任意开口薄壁截面圆弧曲梁的通用线性理论

在曲梁精确的翘曲位移基础上,根据变分原理,提出了对任意开口薄壁截面圆弧曲梁通用的线性理论,给出平衡微分方程和相应的边界条件。定义了两个新的变量vH和qH,借助它们可以很方便地计算曲梁中的剪力和扭矩。最后就该理论在常见截面形式(工字形,槽形及无对称轴H形)水平曲梁中的应用进行说明,并与已有理论进行比较。     

均布荷载作用下工字形截面单跨超静定深梁应力计算方法研究

工字形截面单跨超静定深梁因截面形式复杂目前其应力计算仍无解析方法。该文对均布荷载作用下工字形截面单跨超静定深梁应力计算进行研究,通过引入合理的参数并对工字形截面作恰当的拆分和组合及合理假设,建立了力学模型,应用半逆解法求解,并考虑了弯剪耦合效应,给出了各应力分量的计算公式。分析了弯应力沿梁高及跨长的分布规律及剪力对弯应力的影响,结果表明剪力对弯应力的影响较大,这种影响随跨高比的减小或翼缘与腹板面积比的增大而增大。结果可为工字形截面单跨超静定深梁的设计提供概念明确的理论计算方法。     

Particle beam polarization in storage rings controlled by longitudinal magnetic field

It is suggested to use a longitudinal magnetic field for changing the polarization of particles (electrons, positrons, muons) in the beam of a storage ring. The change from vertical to longitudinal polarization by single or multiple switching of the longitudinal field for a certain time interval is considered.     

The dynamic response of end-clamped sandwich beams with a Y-frame or corrugated core

The dynamic response of end-clamped monolithic beams and sandwich beams has been measured by loading the beams at mid-span using metal foam projectiles. The AISI 304 stainless-steel sandwich beams comprise two identical face sheets and either prismatic Y-frame or corrugated cores. The resistance to shock loading is quantified by the permanent transverse deflection at mid-span of the beams as a function of projectile momentum. The prismatic cores are aligned either longitudinally along the beam length or transversely. It is found that the sandwich beams with a longitudinal core orientation have a higher shock resistance than the monolithic beams of equal mass. In contrast, the performance of the sandwich beams with a transverse core orientation is very similar to that of the monolithic beams. Three-dimensional finite element (FE) simulations are in good agreement with the measured responses. The FE calculations indicate that strain concentrations in the sandwich beams occur at joints within the cores and between the core and face sheets; the level of maximum strain is similar for the Y-frame and corrugated core beams for a given value of projectile momentum. The experimental and FE results taken together reveal that Y-frame and corrugated core sandwich beams of equal mass have similar dynamic performances in terms of rear-face deflection, degree of core compression and level of strain within the beam.     

On the importance of cross-sectional warping in solid composite beams

The relative importance of the cross-sectional warping components in composite beams is studied and demonstrated using an exact solution for solid orthotropic beam of arbitrary cross-sectional geometry that undergoes a bending moment. In light of the effort required for warping modeling in general numerical schemes of composite beams, the present study contributes to the understanding of the importance of modeling the in-plane and the out-of-plane warping components.     

Space-charge effects on bunching of electrons in the CEBAF injector

The main injector for the 4 GeV CEBAF accelerator at Thomas Jefferson National Accelerator Facility was designed to deliver simultaneously three CW electron beams for nuclear physics research. The maximum design current for a single beam from the injector is 100 μA, or 0.2 pC per microbunch at a repetition rate of 499 MHz. It was found through computer simulation that space charge even at a subpicocoulomb level can spoil the bunching of electrons significantly, and some unexpected phenomena observed experimentally could be explained accordingly. This problem arises because of the low-momentum tilt allowed for bunching to preserve low-momentum spread. In this paper, we analyze in detail the space-charge effects on bunching of electrons with the CEBAF injector as an example. Conditions for effective matching of longitudinal phase space in the presence of space charge are discussed.     

Size effect on failure of overreinforced concrete beams

The results of full-scale failure of singly reinforced four-point-bend beams of different sizes containing deformed longitudinal reinforcing bars are reported. The tests consisted of four groups with one, two and three different size combinations. The specimens were made of concrete with a maximum aggregate size of 10 mm. The beams were geometrically similar in one, two and three-dimensions, and even the bar diameter and cover thicknesses were scaled in proportion. The reinforcement ratio was 3%. The results revealed the existence of a significant size effect, which can approximately be described by the size effect law previously proposed by Bazant. The size effect is found to be stronger in two-dimensional similarities than for one and three-dimensional similarities.     

Propagation properties of cosh-Airy beams

We theoretically investigate the propagation properties of cosh-Airy beams, which can be considered as a superposition of two Airy beams with different decay factors. We find that the field distribution of cosh-Airy beams is the same as that of Airy beams. Moreover, we find that the evolution of cosh-Airy beams is determined by the parameters of the cosh modulation function, in addition to the transverse scale factor and decay factor of the Airy beams. Our results demonstrate a possible method of manipulating Airy beams in free space. They can also be extended to the study of the propagation properties of cosine-Airy beams (or sine-Airy beams).     

