A novel method for crack detection in beam-like structures by measurements of natural frequencies

A novel method is proposed for calculating the natural frequencies of a multiple cracked beam and detecting unknown number of multiple cracks from the measured natural frequencies. First, an explicit expression of the natural frequencies through crack parameters is derived as a modification of the Rayleigh quotient for the multiple cracked beams that differ from the earlier ones by including nonlinear terms with respect to crack severity. This expression provides a simple tool for calculating the natural frequencies of the beam with arbitrary number of cracks instead of solving the complicated characteristic equation. The obtained nonlinear expression for natural frequencies in combination with the so-called crack scanning method proposed recently by the authors allowed the development of a novel procedure for consistent identification of unknown amount of cracks in the beam with a limited number of measured natural frequencies. The developed theory has been illustrated and validated by both numerical and experimental results.     

Ultrasonic methods for measuring crack location,crack height and crack angle

Two ultrasonic methods are proposed to measure the crack location and the crack height for vertical and inclined defects. The methods measure the time traversed between the entry point of the sound beam in the material and the defect. Differing from most previous methods, they do not depend on knowing the probe angle, a parameter difficult to measure accurately and which varies from material to material being inspected. Accordingly, the crack can be viewed from different angles, thus reducing the random error in the measurement.     

Natural frequencies of a shaft with periodically placed rotors and bearings

Flexural and torsional natural frequencies of a shaft with periodically placed rotors and bearings are investigated by using a “wave approach”. By a judicious choice of parameters, it is possible to obtain ranges of operating speed that are free from both flexural and torsional resonances. Suggestions for widening such speed ranges are included.     

确定声源频率和位置的超分辨阵列处理方法

基于均匀线阵接收一定距离的声源得到的时间和空间维的样本数据提出分离处理:用时间协方差矩阵确定声源频率;通过对近场空间协方差矩阵进行修正,用重构在相同方位上的远场近似空间协方差矩阵确定声源方位角。并在此基础上,通过改变阵列中心位置确定声源位置。计算机模拟和声学实验证实了上述方法的可行性.     

The natural frequencies of cylindrical cavities with a magnetoactive plasma

Radiophysics and Quantum Electronics -     

Intensity and frequency resolution: masking of absolute identification and fixed and roving discrimination.

Auditory intensity and frequency resolution were studied in three paradigms under masking conditions. Absolute identifications of single stimuli (one-interval paradigm) and 2IFC judgments of fixed- and roving-level pairs of stimuli (two-interval paradigm) were obtained from the same experienced observers. Judgments were made under optimal (no mask) conditions, in the presence of a broadband noise mask (simultaneous mask), and when the stimulus(i) to be judged were either preceded (forward mask) or followed (backward mask) by a broadband noise mask. Substantial masking of intensity resolution was found in all mask conditions. Only a simultaneous mask affected frequency resolution. In the no mask condition, performance was best for fixed-level (or frequency) 2IFC discrimination, followed by roving-level (frequency) 2IFC, and finally absolute identification. These differences were maintained under masking for frequency resolution, but not for intensity resolution. The results are discussed in terms of the Braida and Durlach (1988) model of intensity resolution. A similar model is suggested for frequency resolution with differences suggested by the differences in neural coding of sound intensity and frequency.     

A stiffness equation transfer method for natural frequencies of structures

A stiffness equation transfer method is proposed for obtaining vibration frequencies of structures. This method is an extension of the finite element-transfer matrix (FE-TM) method. In the present method, the transfer of state vectors from left to right in the ordinary FE-TM method is changed into the transfer of stiffness equations of every section from left to right. This method reduces the propagation of round-off errors produced in the ordinary transfer matrix method. Furthermore, the drawback that the number of degrees of freedom on the left boundary must be the same as that on the right boundary in the ordinary FE-TM method, is now avoided. Besides, this method finds out the values of the frequency by Newton-Raphson iteration method, so no plotting of the value of the determinant versus assumed frequency is necessary. An IFETM—W program based on this method for use on an IBM PC586 microcomputer is developed. Finally, numerical examples are presented to demonstrate the accuracy as well as the potential of the proposed method for free vibration analysis of structures.     

Galerkin methods for natural frequencies of high-speed axially moving beams

In this paper, natural frequencies of planar vibration of axially moving beams are numerically investigated in the supercritical ranges. In the supercritical transport speed regime, the straight equilibrium configuration becomes unstable and bifurcate in multiple equilibrium positions. The governing equations of coupled planar is reduced to two nonlinear models of transverse vibration. For motion about each bifurcated solution, those nonlinear equations are cast in the standard form of continuous gyroscopic systems by introducing a coordinate transform. The natural frequencies are investigated for the beams via the Galerkin method to truncate the corresponding governing equations without nonlinear parts into an infinite set of ordinary-differential equations under the simple support boundary. Numerical results indicate that the nonlinear coefficient has little effects on the natural frequency, and the three models predict qualitatively the same tendencies of the natural frequencies with the changing parameters and the integro-partial-differential equation yields results quantitatively closer to those of the coupled equations.     

