Nonlinear dynamics of the spatio-timporal pattern of a macroscopically localized deformation

High-speed video recording has been used in the in situ investigation of the spatio-temporal bands of a macroscopically localized deformation in the unsteady plastic flow of an Al-Mg alloy subjected to a constant stress increase rate. It is shown that the main mechanism of the development of deformation jumps is the cascade multiplication of the Savart-Masson deformation bands. This mechanism is compared to the discontinuous deformation models.     

Fractal characteristics of the surface of tungsten carbide-manganese steel composite after plastic deformation

The structure evolution in WC-80G4 steel composite heavily loaded by compression is studied. The fractal properties of the deformation relief appearing on the surface are determined. The fractal dimension of the deformed surface profile is shown to depend on the mean spacing between bands of localized deformation and on the parameters of the material fine crystal structure.     

Turbulence and dissipative structures in shock-loaded copper

Shock-loading tests of polycrystalline copper M3 under conditions of uniaxial deformation at impact velocities of 100 to 700 m/s were performed. It was established that a threshold deformation rate exists above which dissipative structures in the dynamically deformed material arise in the form of local regions of cellular type, with a size of 15–25 μm, separated by shear plastic bands. The basic size of cellular structure domains is on the nanometer scale. The microhardness of the material within the cellular structures is somewhat higher than in the bands of plastic deformation that separate these structures. At threshold deformation rates and above it, the defect of the mass velocity, the difference between the impactor velocity for symmetrical collision and the free surface velocity at the plateau of the compression pulse, increases sharply as does the spall strength of the material.     

Localization of plastic deformation in calcium fluoride crystals at elevated temperatures

The strain distribution was experimentally studied in CaF₂ crystals subjected to compression tests along [110] and [112] at a constant strain rate at temperatures T = 373–1253 K. At T > 845 K, the plastic deformation in deformed samples is found to be strongly localized in narrow bands, where the shear strain reaches several hundred percent. The physical deformation conditions are determined under which the plastic flow loses its stability and, as a result, the deformation is localized. The temperature dependence of the critical stress of the transition to a localized flow is found. A scenario is proposed for the nucleation and development of large localized shears during high-temperature deformation of single crystals.     

Self-blocking of shear bands and the delocalization of plastic flows in amorphous alloys upon megaplastic deformation

Features of the formation of shear bands and nanocrystalline phases upon the megaplastic deformation of amorphous alloys based on iron, nickel, and titanium at room temperature in a Bridgman chamber are analyzed via transmission electron microscopy. It is shown that the transition from strongly localized to quasi-homogeneous plastic deformation occurs at a definite stage of the inhomogeneous plastic flow. Mechanisms based on the self-blocking of propagating shear bands by particles of the nanocrystalline phase that emerge due to a dissipative increase in the temperature along the front of shear bands are proposed for the delocalization of plastic flow.     

A crystal plasticity model of a formation of a deformation band structure

The formation of deformation bands with the typically alternating sign of the misorientation across their boundaries is interpreted as spontaneous deformation instability caused by anisotropy of hardening. To analyse the nature of the fragmentation, a model of a rigid-plastic crystal domain deformed by symmetric double slip in a plane-strain compression is considered. The basic reason for the deformation band existence is that a local decrease in number of active slip systems in the bands is energetically less costly than a homogeneous deformation by multislip. However, such model of the bands predicts their extreme orientation and their width tends to zero. This trend is modified by hardening caused by a build up of the band boundaries and by a dislocation bowing (Orowan) stress. The model provides an explanation of observed orientation of the bands, their width and the significant change in the structural morphology seen as the band reorientation occurs at large strains. The predictions are in a favourable agreement with the available observations.     

