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CONTENTS
Volume 25, Number 3, March 2020
 

Abstract
Recently, the concept of"drive-by" bridge monitoring system using indirect measurements from a passing vehicle to extract key parameters of a bridge has been rapidly developed. As one of the most key parameters of a bridge, the natural frequency has been successfully extracted theoretically and in practice using indirect measurements. The frequency of bridge is generally calculated applying Fast Fourier Transform (FFT) directly. However, it has been demonstrated that with the increase in vehicle velocity, the estimated frequency resolution of FFT will be very low causing a great extracted error. Moreover, because of the low frequency resolution, it is hard to detect the frequency drop caused by any damages or degradation of the bridge structural integrity. This paper will introduce a new technique of bridge frequency extraction based on Hilbert Transform (HT) that is not restricted to frequency resolution and can, therefore, improve identification accuracy. In this paper, deriving from the vehicle response, the closed-form solution associated with bridge frequency removing the effect of vehicle velocity is discussed in the analy tical study. Then a numerical Vehicle-Bridge Interaction (VBI) model with a quarter car model is adopted to demonstrate the proposedapproach. Finally, factors that affect the proposed approach are studied, including vehicle velocity, signal noise, and road roughness profile.

Key Words
Hilbert Transform; bridge frequency; drive-by bridge inspection; bridge health monitoring; non-destructive evaluation

Address
(1) Chengjun Tan:
Civil engineering Department, Hunan University, Changsha 41000, Hunan, China;
(2) Nasim Uddin:
Civil & Environmental Engineering Department, The University of Alabama at Birmingham, 1075 13th St S, Birmingham, AL 35205 USA.

Abstract
Electrical current is usually used to change the damping force of Magnetorheological Dampers (MRDs). However, changing the electrical current could shift the stiffness of the system, the phenomenon that was not considered carefully. This study aims to evaluate this shift. A typical MRD was designed, optimized, and fabricated to do some accurate and detailed experimental tests to examine the stiffness variation. The damper is equipped with a circulating system to prevent the deposition of particles when it is at rest. Besides that, a vibration setup was developed for the experimental study. It is capable of generating vibration with either constant frequency or frequency sweep and measure the amplitude of vibration. The damper was tested by the vibrating setup, and it was concluded that with a change in electrical current from 0 to 1.4 A, resonant frequency would change from 13.8 Hz to 16 Hz. Considering the unchanging mass of 85.1 kg, the change in resonant frequency translates as a shift in stiffness, which changes from 640 kN/m to 860 kN/m.

Key Words
magnetorheological damper; stiffness shift; magnetorheological fluid; magnetorheological valve

Address
Department of Mechanical Engineering, Tarbiat Modares University, Jalal Ale Ahmad, No. 7, Iran.


Abstract
Long term structural health monitoring has gained wide attention among civil engineers in recent years due to the scale and severity of infrastructure deterioration. Establishing effective damage indicators and proposing enhanced monitoring methods are of great interests to the engineering practices. In the case of bridge health monitoring, long term structural vibration measurement has been acknowledged to be quite useful and utilized in the planning of maintenance works. Previous researches are majorly concentrated on linear time series models for the measurement, whereas nonlinear dependences among the measurement are not carefully considered. In this paper, a new bridge health monitoring method is proposed based on the use of long term vibration measurement. A combination of the fundamental ARMA model and copula theory is investigated for the first time in detecting bridge structural damages. The concept is applied to a real engineering practice in Japan. The efficiency and accuracy of the copula based damage indicator is analyzed and compared in different window sizes. The performance of the copula based indicator is discussed based on the damage detection rate between the intact structural condition and the damaged structural condition.

Key Words
structural health monitoring; copula; long term assessment; bridge structure; ARMA model

Address
(1) Yi Zhang:
School of Civil Engineering, Tsinghua University, Beijing 100084, China;
(2) Chul-Woo Kim, Lian Zhang:
Department of Civil and Earth Resources Engineering, Kyoto University, Kyoto 615-8540, Japan;
(3) Yongtao Bai:
College of Civil Engineering, Chongqing University, Chongqing, China;
(4) Hao Yang, Xiangyang Xu:
Geodetic Institute, Faculty of Civil Engineering and Geodetic Science, Leibniz University Hanover, Germany;
(5) Zhenhao Zhang:
School of Civil Engineering, Changsha University of Science and Technology, Changsha, China.


