Experimental study of delayed positive feedback control for a flexible beam

Recently, some researches indicate that positive feedback can benefit the control if appropriate time delay is intentionally introduced into control system. However, most work is theoretical one but few are experimental. This paper presents theoretical and experimental studies of delayed positive feedback control technique using a flexible beam as research object. The positive feedback weighting coefficient is designed by using the optimal control method. The available time delay is determined by analyzing the maximal real part of characteristic roots of the system. A DSP-based experiment system is introduced. Simulation and experimental results indicate that the delayed positive feedback control may effectively reduce the beam vibration if time delay is appropriately selected.     

Experimental two-phase flow measurement using ultra fast limited-angle-type electron beam X-ray computed tomography

An experimental evaluation of a novel limited-angle-type ultra fast electron beam X-ray computed tomography approach for the visualization and measurement of a gas–liquid two-phase flow is reported here. With this method, a simple linear electron beam scan is used to produce instantaneous radiographic views of a two-phase flow in a pipe segment of a flow loop. Electron beam scanning can be performed very rapidly, thus a frame rate of 5 kHz is achieved. Radiographic projections are recorded by a very fast detector arc made of zink–cadmium–telluride elements. This detector records the X-ray radiation passing through the object with a sampling rate of 1 MHz. The reconstruction of slice images from the recorded detector data is a limited-angle problem since in our scanning geometry the object’s Radon space is only incompletely sampled. It was investigated here, whether this technology is able to produce accurate gas fraction data from bubbly two-phase flow. Experiments were performed both on a Perspex phantom with known geometry and an experimental flow loop operated under vacuum conditions in an electron beam processing box.     

Acoustical-speckle interferometry

When a continuous or pulsed wave of ultrasound is scanned over an object, an echo emanating from the object may be detected. If the echo has a random variation in amplitude vs. position of the beam, the resulting variation is referred to as acoustical speckle. An analysis is presented for utilizing acoustical speckle to determine displacements on the surface or interior of an object. Two experimental configurations, pulse-echo and continuous wave scanning, are treated in the verification of the analysis.     

Holographic moiré for strain fringe patterns

New holographic-moiré methods are presented to obtain strain fringe patterns. To introduce two carriers of different directions, two kinds of recording procedures are proposed: (1) triple-exposure recording by two object beams and one reference beam (all of these beams are identical in polarization state) and (2) double exposure or real-time recording by two orthogonally polarized object beams and two corresponding reference beams. In these recordings, the carriers are introduced by changing the directions of the reference beams and are modulated by deformation. Separated in-plane displacement derivative patterns are then obtained by optical processings. Both the principles and the experimental results are shown in the paper.     

先张预应力CFL加固RC梁预应力损失的实验研究

由于预应力碳纤维增强复合材料(CFRP)加固钢筋混凝土(RC)结构技术能够充分发挥其优异的力学性能而备受关注。本文以预应力碳纤维薄板(CFL)加固RC桥梁结构为研究对象,研制了采用先张法对CFL施加预应力的实验装置,提出了先张预应力CFL加固RC梁的实验方法及其预应力损失的监测方法,并在3个预应力水平下对RC梁实施了预应力CFL加固,测试和分析了CFL的预应力损失演化规律。研究结果表明,本文提出的先张预应力CFL加固RC梁以及预应力损失测试的方法是可行和有效的。     

时间序列散斑干涉技术的发展及应用

当用相干光照射一个连续变形的漫射物体表达时，由漫射物体表面散射所形成的物光与一参考光干涉，在其干涉的区域即可形成一随时间变化的散斑场，连续采集这一时变散斑场，并通过时间域分析，即可获得被测物质表面所对应的时变位移场，本文在作者研究的基础上，介绍序列散斑计量技术中所发展起来的几种计量方法，探讨它们的计量特性及其发展方向，并同时给出这些计量方法在时变场检测中的一些应用成果。     

On the Investigation of Some Parameter Identification and Experimental Modal Filtering Issues for Nonlinear Reduced Order Models

