DYNAMIC NUCLEAR POLARIZATION OF ORGANIC COMPOUND DOPED WITH FREE RADICALS

For those organic compounds which lack free radicals, a doping approach named melting-liquid-nitrogen-quench was developed in order to perform dynamic nuclear polarization(DNP)-nuclear magnetic resonance experiments. By using this method, the ¹H, ¹³C, ¹⁵N DNP enhancements were observed with free-radical-doped dibenzofuran and benzamide. The enhancement mechanism and relation between the concentration of unpaired electrons and the maximum DNP enhancement are discussed.     

功能梯度材料研究的现状与将来发展

功能梯度材料是近年来材料科学的研究热点之一,文章评述了ＦＧＭ的基本概念、制备技术、理论研究和应用情况等发展现状,同时对可能的发展进行了讨论,强调了对电磁性能研究的重要性。     

FREE VIBRATIONS OF SELF-STRAINED ASSEMBLIES OF BEAMS

This paper examines the natural frequencies and modes of transverse vibration of two simple redundant systems comprising straight uniform Euler-Bernoulli beams in which there are internal self-balancing axial loads (e.g., loads due to non-uniform thermal strains). The simplest system consists of two parallel beams joined at their ends and the other is a 6-beam rectangular plane frame. Symmetric mode vibration normal to the plane of the frame is studied. Transcendental frequency equations are established for the different systems. Computed frequencies and modes are presented which show the effect of (1) varying the axial loads over a wide range, up to and beyond the values which cause individual members to buckle (2) pinning or fixing the beam joints (3) varying the relative flexural stiffness of the component beams. When the internal axial loads first cause any one of the component beams to buckle, the fundamental frequency of the whole system vanishes. The critical axial loads required for this are determined. A simple criterion has been identified to predict whether a small increase from zero in the axial compressive load in any one member causes the natural frequencies of the whole system to rise or fall. It is shown that this depends on the relative flexural stiffnesses and buckling loads of the different members. Computed modes of vibration show that when the axial modes reach their critical values, the buckled beam(s) distort with large amplitudes while the unbuckled beam(s) move either as rigid bodies or with bending which decays rapidly from the ends to a near-rigid-body movement over the central part of the beam. The modes of the systems with fixed joints change very little (if at all) with changing axial load, except when the load is close to the value which maximizes or minimizes the frequency. In a narrow range around this load the mode changes rapidly. The results provide an explanation for some computed results (as yet unpublished) for the flexural modes and frequencies of flat plates with non-uniform thermal stress distributions.     

FORCED AND FREE VIBRATIONS OF RECTANGULAR SANDWICH PLATES WITH PARAMETRIC STIFFNESS MODULATION

An active control of the resonant vibrations of a rectangular sandwich plate performed by the parametric stiffness modulation is analyzed. The controlled vibrations are those of the dominantly flexural type excited by the transverse force acting at the first resonant frequency of dominantly flexural vibrations. The stiffness modulation is performed at a comparatively high frequency identified by the resonance of a mode of the dominantly shear type. The method of direct partition of motions is used that predicts an existence of the modal interaction between these two modes of vibrations due to the parametric stiffness modulation. It is shown that such a parametric control can provide a significant shift of the first eigenfrequency of a controlled plate (the one subjected to the stiffness modulation) from its nominal value for an uncontrolled plate. Heavy fluid loading conditions are accounted for as well as the energy dissipation in the material of a plate. It is demonstrated that although heavy fluid loading reduces resonant frequencies of forced vibrations, the suggested mechanism of control remains valid in these cases. Dynamics of an elementary two-degree-of-freedom model mechanical system is considered to illustrate the mechanism of modal interaction, which is involved in the suggested way of an active control of vibrations of sandwich plates.     

W/Cu梯度功能材料的高热负荷性能研究

用等离子体喷涂和热压方法制作了W/Cu梯度功能材料(FGM)样品,用大功率ND∶YAG激光对其进行了高热负载模拟实验.结果表明,在100～400MW@m-2的瞬时(脉冲宽度为4ms)热负载下,经过200～700次热循环,未发现有W-Cu复合体开裂.在123MW@m-2的功率密度下作用700次,发现钨表面有再结晶现象及严重的晶界腐蚀和裂纹,再结晶的平均晶粒尺寸约为5～10μm,垂直于表面呈柱状结构,再结晶层厚度约20～30μm.由于激光的淬冷效应,晶粒生长的趋势并不明显.在398MW@m-2功率密度下出现了明显的腐蚀坑,坑内呈疏松的蜂窝结构,坑的边缘出现了明显沉积区,能谱分析表明沉积区集聚了大量的金属杂质.等离子体喷涂试样比热压试样更易产生晶界的断裂的裂纹.在相同的热负荷条件下,W/Cu FGM的重量损失低于石墨材料的重量损失.     

