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CONTENTS
Volume 8, Number 3, September 2011
 

Abstract
In this study, the performances of a passive tuned mass damper (TMD) and a semi-active TMD (STMD) were evaluated in terms of seismic response control of elastic and inelastic structures under seismic loads. First, elastic displacement spectra were obtained for damped structures with a passive TMD and with a STMD proposed in this study. The displacement spectra confirmed that the STMD provided much better control performance than passive TMD and the STMD had less stroke requirement. Also, the robustness of the TMD was evaluated by off-tuning the frequency of the TMD to that of the structure. Finally, numerical analyses were conducted for an inelastic structure of hysteresis described by the Bouc-Wen model. The results indicated that the performance of the passive TMD whose design parameters were optimized for an elastic structure considerably deteriorated when the hysteretic portion of the structural responses increased, and that the STMD showed about 15-40% more response reduction than the TMD.

Key Words
semi-active TMD; inelastic structure; displacement spectra; stroke; robustness; Bouc-Wen Model.

Address
Sung-Sik Woo, Sang-Hyun Lee and Lan Chung : Department of Architectural Engineering, Dankook University, 126, Yongin-city, Korea

Abstract
In this paper, the harmonic displacement response of a damaged square plate with all-over partthrough damage parallel to one edge is utilized as the input signal function in wavelet analysis. The method requires the properties of the damaged plate, i.e., no information about the original undamaged structure is required. The location of damage is identified by sudden changes in the spatial variation of transformed response. The incurred damage causes a change in the stiffness or mass of the plate. This causes a localized singularity which can be identified by a wavelet analysis of the displacement response. In this study via numerical examples shown by using harmonic response is more versatile and effective compared with the static deflection response, specially in the presence of noise. In the light of the obtained results, suggestions for future work are presented and discussed.

Key Words
damage detection; Wavelet Transform; harmonic excitation.

Address
S.B. Beheshti-Aval and M. Taherinasab : Faculty of Civil Engineering, Earthquake Department, K.N.Toosi University of Technology, Tehran, Iran
M. Noori:College of Engineering, California Polytechnic State University, San Luis Obispo, California, USA

Abstract
This is a review paper on mathematical modeling of actively controlled piezo smart structures. Paper has four sections to discuss the techniques to: (i) write the equations of motion (ii) implement sensoractuator design (iii) model real life environmental effects and, (iv) control structural vibrations. In section (i), methods of writing equations of motion using equilibrium relations, Hamilton

Key Words
smart structure; piezoelectric materials; mathematical modeling; active vibration control; review.

Address
Vivek Gupta and Nagesh Thakur : Department of Physics, Himachal Pradesh University, Shimla-171005, India
Manu Sharma : Mechanical Engineering Branch, UIET, Panjab University, Chandigarh -160025, India

Abstract
Comparing to active damage monitoring, impact localization on composite by using time reversal focusing method has several difficulties. First, the transfer function of the actuator-sensor path is difficult to be obtained because of the limitation that no impact experiment is permitted to perform on the real structure and the difficulty to model it because the performance of real aircraft composite is much more complicated comparing to metal structure. Second, the position of impact is unknown and can not be controlled as the excitation signal used in the active monitoring. This makes it not applicable to compare the difference between the excitation and the focused signal. Another difficulty is that impact signal is frequency broadband, giving rise to the difficulty to process virtual synthesis because of the highly dispersion nature of frequency broadband Lamb wave in plate-like structure. Aiming at developing a practical method for on-line localization of impact on aircraft composite structure which can take advantage of time reversal focusing and does not rely on the transfer function, a PZT sensor array based phase synthesis time reversal impact imaging method is proposed. The complex Shannon wavelet transform is presented to extract the frequency narrow-band signals from the impact responded signals of PZT sensors. A phase synthesis process of the frequency narrow-band signals is implemented to search the time reversal focusing position on the structure which represents the impact position. Evaluation experiments on a carbon fiber composite structure show that the proposed method realizes the impact imaging and localization with an error less than 1.5 cm. Discussion of the influence of velocity errors and measurement noise is also given in detail.

Key Words
structural health monitoring; time reversal; phase synthesis; impact imaging; composite structure; complex Shannon wavelet transform.

Address
Lei Qiu and Shenfang Yuan : The Aeronautic Key Lab for Smart Materials and Structures, Nanjing University of
Aeronautics and Astronautics, Nanjing, P.R China

Abstract
Acoustic emission (AE) monitoring was conducted for mortar specimens under three types of static loading patterns (cubic-splitting, direct-shear and pull-out). Each of the applied loading patterns was expected to produce a particular fracture process. Subsequently, the AEs generated by various fracture or damage processes carried specific information on temporal micro-crack behaviors of concrete for post analysis, which was represented in the form of detected AE signal characteristics. Among various available characteristics of acquired AE signals, frequency content was of great interest. In this study, cement-based piezoelectric sensor (as AE transducer) and home-programmed DEcLIN monitoring system were utilized for AE monitoring on mortar. The cement-based piezoelectric sensor demonstrated enhanced sensitivity and broad frequency domain response range after being embedded into mortar specimens. This broad band characteristic of cement-based piezoelectric sensor in frequency domain response benefited the analysis of frequency content of AE. Various evaluation methods were introduced and employed to clarify the variation characteristics of AE frequency content in each test. It was found that the variation behaviors of AE frequency content exhibited a close relationship with the applied loading processes during the tests.

Key Words
mortar; acoustic emission; cement-based piezoelectric sensor; frequency domain; energy; nondestructive testing.

Address
Youyuan Lu and Zongjin Li: Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology,Clear Water Bay, Hong Kong


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