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CONTENTS
Volume 19, Number 1, January 2017
 

Abstract
The operation of subway trains induces secondary structure-borne vibrations in the nearby underground spaces. The vibration, along with the associated noise, can cause annoyance and adverse physical, physiological, and psychological effects on humans in dense urban environments. Traditional tethered instruments restrict the rapid measurement and assessment on such vibration effect. This paper presents a novel approach for Wireless Smart Sensor (WSS)-based autonomous evaluation system for the subway train-induced vibrations. The system was implemented on a MEMSIC\'s Imote2 platform, using a SHM-H high-sensitivity accelerometer board stacked on top. A new embedded application VibrationLevelCalculation, which determines the International Organization for Standardization defined weighted acceleration level, was added into the Illinois Structural Health Monitoring Project Service Toolsuite. The system was verified in a large underground space, where a nearby subway station is a good source of ground excitation caused by the running subway trains. Using an on-board processor, each sensor calculated the distribution of vibration levels within the testing zone, and sent the distribution of vibration level by radio to display it on the central server. Also, the raw time-histories and frequency spectrum were retrieved from the WSS leaf nodes. Subsequently, spectral vibration levels in the one-third octave band, characterizing the vibrating influence of different frequency components on human bodies, was also calculated from each sensor node. Experimental validation demonstrates that the proposed system is efficient for autonomously evaluating the subway train-induced ambient vibration of underground spaces, and the system holds the potential of greatly reducing the laboring of dynamic field testing.

Key Words
underground space; ambient vibration; subway train; wireless smart sensors

Address
Ke Sun: School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210046, China
Wei Zhang: School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210046, China;
High-Tech Research Institute of Nanjing University (Suzhou), Suzhou, Jiangsu 215123, China
Huaping Ding: School of Electronic Science and Engineering, Nanjing University, Nanjing, Jiangsu, 210046 China
Robin E. Kim: Center for Integrated Smart Sensors, KAIST, Daejeon 305-701, South Korea
Billie F. Spencer Jr.: Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA


Abstract
For structural damage detection of shear buildings, this paper proposes a new concept using structural element mass-stiffness vector (SEMV) based on special mass and stiffness distribution characteristics. A corresponding damage identification method is developed combining the SEMV with the cross-model cross-mode (CMCM) model updating algorithm. For a shear building, a model is assumed at the beginning based on the building\'s distribution characteristics. The model is updated into two models corresponding to the healthy and damaged conditions, respectively, using the CMCM method according to the modal parameters of actual structure identified from the measured acceleration signals. Subsequently, the structural SEMV for each condition can be calculated from the updated model using the corresponding stiffness and mass correction factors, and then is utilized to form a new feature vector in which each element is calculated by dividing one element of SEMV in health condition by the corresponding element of SEMV in damage condition. Thus this vector can be viewed as a damage detection feature for its ability to identify the mass or stiffness variation between the healthy and damaged conditions. Finally, a numerical simulation and the laboratory experimental data from a test-bed structure at the Los Alamos National Laboratory were analyzed to verify the effectiveness and reliability of the proposed method. Both simulated and experimental results show that the proposed approach is able to detect the presence of structural mass and stiffness variation and to quantify the level of such changes.

Key Words
structural element mass-stiffness vector; damage identification; cross-model cross-mode method; shear building

Address
Yabin Liang and Gangbing Song: Faculty of Infrastructure Engineering, Dalian University of Technology, Dalian, Liaoning, China;
Department of Mechanical Engineering, University of Houston, Houston, TX, USA
Dongsheng Li and Chao Zhan: Faculty of Infrastructure Engineering, Dalian University of Technology, Dalian, Liaoning, China