Shear strengthening of reinforced concrete beams with PBO-FRCM composites

The work analyses the functioning of reinforced concrete beams which are shear reinforced with FRCM (Fibre Reinforced Cementitious Matrix) composite mesh on mineral mortar. The analysis of previous tests provides an overview on the efficiency of the related FRP (Fibre Reinforced Polymers) system used for strengthening the beams against shearing forces. The information gathered on the functioning of FRCM strengthened concrete elements show that the key factor in this technology is ensuring appropriate carrying capacity of anchoring of the mesh. The tests were carried out on reinforced concrete beams with different design of lateral reinforcement. The beams differed in terms of the angle of inclination of the reinforcement to the longitudinal axis, as well as in terms of the manner of anchoring the mesh at the top and bottom surface of beams. The destruction mechanisms of beams were analysed and described. Value of deflection measured at the maximum load for all of the beams is significantly greater than the allowable value. The distribution of strains is presented as a function of the load, where the phases of operation in relation to shear of FRCM reinforced beams are visible.     

A firefly algorithm for optimum design of new-generation beams

This research addresses the minimum weight design of new-generation steel beams with sinusoidal openings using a metaheuristic search technique, namely the firefly method. The proposed algorithm is also used to compare the optimum design results of sinusoidal web-expanded beams with steel castellated and cellular beams. Optimum design problems of all beams are formulated according to the design limitations stipulated by the Steel Construction Institute. The design methods adopted in these publications are consistent with BS 5950 specifications. The formulation of the design problem considering the above-mentioned limitations turns out to be a discrete programming problem. The design algorithms based on the technique select the optimum universal beam sections, dimensional properties of sinusoidal, hexagonal and circular holes, and the total number of openings along the beam as design variables. Furthermore, this selection is also carried out such that the behavioural limitations are satisfied. Numerical examples are presented, where the suggested algorithm is implemented to achieve the minimum weight design of these beams subjected to loading combinations.     

Representation and focusing properties of higher-order radially polarized Laguerre–Gaussian beams

A method based on higher-order Poincaré sphere was proposed to represent the states of polarization of higher-order radially polarized Laguerre–Gaussian (LG) vectorial beams (VBs). And the focusing properties of such LG beams of different radial orders focused by a high-NA lens were discussed. By tuning the ratio of the pupil radius to the waist of the incident beams, some cage-like or needlelike electric intensity field is generated in the focal region for several specific LG VBs with high order. Modulated by diffractive optical elements, the shape of the focal field shows novel behaviors such as splitting of cage-like modes, which provides potentially a more flexible control over micro-particles.     

Deflection and stress behaviour of nanocomposite reinforced beams using a multiscale analysis

Bending behaviour of one-dimensional structures is an important consideration in the design of structural components. In the present study a multiscale analysis of the deflection and stress behaviour of carbon nanotube (CNT) reinforced polymer composite beams is presented. The micromechanics models used in the study include straight CNTs aligned in one direction, randomly oriented CNTs and a two parameter model of agglomeration. The effects of volume fraction of CNTs and the nanotube diameter are investigated on the beam deflection and comparisons are made with carbon fibre reinforced composites. The main purpose of the study is to observe the stiffening effect of CNTs when used in structural applications such as beams.     

Nonparaxial analyses of cylindrical vector beams with arbitrary polarization order in the far field

Abstract

Based on the vector angular spectrum representation and the method of stationary phase, the analytical expressions for the electromagnetic fields of the cylindrical vector Laguerre–Gaussian beams with arbitrary polarization order are derived in the far field. The radially, anti-vortex and linearly polarized beams can be viewed as the special cases of our general result. The analyses indicate that the beam evolution properties and nonparaxiality are closely related to the radial mode number, the polarization order number and the ratio of the waist width to the wavelength. The high polarization order cylindrical vector beams compared with the radially polarized beams are more influenced by the nonparaxiality. This research provides a convenient approach to manipulate the cylindrical vector Laguerre–Gaussian beams by choosing the special state of polarization.     

Failure modes and serviceability of high strength self compacting concrete deep beams

The behaviour of deep beams is significantly different from shallow beams. In deep beams, the plane section does not remain plane after deformation. The main purpose of this study is to facilitate the prediction of deep beam failure related to tensile bar and web reinforcement percentage variations. Six high strength self compacting concrete (HSSCC) deep beams were tested until failure. Strains were measured on concrete surface along mid span, tensile bar and compression strut trajectory. The load was incrementally applied and at each load increment new cracks, their widths and propagation were monitored. The results clearly show that, at ultimate limit condition, the strain distribution on concrete surface along mid-span is no longer parabolic. In deep beams several neutral axes were obtained before ultimate failure is reached. As the load increases, the number of neutral axis decreases and at failure load it reduces to one. The failure of deep beams with longitudinal tensile steel reinforcement less than that suggested by ACI codes is flexural and is accompanied by large deflections without any inclined cracks. As the longitudinal tensile steel reinforcement increases, the failure due to crushing of concrete at nodal zones was clearly observed. The first flexural crack at mid-span region was always vertical. It appeared at 25–42% of peak load. The crack length was in the range of 0.24–0.6 times the height of section. As the tensile bar percentage increases number of cracks increases with reduced crack length and crack width. The appearance of first inclined crack in compression strut trajectory is independent of tensile and web bar percentage variations.     

