Papers by Author: Chung Bang Yun

Abstract: In this paper, in-field ambient vibration testing of a highway bridge in South Korea under traffic loadings has been conducted to update its finite element model for future predictive analysis and diagnosis purpose. The research results presented in this paper are outcomes from an international REU (Research Experience for Undergraduates) program in smart structures funded by US-NSF (National Science Foundation) and hosted abroad by the Korean Advanced Institute of Science and Technology (KAIST). The monitoring, modeling, and model updating of civil infrastructures are vital in maintaining new design and maintenance standards. Using the frequency domain decomposition (FDD), experimental modal properties of the structure were found and, after a finite element model was created and updated based on the modal properties. From the results, it has been concluded that (a) the FDD method successfully identified the modal characteristics of the structure from ambient vibration, (b) that model updating improved the accuracy of the finite element model, (c) Representing the structural supports as springs in the FEM improved the results from the ideally supported model.

Abstract: In this study, a hybrid health monitoring technique that combines wireless sensing and embedded monitoring algorithms is proposed to realize the diagnosis of damage in PSC girder bridges. Firstly, a hybrid damage monitoring system that can alarm damage occurrence and classify damage-types is designed for PSC girder bridges. Secondly, smart sensor nodes that have wireless, stand-alone, sensing and monitoring capacities of acceleration and impedance are developed for hybrid health monitoring of the structures. Finally, the performance of the smart sensor nodes is evaluated using a laboratory-scale PSC girder bridge model for which acceleration and impedance signals were measured for prestress-loss and stiffness-loss cases.

Abstract: In bridge structures, damage may induce an additional deflection which may naturally contain essential information about the damage. However, inverse mapping from the damage-induced deflection to the actual damage location and severity is generally complex, particularly for statically indeterminate systems. In this paper, a new load concept, called the positive-bending-inspection-load (PBIL) is proposed to construct a simple inverse mapping from the damage-induced deflection to the actual damage location. A PBIL for an inspection region is defined as a load or a system of load which guarantees bending moment to be positive in the inspection region. From the theoretical investigations, it was proven that the damage-induced chord-wise deflection (DI-CD) has the maximum value with the abrupt change in its slope at the damage location under a PBIL. Hence, a novel damage localization method is proposed based on the DI-CD under a PBIL. Experimental verification study was also carried out on a two-span continuous steel box-girder model with a steel box-girder. It was found that the proposed method clearly identified the damage existence and damage location for small damage cases with narrow cuts at the bottom flange.

Abstract: This paper presents novel structural health monitoring techniques for critical members of civil structures using electro-mechanical impedance sensors. The basic concept of this technique is to monitor critical locations of a structure for changes in structural impedance that would indicate imminent damage. In this paper, principal hardware and software issues on this topic are reviewed. An active sensing node incorporating on-board microprocessor and radio frequency telemetry is introduced in a sense of tailoring wireless sensing technology to the impedance method. A data compression algorithm using a principal component analysis is embedded into the on-board chip of the active sensing node. Finally, a method for compensating the temperature effects on the impedance measurements using cross-correlation analysis with effective frequency shifts is presented.

Abstract: Many bridges are located in rural areas where conventional Internet services are not readily available. 3.5 generation mobile telecommunication technology, HSDPA (High Speed Downlink Packet Access), is currently commercialized in 55 countries to make the Internet access possible in anywhere the mobile phone service is available. In this study, a novel framework of remotely controllable SHM systems is proposed for bridges without the conventional Internet service utilizing HSDPA technology and remote desktop software. The proposed SHM system has benefits such as remote maintenance of SHM systems and a low cost network implementation. The feasibility of the proposed framework has been demonstrated by field tests on a concrete box-girder highway bridge in operation.

Abstract: A modified one-sided Rayleigh wave (R-wave) velocity measurement technique is applied to monitor the strength gain of early-age concrete. A series of experiments were performed on early-age concrete specimens with various curing conditions. The results show that the R-wave velocity and strength gain has a strong relationship and that the R-wave velocity can be effectively used to monitor the strength development of the early-age concrete.

Abstract: A modified one-sided measurement technique is proposed for Rayleigh wave (R-wave) velocity measurement in concrete. In this study, the maximum energy arrival concept is adopted to determine the wave velocity by employing its continuous wavelet transform. Experimental study was performed to show the effectiveness of the proposed method. Results reveal that the proposed method can be effectively used to measure the R-wave velocity in concrete structures.

Abstract: The integrity assessment method for beam-like structures using continuous wavelet transform (CWT) is proposed. CWT is applied to the response acceleration of a structure to decompose the response acceleration correspond to each scale. The curvature difference of the normalized energy for each member caused by structural degradation clearly indicated the location of damaged members in the structure. It is shown that the proposed method can identify damaged members without performing complicated dynamic analyses which usually require significant effort and time.

Abstract: This paper presents a feasibility study of an impedance-based damage detection technique using PZT (Lead-Zirconate-Titanate) patches for real-time health monitoring of concrete structures. The PZT patches are used to detect progressive surface damage on a plain concrete beam. Both experimental and analytical studies are carried out. For damage quantification, root-mean square deviations (RMSD) before and after damage are used as a damage indicator.

Abstract: This paper presents a non-destructive evaluation (NDE) technique for detecting damages on a jointed steel plate on the basis of the time of flight and wavelet coefficient, obtained from wavelet transforms of Lamb wave signals. Probabilistic neural networks (PNNs) and support vector machines (SVMs) were applied for pattern classification. In this study, the applicability of the PNNs and SVMs was investigated for the damages in and out of the Lamb wave path. It has been found that the present methods are very efficient in detecting the damages simulated by the loose bolts on the jointed steel plate.