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CONTENTS
Volume 14, Number 3, September 2014
 

Abstract
This paper aims to present the analytical solutions of 2-2 cement based piezoelectric transducers in series electrically based on the theory of piezo-elastic dynamics. The solutions of two different kinds of 2-2 cement based piezoelectric transducers under external harmonic load are obtained by using the displacement method. The effects of electrical connection of piezoelectric layers, loading frequency, thickness and distance of piezoelectric layers on the characteristics of the transducers are discussed. Comparisons with other related experimental investigations are also given, and good agreement is found. The proposed 2-2 cement based piezoelectric transducers have a great potential application in monitoring structural health in civil engineering and capturing mechanical energy or monitoring train-running safety in railway system and traffic safety in road system.

Key Words
2-2 cement composite; piezoelectric transducer; actuator; series; dynamic behavior

Address
Jianjun Wang and Zhifei Shi: School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, P. R. China

Abstract
This study employs a novel beam-type wavelet finite element model (WFEM) to fulfill an adaptive-scale damage detection strategy in which structural modeling scales are not only spatially varying but also dynamically changed according to actual needs. Dynamical equations of beam structures are derived in the context of WFEM by using the second-generation cubic Hermite multiwavelets as interpolation functions. Based on the concept of modal strain energy, damage in beam structures can be detected in a progressive manner: the suspected region is first identified using a low-scale structural model and the more accurate location and severity of the damage can be estimated using a multi-scale model with local refinement in the suspected region. Although this strategy can be implemented using traditional finite element methods, the multi-scale and localization properties of the WFEM considerably facilitate the adaptive change of modeling scales in a multi-stage process. The numerical examples in this study clearly demonstrate that the proposed damage detection strategy can progressively and efficiently locate and quantify damage with minimal computation effort and a limited number of sensors.

Key Words
damage detection; adaptive-scale; wavelet finite element model; cubic Hermite multiwavelets; modal strain energy

Address
Wen-Yu He and Songye Zhu: Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
Wei-Xin Ren: School of Civil Engineering, Hefei University of Technology, Hefei 230009, China

Abstract
A newly developed method based on energy is presented to study the damage pattern of FRP material. Basalt fiber reinforced polymer (BFRP) is employed to monitor the damage under fatigue loading. In this study, acoustic emission technique (AE) combined with scanning electronic microscope (SEM) technique is employed to monitor the damage evolution of the BFRP specimen in an approximate continuous scanning way. The AE signals are analyzed based on the wavelet transform, and the analyses are confirmed by SEM images. Several damage patterns of BFRP material, such as matrix cracking, delamination, fiber fracture and their combinations, are identified through the experiment. According to the results, the cumulative energy (obtained from wavelet coefficients) of various damage patterns are closely related to the damage evolution of the BFRP specimens during the entire fatigue tests. It has been found that the proposed technique can effectively distinguish different damage patterns of FRP materials and describe the fatigue damage evolution.

Key Words
fatigue damage; acoustic emission; wavelet transform; damage pattern; damage evolution

Address
Hui Li, Wentao Wang and Wensong Zhou: Research Center of Structural Monitoring and Control, School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China

Abstract
This paper presents a composite control strategy for the active suppression of vibration due to the unknown disturbances, such as external excitation, harmonic effects and control spillover, as well as high-frequency accelerometer measurement noise in the all-clamped stiffened plate. The proposed composite control action based on the modal approach, consists of two contributions including feedback part and feedforward part. The feedback part is the well-known PID controller, which is widely used to increase the structure damping and improve its dynamic performance close to the resonance frequencies. In order to get better performance for vibration suppression, the weight matrixes is optimized by chaos sequence. Then an improved disturbance observer (IDOB) as the feedforward compensation part is developed to enhance the vibration suppression performance of PID under various disturbances and uncertainties. The proposed IDOB can simultaneously estimate the various disturbances dynamically as well as measurement noise acting on the system and suppress them by feedforward compensation design. A rigorous analysis is also given to show why the IDOB can effectively suppress the unknown disturbances and measurement noise. In order to verify the proposed composite control algorithm (IDOB-PID), the dSPACE real-time simulation platform is used and an experimental platform for the all-clamped stiffened plate active vibration control system is set up. The experimental results demonstrate the effectiveness, practicality and strong anti-disturbances ability of the proposed control strategy.