Abstract
To control the stochastic vibration of a vibration-sensitive instrument supported on a beam, the beam is designed as a sandwich structure with magneto-rheological visco-elastomer (MRVE) core. The MRVE has dynamic properties such as stiffness and damping adjustable by applied magnetic fields. To achieve better vibration control effectiveness, the optimal bounded parametric control for the MRVE sandwich beam with supported mass under stochastic and deterministic support motion excitations is proposed, and the stochastic and shock vibration suppression capability of the optimally controlled beam with multi-mode coupling is studied. The dynamic behavior of MRVE core is described by the visco-elastic Kelvin-Voigt model with a controllable parameter dependent on applied magnetic fields, and the parameter is considered as an active bounded control. The partial differential equations for horizontal and vertical coupling motions of the sandwich beam are obtained and converted into the multi-mode coupling vibration equations with the bounded nonlinear parametric control according to the Galerkin method. The vibration equations and corresponding performance index construct the optimal bounded parametric control problem. Then the dynamical programming equation for the control problem is derived based on the dynamical programming principle. The optimal bounded parametric control law is obtained by solving the programming equation with the bounded control constraint. The controlled vibration responses of the MRVE sandwich beam under stochastic and shock excitations are obtained by substituting the optimal bounded control into the vibration equations and solving them. The further remarkable vibration suppression capability of the optimal bounded control compared with the passive control and the influence of the control parameters on the stochastic vibration suppression effectiveness are illustrated with numerical results. The proposed optimal bounded parametric control strategy is applicable to smart visco-elastic composite structures under deterministic and stochastic excitations for improving vibration control effectiveness.

Key Words
stochastic vibration; optimal bounded control; sandwich beam; magneto-rheological visco-elastomer; stochastic response reduction

Address
Z.G. Ying: Department of Mechanics, School of Aeronautics and Astronautics, Zhejiang University, Hangzhou 310027, P.R. China
Y.Q. Ni: Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
Y.F. Duan: Department of Civil Engineering, College of Civil Engineering and Architecture, Zhejiang University,
Hangzhou 310058, P.R. China


Abstract
This research tries to present a nonlinear thermo-elastic solution for a functionally graded spherical shell subjected to mechanical and thermal loads. Geometric nonlinearity is considered using the Lagrange or finite strain tensor. Non-homogeneous material properties are considered based on a power function. Adomian\'s decomposition method is used for calculation of nonlinear results. Nonlinear results such as displacement can be evaluated for sphere in terms of different indexes of non-homogeneity. A comprehensive comparison between linear and nonlinear results and evaluation of the percentage of difference between them can be performed in this paper. The obtained results indicate that the improvement of the results due to usage of nonlinear analysis is depending on the non-homogeneous index.

Key Words
thermo-elastic; nonlinear analysis; non-homogenous index; mechanical loads; strain

Address
Mohammad Arefi: Department of Solid Mechanics, Faculty of Mechanical Engineering, University of Kashan, Kashan 87317-51167, I.R. Iran
Ashraf M. Zenkour: Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia;
Department of Mathematics, Faculty of Science, Kafrelsheikh University, Kafrelsheikh 33516, Egypt


Abstract
Finite element model updating is very effective procedure to determine the uncertainty parameters in structural model and minimize the differences between experimentally and numerically identified dynamic characteristics. This procedure can be practiced with manual and automatic model updating procedures. The manual model updating involves manual changes of geometry and analyses parameters by trial and error, guided by engineering judgement. Besides, the automated updating is performed by constructing a series of loops based on optimization procedures. This paper addresses the ambient vibration based finite element model updating of long span reinforced concrete highway bridges using manual model updating procedure. Birecik Highway Bridge located on the 81stkm of Şanliurfa-Gaziantep state highway over Firat River in Turkey is selected as a case study. The structural carrier system of the bridge consists of two main parts: Arch and Beam Compartments. In this part of the paper, the arch compartment is investigated. Three dimensional finite element model of the arch compartment of the bridge is constructed using SAP2000 software to determine the dynamic characteristics, numerically. Operational Modal Analysis method is used to extract dynamic characteristics using Enhanced Frequency Domain Decomposition method. Numerically and experimentally identified dynamic characteristics are compared with each other and finite element model of the arch compartment of the bridge is updated manually by changing some uncertain parameters such as section properties, damages, boundary conditions and material properties to reduce the difference between the results. It is demonstrated that the ambient vibration measurements are enough to identify the most significant modes of long span highway bridges. Maximum differences between the natural frequencies are reduced averagely from %49.1 to %0.6 by model updating. Also, a good harmony is found between mode shapes after finite element model updating.