Abstract
The laser ultrasonic technique is gaining popularity for nondestructive evaluation (NDE) applications because it is a noncontact and couplant-free method and can inspect a target from a remote distance. For the conventional laser ultrasonic techniques, a pulsed laser is often used to generate broadband ultrasonic waves in a target structure. However, for crack detection using nonlinear ultrasonic modulation, it is necessary to generate narrowband ultrasonic waves. In this study, a pulsed laser is shaped into dual-line arrays using a spatial mask and used to simultaneously excite narrowband ultrasonic waves in the target structure at two distinct frequencies. Nonlinear ultrasonic modulation will occur between the two input frequencies when they encounter a fatigue crack existing in the target structure. Then, a nonlinear damage index (DI) is defined as a function of the magnitude of the modulation components and computed over the target structure by taking advantage of laser scanning. Finally, the fatigue crack is detected and localized by visualizing the nonlinear DI over the target structure. Numerical simulations and experimental tests are performed to examine the possibility of generating narrowband ultrasonic waves using the spatial mask. The performance of the proposed fatigue crack localization technique is validated by conducting an experiment with aluminum plates containing real fatigue cracks.

Key Words
fatigue crack localization; laser scanning; laser ultrasonic; narrowband excitation; nonlinear ultrasonic modulation; spatial mask

Address
Department of Civil and Environmental Engineering, Korean Advanced Institute for Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea.


Abstract
This paper studies the influence of parameters of a novel SMA helical spring energy dissipation brace on the seismic resistance of a frame structure. The force-displacement relationship of the SMA springs is established mathematically based on a multilinear constitutive model of the SMA material. Four SMA helical springs are fabricated, and the forcedisplacement relationship curves of the SMA springs are obtained via tension tests. A numerical dynamic model of a twofloor frame with spring energy dissipation braces is constructed and evaluated via vibration table tests. Then, two spring parameters, namely, the ratio of the helical spring diameter to the wire diameter and the pre-stretch length, are selected to investigate their influences on the seismic responses of the frame structure. The simulation results demonstrate that the optimal ratio of the helical spring diameter to the wire diameter can be found to minimize the absolute acceleration and the relative displacement of the frame structure. Meanwhile, if the pre-stretch length is assigned a suitable value, excellent vibration reduction performance can be realized. Compared with the frame structure without braces, the frames with spring braces exhibit highly satisfactory seismic resistance performance under various earthquake waves. However, it is necessary to select an SMA spring with optimal parameters for realizing optimal vibration reduction performance.

Key Words
dissipation brace; Shape memory alloy(SMA); frame structure; parametric study; vibration test; seismic response

Address
School of Civil Engineering and Architecture, Wuhan University of Technology, 122 Luoshi Road, Hongshan District, Wuhan, Hubei Province, China.


Abstract
Magnetorheological elastomer (MRE), a smart material, is an innovative material for base isolation system. It has magnetorheological (MR) effect that can control the stiffness in real-time. In this paper, a new hybrid base isolation system combining two electromagnetic closed circuits and the roller bearing is proposed. In the proposed system, the roller part can support the vertical load. Thus, the MRE part is free from the vertical load and can exhibit the maximum MR effect. The MRE magnetic loop is constructed in the free space of the roller bearing and forms a strong magnetic field. To demonstrate the performance of the proposed hybrid base isolation system, dynamic characteristic tests and performance evaluation were carried out. Dynamic characteristic tests were performed under the extensive range of strain of the MRE and the change of the applied current. Performance evaluation was carried out using the hybrid simulation under five earthquakes (i.e., El Centro, Kobe, Hachinohe, Northridge, and Loma Prieta). Especially, semi-active fuzzy control algorithm was applied and compared with passive type. From the performance evaluation, the comparison shows that the new hybrid base isolation system using fuzzy control algorithm is superior to passive type in reducing the acceleration and displacement responses of a target structure.

Key Words
Magnetorheological Elastomer (MRE); Magnetorheological (MR) effect; hybrid base isolation system; semiactive control; hybrid simulation

Address
(1) Yongmoon Hwang, Junghoon Lee, Hyung-Jo Jung:
Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea;
(2) Chan Woo Lee:
Department of Civil and Environmental Engineering, Korea Army Academy at Yeong-cheon, Yeong-cheon 38900, Republic of Korea.

Abstract
In this paper, we focused on the self-healing concrete using new nano-capsules. Three types of nano-capsules with respect to availability, high strength and temperature tolerance are used; type 1 is URF and polyethylene (PE) as shell and nano titanium oxide (TiO2) as core, type 2 is URF and PE as shell and nano silica oxide (SiO2) as core, type 3 is PE as shell and nano silica oxide (SiO2) as core. The concrete samples mixed by nano-capsules with three percents of 0.5, 1 and 1.5. Based on experimental tests and the compressive strength of samples, the URF-PE-SiO2 is selected for additional tests of compressive strength before and after recovery, ultrasonic test, ion chlorine and water penetration depths. After careful investigation, it is concluded that the optimum value of URF-PE-SiO2 nano-capsules is 0.5% since leads to higher compressive strength, ultrasonic test, ion chlorine and water penetration depths.

