fine dispersed Ni3Al gamma' phase and Laves particles as well as MC needle-like carbides distributed in the interdendritic regions. Although the microhardness of the laser-welded seam was lower than that of the base metal, the strength of the joint was equal to that of the base metal and the fracture mechanism showed fine ductility. (c) 2007 Elsevier B.V. All rights reserved.

The interlaminar fracture-behavior and toughening mechanisms of new carbon-fiber-reinforced bismaleimide composites

The interlaminar fracture behaviour of carbon fibre-reinforced bismaleimide (BMI) composites prepared by using a new modified BMI matrix has been investigated by various methods. Laminates of three typical stacking sequences were evaluated. Double cantilever beam, end-notch flexure and edge-delamination tension tests were conducted under conventional conditions and in a scanning electron microscope. The strain energy release rates in Mode I and Mode III G(lc) and G(llc), as well as the total strain energy release rate, G(mc), have been determined and found to be higher than those for laminates with an epoxy matrix. Dynamic delamination propagation was also studied. The toughening mechanisms are discussed.

K418高温合金和42CrMo合金钢的激光焊接

进行了K418高温合金和42CrMo钢的激光焊接实验，分别采用OM、SEM、XRD和EDS等手段分析了焊缝的金相组织和物相组成，评价了焊缝的显微硬度和拉伸强度。结果表明：焊缝主要由树枝状非平衡凝固的FeCrNiC（力固溶体组成，此外，还有少量细小、弥散的Ni_3Al（γ′）相、Laves颗粒和少量MC碳化物分布在树枝晶间区域。由于主要强化相γ′在激光辐照后的部分溶解和随后快速凝固的抑制作用，焊缝的硬度虽分布较均匀但低于母材。由于没有获得穿透的焊接接头，焊接接头的强度约只有母材的88．5％，焊缝的断裂机制是塑性和脆性断裂的混合机制。由于在焊缝中存在一些Laves颗粒，这促进了微裂纹和微孔的形成和扩展，降低了焊接接头的抗拉强度。

粘性可压混合层时间稳定性对称紧致差分求解

基于可压扰动方程组的一阶改型，将高精度对称紧致格式引入边值法数值线性稳定分析。对所获非线性离散特征值问题给出了一个通用形式二阶迭代局部算法，实现了时间模式和空间模式的统一求解，并将扰动特征及其特征函数同时得到。据此分析了可压平面自由混合层时间稳定性，涉及二维/三维扰动波、粘性/无粘扰动波、第一/第二模态、特征函数、伪特征值谱等。研究表明，压缩性效应和粘性效应对最不稳定扰动波数和增长率呈相似的减抑作用；在Mc = 1附近，从高波数段开始，粘性效应可强化二维不稳定扰动波由第一模态向第二模态的过渡。

铁基合金激光熔覆组织及其冲击磨损性能

结果表明熔覆组织为过共晶组织,凝固领先相为M_7C_3碳化物,枝晶间共晶为γ-奥氏体M_7C_3,γ为具有较高合金元素过饱和含量的亚稳相,M_7C_3(M=Cr,Fe,W等)为六方结构的Cr基合金碳化物.高温时效过程中熔覆组织存在M_7C_3+γ→M_(23)C_6及M_7C_3+γ→M_6C的原位转变,同时在非平衡γ中析出M_(23)C_6、M_2C与MC碳化物.熔覆组织具有较高的显微硬度(HV920_(0.2)并存在显著的二次硬化特征,二次硬化峰值温度及峰值硬度分别为700℃及HV1160_(0.2).此外,熔覆组织还具有较高的冲击磨损性能.

固体力学进展及应用: 庆贺李敏华院士90华诞文集

《固体力学进展及应用：庆贺李敏华院士90华诞文集》收录了近代固体力学基础理论及其应用领域的重要科技成果和最新进展。作者是在同体力学领域工作多年的资深研究员，他们来自各行各业，有丰富的科研与丁作经验。他们提供的论文在相当程度上反映当前同体力学的发展现状与成就，并能看出发展趋势，对未来研究的课题选择有参考价值。《固体力学进展及应用：庆贺李敏华院士90华诞文集》还收集了李敏华院士的珍贵照片和纪念李敏华院士90华诞的庆贺和回忆文章，具有重要的史料价值。

材料和结构的动态响应

《材料和结构的动态响应》是我国爆炸力学、冲击动力学领域的专家、教授、学者们为祝贺爆炸力学、冲击动力学专家王礼立教授七十华诞而撰写的一部学术著作，从不同的角度展示了我国乃至国际上近年来在该领域所取得的知识创新成果及学科发展动向，为促进爆炸力学、冲击动力学的理论创新和科技进步，造福人类，做出积极努力。