Key Words
ambient vibration test; enhanced frequency domain decomposition; finite element model; highway bridge; manual model updating; operational modal analysis

Address
Ahmet C. Altunisik and Alemdar Bayraktar: Department of Civil Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey

Abstract
Considerable achievements in developing structural regulators as an important method for vibration control have been made over the last few decades. The use of large quantities of cables in traditional wired control systems to connect sensors, controllers, and actuators makes the structural regulators complicated and expensive. A wireless decentralized control experimental platform based on Wi-Fi unit is designed and implemented in this study. Centralized and decentralized control strategies as sample controllers are employed in this control system. An optimal control algorithm based on Kalman estimator is embedded in the dSPACE controller and the DSP controller. To examine the performance of this control scheme, a three-story steel structure is developed with active mass dampers installed on each floor as the wireless communication platform. Experimental results show that the wireless decentralized control exhibits good control performance and has various potential applications in industrial control systems. The proposed experimental system may become a benchmark platform for the validation of the corresponding wireless control algorithm.

Key Words
active mass damper; wireless decentralized control; benchmark; DSP; Wi-Fi

Address
Yan Yu and Xiaozhi Leng: School of Electronic Science and Technology, Dalian University of Technology, Dalian, Liaoning, China
Luyu Li and Jinping Ou: School of Civil Engineering, Dalian University of Technology, Dalian, Liaoning, China
Gangbing Song: Department of Mechanical Engineering, University of Houston, Houston TX, USA
Zhiqiang Liu: CCCC Highway Consultants Co., Ltd., Beijing 100088, P.R. China

Abstract
Nanocomposites reinforced with carbon nanotube fibers exhibit greater stiffness, strength and damping properties in comparison to conventional composites reinforced with carbon/glass fibers. Consequently, most of the nanocomposite research is focused in understanding the dynamic characteristics, which are highly useful in applications such as vibration control and energy harvesting. It has been observed that those nanocomposites show better stiffness when the geometry of nanotubes is straight as compared to curvilinear although nanotube agglomeration may exist. In this work the damping behavior of the nanocomposite is characterized in terms of loss factor under the presence of nanotube agglomerations. A micro stick-slip damping model is used to compute the damping properties of the nanocomposites with multiwall carbon nanotubes. The present formulation considers the slippage between the interface of the matrix and the nanotubes as well as the slippage between the interlayers in the nanotubes. The nanotube agglomerations model is also presented. Results are computed based on the loss factor expressed in terms of strain amplitude and nanotube agglomerations. The results show that although – among the various factors such as the material properties (moduli of nanotubes and polymer matrix) and the geometric properties (number of nanotubes, volume fraction of nanotubes, and critical interfacial shear stresses), the agglomeration of nanotubes significantly influences the damping properties of the nanocomposites. Therefore the full potential of nanocomposites to be used for damping applications needs to be analyzed under the influence of nanotube agglomerations.

Key Words
numerical material modeling; nanocomposites; damping; hysteresis, agglomeration

Address
Chetan S. Jarali: Structural Technologies Division, CSIR National Aerospace Laboratories, Bangalore -560017, India
M. Madhusudan: Ph.D. Research Centre, Visvesvaraya Technological University, Belgaum-590008, India;
Department of Mechanical Engineering, Bangalore Institute of Technology, Bangalore -560004, India
S. Vidyashankar: Department of Mechanical Engineering, Bangalore Institute of Technology, Bangalore -560004, India
Charles Lu: Department of Mechanical Engineering, University of Kentucky, Lexington, KY 40506, USA

