Multiscale approach to micro/macro fatigue crack growth in 2024-T3 aluminum panel

When two contacting solid surfaces are tightly closed and invisible to the naked eye,the discontinuity is said to be microscopic regardless of whether its length is short or long.By this definition,it is not sufficient to distinguish the difference between a micro-and macro-crack by using the length parameter.Microcracks in high strength metal alloys have been known to be several centimeters or longer.Considered in this work is a dual scale fatigue crack growth model where the main crack can be micro or macro but there prevails an inherent microscopic tip region that is damaged depending on the irregularities of the microstructure.This region is referred to as the"micro-tip"and can be simulated by a sharp wedge with different angles in addition to mixed boundary conditions.The combination is sufficient to model microscopic entities in the form of voids,inclusions,precipitations,interfaces,in addition to subgrain imperfections,or cluster of dislocations.This is accomplished by using the method of"singularity representation"such that closed form asymptotic solutions can be obtained for the development of fatigue crack growth rate relations with three parameters.They include:(1)the crack surface tightness*represented by o/=0.3-0.5 for short cracks in region I,and 0.1-0.2 for long cracks in region II,(2)the micro/macro material properties reflected by the shear modulus ratio*(=micro/macro varying between 2 and 5)and(3)the most sensitive parameter d*being the micro-tip characteristic length d*(=d/do)whose magnitude decreases in the direction of region I II.The existing fatigue crack growth data for 2024-T3 and 7075-T6 aluminum sheets are used to reinterpret the two-parameter da/dN=C(K)nrelation where K has now been re-derived for a microcrack with surfaces tightly in contact.The contact force will depend on the mean stress m or mean stress ratio R as the primary parameter and on the stress amplitude a as the secondary parameter.     

Simulation of the inherent turbulence and wake interaction inside an infinitely long row of wind turbines

The turbulence in the interior of a wind farm is simulated using large eddy simulation and the actuator line technique implemented in the Navier–Stokes equations. The simulations are carried out for an infinitely long row of turbines simulated by applying cyclic boundary conditions at the inlet and outlet. The simulations investigate the turbulence inherent to the wind turbines as no ambient turbulence or shear is added to this idealised case. The simulated data give insight into the performance of the wind turbines operating in the wake of others as well as details on key turbulent quantities. One of the key features of wakes behind wind turbines is the dynamic wake meandering, which is shown to be related to the wind turbine spacing and the vortex shedding from the turbine as a bluff body. The flow is analysed and reconstructed by applying proper orthogonal decomposition.     

Polarization-dependent Goos-Hänchen shift at a graded dielectric interface

We examine the polarization differential Goos-Hänchen beam shift upon total internal reflection, for a graded-index dielectric interface. We find a generic scaling law where the magnitude of this shift depends solely on the product of wavelength and gradient steepness. The analytic results are extended using transmission matrix calculations in cases where the assumptions made to allow analytical treatment might become questionable. Two important cases in this category are: (i) incident angle close to the critical angle and (ii) gradients with an overall thickness of the order of a wavelength. We demonstrate this effect experimentally using a polymer-blend sample with a gradual refractive-index transition induced by diffusion.     

大平面二极管虚阴极振荡器的宽脉冲微波辐射

给出了产生宽脉冲微波辐射的大平面天鹅绒二极管虚阴极振荡器的结构和测试装置,测得了二极管中阴–阳极等离子体的闭合速率为30~43km/s,获得了微波辐射的最大脉宽为1.5μs,产生的微波峰值功率约为15MW,频率为0.5GHz,指出这种平面二极管型虚阴极振荡器在辐射脉宽,辐射频率和辐射功率三者之间存在着物理上的制约关系,认为同轴型虚阴极振荡器是突破这种制约关系的一个新途径。     

Shift‐invert Lanczos method for the symmetric positive semidefinite Toeplitz matrix exponential

The Lanczos method with shift‐invert technique is exploited to approximate the symmetric positive semidefinite Toeplitz matrix exponential. The complexity is lowered by the Gohberg–Semencul formula and the fast Fourier transform. Application to the numerical solution of an integral equation is studied. Numerical experiments are carried out to demonstrate the effectiveness of the proposed method. Copyright © 2010 John Wiley & Sons, Ltd.     