短脉冲强激光在稀薄等离子体中传播时的电磁类孤立子

 采用二维粒子模拟方法，研究了短脉冲强激光在稀薄等离子体中传播时电磁类孤立子的产生和时空演化过程。通过分析电磁场与等离子体波的非线性能量交换和激光场的频率谱结构等，给出了电磁类孤立子形成的基本物理图像，讨论了等离子体参数对电磁类孤立子形成的影响。模拟结果表明：类孤立子的形成是由于局部电磁波振荡频率减小至等离子体频率引起的，初始等离子体密度越高越容易形成空间局域结构。     

A mechanism for cutting carbon nanotubes with a scanning tunneling microscope

We discuss the local cutting of single-walled carbon nanotubes by a voltage pulse to the tip of a scanning tunneling microscope. The tip voltage ( V) is the key physical quantity in the cutting process. After reviewing several possible physical mechanisms we conclude that the cutting process relies on the weakening of the carbon-carbon bonds through a combination of localized particle-hole excitations induced by inelastically tunneling electrons and elastic deformation due to the electric field between tip and sample. The carbon network releases part of the induced mechanical stress by forming topological defects that act as nucleation centers for the formation of dislocations that dynamically propagate towards bond-breaking. Received 6 April 2000     

NAB对撞增强相位共轭腔快锁模特性

本文分析了NAB[NdAl_3(BO_3)_4]自激活激光晶体脉冲被动锁模的优越性。研究了对撞增强自起动自泵浦近简并四波混频相位共轭腔的形成条件及其在锁模过程中的作用。首次演示了NAB对撞增增强快速压缩脉冲锁模过程。获得了平均7ps的超短脉冲的实验结果。     

脉冲激励混响室测试区域电磁场均匀性

研究了电磁脉冲激励混响室内电磁波传播规律,通过仿真建模和实验两方面对脉冲激励混响室测试区域内电磁场均匀性进行了研究。提出了混响室时域均匀性的概念及其计算方法,并利用此方法计算了仿真和实验结果。结果表明:脉冲激励混响室内电磁环境无论在时域上还是频域上,均满足国际标准规定的均匀性要求;在一定空间内,混响室内也可以形成统计平均的脉冲场电磁环境;在混响室内进行电磁脉冲场的电磁敏感度测试是可行的。     

Dynamical Bragg diffraction of optical pulses in photonic crystals in the Laue geometry: Diffraction-induced splitting, selective compression, and focusing of pulses

A theory for the dynamical Bragg diffraction of a spatially confined laser pulse in a linear photonic crystal with a significant modulation of the refractive index in the Laue geometry has been developed. The diffraction-induced splitting of a spatially confined pulse into the Borrmann and anti-Borrmann pulses localized in different regions of the photonic crystal and characterized by different dispersion laws is predicted. The selective compression or focusing of one of these pulses with the simultaneous broadening or defocusing of the other pulse is shown to be possible.     

一种改进的水下爆炸声混响模型

为了研究水下爆炸声脉冲引起的混响,基于射线理论,利用椭球的几何性质,建立了改进的三维近程双基地混响平均强度模型,推导出爆炸深度任意的双基混响平均强度预报公式。计算分析了爆炸深度和脉冲宽度对混响强度的影响,并与实验结果进行了比较,结果表明:爆炸深度对混响强度的影响相对较小,脉冲宽度越大,混响强度越大。      

Author index to volume 54

It has been experimentally shown that when light pulses propagate in active neodymium glass fibers, a decrease of amplified pulse length almost by a factor of two is observed at a fiber length of 1 m. The study of the amplification process of an ultrashort laser pulse train by a glass fiber quantum amplifier showed that the main reason of the nanosecond light pulse compression is a recombination of the short-lifetime colouring centers due to the absorption of the front of the light pulse. Experimental results are presented concerning light pulse compression at various excitation levels of active glass fibers.     