FREE VIBRATIONS OF SIMPLY SUPPORTED AND MULTILAYERED MAGNETO-ELECTRO-ELASTIC PLATES

Analytical solutions are derived for free vibrations of three-dimensional, linear anisotropic, magneto-electro-elastic, and multilayered rectangular plates under simply supported edge conditions. For any homogeneous layer, we construct the general solution in terms of a simple formalism that resembles the Stroh formalism, from which any physical quantities can be solved for given boundary conditions. In particular, the dispersion equation that characterizes the relationship between the natural frequency and wavenumber can be obtained in a simple form. For multilayered plates, we derive the dispersion relation in terms of the propagator matrices. The present solution includes all previous solutions, such as piezoelectric, piezomagnetic, and purely elastic solutions as special cases, and can serve as benchmarks to various thick plate theories and numerical methods used for the modelling of layered composite structures. Typical natural frequencies and mode shapes are presented for sandwich piezoelectric/piezomagnetic plates. It is shown clearly that some of the modes are purely elastic while others are fully coupled with piezoelectric/piezomagnetic quantities, with the latter depending strongly upon the material property and stacking sequence. These frequency and mode shape features could be of particular interest to the analysis and design of various “smart” sensors/actuators constructed from magneto-electro-elastic composite laminates.     

FREE VIBRATIONS OF A RECTANGULAR PLATE CARRYING A DISTRIBUTED MASS

In this paper, an analytical method is presented to find the eigenfrequencies of a rectangular plate carrying a uniformly distributed mass. Using the standard Galerkin procedure, the equation of motion is reduced to a set of ordinary differential equations. From this set, the frequency equation is obtained. This polynomial equation is solved numerically. Due to the significance of the fundamental frequency of the system, its variation with respect to the non-dimensional parameters associated with the location, the area density and the distribution area of the mass attached to the plate, is investigated. Furthermore, it is shown by a numerical example that the method can be used to study plates with concentrated mass as a special case. Finally, an analysis to obtain the modal surfaces and the related nodal lines is carried out. It is demonstrated that the location of the attachment significantly affects the nodal lines, and modal interchanges may occur.     

FREE TRANSVERSE VIBRATIONS OF AN ELASTICALLY CONNECTED COMPLEX BEAM-STRING SYSTEM

The present work is devoted to theoretical vibration analysis of a complex mixed continuous system. Undamped free transverse vibrations of an elastically connected beam-string system are considered. Solutions of the problem are formulated by using the modal expansion method. Two infinite sequences of the natural frequencies corresponding to two possible kinds of vibration motions: synchronous and asynchronous are determined. In a numerical example, illustrating the theory presented, the effect of string tension force on the natural frequencies of the system is investigated in detail.     

VIBRATION CHARACTERISTICS AND TRANSIENT RESPONSE OF SHEAR-DEFORMABLE FUNCTIONALLY GRADED PLATES IN THERMAL ENVIRONMENTS

Free and forced vibration analyses for initially stressed functionally graded plates in thermal environment are presented. Material properties are assumed to be temperature dependent, and graded in the thickness direction according to a simple power law distribution in terms of the volume fractions of the constituents. Theoretical formulations are based on Reddy's higher order shear deformation plate theory and include the thermal effects due to uniform temperature variation. The plate is assumed to be clamped on two opposite edges with the remaining two others either free, simply supported or clamped. One-dimensional differential quadrature technique, Galerkin approach, and the modal superposition method are used to determine the transient response of the plate subjected to lateral dynamic loads. Comprehensive numerical results for silicon nitride/stainless-steel rectangular plates are presented in dimensionless tabular and graphical forms. The roles played by the constituent volume fraction index, temperature rise, shape and duration of dynamic loads, initial membrane stresses as well as the character of boundary conditions are studied. The results reveal that, when thermal effects are considered, functionally graded plates with material properties intermediate to those of isotropic ones do not necessarily have intermediate natural frequencies and dynamic responses.     

