Papers by Author: Sang Oh Park

Abstract: We carried out experiments to detect impact locations on a composite plate using two types of composite plates, a composite flat plate with a constant thickness of 5 mm and a composite stiffened panel with stringers. Four multiplexed FBG sensors were attached to the bottom surface of the composite plates to acquire impact signals. The FBG sensor wavelength shift data were collected at a sampling frequency of 40 kHz using a high-speed FBG interrogator (SFI-710, Fiberpro Inc., Korea). The arrival times of the impact signals at each FBG sensor were obtained using a signal processing procedure. The arrival times were affected by noise level and signal-to-noise ratio. In order to overcome this weakness, signal processing techniques such as wavelet decomposition, normalization using each noise level and filtering with a moving average were adopted. To calculate the impact locations of the composite plate, a neural network algorithm was applied.

Abstract: Most accelerometers are composed of a mass, a spring, and a damper. The quantification of the acceleration could be obtained from the equation of dynamic motion expressed in terms of the relative moving displacement. Therefore, if the moving displacement of a seismic mass is inferred from the output signal of a single-degree-of-freedom (SDOF) system, the acceleration could theoretically be obtained by a deformation response factor expressed in the relation between relative displacement and excited acceleration. A reflective grating panel-optical fiber sensor can measure the relative displacement by the movement of the grating panel that is attached to the seismic mass. Ultimately, this research is intended for application to grating panel-optical fiber sensors. However, in order to fabricate the adequate probe for this sensing principle, microscopic rotation (< 0.007 degree) should be prevented for stable reflectivity. Therefore, this paper describes the optimum design of the mass-spring structure which features a larger linear motion range of the leaf spring and no rotation of the seismic mass. Accordingly, finite element analyses (FEA) were accomplished for reflecting the grating panel-optical fiber sensor probe. The leaf spring was employed for the strict linear motion, and some parametric studies on the design of the leaf spring was conducted. First of all, in order to have flexibility in the leaf spring, the leaf spring is divided into N equal parts (N= 3, 4, 5). Furthermore, some parametric studies were carried out, including the length of the seismic mass, arm width, and length of the leaf spring, as well as the moment balancing design. Through the comparisons of each FEA result, the best design of the leaf spring was determined to obtain the optimum features over the measurement range of acceleration and frequency within the linear elastic of the leaf spring.

Abstract: Composite pressure tanks are rapidly expanding in their range of use. However, for high pressure flammable or toxic gases, there is less confidence about their reliability. In this study, fiber Bragg grating (FBG) sensors embedded into a composite pressure tank monitored strain and detected damage. The sensor heads and fiber optic lines were protected with an acrylate recoating, adhesive films, PVC tubes and Teflon
film to survive under the harsh environment of the filament winding process. During the quasi-static loading test, the internal strain was measured with embedded FBG sensor arrays. In order to detect damage occurred during the test, impacts were applied on three different positions in each side by an impact hammer. The difference between damaged and undamaged sides in impact response was analyzed. An interrogation system using a wavelength-swept fiber laser (WSFL) was used for measuring strain, and an erbium-doped fiber amplifier (EDFA) laser source with a tunable Fabry-Perot filter was used for measuring impact response.