This paper discusses modal filtering of experimental data and the corresponding identification of linear and nonlinear parameters in reduced order space. Specifically, several experimental configurations will be discussed in order to provide insight into such identification issues as spatial discretization, observability, and the linear independence of the assumed filter or basis. The two experiments considered herein represent different measurement configurations of the same clamped–clamped beam. First, asymmetric inertial loading via asymmetric sensor location was considered, while the second scenario presents a symmetric sensor configuration. Several important conclusions can be drawn from the two experimental scenarios. First, by asymmetrically loading the beam, a corresponding asymmetric beam mode was excited yet not observable. In the second scenario, the symmetric distribution of sensors minimized the impact of the respective asymmetric mode. The resulting spatial information allowed for the proper filtering of the remnants of the asymmetric mode. Nonlinear parameters in modal space as well as the underlying linear parameters were successfully identified simultaneously in both experimental scenarios, although the usefulness of the asymmetrically loaded beam was limited. Finally, successful comparisons were made between the identified reduced order model and experimental response at the beam quarter point using the symmetric case and the beam midpoint using both experimental scenarios.     

基于模态数据的长细结构小损伤精确识别研究

本文针对现有的损伤识别方法不能满足部分结构损伤识别精度要求的现状,对结构的小损伤精确识别方法开展研究．以长细结构为研究对象,对具有不同损伤位置和损伤程度的圆柱形的轻阻尼梁结构进行了数值分析和实验研究,应用数值计算方法和实验确定的特征向量和特征频率对长细结构裂缝参数进行识别计算．本文在研究过程中编制了一个创新性的预测程序,通过其一次性生成目标函数图来选择合适的初始参数,从而对识别结果进行分析．研究结果表明,应用本文提出的识别方法,裂缝位置的识别误差可以控制在0.05 %~0.28 %范围内,裂缝深度识别误差低于7 %．     

温度-随机载荷下CFL加固RC梁疲劳性能的实验研究

考虑亚热带地区公路桥梁的服役环境温度与车辆载荷的作用效应,本文以碳纤维薄板(CFL)加固钢筋混凝土(RC)桥梁结构为研究对象,利用本课题组构建的实验平台,提出了温度与车辆随机载荷耦合作用下CFL加固RC梁的疲劳实验方法。在3个温度和3个载荷水平下实施了温度-随机载荷耦合作用下的三点弯曲疲劳实验,初步探讨了温度-随机载荷作用下加固梁的疲劳破坏机理,并提出了温度-随机载荷耦合下加固梁疲劳寿命的半经验公式。     

Motion of an object along a one-dimensional guide under elastic wave radiation reaction

We consider nonseparated motion of an object along a one-dimensional elastic guide (a beam or a string) under the radiated wave pressure. Conditions on the parameters of the vibration sources acting on the object and providing directional radiation are obtained. Using the exact solutions obtained under the assumption that the law of motion is uniform, we study the dependencies of the motive force and the vibration-source-to-object-translational-motion energy conversion factor (efficiency) on the body velocity. It is shown that an object moving at a supercritical velocity for the case in which only a single wave is excited to the left of it must be distributed; i.e., its dimensions must be comparable with the radiated wave length. In this case, the efficiency can be arbitrarily close to unity.     

Prediction of buckling characteristics of carbon nanotubes

In this paper, to investigate the buckling characteristics of carbon nanotubes, an equivalent beam model is first constructed. The molecular mechanics potentials in a C–C covalent bond are transformed into the form of equivalent strain energy stored in a three dimensional (3D) virtual beam element connecting two carbon atoms. Then, the equivalent stiffness parameters of the beam element can be estimated from the force field constants of the molecular mechanics theory. To evaluate the buckling loads of multi-walled carbon nanotubes, the effects of van-der Waals forces are further modeled using a newly proposed rod element. Then, the buckling characteristics of nanotubes can be easily obtained using a 3D beam and rod model of the traditional finite element method (FEM). The results of this numerical model are in good agreement with some previous results, such as those obtained from molecular dynamics computations. This method, designated as molecular structural mechanics approach, is thus proved to be an efficient means to predict the buckling characteristics of carbon nanotubes. Moreover, in the case of nanotubes with large length/diameter, the validity of Euler’s beam buckling theory and a shell model with the proper material properties defined from the results of present 3D FEM beam model is investigated to reduce the computational cost. The results of these simple theoretical models are found to agree well with the existing experimental results.     

参强联合作用非线性结构动力学实验建模

搭建以L 型梁为实验研究对象的参强联合作用多自由度非线性振动实验系统, 将增量谐波平衡非线性识别理论运用到实验建模方法中, 建立了L 型梁的动力学控制方程. 通过对不同激励频率和不同响应情况下的数值模拟与实验数据的比较, 验证了基于增量谐波平衡识别的实验建模方法对多自由度参强联合作用非线性动力学结构的有效性, 以及动力学控制方程的普适性.     

