The propellant tank, which account for about 80 percent of the structural weight of the space launch vehicle, is designed and manufactured in a thin walled cylindrical shell structure to improve performance of launch vehicle and reduce manufacturing cost. The propellant tank structure is vulnerable to buckling due to compression load in axial direction because of the long length in the axial direction and its thin skin. Buckling strength of the structure is used as a major indicator in the design of the lightweight space launch vehicle structure. Geometric imperfection in the structure of the propellant tank and the internal pressure of the propellant tank affect the buckling stability of the structure. Therefore, geometric imperfection and internal pressure of the structure should be considered in the design of propellant tank.
In this thesis, a buckling experiment was performed to understand the effects of geometric imperfections and internal pressure in the metallic pressure vessel which is scale-down propellant tank structure. Geometric imperfections were considered in two aspects: the geometric initial imperfections of pressure vessel and artificially assigned imperfections. In order to measure the geometric initial imperfections, an external shape measuring device was made, and the geometric initial imperfection of the specimen was measured. The results of the measurement confirmed the slope of the specimen and confirmed that uniform geometric initial imperfection were distributed throughout the specimen.
Buckling experiment was performed after applying artificial imperfection and internal pressure to the specimen which was measure the geometric initial imperfections, and the corresponding buckling behavior of the specimen was investigated.
Nonlinear buckling analysis using SPLA (Single Perturbation Load Approach) was performed for the metallic cylinder model without geometric initial imperfections and the metallic cylinder model with geometric initial imperfections based on the results of external shape measurement results to observe the buckling behavior of the finite element model and compare with the results of buckling experiment.
By comparing the results of the buckling experiment and finite element analysis, it was confirmed that the effect of the geometric imperfections was reduced when applying internal pressure. The nondimensional displacement were decreased as internal pressure was applied.
As the size of artificial imperfections increases, the global buckling load was converged. In general, the convergent perturbation load of a model with a geometrically perfect initial shape is defined as the minimum perturbation load or design load of structure. In this study, the perturbation load, which was converged as the results of the buckling experiment, is proposed as the damage tolerance load, and the buckling knockdown factor between the minimum perturbation load and the damage tolerance load as the design tolerance range.