Al—12.7%Si共晶中硅相的形态与分枝特征

用SEM考察了生长速度R=30～2500μm/s时Al-12.7％Si共晶中硅相的形态与分枝特征。对深腐蚀样品的观察表明,共晶硅相具有连续生长、成束分布特征,随生长速度R增大,共晶硅逐渐由粗大片状向片状、条状转变。硅相生长与分枝特征与局部生长条件密切相关,从整体上看,随生长速度增大,晶体学生长与分枝特征减弱。     

Simulations of instability in dynamic fracture by the cracking particles method

Crack instabilities and the phenomenon of crack speed saturation in a brittle material (PMMA) are studied with a meshfree cracking particle method. We reproduce the experimental observation that the computed terminal crack speeds attained in PMMA specimens are substantially lower than the Rayleigh wave speed; the computed crack speeds agree quite well with the reported experimental results. We also replicate repetitive microcrack branching along with the increased rate of energy dissipation after attainment of a critical crack speed, even in the absence of microstructural defects. We show that the presence of microdefects changes the response only a little. The computations reproduce many of the salient features of experimental observations.     

一类两性分枝过程条件均值增长率的极限性质

本文主要研究随机环境中配对依赖人口数两性 Galton-Watson 分枝过程的条件均值增长率的极限性质．利用上可加函数的性质,得到配对单元平均增长率的极限性质和该过程条件均值的上界和下界．文中给出了关于过程条件均值增长率的两个序列,利用配对单元平均增长率的性质,获得了这两个序列的极限性质．随机环境中配对依赖人口数两性分枝过程比较复杂,本文的结论推广了现有的研究成果．     

Extrinsic cohesive modelling of dynamic fracture and microbranching instability in brittle materials

Dynamic crack microbranching processes in brittle materials are investigated by means of a computational fracture mechanics approach using the finite element method with special interface elements and a topological data structure representation. Experiments indicate presence of a limiting crack speed for dynamic crack in brittle materials as well as increasing fracture resistance with crack speed. These phenomena are numerically investigated by means of a cohesive zone model (CZM) to characterize the fracture process. A critical evaluation of intrinsic versus extrinsic CZMs is briefly presented, which highlights the necessity of adopting an extrinsic approach in the current analysis. A novel topology‐based data structure is employed to enable fast and robust manipulation of evolving mesh information when extrinsic cohesive elements are inserted adaptively. Compared to intrinsic CZMs, which include an initial hardening segment in the traction–separation curve, extrinsic CZMs involve additional issues both in implementing the procedure and in interpreting simulation results. These include time discontinuity in stress history, fracture pattern dependence on time step control, and numerical energy balance. These issues are investigated in detail through a ‘quasi‐steady‐state’ crack propagation problem in polymethylmethacrylate. The simulation results compare reasonably well with experimental observations both globally and locally, and demonstrate certain advantageous features of the extrinsic CZM with respect to the intrinsic CZM. Copyright © 2007 John Wiley & Sons, Ltd.     

Modeling study of migration-driven lateral instability in autocatalytic systems

The lateral stability of reaction fronts in simple autocatalytic models with the components carrying various charges is investigated when the system is exposed to an inhomogeneous electric field parallel to the direction of propagation. The enhanced migrational flux of the reactant destabilizes the planar front giving rise to a cellular structure because the electric field strength is greater on the reactant side of the reaction front. The onset of instability depends not only on the charge difference between the reactant and the autocatalyst but also on the variation of specific conductance in the course of the reaction, which results in a difference in electric field strength on the opposite sides of the reaction front.     

