大气压力对激光辐照双层板接触传热的影响

考虑激光辐照下结构变形对双层板接触传热影响可以更真实地反映两板间的传热情况,通过实验和数值模拟分析了大气压力对接触传热的影响。结果表明:大气压力对接触热阻影响非常明显,当大气压力超过一定值后,双层板界面始终接触,后板中心的温升阶段成类抛物形状;当大气压力在某一范围内,大气压力与温度矩产生的靶板挠度相当,接触面时分时合,后板的温升阶段成振荡式类线性增加;当大气压力小于某一特定值时,后板温度有一突跃过程,通过适当的设计有可能观测到该现象。     

基于黎曼解的粒子间接触算法在SPH中的应用

 采用光滑粒子流体动力学（SPH）方法模拟大变形问题具有明显的优点,但传统的SPH方法在模拟冲击波与接触界面的作用时,往往会出现压力的反常跳动。采用黎曼解描述粒子间相互作用的接触算法对传统SPH方法进行修正,计算了激波管和飞片碰撞（包含接触界面）问题中波的传播,并将计算结果与解析解作比较。结果表明,与传统的光滑粒子法相比,该改进的光滑粒子法无需引入人工粘性项和人工热流项,程序结构简洁,且能较好地处理接触界面问题,从而能有效提高计算精度。     

大气压力对激光辐照双层板接触传热的影响

考虑激光辐照下结构变形对双层板接触传热影响可以更真实地反映两板间的传热情况,通过实验和数值模拟分析了大气压力对接触传热的影响。结果表明：大气压力对接触热阻影响非常明显,当大气压力超过一定值后,双层板界面始终接触,后板中心的温升阶段成类抛物形状;当大气压力在某一范围内,大气压力与温度矩产生的靶板挠度相当,接触面时分时合,后板的温升阶段成振荡式类线性增加;当大气压力小于某一特定值时,后板温度有一突跃过程,通过适当的设计有可能观测到该现象。     

Deformation and Spallation of Explosive Welded Steels under Gas Gun Shock Loading

We investigate deformation and spallation of explosive welded bi-steel plates under gas gun shock loading. Free surface histories are measured to obtain the Hugoniot elastic limit and spall strengths at different impact velocities.Pre-and post-shock microstructures are characterized with optical metallography, scanning electron microscopy,and electron backscatter diffraction. In addition, the Vickers hardness test is conducted. Explosive welding can result in a wavy steel/steel interface, an ultrafine grain region centered at the interface, and a neighboring high deformation region, accompanied by a hardness with the highest value at the interface. Additional shock compression induces a further increase in hardness, and shock-induced deformation occurs in the form of twinning and dislocation slip and depends on the local substructure. Spall damage nucleates and propagates along the ultrafine grain region, due to the initial cracks or weak interface bonding. Spall strengths of bimetal plates can be higher than its constituents. Plate impact offers a promising method for improving explosive welding.     

接触缺陷的振动调制超声导波检测技术研究

针对常规线性超声检测方法无法实现板结构接触类缺陷(如微裂纹、脱粘等)检测问题,将超声导波技术与振动声调制技术相结合,利用稀疏分布传感器发展了一种板结构中接触缺陷非线性超声检测方法。通过低频振动改变缺陷的接触状况,使得通过接触面的高频导波信号的相位和幅值受到调制。对受低频振动调制的超声导波二维时间序列进行时频分析,由于接触类缺陷的存在,在振动调制超声导波序列的时频分布上出现明显的低频振动频率分量。利用提取出的低频振动频率下的超声导波信号,进行了结构接触缺陷成像处理。检测试验表明,基于振动声调制的超声导波缺陷成像方法可以实现结构中的接触类缺陷检测。     

冲击射流激波振荡与抑制

欠膨胀冲击射流具有复杂的激波结构,并伴随产生高幅值的离散频率单音.通过高速摄像获取的纹影图像并结合噪声测量,对欠膨胀冲击射流激波振荡过程、剪切层不稳定波的模态和离散频率单音的产生进行了系列研究.给出了冲击距离为5倍喷嘴出口直径的复杂流动实验结果分析,射流剪切层不稳定波有对称和非对称两种模态,发现不同模态下的离散频率单音...     