Key Words
self-healing concrete; nano-capsules; compressive strength; ultrasonic test; ion chlorine and water penetration depths

Address
(1) Mojtaba Naseri Taheri, Seyyed Ali Sabet:
Department of Civil Engineering, Kashan Branch, Islamic Azad University, Kashan, Iran;
(2) Reza Kolahchi:
Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam.

Abstract
The present work proposes a self-controlled multi-stage optimization method for damage identification of structures utilizing standard particle swarm optimization (PSO) algorithm. Damage identification problem is formulated as an inverse optimization problem where damage severity in each element of the structure is considered as optimization variables. An efficient objective function is formed using the first few frequencies and mode shapes of the structure. This objective function is minimized by a self-controlled multi-stage strategy to identify and quantify the damage extent of the structural members. In the first stage, standard PSO is utilized to get an initial solution to the problem. Subsequently, the algorithm identifies the most damage-prone elements of the structure using an adaptable threshold value of damage severity. These identified elements are included in the search space of the standard PSO at the next stage. Thus, the algorithm reduces the dimension of the search space and subsequently increases the accuracy of damage prediction with a considerable reduction in computational cost. The efficiency of the proposed method is investigated and compared with available results through three numerical examples considering both with and without noise. The obtained results demonstrate the accuracy of the present method can accurately estimate the location and severity of multi-damage cases in the structural systems with less computational cost.

Key Words
damage identification; self-controlled multi-stage optimization; particle swarm optimization; mode shape and natural frequency; inverse problem

Address
(1) Subhajit Das, Nirjhar Dhang:
Department of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, West Bengal, India;
(2) Subhajit Das:
CSIR-Structural Engineering Research Centre, Chennai, Tamil Nadu 600113, India.

Abstract
A wavefield sparse reconstruction technique based on compressed sensing is developed in this work to dramatically reduce the number of measurements. Firstly, a severely underdetermined representation of guided wavefield at a snapshot is established in the spatial domain. Secondly, an optimal compressed sensing model of guided wavefield sparse reconstruction is established based on l1-norm penalty, where a suite of discrete cosine functions is selected as the dictionary to promote the sparsity. The regular, random and jittered undersampling schemes are compared and selected as the undersampling matrix of compressed sensing. Thirdly, a gradient projection method is employed to solve the compressed sensing model of wavefield sparse reconstruction from highly incomplete measurements. Finally, experiments with different excitation frequencies are conducted on an aluminum plate to verify the effectiveness of the proposed sparse reconstruction method, where a scanning laser Doppler vibrometer as the true benchmark is used to measure the original wavefield in a given inspection region. Experiments demonstrate that the missing wavefield data can be accurately reconstructed from less than 12% of the original measurements; The reconstruction accuracy of the jittered undersampling scheme is slightly higher than that of the random undersampling scheme in high probability, but the regular undersampling scheme fails to reconstruct the wavefield image; A quantified mapping relationship between the sparsity ratio and the recovery error over a special interval is established with respect to statistical modeling and analysis.

Key Words
guided wavefield; compressed sensing; sparse measurements; laser Doppler vibrometer; full-field reconstruction; piezoelectric structural health monitoring

Address
(1) Baijie Qiao, Xuefeng Chen:
The State Key Laboratory for Manufacturing Systems Engineering, Xi\'an, 710061, PR China.
(2) Baijie Qiao, Zhu Mao, Songmao Chen:
Department of Mechanical Engineering, University of Massachusetts Lowell, Lowell, MA 01854, USA;
(3) Hao Sun:
Department of Civil Engineering, Northeastern University, Boston, MA 02115, USA.

Abstract
In this paper, a new passive lossless snubber circuit with energy transfer capability is proposed. The proposed lossless snubber circuit provides Zero-Current Switching (ZCS) condition for turn-on instants and Zero-Voltage Switching (ZVS) condition for turn-off instants. In addition, its diodes operate under soft switching condition. Therefore, no significant switching losses occur in the converter. Since the energy of the snubber circuit is transferred to the output, there are no significant conduction losses. The proposed snubber circuit can be applied on isolated and non-isolated converters. To verify the operation of the snubber circuit, a boost converter using the proposed snubber is implemented at 70W. Also, the measured conducted Efficiency Electromagnetic Interference (EMI) of the proposed boost converter and conventional ones are presented which show the effects of proposed snubber on EMI reduction. The experimental results confirm the presented theoretical analysis.

Key Words
lossless passive snubber; proposed converter; soft switching; zero-current switching (ZCS); zero-voltage switching (ZVS); Efficiency electromagnetic interference (EMI)

Address
(1) Shabnam Nasr Esfahani, Mohhamad Bagher Tavakoli:
Department of Electrical Engineering, Arak Branch, Islamic Azad University, Arak, Iran;
(2) Majid Delshad:
Department of Electrical Engineering, Isfahan(Khorasgan) Branch, Islamic Azad University, Isfahan, Iran.


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