Polysaccharide metabolism regulates structural colour in bacterial colonies

The brightest colours in nature often originate from the interaction of light with materials structured at the nanoscale. Different organisms produce such coloration with a wide variety of materials and architectures. In the case of bacterial colonies, structural colours stem for the periodic organization of the cells within the colony, and while considerable efforts have been spent on elucidating the mechanisms responsible for such coloration, the biochemical processes determining the development of this effect have not been explored. Here, we study the influence of nutrients on the organization of cells from the structurally coloured bacteria Flavobacterium strain IR1. By analysing the optical properties of the colonies grown with and without specific polysaccharides, we found that the highly ordered organization of the cells can be altered by the presence of fucoidans. Additionally, by comparing the organization of the wild-type strain with mutants grown in different nutrient conditions, we deduced that this regulation of cell ordering is linked to a specific region of the IR1 chromosome. This region encodes a mechanism for the uptake and metabolism of polysaccharides, including a polysaccharide utilization locus (PUL operon) that appears specific to fucoidan, providing new insight into the biochemical pathways regulating structural colour in bacteria.     

Short-term instability in stochastic systems

Dynamic systems with lumped parameters which experience random temporal variations are considered in this paper. These variations may lead to “short-term” dynamic instability that is reflected in the system’s response as alternating periods of zero or almost zero response and rare short outbreaks. As long as it may be impractical to preclude completely such outbreaks for a designed system, the corresponding response should be analyzed to evaluate the system’s reliability.Results of such analyses are presented separately for cases of slow and rapid parameter variations. Linear models of the systems are studied in the former case using parabolic approximation (PA) for the variations in the vicinity of their peaks together with Krylov-Bogoliubov (KB) averaging for the transient response. This results in a solution for the response probability density function (PDF).The case of rapid broadband parameter variations is studied using theory of Markov processes. The system is assumed to operate beyond its stochastic instability threshold-although only slightly-and its nonlinear model is used accordingly. The analysis is based on solution of the Fokker-Planck-Kolmogorov (FPK) partial differential equation for stationary PDF of the response. Several such PDFs are analyzed; they are found to have integrable singularities at the origin indicating an intermittent nature of the response.     

Strain localization in a continuum as an instability event

The paper presents a novel physical model, based on perturbation theory, to describe localization pattern formation in a solid material as a result of system instabilities. Such kind of approach has been inspired by the theory of population dynamics. In particular, the sinergetic phenomenon of strain localization into a stressed continuum, and its subsequent evolution to cohesive cracking, is obtained through the competition of an external source of energy (e.g., strain energy) and of the internal behavior of the material. The hypothesis of mobile energy entities within material bulk is put forward. These entities, which under low strain conditions are evenly distributed throughout the body, can be considered as strain quanta. The quantization of mechanical quantities is not new in continuum and fracture mechanics, [see, e.g., Novozhilov (1969, Prik Mat Mek 33:212–222)]. With increasing strain, a certain critical point is reached when the homogeneous situation becomes unstable and the strain quanta begin to aggregate into bands, leading to periodic strain localization patterns. The model, which is only theoretical at this stage, can be applied to the particular case of dry snow avalanches. In these cases, snow avalanche triggering is due toinstability (onset of sliding onto a weak plane) and is controlled by external loading (e.g., weight of the slope, load by skiers) and by internal factors (e.g., temperature changes, snow phase transformations etc.).     

Electronically driven short-range lattice instability: Cluster effects in superconductors

In the first part of this series [1] short- and medium-range interactions in superconductors were investigated. In the present paper a discussion is made on the cluster-mass dependence of the superconductive transition temperature and the relevant phenomenon of electron localization. A comparison with experiments is given; the predictions fit well to the observations.     

Factors affecting schedule instability in manufacturing companies

Schedule instability is a major issue in many manufacturing companies. It results in a low service level to customers, high inventory levels, and high costs associated with production changeovers. Much has been written on schedule instability in the literature, but most studies have used simulation and mathematical modelling to look at the internal operations of manufacturing companies. While schedule instability has always been very much a practical problem, there have been few empirical studies presented in the literature. The aim of this research was to obtain the opinions of practitioners on schedule instability, and to identify factors that affect the perceived level of instability experienced by manufacturing companies. An e-mail/web-based survey was administered to practitioners working in the planning and scheduling area. The results suggest that the majority of the respondents perceive schedule instability to be either an important or a very important issue in their operations. Analysis of the responses also shows that schedule instability is affected mainly by external factors, notably relationships with buyers and relationships with suppliers; internal operations have a lower impact. The research moves the debate about schedule instability beyond the current concentration on simulation and mathematical modelling, and from a focus on internal operations to a supply-chain context.     

