Sang-Wuk Park

Damage Analysis of a Type 3 Cryogenic Propellant Tank After LN2 Storage Test

The application of composites to cryotanks has been one of the major concerns for lightweight launch vehicles. In this study, a prototype of a Type 3 cryotank was fabricated with the composite developed for cryogenic application and aluminum liner, and the cryogenic conditions were applied by filling the prototype with liquid nitrogen and then pressurizing it with gaseous nitrogen. During the experiment, delamination inside the cryotank happened. This article describes several attempts made to investigate failure through both analytical approach with thermo-elastic analysis accompanied by progressive failure and experimental approach with LN2 immersion of composite/aluminum ring specimens.

Impact monitoring in smart composites using stabilization-controlled FBG sensor system

Impact location monitoring is one of the major concerns of the smart health monitoring. For this application, multipoint ultrasonic sensors are to be employed. In this study, a multiplexed FBG sensor system with wide dynamic range was proposed and stabilization controlling system was also developed for the maintenance of maximum sensitivity of sensors. For the intensity demodulation system of FBG sensors, Fabry-Perot tunable filter (FP-TF) with 23.8 nm FSR (free spectral range) was used, which behaves as two separate filters between 1530 ~ 1560 nm range. Two FBG sensors were attached on the bottom side of the graphite/epoxy composite beam specimen, and low velocity impact tests were performed to detect the one-dimensional impact locations. Impact locations were calculated by the arrival time differences of the impact longitudinal waves acquired by the two FBGs. As a result, multiplexed in-line FBG sensors could detect the moment of impact precisely and found the impact locations with the average error of 1.32 mm.

Feedback controlled nano-positioner using fiber optic EFPI sensor with novel demodulation technique

For the translation stage of nanometer scale, fiber optic EFPI sensor is suggested for the feedback control system on account of its high sensitivity, small size, simple system and relatively low cost. The novel signal processing algorithm for the real-time demodulation of EFPI output signal was developed and verified. The local linearity in the adjacent fringe values was shown, and used for the sinusoidal approximation of the nonlinear output signal. The real-time signal processing program was designed and the intensity signal of the EFPI sensor was demodulated to the phase shift with this program. The theoretical resolution of 0.36~8.6 nm in the displacement range of 0~200 μm was obtained. The sensor system was applied to the 1-D nano-positioner with a Piezo-electric actuator. The positioner successfully reached to the desired destination within 1 nm accuracy.

In-situ Health Monitoring of Filament Wound Pressure Tanks using Embedded FBG Sensors

ABSTRACT In this research, in-situ structural health monitoring of filament wound pressure tanks were conducted during water-pressurizing test using embedded fiber Bragg grating (FBG) sensors. We need to monitor inner strains during working in order to verity the health condition of pressure tanks more accurately because finite element analyses on filament wound pressure tanks usually show large differences between inner and outer strains. Fiber optic sensors, especially FBG sensors can be easily embedded into the composite structures contrary to conventional electric strain gages (ESGs). In addition, many FBG sensors can be multiplexed in single optical fiber using wavelength division multiplexing (WDM) techniques. We fabricated a standard testing and evaluation bottle (STEB) with embedded FBG sensors and performed water-pressurizing test. In order to increase the survivability of FBG sensor during cure, we suggested a novel fabrication process for embedding FBG sensors into a filament wound pressure tank, which includes a new protecting technique of sensor heads, the grating parts. From the experimental results, it was demonstrated that FBG sensors can be successfully adapted to filament wound pressure tanks by embedding.

LN 2 storage test and damage analysis for a Type 3 cryogenic propellant tank

Nowadays, researches for replacing material systems for cryotanks by composites have been being performed for the purpose of lightweight launch vehicle. In this paper, a type 3 propellant tank, which is composed of the composite developed for cryogenic use and an aluminum liner, was fabricated and tested considering actual operating environment, that is, cryogenic temperature and pressure. For this aim, liquid nitrogen (LN2) was injected into the fabricated tank and in turn, gaseous nitrogen (GN2) was used for pressurization. During this test procedure, strains and temperatures on the tank surface were measured. The delamination between hoop layer and helical one, was detected during the experiment. Several attempts were followed to investigate the cause analytically and experimentally. Thermo-elastic analysis in consideration of the progressive failure was done to evaluate the failure index. Experimental approach through a LN2 immersion test of composite/aluminum ring specimens suitable for simulating the Type 3 tank structure.

Development of nano-positioner using fiber optic EFPI sensor

Precision displacement of less than a few nm resolution was measured in real-time using fiber optic EFPI sensor. The novel method for real-time processing of analyzing EFPI output signal was developed and verified. Linearity in the mean values of interferometric light intensity among adjacent fringes was shown and verified the sinusoidal approximation algorithm that estimates past and coming fringe values. Real-time signal processing program was developed and the intensity signal of the EFPI sensor was transformed to the phase shift with this program. The resolution below nm in the displacement range of was obtained. The nano-positioner with a piezoelectric actuator and the EFPI sensor system was designed and tested. The positioner successfully reached to the desired destination within 1 nm accuracy.