Dynamic gain induced pulse shortening in Q-switched lasers

We describe a novel mechanism of pulse shortening in a Q-switched laser induced by the gain compression effect under strong pumping conditions. The pulse shortening requires a large variation of the gain excursion during the saturation process and benefits from the large volume of the gain medium. The effect has been experimentally demonstrated using a passive Q-switched Tm/Ho-doped fiber laser that shows gain-induced pulse compression from 800 ns down to 160 ns when the pump threshold is exceeded by 15 times.     

浅海近程混响的振荡现象*

研究浅海近程混响特性对于评估和提高主动声纳性能具有重要意义。多次浅海混响实验显示,近程混响强度存在稳定的振荡现象,脉宽基本对振荡的幅度和周期没有影响。为解释这一现象,本文基于射线理论和小斜率近似给出了浅海近程混响模型,仿真与实测数据结果基本吻合。数值仿真结果表明：海底反射声场对单站声纳接收到回声信号的贡献远小于海底近垂向大掠射角散射声场的作用;混响强度振荡现象是海底近程散射声场的多途现象造成的,并由此给出了振荡周期与海深及收发深度的关系。     

Instability of plastic flow in the microstructure of alkali halide crystals

Spatial localization of deformation bands in LiF and KCl single crystals caused by instability of plastic flow in the strain rate range from 5 × 10^?6 to 2 × 10^?4 s^?1 was studied experimentally. The geometrical parameters of localized shift bands (LSB) were studied as a function of strain rate and temperature. To study the LSB relief, a surface profilometry technique was used for the first time, which made it possible to determine the LSB parameters at the early stages of plastic flow (for strains in the range from 0.5 to 2%). The formation and branching of LSB steps on the surface of a deformed crystal due to the generation and motion of dislocations were found to be scaled. It was shown experimentally that the LSB formation is a thermally activated process that occurs through dislocation glide and is limited by dislocation creep.     

激光脉冲在等离子体中的压缩分裂

通过数值求解一维非线性薛定谔方程,研究了圆偏振入射激光脉冲在初始密度范围为1/4到略低于1倍临界密度的等离子体中的自压缩和分裂现象. 提高等离子体密度和入射激光强度以及减小脉冲宽度可以在更短的传输距离获得有效的激光脉冲压缩,压缩后的脉冲半高宽度可达到初始脉冲半高宽度的1/35,甚至更小. 这种压缩是激光脉冲在等离子体中形成高阶孤子的过程中产生的,可以获得比在稀薄等离子体中更好的压缩比例. 数值计算的结果给出了该情况下激光脉冲在等离子体中自压缩后形成的高阶孤子分裂. 利用一维粒子数值模拟程序(particle-in-cell,PIC)也观察到了脉冲的压缩和分裂现象,得到了与数值计算一致的结果. 关键词： 非线性薛定谔方程 自压缩 脉冲分裂 粒子模拟     

On the deformation waves excitation in the process of non-stationary ballistic phonon heat conductivity

Theoretical investigation of deformation waves generation in the process of ballistic phonon heat-pulse propagation is presented. The regime of synchronous excitation of deformation pulse in the zone localized at the moving front of nonstationary phonon flux is forecast.     

Nature of nanocrystallization in shear bands created by megaplastic deformation in amorphous alloys

A theoretical study is made of the process of nanocrystallization upon the formation of shear bands created by megaplastic deformation in amorphous metallic alloys. Such nanocrystallization is shown to be caused by a considerable increase in temperature inside the shear bands, which in turn is associated with the stored energy of megaplastic deformation. The temperature increment depends on the degree of deformation, the rate of propagation of the shear band, and the physical parameters that determine the thermal characteristics of an amorphous matrix in the range of the shear band.     

Amorphous shear bands in deformed TiNi alloy

The structure and strain relief of TiNi alloy are examined following combined deformation consisting of quasi-hydro-extrusion followed by uniaxial compression. The shear nature of the amorphous bands resulting from such strain is demonstrated. A connection between the amorphous bands and the strain relief has been found. Fiz. Tverd. Tela (St. Petersburg) 39, 1237–1239 (July 1997)