Implementation of ordinary and extraordinary beams interference by application of diffractive optical elements

Abstract

We apply diffractive optical elements in problems of transformation of Bessel beams in a birefringent crystal. Using plane waves expansion we show a significant interference between the ordinary and extraordinary beams due to the energy transfer in the orthogonal transverse components in the nonparaxial mode. A comparative analysis of the merits and lack of diffractive and refractive axicons in problems of formation non-paraxial Bessel beams has shown the preferability of diffractive optics application in crystal optics. The transformation of uniformly polarised Bessel beams in the crystal of Iceland spar in the nonparaxial mode by application of a diffractive axicon is investigated numerically and experimentally.     

Measurement of the longitudinal and transverse diffusion of single electrons drifting in gases

The transit time of single drifting electrons has been measured in several gases, over distances ranging from 5 cm up to 80 cm. The longitudinal diffusion coefficient is deduced from the fluctuations of this transit time. The transverse diffusion has been measured directly by scanning the electron impact density at the end of the drift zone. The effect of a longitudinal magnetic field has also been studied.     

Scintillation index for two Gaussian laser beams with different wavelengths in weak atmospheric turbulence

We study the propagation of the two lowest-order Gaussian laser beams with different wavelengths in weak atmospheric turbulence. Using the Rytov approximation and assuming a slow detector, we calculate the longitudinal and radial components of the scintillation index for a typical free-space laser communication setup. We find the optimal configuration of the two laser beams with respect to the longitudinal scintillation index. We show that the value of the longitudinal scintillation for the optimal two-beam configuration is smaller by more than 50% compared with the value for a single lowest-order Gaussian beam with the same total power. Furthermore, the radial scintillation for the optimal two-beam system is smaller by 35%-40% compared with the radial scintillation in the single-beam case. Further insight into the reduction of intensity fluctuations is gained by analyzing the self- and cross-intensity contributions to the scintillation index.     

带大边梁的桥梁结构实用空间分析的虚拟纵梁法

通过设置虚拟纵梁的途径分析大边梁桥的荷载横向分布,推导了考虑大边梁影响的横向分布基本公式,数值算例验证了所推公式的正确性。为了进一步掌握大边梁桥的横向分布特性,以所推公式为依据,编制了大边梁桥横向分布电算程序,分析了大边梁与内梁的刚度比及设置内横梁对横向分布的影响,绘制的大量图表显示了其影响规律。研究结果表明:边梁刚度增大后,会显著影响荷载横向分布,使边梁的横向分布系数大为增加,而内梁则大为减小,但当刚度比增至一定值后,这种影响的程度就会变得很小;设置内横梁会使大边梁的横向分布系数有一定增加,但却使内梁的横向分布系数大大减小。     

Paraxial propagation of Lorentz and Lorentz–Gauss beams in uniaxial crystals orthogonal to the optical axis

Paraxial propagation of Lorentz and Lorentz–Gauss beams in uniaxial crystals orthogonal to the optical axis is investigated based on the beam propagation equations. Analytical propagation formulae for Lorentz and Lorentz–Gauss beams in uniaxial crystals are derived. The derived propagation formulae reduce to the propagation formulae for Lorentz and Lorentz–Gauss beams in free space under certain conditions. The propagation properties of Lorentz and Lorentz–Gauss beams in uniaxial crystals and in free space are studied numerically and comparatively. It is found that the propagation properties of Lorentz and Lorentz–Gauss beams in uniaxial crystals behave very differently from those in free space, and are closely determined by the parameters of the uniaxial crystals. The uniaxial crystals provide an effective way for generating astigmatic beams.     

简支梁转变为连续梁的弯矩调幅试验研究及其简化计算方法

提出了一种通过在负弯矩区施加预应力将简支梁转变为连续梁的方法。为深入了解连续梁的内力重分布特性,进行了2根连续梁及1根对比梁的试验研究。试验中考虑的参数为预应力钢绞线的长度、单双支座。试验结果表明,简支梁转变为连续梁后,开裂荷载、极限荷载得到了较大的提高。弯矩重分布的规律为跨中截面开裂后,跨中截面的弯矩向中支座截面传递,中支座截面开裂后,弯矩反向向跨中截面传递。在试验研究的基础上,提出了两跨连续梁的弯矩调幅分析方法。计算结果表明:提出的计算方法与试验值吻合较好,可以用来计算连续梁的弯矩调幅。