Key Words
active vibration control; all-clamped stiffened plate; disturbance observer; piezoelectric; high frequency measurement noise; composite control

Address
Shengquan Li, Rong Zhao, Yueping Mo and Zhenyu Sun: School of Hydraulic, Energy and Power Engineering, Yangzhou University, Yangzhou 225127, China
Juan Li: School of Automation, Southeast University, Nanjing 210096, China

Abstract
The main objective of this paper is to develop an innovative methodology for the vibration suppression control of the multiple degrees-of-freedom (MDOF) flexible structure. The proposed structure represented in this research as a clamped-free-free-free truss type plate is rotated by motors. The controller has two loops for tracking and vibration suppression. In addition to stabilizing the actual system, the proposed feedback control is based on a genetic algorithm (GA) to seek the primary optimal control gain for tracking and stabilization purposes. Moreover, input shaping is introduced for the control scheme that limits motion-induced elastic vibration by shaping the reference command. Experimental results are presented, demonstrating that, in the control loop, roll and yaw angles track control and elastic mode stabilization. It was also demonstrated that combining the input shaper with the proportional-integral-derivative (PID) feedback method has been shown to yield improved performance in controlling the flexible structure system. The broad range of problems discussed in this research is valuable in civil, mechanical, and aerospace engineering for flexible structures with MDOM motion.

Key Words
large flexible structures; genetic algorithms; input shaping; multiple degrees of freedom; vibration suppression

Address
J. Lin and C.B. Chiang: Department of Mechanical Engineering, Chien Hsin University of Science and Technology
229, Chien-Hsin Rd., Jung-Li City, Taiwan 320, R.O.C.


Abstract
Ambient vibration techniques are nowadays a very popular tool to assess dynamic properties of buildings. Due to its non destructive character, this method is particularly valuable, especially for health monitoring of historical monuments. The present ambient vibration experiment consists on the evaluation of vibration modes of a Medieval tower. Situated in Soncino (close to Cremona, in the Northern Italian region named Lombardia), the tower of 41.5 meters height has been monitored by seismometers located at different points inside the structure. Spectral ratios of the recorded ambient vibrations clearly identify a fundamental mode at about 1 Hz, with a slight difference in the two horizontal components. A second mode is also evidenced at approx 4-5 Hz, with a moderate degree of uncertainty. The records of a ML 4.4 earthquake, occurred during the monitoring period, confirm the information obtained by microtremor analysis. Daily variations of both 1st and 2nd mode were detected: these variations, of an amount up to 2%, seem to be well related with the temperature.

Key Words
modeling; old masonry structure; retrofit; structural analyses; temperature effects

Address
S. Casciati: DICA, University of Catania at Siracusa, P. Federico di Svevia, 96100 Siracusa, Italy
A. Tento, A. Marcellini and R. Daminell: Istituto per la Dinamica dei Processi Ambientali - CNR, Via Mario Bianco 9, 20131 Milano, Italy

Abstract
Vibration-based fault detection and condition monitoring of rotating machinery, using statistical process control (SPC) combined with statistical pattern recognition methodology, has been widely investigated by many researchers. In particular, the discrete wavelet transform (DWT) is considered as a powerful tool for feature extraction in detecting fault on rotating machinery. Although DWT significantly reduces the dimensionality of the data, the number of retained wavelet features can still be significantly large. Then, the use of standard multivariate SPC techniques is not advised, because the sample covariance matrix is likely to be singular, so that the common multivariate statistics cannot be calculated. Even though many feature-based SPC methods have been introduced to tackle this deficiency, most methods require a parametric distributional assumption that restricts their feasibility to specific problems of process control, and thus limit their application. This study proposes a nonparametric multivariate control chart method, based on multiscale wavelet scalogram (MWS) features, that overcomes the limitation posed by the parametric assumption in existing SPC methods. The presented approach takes advantage of multi-resolution analysis using DWT, and obtains MWS features with significantly low dimensionality. We calculate Hotelling\' s T2-type monitoring statistic using MWS, which has enough damage-discrimination ability. A bootstrap approach is used to determine the upper control limit of the monitoring statistic, without any distributional assumption. Numerical simulations demonstrate the performance of the proposed control charting method, under various damage-level scenarios for a bearing system.