FREE VIBRATIONS OF MULTILAYERED PLATES BASED ON A MIXED VARIATIONAL APPROACH IN CONJUNCTION WITH GLOBAL PIECEWISE-SMOOTH FUNCTIONS

This paper presents a two-dimensional model for the analysis of freely vibrating laminated plates. The governing differential equations, associated boundary conditions and constitutive equations are derived from Reissner's mixed variational theorem. Both the governing differential equations and the related boundary conditions are presented in terms of resultant stresses and displacements. The model is able to provide the results for the corresponding three-dimensional theory. Such a performance is guaranteed from an appropriate expansion of relevant kinetic and stress quantities through the thickness of the multilayered plate. The expansion is realized by using a novel selection of global piecewise-smooth functions (GPSFs). The number of GPSFs can be arbitrarily increased to achieve a two-dimensional plate theory which is, at least, as accurate as that of a full layerwise theory. It is also shown that GPSFs permit to deal a multilayered plate as if it was virtually made of a single layer. Indeed, the theory need not explicitly introduce continuity conditions for both displacements and relevant stresses. The performance of the present two-dimensional model in conjunction with the global piecewise-smooth functions is tested and discussed by comparing its resulting eigen-parameters, for a class of simply supported plates, with those of other two-dimensional models and with those existing of the exact three-dimensional theory.     

同频率互相垂直简谐振动的合成

讨论了质点参与两个同频率互相垂直简谐振动时的角动量、速率、能量以及同频率互相垂直简谐振动的合成推广到三维的情况。     

压电陶瓷和金属构成的复合薄圆板的强迫振动——A电压策动

本文报告了电压策动下的复合薄圆板的强迫弯曲振动的严格解和近似解。复合薄圆板是由两片半径为b的压电陶瓷片和半径为a（a≥b）的金属薄圆板构成的。我们把它称作一般复合薄圆板,因为它可简化为各种特殊薄圆板.在压电陶瓷的极性做适当的安排后,交流策动电压将使一般复合薄圆板产生弯曲振动。从哈密顿原理出发推导了该系统的运动微分方程,连接条件和边界条件。并给出了一般解和严格共振频率方程。对于b=a的特殊复合薄圆板作了特殊的讨论。在简支边界情况下,策动电压的作用相当于在边界上加一个策动弯矩。当边界固定时,b=a的复合薄圆板表现为纯弹性,电压策动不能使之发生振动。本文还用里兹方法求出了一般复合圆板在电压策动下的强迫振动的近似解和近似共振频率方程。共振频率的近似值与实验值进行了比较,结果表明近似结果在工程上是可以使用的。     

VIBRATIONS OF PARTIALLY FILLED CYLINDRICAL TANKS WITH RING-STIFFENERS AND FLEXIBLE BOTTOM

The dynamics of a tank partially filled with a liquid having a free surface is investigated. In the analysis, the effect of free surface waves is taken into account, so that both bulging and sloshing modes are studied. The structure is completely flexible, and is composed of a vertically standing circular cylindrical shell, with ring-stiffeners, and a flexible bottom that consists of a circular plate generally resting on an elastic Winkler foundation. The plate edge is joined to the shell by coupling rotational springs distributed around the edge. The interaction between the plate and the shell via the fluid is included. Application to the particular cases of (i) a flexible shell with a rigid bottom and (ii) a flexible bottom plate with a rigid cylinder are also presented, as well as a comparison with approximate theories. Ring-stiffeners on the shell are also considered in order to investigate their influence on the modal behaviour of the tank. Solution of this problem is achieved via the method of artificial springs within the framework of the Rayleigh–Ritz theory, while considering strong fluid–structure interaction.     

HIGH HARMONIC GENERATION OF TWO-COLOUR FIELDS WITH ORTHOGONAL LINEAR POLARIZATION

We studied the high-order harmonic generation of two linearly-polarized orthogonal fields.The results showed that the frequency of the harmonics can be controlled by controlling the relative frequency of the two incident fields,especially, when the frequency of one field is an integral mutiple of the frequency difference between the two incident fields,the fractional times of harmonic can be obtained.     

NON-LINEAR VIBRATIONS OF SHELL-TYPE STRUCTURES: A REVIEW WITH BIBLIOGRAPHY

The aim of this paper is to provide a contemporarily relevant survey of studies on non-linear vibrations of shell-type structures. The effects of geometrical non-linearity, and specific difficulties encountered in non-linear dynamic analysis of shell-type structures are presented and discussed. Studies on non-linear vibrations of shells are categorized by different shell configurations (shapes) in a chronological order. Also, the most commonly used methods of modelling and solution are reviewed and commented. Published reviews on non-linear vibrations of shell-type structures including complicating effects of anisotropy, initial stress, added mass, elastic foundation, stiffeners, open geometry (singly and doubly curved), transverse shear deformations, torsion, and interaction with fluid are also surveyed. Comments on the previous non-linear works are presented and some orientations for future research are suggested. Another purpose of this paper is to provide engineers, scientists and researchers with a list of 175 references, which should be very useful for locating relevant existing literature quickly.