Vibrational Branching Ratios and Asymmetry Parameters in the Photoionization of CO2 in the Region Between 650 ? and 840 ?

The vibrational branching ratios and asymmetry parameters for CO₂ have been determined in the wavelength region of 650 Å to near the ionization onset at about 840 Å. The study was performed using synchrotron radiation from the Daresbury storage ring that was dispersed with a 5 m grating monochomator that afforded resolution of 0.1 Å to 0.2 Å. This resolution allowed the study of the branching ratios and asymmetry parameters with enough detail to see the changes in the parameters within the pronounced autoionization structure in CO₂ in this wavelength region. While the electron spectrometer resolution was not sufficient to resolve the spin orbit and Renner-Teller splitting in the photoelectron spectra, we are able to fit the data with a model that identifies the major structure in terms of the symmetric stretch and elements of the asymmetric stretch and bending modes. A calculation of the expected relative vibrational excitations based upon the Franck-Condon principle clearly showed non-Franck-Condon behavior in some of the vibrational-electronic transitions.     

Self-similarity and crack growth instability in the correlation between the Paris’ constants

In this note the question about the existence of a correlation between the parameters C and m of the Paris’ law is re-examined. According to dimensional analysis and incomplete self-similarity concepts applied to the linear range of fatigue crack growth, a power-law asymptotic representation relating the parameter C to m and to the governing variables of fatigue is derived. Then, from the observation that the Griffith-Irwin instability must coincide with the Paris’ instability at the onset of rapid crack growth, the exponents entering this correlation are determined. A fair good agreement is found between the proposed theory and extensive experimental data.     

Local lattice instability and ionic transport in high-temperature superconductors

Ionic transport, a characteristic feature of superionic conductors, is analyzed microscopically for the RBa₂Cu₃O{7-Σ} (R = rare earth) high-temperature superconductors. The electronic correlation effects are considered to be responsible for the formation of a local double-well potential for apical oxygen, which in its turn results in a decrease of the activation energy for interstitial chain oxygen defects. The behavior of the double well upon reduction of the oxygen content is discussed and its possible manifestation in the low-frequency dynamical response is considered.     

Modulation instability scenario in negative index materials

We present an investigation of the critical frequency windows permitting modulation instability in negative index materials. The principal motivation for our analysis stems from the impact of the inevitable presence of the effective dispersive magnetic permeability in addition to the effective dielectric permittivity determining the propagation model for ultrashort pulses in negative index materials. We emphasize the influence of nonlinear dispersion terms, arising out of the combinatorial effect of the dispersive permeability with the nonlinear polarization, over the MI phenomena, the outcome of its development achieved by using linear stability analysis. Gain spectrum investigation has been carried out for both anomalous and normal dispersion regime in the focusing and defocusing cases of nonlinearity and near zero dispersion regime where higher order linear dispersive effects emerge. The results of linear stability analysis have been validated by direct numerical simulation of the governing equation using the split-step Fourier transform method. We show that second-order nonlinear dispersion opens up distinct MI windows with their appropriate conditions and unlike the first-order nonlinear dispersion term, the sign of it has got deep consequences in the modulation instability scenario.     