Key Words
statistical process control; fault detection; bootstrap; wavelet; scalogram

Address
Uk Jung: Production and Operations Division, School of Business, Dongguk University-Seoul, Republic of Korea
Bong-Hwan Koh: Department of Mechanical, Robotics, and Energy Engineering, Dongguk University-Seoul,
30 Pildong-ro, 1 gil Jung-gu, Seoul 100-715, Republic of Korea


Abstract
In this paper, a simple two-step method for structural vibration-based health monitoring for beam-like structures have been extended to plate-like structures with though-thickness cracks. Crack locations and severities of plate-like structures are detected using a hybrid approach. The interval wavelet transform is employed to extract crack singularity locations from mode shape and support vector regression (SVR) is applied to predict crack serviettes form crack severity detection database (the relationship of natural frequencies and crack serviettes) using several natural frequencies as inputs. Of particular interest is the natural frequencies estimation for cracked plate-like structures using Rayleigh quotient. Only the natural frequencies and mode shapes of intact structures are needed to calculate the natural frequencies of cracked plate-like structures using a simple formula. The crack severity detection database can be easily obtained with this formula. The hybrid method is investigated using numerical simulation and its validity of the usage of interval wavelet transform and SVR are addressed.

Key Words
plate-like structures; Rayleigh quotient; interval wavelet transform; natural frequency estimation; crack detection; support vector regression

Address
Jiawei Xiang, Yongying Jiang and Haifeng Gao: College of Mechanical & Electrical Engineering, Wenzhou University, Wenzhou, 325035, China
Jiawei Xiang, Udo Nackenhorst: Institute of Mechanics and Computational Mechanics, Leibniz Universität Hannover,
30167 Hannover, Germany
Yanxue Wang : School of Mechanical and Electrical Engineering, Guilin University of Electronic Technology, Guilin, 541004, China
Yumin He: College of Mechanical and Electrical Engineering, Xi\'an University of Architecture and Technology, 710055, Xi\'an, shaanxi, 710055, China

Abstract
Within this paper a new approach for early damage detection in rotor blades of wind energy converters is presented, which is shown to have a more sensitive reaction to damage than eigenfrequency-based methods. The new approach is based on the extension of Gasch\' s proportionality method, according to which maximum oscillation velocity and maximum stress are proportional by a factor, which describes the dynamic behavior of the structure. A change in the proportionality factor can be used as damage indicator. In addition, a novel deflection sensor was developed, which was specifically designed for use in wind turbine rotor blades. This deflection sensor was used during the experimental tests conducted for the measurement of the blade deflection. The method was applied on numerical models for different damage cases and damage extents. Additionally, the method and the sensing concept were applied on a real 50.8 m blade during a fatigue test in the edgewise direction. During the test, a damage of 1.5 m length was induced on the upper trailing edge bondline. Both the initial damage and the increase of its length were successfully detected by the decrease of the proportionality factor. This decrease coincided significantly with the decrease of the factor calculated from the numerical analyses.

Key Words
structural health monitoring; condition monitoring; wind turbine rotor blades

Address
Stephan Zerbst, Stavroula Tsiapoki and Raimund Rolfes: Institute of Structural Analysis, Leibniz Universität Hannover, Appelstr. 9A, Hanover, Germany

Abstract
The objective in this research is to determine the feasibility of using changes on the dynamic properties of a reinforced concrete (RC) structure to identify different levels of seismic induced damage. Damping ratio and natural frequency changes in a RC bridge column are analyzed using different signal processing techniques like Hilbert Transforms, Random Decrement and Wavelet Transforms. The data used in the analysis was recorded during a full-scale RC bridge column shake table test. The structure was subjected to ten earthquake excitations that induced different levels of inelastic demand on the column. In addition, low-intensity white noises were applied to the column in-between earthquakes. The results obtained show that the use of the damping ratio and natural frequency of vibration as damage indicators is arguable.