机动目标的噪声识别和定位系统的研究

本文介绍了一种机动目标噪声识别和定位系统硬件组成，工作原理及系统的软件设计方法，对于在系统中解决的几个主要技术问题进行了论述。     

Radio frequencies in EPR: Conventional and advanced use

This mini-review focuses on various aspects of the application of radio frequency (rf) irradiation in electron paramagnetic resonance (EPR). The development of the electron-nuclear double resonance (ENDOR) technique is briefly described, and we highlight the use of circularly polarized rf fields and pulse ENDOR methodology in one- and two-dimensional experiments. The capability of pulse ENDOR at Q-band is illustrated with interesting experimental examples. Electron spin echo envelope modulation effects induced by an rf field in liquid samples demonstrate another role which rf fields can play. Technical achievements in the design of ENDOR resonators are illustrated by the example of a bridged loop-gap resonator. Finally, the influence of longitudinal rf fields on the dynamics of EPR transitions is explained using a dressed spin resonance treatment.     

Identification of multiple cracks in a beam using natural frequencies

A simple method to identify multiple cracks in a beam is presented. The cracks are modeled as rotational springs and the forward problem is solved using the finite element method. The inverse problem is solved iteratively for the locations and sizes of the cracks using the Newton-Raphson method. Numerical examples are provided for the identification of triple cracks in a cantilever beam as well as double cracks. The detected crack locations and sizes are in excellent agreement with the actual ones.     

Theory of orthogonal aberrations and its use in lens design

The author introduces a complete set of polynomials that are orthogonal in the three-dimensional region (generalized Zernike polynomials). These polynomials make it possible to obtain orthogonal expansion of the wave aberration in the three-dimensional region of field-pupil 0 ≤ r ≤ 1, 0 ≤ ρ ≤ 1, 0 ≤ φ ≤ 2π. This permits us to determine the orthogonal system of individual aberrations and introduce a classification of individual aberrations depending on the degree of field r and pupil ρ, φ variables. The author shows that orthogonal aberrations have a number of unique properties. The developed approach, describing the aberration properties of optical systems by means of orthogonal aberrations, and its use in the construction of new methods and techniques for the design of optical systems form a new section of lens design, namely, “Theory of orthogonal aberrations and its applications in the design of optical systems”.     

摇摆条件下自然循环系统流量混沌脉动的检验与预测

利用替代数据法检验了摇摆条件下自然循环系统不规则复合型脉动的混沌特性, 并在此基础上进行混沌预测. 关联维数、最大Lyapunov指数等几何不变量计算结果表明不规则复合型脉动具有混沌特性, 但是由于计算结果受实验时间序列长度的限制和噪声的影响, 可能会出现错误的判断结果. 为了避免出现误判, 在提取流量脉动的非线性特征的同时, 需要用替代数据法进一步检验混沌特性是否来自于确定性的非线性系统. 本文用迭代的幅度调节Fourier 算法进行混沌检验, 在此基础上用加权一阶局域法进行混沌脉动的预测. 计算结果表明: 不规则复合型脉动是来自于确定性系统的混沌脉动, 加权一阶局域法对流量脉动进行混沌预测效果较好, 并提出动态预测方法. 关键词： 混沌时间序列 替代数据法 实时预测 两相流动不稳定性     

Enhanced mechanism of catalytic ozonation by ultrasound with orthogonal dual frequencies for the degradation of nitrobenzene in aqueous solution

The experiments have been performed with a semi-continuous batch reactor to investigate the degradation efficiency of nitrobenzene in aqueous solution by ultrasound with the different orthogonal dual frequencies catalytic ozonation. The introduction of ultrasound can enhance the degradation efficiency of nitrobenzene compared to the results obtained from the processes of ozonation alone and ultrasound alone. The degradation of nitrobenzene is found to be zero-order in the two systems of ultrasound alone, and the reactions follow the pseudo-first-order kinetic model in the processes of ozone alone and ozone/ultrasound. The investigation confirms that the degradation of nitrobenzene follows the mechanism of hydroxyl radical (^•OH) oxidation, and the enhancement function is even more pronounced in the presence of ultrasound with the greater difference between the orthogonal dual frequencies due to the obvious synergetic effect between ozone and ultrasound, which increases the utilization efficiency of ozone, and accelerates the initiation of ^•OH and the formation of H₂O₂, resulting in the rapid formation of an increasing diversity of byproducts and the advancement degree of mineralization of total organic carbon (TOC). The oxidative byproducts have been, respectively identified in the different processes selected, including o, p, m-nitrophenols, phenol, malonic acid, 4-nitrocatechol, nitrate ion, maleic acid, oxalic acid, hydroquinone, p-quinone, 1,2,3-trihydroxy-5-nitrobenzene and acetic acid.