Abstract
The interdisciplinary research area of small scale energy harvesting has attracted tremendous interests in the past decades, with a goal of ultimately realizing self-powered electronic systems. Among the various available ambient energy sources which can be converted into electricity, wind energy is a most promising and ubiquitous source in both outdoor and indoor environments. Significant research outcomes have been produced on small scale wind energy harvesting in the literature, mostly based on piezoelectric conversion. Especially, modeling methods of wind energy harvesting techniques plays a greatly important role in accurate performance evaluations as well as efficient parameter optimizations. The purpose of this paper is to present a guideline on the modeling methods of small-scale wind energy harvesters. The mechanisms and characteristics of different types of aeroelastic instabilities are presented first, including the vortex-induced vibration, galloping, flutter, wake galloping and turbulence-induced vibration. Next, the modeling methods are reviewed in detail, which are classified into three categories: the mathematical modeling method, the equivalent circuit modeling method, and the computational fluid dynamics (CFD) method. This paper aims to provide useful guidance to researchers from various disciplines when they want to develop and model a multi-way coupled wind piezoelectric energy harvester.

Key Words
energy harvesting; wind energy; modeling; aeroelasticity; piezoelectric material

Address
Liya Zhao and Yaowen Yang: School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

Abstract
Generalized fractal dimension is used to detect the presence of partial delamination in a composite laminated beam. The effect of boundary conditions and location of delamination on the fractal dimension curve is studied. Appropriability of higher mode shape data for detection of delamination in the beam is evaluated. It is shown that fractal dimension measure can be used to detect the presence of partial delamination in composite beams. It is found that the torsional mode shape is well suited for delamination detection in beams. First natural frequency of delaminated beam is found to be higher than the healthy beam for certain small and partial width delaminations and some boundary conditions. An explanation towards this counter intuitive phenomenon is provided.

Key Words
partial delamination; finite elements; frequency shift; fractal dimension; damage detection

Address
S. Keshava Kumar, Ranjan Ganguli and Dineshkumar Harursampath: Department of Aerospace Engineering, Indian Institute of Science Bangalore, India

Abstract
A microstructure-dependent dynamic model for silicon nanobeams with axial motion is developed by considering the effects of nonlocal elasticity and surface energy. The nanobeam is considered to subject to both transverse and longitudinal loads arising from nanostructural surface effect and all positive directions of physical quantities are defined clearly prior to modeling so as to clarify the confusions of sign in governing equations of previous work. The nonlocal and surface effects are taken into consideration in the dynamic behaviors of silicon nanobeams with axial motion including circular natural frequency, vibration mode, transverse displacement and critical speed. Various supporting conditions are presented to investigate the circular frequencies by a numerical method and the effects of many variables such as nonlocal nanoscale, axial velocity and external loads on non-dimensional circular frequencies are addressed. It is found that both nonlocal and surface effects play remarkable roles on the dynamics of nanobeams with axial motion and cause the frequencies and critical speed to decrease compared with the classical continuum results. The comparisons of the non-dimensional calculation values by present and previous studies validate the correctness of the present work. Additionally, numerical examples for silicon nanobeams with axial motion are addressed to show the nonlocal and surface effects on circular frequencies intuitively. Results obtained in this paper are helpful for the design and optimization of nanobeam-like microstructures based sensors and oscillators at nanoscale with desired dynamic mechanical properties.

Key Words
silicon nanobeam; nonlocal elasticity; surface effect; circular frequency; critical speed; axial motion

Address
J.P. Shen and X.L. Fan: School of Urban Rail Transportation, Soochow University, 8 Jixue Road,
Xiangcheng District, Suzhou 215131, China
C. Li: School of Urban Rail Transportation, Soochow University, 8 Jixue Road,Xiangcheng District, Suzhou 215131, China;
Department of Railroad Civil Engineering, College of Railroad and Logistics, Woosong University,171 Dongdaejeon-ro, Dong-gu, Daejeon, Republic of Korea
C.M. Jung: Department of Railroad Civil Engineering, College of Railroad and Logistics, Woosong University,
171 Dongdaejeon-ro, Dong-gu, Daejeon, Republic of Korea





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