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CONTENTS
Volume 9, Number 4, April 2012
 

Abstract
The possibility of suppressing critical speeds by using electromagnetic actuators (EMAs) is assessed experimentally in this paper. The system studied is composed of a horizontal flexible shaft supported by two ball bearings at one end and one roller bearing that is located in a squirrel cage at the other end. Four identical EMAs supplied with constant current are utilized. The EMAs associated to the squirrel cage constitutes the hybrid bearing. Results obtained, show that the constant current, when applied to the EMAs, produces a shift of the first critical speed toward lower values. Moreover, the application of constant current for a speed interval around the critical speed enables a smooth run-up or run-down without crossing any resonance.

Key Words
critical speeds; electromagnetic actuators; rotordynamics; hybrid bearings; experiments

Address
Jarir Mahfoud and Johan Der Hagopian : Universite de Lyon - Laboratoire de Mecanique des Contacts et des Structures - UMR CNRS 5259, Institut National des Sciences Appliquees de Lyon, France

Abstract
Reinforced concrete (RC) framed buildings dissipate the seismic energy through yielding of the reinforcing bars. This yielding jeopardizes the serviceability of these buildings as it results in residual lateral deformations. Superelastic Shape Memory Alloys (SMAs) can recover inelastic strains by stress removal. Since SMA is a costly material, this paper defines the required locations of SMA bars in a typical RC frame to optimize its seismic performance in terms of damage scheme and seismic residual deformations. The intensities of five earthquakes causing failure to a typical RC six-storey building are defined and used to evaluate seven SMA design alternatives.

Key Words
seismic damage; seismic residual deformations; shape memory alloy; superelasticity; moment frame; reinforced concrete

Address
M.A. Youssef and M.A. Elfeki : The University of Western Ontario, Department of Civil and Environmental Engineering, London, ON N6A 5B9, Canada

Abstract
This study suggests a simple two-step method for structural vibration-based health monitoring for beam-like structures which only utilizes mode shape curvature and few natural frequencies of the structures in order to detect and localize cracks. The method is firstly based on the application of wavelet transform to detect crack locations from mode shape curvature. Then particle swarm optimization is applied to evaluate crack depth. As the Rayleigh quotient is introduced to estimate natural frequencies of cracked beams, the relationship of natural frequencies and crack depths can be easily obtained with only a simple formula. The method is demonstrated and validated numerically, using the numerical examples (cantilever beam and simply supported shaft) in the literature, and experimentally for a cantilever beam. Our results show that mode shape curvature and few estimated natural frequencies can be used to detect crack locations and depths precisely even under a certain level of noise. The method can be extended for health monitoring of other more complicated structures.

Key Words
beam-like structures; rayleigh quotient; wavelet transform; natural frequency estimation; crack detection; particle swarm optimization

Address
Jiawei Xiang: Department of Mechanical Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku,Nagoya, 464-8603, Japan, College of Mechanical and Electrical Engineering, Wenzhou University, 325035, China
Toshiro Matsumoto : Department of Mechanical Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku,Nagoya, 464-8603, Japan
Jiangqi Long: College of Mechanical and Electrical Engineering, Wenzhou University, 325035, China
Yanxue Wang and Zhansi Jiang : School of Mechanical and Electrical Engineering, Guilin University of Electronic Technology, Guilin, 541004, China

Abstract
A composite element method (CEM) is presented to analyze the free and forced vibrations of a cracked Euler-Bernoulli beam with axial force. The cracks are introduced by using Christides and Barr crack model with an adjustment on one crack parameter. The effects of the cracks and axial force on the reduction of natural frequencies and the dynamic responses of the beam are investigated. The time response sensitivities with respect to the crack parameters (i.e., crack location, crack depth) and the axial force are calculated. The natural frequencies obtained from the proposed method are compared with the analytical results in the literature, and good agreement is found. This study shows that the cracks in the beam may have significant effects on the dynamic responses of the beam. In the inverse problem, a response sensitivity-based model updating method is proposed to identify both a single crack and multiple cracks from measured dynamic responses. The cracks can be identified successfully even using simulated noisy acceleration responses.

Key Words
composite element; crack; axial force; response sensitivity; crack identification

Address
Z.R. Lu and J.K. Liu : Department of Mechanics, Sun Yat-sen University, Guangzhou, P.R. China

Abstract
Tracking control of systems with variable stiffness hysteresis using a gain-scheduled (GS) controller is developed in this paper. Variable stiffness hysteretic system is represented as quasi linear parameter dependent system with known bounds on parameters. Assuming that the parameters can be measured or estimated in real-time, a GS controller that ensures the performance and the stability of the closed-loop system over the entire range of parameter variation is designed. The proposed method is implemented on a springmass system which consists of a semi-active independently variable stiffness (SAIVS) device that exhibits hysteresis and precisely controllable stiffness change in real-time. The SAIVS system with variable stiffness hysteresis is represented as quasi linear parameter varying (LPV) system with two parameters: linear timevarying stiffness (parameter with slow variation rate) and stiffness of the friction-hysteresis (parameter with high variation rate). The proposed LPV-GS controller can accommodate both slow and fast varying parameter, which was not possible with the controllers proposed in the prior studies. Effectiveness of the proposed controller is demonstrated by comparing the results with a fixed robust H

Key Words
tracking control; quasi linear parameter varying system; linear parameter varying controller; gain-scheduled controller; robust H

Address
D.T.R. Pasala : Department of Civil and Environmental Engineering, Rice University, Houston, USA
S. Nagarajaiah : Department of Civil and Envi. Engrg. and Mechanical Engrg. and Mat. Sci., Rice University, Houston, USA
K. M. Grigoriadis: Department of Mechanical Engineering, University of Houston, Houston, USA


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