A new holographic interferometer for stress analysis

In holographic interferometry with a photoelastic model, two families of fringes are generated simultaneously when the model is stressed. One family represents the isochromatic-fringe pattern normally associated with photoelasticity which yields the difference between the principal stresses. The other family represents the isopachic-fringe pattern associated with interferometry which yields the sum of the principal stresses. From these complementary patterns, the magnitudes of the principal stresses can readily be determined throughout the field of observation. Unfortunately, these fringe patterns are not completely independent but interact in such a way as to make interpretation difficult in critical regions of the model. A new system has been developed which readily permits simultaneous acquisition of these fringe patterns without their undesirable mutual interaction, as well as providing increased sensitivity. This new interferometer uses a double-pass object beam and an optical rotator to eliminate the isochromatic-fringe pattern and its effect from the isopachic interferogram. Such a system has considerable value in experimental mechanics for applications to both static and dynamic model studies and to materials investigation.     

Experimental verification of the steady-state behavior of a beam system with discontinuous support

This article deals with the experimental verification of the long-term behavior of a periodically excited linear beam supported by a one-sided spring. Numerical analysis of the beam showed subharmonic, quasi-periodic, and chaotic behavior. Further, three different routes leading to chaos were found. Because of the relative simplicity of the beam system and the variety of calculated nonlinear phenomena, an experimental setup is made of this beam system to verify the numerical results. The experimental results correspond very well with the numerical results as far as the subharmonic behavior is concerned. Measured chaotic behavior is proved to be chaotic by calculating Lyapunov exponents of experimental data.     

高速列车引起的地基振动半解析解

随着列车速度的提高,地基振动的反应越来越大。根据有限元理论、轨道动力学及地基土振动Green函数,建立了轨道-无限地基土相互作用理论分析模型。为了组装系统的动力方程,通过引入静态自由度凝聚模式来消除梁存在转动自由度。借助文献[12]的柔度矩阵定义并通过Hankel变换获得了系统的刚度矩阵,进而采用Newmark法求出了系统动力反应。最后,以瑞典X2000高速旅客列车为对象进行仿真,通过与试验结果的比较来说明本文方法的可行性。     

A new application of differential interferometry for stress analysis

In the past, differential interferometry has found interesting applications in gas dynamics. The gradients of density could be measured in gas flows. Now, a first trial is made to extend this method to the experimental treatment of stress problems. A Wollaston prism with polarizing elements is used in the optical arrangement. This prism combines two beams of light which have penetrated the model at locally separated points. A field of interference fringes can be produced behind the Wollaston prism. The deflections of the different conjugated light beams, which are caused by the deformed elements of the model, lead to a shifting of the interference fringes. A Stress Differential-interferometer Law is derived theoretically in order to interpret the optical data According to this theory, the optical effect caused by the deflection in this arrangement is proportional to the gradient of the sum of principal stresses. A calibration test is performed by using a circular disk, this method is applied to a circular ring for measuring the stress gradients. Under special conditions, interference fringes could be produced which represent the loci of equal stress gradient. Plexiglas plane models are loaded diametrically by single loads. The experimental results verify the statements of the developed theory.     

Model interferometry with air-holograms

An “air-hologram” is formed by the superposition of collimated object and reference light beams in a standard transmission holographic bench. Since no model is initially placed in the system, the reconstruction is simply the collimated beam that had passed through the model space. Insertion of transparent model material changes the optical path-length profile causing interference between the existing beam, which traverses the model, and the reconstructed beam which had traversed air only—hence, the designation “air-hologram”. Using this method, model material can be inspected and selected for optical flatness. Interference in the unloaded models can then be eliminated by simple rotation of the hologram. Real-time holographic interferometry is performed in the same manner as is photoelasticity. Further, it is shown how errors caused by large model displacements can be minimized.     

An experimental method for determining the dynamic contact law

An experimental method is investigated whereby the strain response from an impacted beam is sufficient to determine the contacting force. Once the force is known, it is shown how the contact law can be determined. Experimental results for an impacted aluminum beam are demonstrated.     

A shadow-moiré method with continuously variable sensitivity

This paper presents a shadow moiré method whereby the isostathmics (contours of surface depth) can be studied with continuously variable sensitivity. In the method the sensitivity is varied by simply rotating the grating in front of the object surface. Equations interpreting the isostathmics are derived, and an experimental demonstration is included.