沿平板可压缩湍流边界层的直接数值模拟

湍流广泛存在于自然界和工程应用中。可压平板湍流是一种典型的壁湍流，对其进行直接数值模拟研究是壁湍流机理研究的基础。本文采用高精度紧致型差分格式求解三维可压N-S方程，利用MPI并行程序，直接数值模拟了来流马赫数为M的＝2．25，雷诺数为Re＝635O00／in．的空间发展的可压缩平板湍流边界层。本文主要研究了如下一些问题。方法研究方面：●针对壁湍流流动的特点，文中提出了与基于非等距网格高精度紧致型差分格式相结合的简化的迭代一隐式时间推进法，建立了可压缩平板湍流边界层直接数值模拟方法。提高了计算效率，并给出了迭代收敛的条件。●研究了不同网格加密方法对数值解分辨率的影响，指出在保证近物面附近物理平面内网格是均匀的条件下，采用坐标变换的方法也可得到近物面高分辨率的数值解，纠正了人们认为只有在基于非等距网格上才能得到高分辨率解的观点。●采用包括前缘在内的二维可压平板层流边界层直接数值模拟结果为三维可压平板湍流边界层计算提供上游边界条件，解决了国外采用的局限性很大的经验公式带来的不相容问题。物理问题的模拟研究：●在国内首次给出了超声速平板边界层从层流经bypass转披直到充分发展湍流的整个过程。所得流场统计特征（如平均速度分布，物面摩阻系数，脉动速度的均方根等）与相关理论和实验符合甚好。这些结果充分证实了本文所建立的数值方法的有效性和所得结果的可靠性。●在此基础上，文中分析了可压平板湍流边界层内的拟序结构的形成过程。研究了初始扰动对平板边界层流动转挨特征的影响，指出上游初始扰动中展向的非对称性是形成bypass转挨的关键因素，且较大的时间扰动频率使得边界层转披位置后移。这些结果为壁湍流控制提供一定依据。●在此基础上研究了可压缩效应对平均流动特征以及湍能的生成和耗散特征的影响，指出在Mc∞=2.25的情况下，Morkovin假设基本成立。但可压缩效应使得边界层内层中过渡层加宽；拟序结构有明显差别。与不可压相比上抛、下扫对不同区域的作用有所不同。在近壁区，湍动能方程中的压力-膨胀项和压力-速度相关项仍是不可忽略的量，这导致声的产生和可压缩效应对湍流的发展所产生的抑制作用。

可压平面混合层稳定性分析及数值模拟

可压平面混合层是包含复杂多时空尺度运动的非定常流体力学部问题，具有深刻的理论意义和广泛的应用背景。针对该问题所涉及内容的多面性，本文的目的是，基于高精度、高分辨率数值算法的构造、发展和数值行为分析，采用线性稳定性分析和直接数值模拟方法。从理论和计算两方面集中研究压缩性效应、粘性效应、初值效应以及燃烧反应放热效应等对可压平面混合层早期稳定性行为和大尺度拟序涡结构非线性演化的影响。以混合层已有研究成果的分析和综述为开端，论文主体共包括四部分：第一部分是可压平面混合层时间/空间模式数值线性稳定性分析。实现了高精度对称紧致差分格式（SCD）对可压粘性扰动线性稳定性边值问题的求解，对导出的线性和非线性离散特征值问题，提出了两个高效局部解法。研究涉及二维/三维扰动波、无粘/粘性扰动波、特征函数和特征值谱、第一/第二模态、超声速快/慢模态、速度比和密度比等。验证了对流Mach数Mc为一个合理的压缩性参数。指出压缩性效应和粘性效应对最不稳定扰动波的波数（频率）和增长率呈相拟的抑制作用，且时间模式稳定性分析结果在许多方面是可信的。从随机和线性扰动场出发，采用高精度五阶迎风紧致和六阶对称紧致混合差分算法（UCD5/SCD6）对可压平面混合层的稳定性特征进行了直接数值模拟，揭示了初始主导线性扰动与一些实际涡结构非线性作用形态间的内在关联，印证了线性稳定性分析方法的合理性和有效性。第二部分是高精度迎风紧致差分格式（UCD）时空全离散数值行为分析。导出了其一维/二维一般色散表达式。研究表明，UCD格式在高波数区具有内在的全离散耗散和色散特性；其数值群速度的快/慢特征可因CFL数不同而改变；在稳定CFL数下简单附加人工粘性可强化UCD格式在高波数区的耗散量；提高时间精度可放宽稳定CFL数限制；UCD格式的二维全离散色散介质中存在三个不同性质的数值波，其全离散稳定性由数值声波主控。第三部分实现了高精度UCD5/SCD6差分算法对空间发展可压平面混合层的直接数值模拟。通过亚谐扰动波的个数和扰动频率的控制，捕捉到了基频涡的饱和、一次和二次对并等现象，显示了大尺度涡结构与入中初始扰动方式之间的内在联系。利用参数Mc观察了压缩性效应对大尺度涡空间演化及其相互作用的影响。第四部分实现了高精度UCD5/SCD6差分算法对非预混扩散火焰化学反应平面混合层的直接数值模拟。研究指出，放热效应可抑制和延迟涡的形成，使基频涡卷拉伸甚至丧失，混合层Reynolds 应力ρu'v'和流向速度波动关联项u'v'下降，以致涡结构与外流动量交换和标量输运减少，脉动输运能力被削弱，从而混合效率、产物生成率和混合层增长率下降，放热主要通过膨胀效应和斜压效应来抑制大尺度涡的演化。