(Sepcial) Advances in Health Monitoring and Sensing Technologies for Aerospace System - II
Chair: Prof. Gunjin YUN (Seoul National University)
Effects of the Bonding Length on the Reflected Spectra and Strain Measurement of the Surface Bonded Fiber Bragg Grating Sensor Mr. Hyunseok KWON, Yurim PARK, Pratik SHRESTHA and Chun-Gon KIM (KAIST)
Since fiber Bragg grating (FBG) sensor has a high applicability, it is being widely used in structural health monitoring (SHM) research. There are two ways to install the FBG sensor to a sensing point. One is to bond the sensor to the surface of the structure and another is to embed the sensor into the composite material. Since the surface bonding method is relatively easy and convenient, it is preferred in many researches. However, surface bonded FBG sensors have limitation of signal characteristics being affected by the bonding layer. Previously conducted studies about this limitation used different types of optical fiber sensor making absolute comparisons difficult. Since the FBG sensor uses the reflected spectrum to measure the change in physical quantity, it is necessary to confirm how the adhesive length affects the reflection spectrum. Therefore, in this study, the effects of the bonding length on the reflected spectrum and strain measurement of the surface bonded FBG sensor were experimentally investigated to determine the effective bonding length of the FBG sensor. Acrylate coated FBG sensors having 10mm grating length were used in the experiments. FBG sensors were attached to the surface of the specimen using epoxy adhesives by different bonding lengths. Tensile loads were applied to the sensor installed specimen and reflected spectra and strains were measured. From the experimental results, it was found that reflected spectra of the surface bonded FBG sensor were distorted when it was bonded with short length. As a result of the distortion, strain measurements using distorted reflected spectra showed large errors.
Impact Localization on Composite Wing Using a Single FBG Sensor and Error Outlier Based Impact Localization Algorithm Dr. Pratik SHRESTHA, Yurim PARK, Hyunseok KWON and Chun-Gon KIM (KAIST)
Low velocity impact on composite structure can result in occurrence of barely visible impact damage (BVID) which are difficult to detect. Aircraft structures are vulnerable to such BVID because of low velocity impact due to runway debris, bird strike, tool drop, etc. Therefore, low velocity impact monitoring of composite structure is highly desirable for impact detection and localization. Additionally, SHM of large-scale structures with fewer numbers of sensors is desirable to reduce the complexity involved with the data-acquisition and processing. In this paper, feasibility of localizing low velocity impact on composite structure using a single sensor was investigated. Three FBG sensors were attached on the surface of the full-scale composite wing and the signal from each of the FBG sensor was processed using the error outlier based impact localization algorithm to predict the location of the impact. The localization results demonstrated the feasibility of localizing impact on composite structure using a single FBG sensor; overall, the impacts were localized with average error of about 26.6 mm.
Development of Scanning Free-space Measurement Setup and Its Application to Structural Imaging Mr. Hasan AHMED, Jongmin HYUN, Jung-Ryul LEE and Won-Jun LEE (KAIST, Agency for Defense Development)
This paper purposes a scanning free-space measurement (SFSM) setup for the evaluation of microwave absorption abilities of a structure. The system works by illuminating the specimen with microwave signals of varying frequencies and measuring the power of reflected microwaves. The reflected power is then divided by the emitted power in order to convert the measurements to the standard parameter known as S11. The system comprises of a vector network analyzer (VNA) to measure S11 parameter, a focused horn antenna attached to the VNA for transmitting/receiving microwave signals, a dual-axis automated translation stage for raster scanning of the specimen and a standard personal computer. A graphical user interface (GUI) running on the computer manages the configuration and synchronization of the VNA and the stage system, S11 reception from VNA and compilation of results for display to the user. This GUI is created in C++ using QT framework and is designed with a minimalistic approach to promote usability and adaptability sans intricacies of actual system operation.