Sang-Guk Kang

In-flight health monitoring of a subscale wing using a fiber Bragg grating sensor system

In this paper, fiber Bragg gratings (FBGs) were applied to measure dynamic strains inside a subscale wing under real-time wind tunnel testing. Two re-coated FBGs were embedded in the wing skin. The FBG sensor system includes a wavelength swept fiber laser with a wavelength indicator and fast signal processing modules. The agreement among the three kinds of sensor inside the subscale wing (FBG, electric strain gauge and PZT sensor) was confirmed in the bench test. The optical fiber strain sensors had an excellent resolution (< 5μe) in the time domain and could detect a frequency response up to 100 Hz. Through the wind tunnel test of the subscale smart wing, the flutter was experimentally detected using FBG sensors and their usefulness as an in-flight health monitoring system was demonstrated.

Flexural Stiffness Control of Multilayered Beams

This paper focuses on the ability to introduce change in the flexural bending stiffness of a niultilayered beam. The multilayered beam comprises a base layer with polymer layers on the upper and lower surfaces and stiff cover layers. The flexural stiffness can be reduced by effecting a reduction in the shear modulus of the polymer layers by heating through glass transition. Stiffer polymer layers strongly couple the cover layers to the base beam and the entire multilayered beam bends more as an integral unit On heating, a reduction in the shear modulus of the polymer layer results in its undergoing shear deformation as the base beam undergoes flexural bending and results in the cover layers decoupling from the base beam. This reduces the overall flexural bending stiffness. A finite element analysis is developed for the multilayered beam, and after experimentally verifying its ability to predict change in flexural bending deflection under load with a change in the polymer-layer shear modulus, it is used to conduct parametric studies. The results of the parametric studies provide broad insights into how the achievable change in flexural bending stiffness with a change in the polymer-layer modulus varies with design parameters such as the modulus and thickness of both the cover layers and the polymer layers. Changes in flexural bending stiffness by a factor of over 70 for a clamped-free beam and by a factor of over 130 for a pinned-pinned beam were observed for certain configuration designs.

Thermo Elastic Analysis of a Type 3 Cryogenic Tank Considering Curing Temperature and Autofrettage Pressure

In this study, effects of curing temperature and autofrettage pressure on a Type 3 cryogenic propellant tank, which is composed of composite hoop/helical layers and a metal liner, were investigated by thermo elastic analysis using finite element method. The temperature field of a Type 3 tank was obtained by solving the heat transfer problem and, in turn, was used as the nodal temperature boundary conditions during the elastic analyses for curing temperature and autofrettage pressure effects. Progressive failure analysis was also introduced in this procedure. As a result, it was shown that the higher the curing temperature was, the more residual compressive stress and tensile stress were induced in composites and metal liner, respectively. On the contrary, autofrettage pressure brought the reduction of these residual thermal stresses caused by cryogenic environments to the tank structure and these trends were verified from the composite/aluminum ring specimen tests at cryogenic temperature, which is suitable for preliminary tests of filament wound structures. This tradeoff for curing temperature and autoprettage pressure must be considered in the design and manufacturing stages for a Type 3 cryogenic tank.

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.

Tensile Properties of Carbon Fiber Composites with Different Resin Compositions at Cryogenic Temperatures

In this study, the tensile properties of carbon fiber reinforced polymer (CFRP) composites with different resin compositions were investigated in order to develop advanced composite materials for cryogenic use. Thermo-mechanical cyclic loading (up to 6 cycles) was applied to CFRP unidirectional laminate specimens from room temperature to –150°C. Tensile tests were then performed at –150°C using an environmental test chamber. In addition, matrix-dominant properties such as the transverse and in-plane shear characteristics of each composite model were measured at –150°C to examine the effects of resin formulation on their interfacial properties. The tensile tests showed that the composite models with large amounts of bisphenol-A epoxy and CTBN modified rubber in their resin composition had good mechanical performance at cryogenic temperatures (CTs).

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.