激波诱导液滴变形和破碎现象实验研究

本文介绍了用于激波诱导液滴变形和破碎现象研究的实验系统及方法,详细分析了激波与液滴相互作用以及液滴加速、变形和破碎过程,为进一步研究激波诱导的液滴内流场性质及气液相间相互作用对液滴变形和破碎的影响机制提供了基础实验数据。     

Perturbation evolution at a metal-metal interface subjected to an oblique shock wave: Supersonic velocity of the point of contact

The perturbation evolution at the interface between identical metals (metal plates) that is exposed to high-speed oblique shock waves is observed experimentally for the first time (the waves are attached to the point of contact, so that a cumulative jet cannot form). The experiments are numerically simulated by the two-dimensional Lagrange method. An elastoplastic model where the dynamic yield strength is a function of material state parameters is employed. An analytical technique to treat instability development under given loading conditions is suggested. High strains produce a high-temperature zone near the interface (thermal softening zone). A short-lived shear flow with a high velocity gradient depending on the angle and velocity of plate collision is observed. In this zone, the shear modulus and the yield strength are appreciably lower than under normal conditions, which favors instability development.     

Influence of imperfect bonding on interface waves guided by piezoelectric/piezomagnetic composites

Interface waves, which can be guided by imperfectly bonded piezoelectric and piezomagnetic half-spaces, are studied. The cases of absorbent and permeable interfaces are discussed in detail. It is shown that imperfection of the interface bonding has significant impact on the existence of interface waves and on their velocities of propagation. Some interface waves present in the case of imperfect bonding vanish when the bonding becomes perfect. The waves that are guided by an imperfect interface are shown to be dispersive, although there is no explicit characteristic length in the structure model. The results obtained show that possible imperfections of interface bonding must be considered in the design and fabrication of piezoelectric/piezomagnetic composites.     

An explanation for forking and backbending in rotational spectra

It is shown that symmetric deformed nuclei are unstable to asymmetric deformation at sufficiently high angular momentum. Forking and backbending are natural consequences of this instability.     

Experimental investigation of plasma excitations in asymmetric stripes of two-dimensional electrons

The plasmon microwave response of symmetric and asymmetric stripes of two-dimensional electrons with different boundary conditions has been studied. The symmetric case corresponds to stripes either without ohmic contacts or with ohmic contacts on both sides, while the contact in asymmetric stripes is present only on one side. It has been shown that there is a frequency shift of the observed plasma modes in asymmetric stripes. The lowest-frequency mode becomes a previously unobserved mode for which the quarter wavelength of the plasmon coincides with the length of the stripe. The behavior of the lowest mode and its multiple frequencies in asymmetric stripes has been studied under variation of the magnetic field, electron density, and temperature and size of the stripes. The results indicate that all plasma modes in symmetric and asymmetric stripes have the same physical nature.     

不同对称性下晶界结构演化及微观机理的晶体相场法研究

采用晶体相场法研究了单轴拉伸下三角相双晶变形过程及机理, 并重点分析了小角对称与非对称晶界和大角对称与非对称晶界在变形过程中的演化及微观机理, 变形过程中应力方向与初始晶界方向平行. 结果表明, 小角对称晶界由柏氏矢量夹角呈60°的两种刃型位错组成, 变形过程中不同类型的位错运动方向相反, 并各自与另一晶界上同一类型位错相互吸引以致部分位错发生湮没; 小角非对称晶界上的位错类型单一, 在应力作用下先沿水平方向攀移, 后各自分解成柏氏矢量约呈120°的两位错, 并通过位错运动和湮没最终形成理想单晶; 大角晶界在应力的作用下先保持水平状态而后锯齿化并发射位错, 伴随着位错运动和湮没, 最终大角非对称晶界发生分解, 而大角对称晶界则重新平直化, 表明大角对称晶界比大角非对称晶界更稳定, 这与实验和分子动力学模拟结果一致. 关键词： 晶体相场 双晶 晶界 对称性     

Interaction of Shock Waves in Cement Mortar Plate Investigated by the Digital Speckle Correlation Method

Interaction of shock waves in cement mortar plate is studied by digital speckle correlation method and digital high-speed photography technique. When the plates were destroyed by two detonators exploding at the same time, variation of shock wave field is obtained. Experimental results show that the interaction of shock waves will result in a nonlinear huge increase of local normal strain, leading to large deformation and serious destruction. However, the occurrence of this strongly nonlinear phenomenon sensitively depends on the interval between detonators, and it will only appear when the interval is smaller than the diameter of the region where shock waves exist.     