Information Shift Dynamics Described by Tsallis q = 3 Entropy on a Compact Phase Space

Recent mathematical investigations have shown that under very general conditions, exponential mixing implies the Bernoulli property. As a concrete example of statistical mechanics that are exponentially mixing we consider the Bernoulli shift dynamics by Chebyshev maps of arbitrary order N≥2, which maximizes Tsallis q=3 entropy rather than the ordinary q=1 Boltzmann-Gibbs entropy. Such an information shift dynamics may be relevant in a pre-universe before ordinary space-time is created. We discuss symmetry properties of the coupled Chebyshev systems, which are different for even and odd N. We show that the value of the fine structure constant αel=1/137 is distinguished as a coupling constant in this context, leading to uncorrelated behaviour in the spatial direction of the corresponding coupled map lattice for N=3.     

Effect of substitution and cooperativity on the Cl–F blue shift in single-electron halogen-bonded H3C ··· ClF complex

The effect of substitution and cooperativity on the blue shift of Cl–F stretch vibration in H₃C ··· ClF complex has been studied with quantum chemical calculations at the UMP2(Full)/aug-cc-pVTZ level. The electron-withdrawing group (F atom) in the electron donor decreases the blue shift, whereas the electron-donating group (methyl group) in the electron donor cause it to increase. The cooperativity between two different types of halogen bonds in H₃C ··· ClF ··· ClF complex enhances the strength of single-electron halogen bond and the blue shift. The natural bond orbital (NBO) and atoms in molecules (AIM) analyses have been performed for the halogen-bonded complexes.     

Amount of valence charge of Al- and Zn-based quasicrystals and related approximant crystals evaluated by chemical shift observations and Bader analysis

The amounts of decreased charge at Al sites of Al-based (Al–Pd–Cr–Fe, Al–Si–Mn, and Al–Re–Si) and at Zn sites of Zn-based (Zn–Mg–Zr) quasicrystals and approximant crystals were estimated. The evaluation was done by comparisons between chemical shifts experimentally observed by soft-X-ray emission spectroscopy and the amount of valence charge obtained by Bader analysis for first principle calculations of reference materials (Al, α-Al₂O₃, Zn, and ZnO). Decreased charges at Al sites of Al-based quasicrystals and at Zn sites of Zn-based quasicrystals were evaluated to be 1.0–2.5 e^–/atom and 1.1–1.2 e^–/atom, respectively. A covalent bonding nature alloy of Al–Re–Si also showed a decrease in valence charge at Al sites.     

大气激光通信中的偏振调制性能

为充分证明偏振移位键控可有效抑制大气湍流等因素的影响这一观点,对目前大气激光通信系统中广泛采用的几种强度调制和偏振移位键控技术进行详细的分析研究,从各调制方式的功率利用率、带宽需求、传输容量以及差错性能几个方面进行对比。研究结果表明,各调制方式各有千秋,不能一概而论。综合各方面性能考虑,偏振移位键控较其他几种强度调制方式更具优越性,其具有最大传输容量、最小的带宽需求和接收误码率。由此可见,偏振调制方式更适合应用于大气激光通信。     

Derivation of macroscopic equations for individual cell‐based models: a formal approach

In this paper we review the theory of cells (particles) that evolve according to a dynamics determined by friction and that interact between themselves by means of suitable potentials. We derive by means of elementary arguments several macroscopic equations that describe the evolution of cell density. Some new results are also obtained—a formal derivation of a limit equation in the case of attractive potential as well as in the case of repulsive potential with a hard‐core part are presented. Finally we discuss the possible relevance of those results within the framework of individual cell‐based models. Several classes of potentials, including hard‐core, repulsive and potentials with attractive parts are discussed. The effect of noise terms in the equation is also considered. Copyright © 2005 John Wiley & Sons, Ltd.