Key Words
damping ratio; natural frequency; reinforced concrete; bridge column; random decrement technique; wavelets; Hilbert Transform

Address
Diego A. Aguirre: Department of Civil Engineering and Surveying, University of Puerto Rico at Mayagüez,
Mayagüez PR 00681-9000, Puerto Rico
Luis A. Montejo: Department of Engineering Science and Materials, University of Puerto Rico at Mayagüez
Mayagüez PR 00681-9000, Puerto Rico


Abstract
As a kind of intelligent materials, conductive asphalt concrete has a broad application prospect including melting ice and snow on the pavement, closing cracks in asphalt concrete, sensing pavement damage, and so on. Conductive pavement will be suffered from fatigue failure as conventional pavement in the process of service, and this fatigue damage of internal structure can be induced by electrical signal output. The characteristics of electrical signal variation of conductive asphalt concrete in the process of fatigue cracking were researched in this paper. The whole process was clearly divided into three stages according to resistance changes, and the development of fatigue damage wasn\'t obvious in stage I and stage II, while in stage III, the synchronicity between the resistance and damage began to appear. Thus, fatigue damage variable D and initial damage value D0 represented by the functions of resistance were introduced in stage III. After calculating the initial damage value D0 under different stress levels, it was concluded that the initial damage value D0 had no noticeable change, just ranged between 0.24 and 0.25. This value represented a critical point which could be used to inform the repair time of early fatigue damage in the conductive asphalt pavement.

Key Words
conductive asphalt concrete; electrical signal; fatigue cracking; damage variable; initial damage value

Address
Qun Yang, Xu Li, Ping Wang and Hong-Wei Zhang :Key Laboratory of Road and Traffic Engineering of the Ministry of Education, Tongji University, Shanghai201804, China
Hong-Wei Zhang: Inner Mongolia Communications Construction Engineering Quality Supervision Bureau,Hohhot010020, China


Abstract
This paper illustrates the damages of reinforced concrete and masonry minarets during October 23 (Erciş) and November 9 (Edremit), 2011 Van earthquakes in Turkey. Erciş and Edremit are townships located 90km and 18km from Van city center in Turkey, respectively. Ground accelerations and response spectrums for these earthquakes are given in this paper. A total of 63 reinforced concrete and masonry minarets are heavily damaged or collapsed in the city center and villages nearby after both earthquakes. Because of the fact that there is no Turkish standard and specification directly related to design of minarets, nearly all of the constructions are carried out by workers using only their own technical knowledge. So, all of the non-engineering reinforced concrete and masonry minarets completely collapsed or damaged heavily. From the study, it is seen that the damages are due to several reasons such as site effect, location, and length of the fault, reduction in cross section and formation of the discontinuity, use of plain reinforcement steel, use of concrete with insufficient strength, existence of short lap splices and incorrect end hook angle, larger mass and stiffness concentrations on some region, longitudinal reinforcements discontinuity, cracks at the cylindrical body, and damage of spire and end ornament. In addition to these reasons, the two earthquakes hit the minarets within seventeen days, causing progressive damage. So, the existing design and construction practices should be improved to provide sufficient earthquake performance. Also, it is recommended that there should be a safe distance between the minaret and surrounding structures to reduce the loose of life after earthquake.

Key Words
damages; reinforced concrete and masonry minarets; Van earthquakes 2011 in Turkey

Address
Alemdar Bayraktar and Ahmet Can Altunişik: Karadeniz Technical University, Department of Civil Engineering, 61080, Trabzon, Turkey
Murat Muvafik: Yüzüncü Yil University, Department of Civil Engineering, 65080, Van, Turkey


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