Cheol-Won Kong

Postbuckling Strength of Composite Plate with a Hole

Buckling and postbuckling behaviors were analyzed numerically and experimentally for composite plates with a hole. In the finite element analysis, the updated Lagrangian formulation and the eight-node degenerated shell element were used. For the progressive failure analysis, the maximum stress criterion was applied to the average stress in each layer of all the finite elements and then stiffness and stress corresponding to the failure mode were reduced to zero for the failed layers of all the elements. In the experiments, the shadow Moiré technique was used to monitor the out-of-plane deformation. PVDF sensors were used to detect the events of internal damage. The major finding was that when a plate has low bending stiffness in the axial direction, the local instabilities caused a change of mode shapes in postbuckling range. Experiments showed good agreement with the finite element results in the buckling load and the postbuckling strength. The influence of hole sizes and stacking sequences was investigated on the compression behavior of the plate. The postbuckling strength was dominated by the value of the bending stiffness in the axial direction.

Experimental strength of composite sandwich panels with cores made of aluminum honeycomb and foam

Composite sandwich panels with face sheets made of carbon/epoxy fabric and carbon/epoxy uni-directional prepreg were tested. The sandwich cores made by aluminum honeycomb and foam were cured with the face sheet at 180 °C. The aluminum core sandwich failed at the 40% failure strain of the face skin in the compression test. On the other hand, the foam core sandwich failed at the 100% failure strain of the face skin in the compression test. In the case of the aluminium core sandwich, nonhomogenous load distributions and fiber contents may affect the lower failure strain. Therefore, the specific compression strength of the foam core was 72% higher than the compression strength of the aluminum core. Failure mode analysis of sandwich panels made of aluminium honeycomb cores shows that shear failure leads to final failure. However, the specific flexural strength of the sandwich panels with aluminum cores had a slightly higher value than that of the sandwich panels with foam cores. Because the sandwich panel with aluminium cores had higher critical stress for the face wrinkling than the sandwich panel with foam cores due to higher compression modulus of aluminium cores than foam cores.

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.

Postbuckling Strength of Stiffened Composite Plates with Impact Damage

The buckling and postbuckling strengths for unstiffened and stiffened composite plates with various impact damages were studied numerically and experimentally. To predict the residual strength, a progressive failure model with the degradation of elastic moduli in the damaged regions was adopted in a nonlinear finite element method. The progressive failure analysis used the maximum stress criterion and the complete unloading model. The degradation ratio in each sublaminate was determined by delamination distributions through the thickness of laminates. The ply-by-ply delamination was detected by an improved ultrasonic technique. The predicted buckling and postbuckling strength showed good agreement with experimental results. The correlation between the impact damage and the postbuckling strength is discussed. The delamination growth was also observed for I-stiffened composite plates under the cyclic compressive load.

Development of a Numerical Model for an Expanding Tube with Linear Explosive Using AUTODYN

Pyrotechnic devices have been employed in satellite launch vehicle missions, generally for the separation of structural subsystems such as stage and satellite separation. Expanding tubes are linear explosives enclosed by an oval steel tube and have been widely used for pyrotechnic joint separation systems. A numerical model is proposed for the prediction of the proper load of an expanding tube using a nonlinear dynamic analysis code, AUTODYN 2D and 3D. To compute a proper core load, numerical models of the open-ended steel tube and mild detonating tube encasing a high explosive were developed and compared with experimental results. 2D and 3D computational results showed good correlation with ballistic test results. The model will provide more flexibility in expanding tube design, leading to economic benefits in the overall expanding tube development procedure.

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).

Fully Noncontact Wave Propagation Imaging in an Immersed Metallic Plate with a Crack

This study presents a noncontact sensing technique with ultrasonic wave propagation imaging algorithm, for damage visualization of liquid-immersed structures. An aluminum plate specimen (400 mm × 400 mm × 3 mm) with a 12 mm slit was immersed in water and in glycerin. A 532 nm Q-switched continuous wave laser is used at an energy level of 1.2 mJ to scan an area of 100 mm × 100 mm. A laser Doppler vibrometer is used as a noncontact ultrasonic sensor, which measures guided wave displacement at a fixed point. The tests are performed with two different cases of specimen: without water and filled with water and with glycerin. Lamb wave dispersion curves for the respective cases are calculated, to investigate the velocity-frequency relationship of each wave mode. Experimental propagation velocities of Lamb waves for different cases are compared with the theoretical dispersion curves. This study shows that the dispersion and attenuation of the Lamb wave is affected by the surrounding liquid, and the comparative experimental results are presented to verify it. In addition, it is demonstrated that the developed fully noncontact ultrasonic propagation imaging system is capable of damage sizing in submerged structures.

Testing and Analysis of Downscaled Composite Wing Box

The fabrication, bending test, and analysis of a downscaled composite wing box were presented. Before the fabrication of the downscaled composite wing box, an I-stiffened composite plate, which was made by the same layup method as the skin of the wing box, was tested and analyzed. The graphite/epoxy wing box consisted of two cocured parts. One part had a lower skin and two I stiffeners. The other part had an upper skin, two I stiffeners, and two spars. The mold forthecocuring ofthecompositewing box and bending testmachine of the wing box were

Structural test of KSLV-I Payload fairing

Payload fairing(PLF) protects satellites and related equipment from the external environment. They are separated before the satellite separation. Payload fairing made of composite sandwich materials due to their considerable bending stiffness and strength-to-weight ratio. Payload fairing have compression, shear and bending load during the flight. In this study, To check the strength of PLF and connected part, structural test of PLF accomplished using an actuator and a fixture. Purpose of structural test is to verify the strength of PLF in force of separation spring and combination structural load applied. Test result shows that the PLF have an acceptable margin of safety for the combination structural load and force of separation spring.