Chung-Bang Yun

Smart wireless sensing and assessment for civil infrastructure

Recently, there has been increasing need for adopting smart sensing technologies to structural health monitoring (SHM) applications for civil infrastructure. In this paper, the state of the art in smart wireless sensing and assessment techniques for civil structures are reviewed focusing on full-scale applications. Three types of smart wireless sensing technologies are discussed: wireless acceleration sensor-based SHM, wireless impedance-based SHM and an optics-based non-contact actuation and sensing technique. At first, vibration-based SHM using a dense array of wireless acceleration sensors is implemented to a cable-stayed bridge. The modal identification of the bridge and cable tension estimation are carried out using the ambient acceleration data. Measured data during a typhoon is also discussed. Secondly, impedance-based SHM using piezoelectric active sensors is presented focusing on hardware and software issues. A wireless impedance sensor node is presented for local SHM and neural network-based smart assessment algorithm is proposed to detect multi-type damages. Finally, a wireless power and data transmission method using laser and optoelectronic technologies is presented for non-contact measurement of guided waves and impedance, and subsequent damage detection. This method is embodied in a small printed circuit board, and the performance is validated on a lab-scale steel truss member.

Impedance-based Long-term Structural Health Monitoring for Tidal Current Power Plant Structure in Noisy Environments

In structural health monitoring (SHM) using electro-mechanical impedance signatures, it is a critical issue for extremely large structures to extract the best damage diagnosis results, while minimizing unknown environmental effects, including temperature, humidity, and acoustic vibration. If the impedance signatures fluctuate because of these factors, these fluctuations should be eliminated because they might hide the characteristics of the host structural damages. This paper presents a long-term SHM technique under an unknown noisy environment for tidal current power plant structures. The obtained impedance signatures contained significant variations during the measurements, especially in the audio frequency range. To eliminate these variations, a continuous principal component analysis was applied, and the results were compared with the conventional approach using the RMSD (Root Mean Square Deviation) and CC (Cross-correlation Coefficient) damage indices. Finally, it was found that this approach could be effectively used for long-term SHM in noisy environments.

Evaluation of Cable Tension Forces Using Vibration Method for a Cable-stayed Bridge under Construction

When a cable-stayed bridge is under construction, the cable tension that changes according to the construction phase is the index indicating the proper construction management. In this study, the vibration method using the least-square estimation has been implemented to monitor changing tensions of two multi-strand cables of a cable-stayed bridge under construction. The test bridge is Hwamyung Bridge in Korea with a prestressed concrete box girder. The field tests are executed during the second tensioning stage just after the installation of the key segment. The tensions of two cables are measured before and after the tensioning and 5 days later (i.e., after finishing the tensioning of all cables). The accuracy of the estimated tensions by the vibration method has been improved by employing proper effective lengths of the cables. The measured tensions are compared with the result of the lift-off tests and design tensions. The vibration method shows very good performance in monitoring the changing tensions according to the construction phase with minimal error.

Automated wireless monitoring system for cable tension using smart sensors

Cables are critical load carrying members of cable-stayed bridges; monitoring tension forces of the cables provides valuable information for SHM of the cable-stayed bridges. Monitoring systems for the cable tension can be efficiently realized using wireless smart sensors in conjunction with vibration-based cable tension estimation approaches. This study develops an automated cable tension monitoring system using MEMSIC’s Imote2 smart sensors. An embedded data processing strategy is implemented on the Imote2-based wireless sensor network to calculate cable tensions using a vibration-based method, significantly reducing the wireless data transmission and associated power consumption. The autonomous operation of the monitoring system is achieved by AutoMonitor, a high-level coordinator application provided by the Illinois SHM Project Services Toolsuite. The monitoring system also features power harvesting enabled by solar panels attached to each sensor node and AutoMonitor for charging control. The proposed wireless system has been deployed on the Jindo Bridge, a cable-stayed bridge located in South Korea. Tension forces are autonomously monitored for 12 cables in the east, land side of the bridge, proving the validity and potential of the presented tension monitoring system for real-world applications.

SPECTRAL ENERGY TRANSMISSION METHOD FOR CARCK DEPTH ESTIMATION IN CONCRETE

Surface cracks in concrete are common defects that can cause significant deterioration and failure of concrete structures. Therefore, the early detection, assessment, and repair of the cracks in concrete are very important for the structural health. Among studies for crack depth assessment, self-calibrating surface wave transmission method seems to be a promising nondestructive technique, though it is still difficult in determination of the crack depth due to the variation of the experimentally obtained transmission functions. In this paper, the spectral energy transmission method is proposed for the crack depth estimation in concrete structures. To verify this method, an experimental study was carried out on a concrete slab with various surface-opening crack depths. Finally, effectiveness of the proposed method is validated by comparing the conventional time-of-flight and cutting frequency based methods. The results show an excellent potential as a practical and reliable in-situ nondestructive method for the crack depth estimation in concrete structures.