Interaction of contact electrohydrodynamic waves in current-carrying semiconductors

It is shown that contact electrohydrodynamic waves similar to the waves on an interface between two immiscible liquids can exist on an interface between two identical semiconductors with different concentrations of free charge carriers. When a current flows, instabilities of such waves, particularly an analog of the Kelvin-Helmholtz hydrodynamic instability, appear in one of the layers. The possibility of practical utilization of contact waves interacting with currents is discussed. Zh. Tekh. Fiz. 69, 1–4 (August 1999)     

激波与SF6球形气泡相互作用的数值研究

本文基于大涡模拟方法, 采用高阶精度格式对平面入射激波以及不同反射距离条件下的反射激波与SF₆重气泡相互作用过程进行了三维数值模拟. 数值结果清晰地显示了SF₆重气泡在激波作用下诱导Richtmyer-Meshkov不稳定性过程, 揭示了入射激波以及反射激波在气泡界面聚焦诱导射流的过程, 详细分析了不同反射距离条件下反射激波与SF₆重气泡作用过程及流场结构.     

Spontaneous phase symmetry breaking in a four-frequency ring gas laser

Lasing regimes of a single-mode four-frequency class-A ring gas laser with elliptical polarization of the emitted waves are studied numerically. Stationary regimes typical of both standing-and traveling-wave lasing are discovered. Self-oscillations exhibiting the properties of asymmetric and symmetric limit cycles are also found. It is shown that transition between cycles with different symmetry may result in the spontaneous phase symmetry breaking and the appearance of chaos arising due to the period doubling bifurcation cascade of the asymmetric limit cycle.     

Analysis of two colliding fractionally damped spherical shells in modelling blunt human head impacts

The collision of two elastic or viscoelastic spherical shells is investigated as a model for the dynamic response of a human head impacted by another head or by some spherical object. Determination of the impact force that is actually being transmitted to bone will require the model for the shock interaction of the impactor and human head. This model is indended to be used in simulating crash scenarios in frontal impacts, and provide an effective tool to estimate the severity of effect on the human head and to estimate brain injury risks. The model developed here suggests that after the moment of impact quasi-longitudinal and quasi-transverse shock waves are generated, which then propagate along the spherical shells. The solution behind the wave fronts is constructed with the help of the theory of discontinuities. It is assumed that the viscoelastic features of the shells are exhibited only in the contact domain, while the remaining parts retain their elastic properties. In this case, the contact spot is assumed to be a plane disk with constant radius, and the viscoelastic features of the shells are described by the fractional derivative standard linear solid model. In the case under consideration, the governing differential equations are solved analytically by the Laplace transform technique. It is shown that the fractional parameter of the fractional derivative model plays very important role, since its variation allows one to take into account the age-related changes in the mechanical properties of bone.     

A two-dimensional analysis of surface acoustic waves in finite elastic plates with eigensolutions

It is generally known that surface acoustic waves, or Rayleigh waves, have different mode shapes in infinite plates. To be precise, there are both exponentially decaying and growing components in plates appearing in pairs, representing symmetric and antisymmentric modes in a plate. As the plate thickness increases, the combined modes will approach the Rayleigh mode in a semi-infinite solid, exhibiting surface acoustic wave deformation and velocity. In this study, the two-dimensional theory for surface acoustic waves in finite plates is extended to include the exponentially growing modes in the expansion function. With these extra equations, we study the surface acoustic waves in a plate with different thickness to examine the coupling of the exponentially decaying and growing modes. It is found that for small thickness, the two groups of waves are strongly coupled, showing the significance of including the effect of thickness in analysis. As the thickness increases to certain values, such as more than five wavelengths, the exponentially decaying modes alone will be able to predict vibrations of surface acoustic wave modes accurately, thus simplifying the equations and solutions significantly. Supported by Qianjiang River Fund established by Zhejiang Provincial Government and Ningbo University and administered by Ningbo University and the National Natural Science Foundation of China (Grant No. 10572065)     

Influence of physical non-linearity on the dynamic behavior of composite plates

Influence of the physically non-linear property is investigated for three types of motion in laminated plates, namely harmonic waves, free vibrations and shock waves. Only the non-linearity in the transverse shear deformation is included.     

A surface wave dispersion relation for non-local media

Conditions are obtained for the existence of surface waves at the interface of vacuum and a semi-infinite non-local medium whose inverse dielectric function is assumed to be symmetric in spatial coordinate in the direction perpendicular to the interface. It is shown that these conditions reduce to those obtained by Maradudin in the appropriate limits: for the isotropic case; and for no retardation.