Diamagnetic levitation enhances growth of liquid bacterial cultures by increasing oxygen availability

Diamagnetic levitation is a technique that uses a strong, spatially varying magnetic field to reproduce aspects of weightlessness, on the Earth. We used a superconducting magnet to levitate growing bacterial cultures for up to 18 h, to determine the effect of diamagnetic levitation on all phases of the bacterial growth cycle. We find that diamagnetic levitation increases the rate of population growth in a liquid culture and reduces the sedimentation rate of the cells. Further experiments and microarray gene analysis show that the increase in growth rate is owing to enhanced oxygen availability. We also demonstrate that the magnetic field that levitates the cells also induces convective stirring in the liquid. We present a simple theoretical model, showing how the paramagnetic force on dissolved oxygen can cause convection during the aerobic phases of bacterial growth. We propose that this convection enhances oxygen availability by transporting oxygen around the liquid culture. Since this process results from the strong magnetic field, it is not present in other weightless environments, e.g. in Earth orbit. Hence, these results are of significance and timely to researchers considering the use of diamagnetic levitation to explore effects of weightlessness on living organisms and on physical phenomena.     

Development of Richtmyer-Meshkov and Rayleigh-Taylor instability in the presence of magnetic field

Fluid instabilities like Rayleigh-Taylor (R-T), Richtmyer-Meshkov (R-M) and Kelvin-Helmholtz (K-H) instability can occur in a wide range of physical phenomenon from astrophysical context to Inertial Confinement Fusion (ICF). Using Layzer's potential flow model, we derive the analytical expressions of growth rate of bubble and spike for ideal magnetized fluid in R-T and R-M cases. In the presence of transverse magnetic field, the R-M and R-T instabilities are suppressed or enhanced depending on the direction of magnetic pressure and hydrodynamic pressure. Again the interface of two fluid may oscillate if both the fluids are conducting. However, it is observed that the magnetic field has no effect in linear case.     

A surface instability of granules under vibration in partitioned containers

This paper describes a surface instability of vibrated granules contained in rectangular vessels. The vessels are partitioned in two equal sections by a vertical plate whose lower end is placed at a certain distance from the vessel base. The free surfaces of granules in two sections, initially at an equal level, start to move when submitted to vertical vibrations, and stabilize at different levels. Measurements are made of the pressure at the bottom of each of the partitioned beds, which reveals the underlying mechanism of the surface instability. Computer simulations are performed to show microscopic structures of this surface instability. Received: 25 February 2000     

Stable supercontinuum pulse generated by modulation instability in a dispersion-managed fiber

We study on non-linear localized waves on continuous wave background in a dispersion and non-linearity management fibre. We find a stable supercontinuum pulse can be generated from a small modulation on continuous wave in a proper management way, for which the pulse spectrum width and its growth rate can be controlled well by the management parameters. Additionally, we demonstrate a Kuznetsov–Ma breather like non-linear localized wave can exist in a periodic dispersion management fibre, and its spectrum evolution is distinctive from the Kuznetsov–Ma breather’s.     

Competition between modulation instability and four-wave mixing in the dispersion-shifted fiber

We demonstrate the modulation instability (MI) and four-wave mixing (FWM) spectra at the input of a pump accompanied with a seed in the dispersion-shifted fiber. By numerically solving the nonlinear Schrodinger equation, it is confirmed that there are two regions as the absolute value of the second-order fiber dispersion decreases, where MI and FWM dominates, respectively. We attribute this phenomenon to the competition between MI and FWM. Furthermore, for the FWM-dominating region, it can be divided into two subregions according to the different power levels of MI, which mainly results from the power transfer between MI and dispersive wave. Our results may lead to a further understanding of nonlinear interactions in optical fiber.     

Modulational instability in metamaterials with saturable nonlinearity and higher-order dispersion

Modulational instability (MI) in negative refractive metamaterials with saturable nonlinearity, fourth-order dispersion (FOD), and second-order nonlinear dispersion (SOND) is investigated by using standard linear stability analysis and the Drude electromagnetic model. The expression for the MI gain spectrum is obtained, which clearly reveals the influence of the saturation of the nonlinearity, FOD, and SOND parameters on the temporal MI. The evolution of the MI in negative refractive metamaterials is numerically investigated. Special attention is paid to study the effects of the higher-order dispersion terms on the formation and evolution of the solitons induced by MI. It is shown that as the third-order dispersion term increases, the solitons travel toward the right. Moreover, the magnitude of the FOD term influences considerably the number of wave trains induced by MI.