Pressure Stiffened Modal Correlation of a Cylindrical Pressure Vessel

Abstract

Acoustoelastic structures are a complex dynamic system that exhibit modal coupling between a structure and its enclosed acoustic fluid. Common structures that exhibit this phenomenon in the aerospace industry are pressure vessels such as a solid rocket boosters or advanced solid rocket motors. When a structural analyst simulates the structural modes of these pressure vessels with shell finite elements, they often apply an internal pressure force in their model to represent the acoustic fluid. In a model free of boundary conditions, the application of this internal pressure force produces modal results that ground some rigid body modes, often going from six to three zero-frequency modes. Therefore, since the finite element modal simulation inappropriately grounds the structure and thus is unable to accurately predict the rigid body modes, it calls into question whether the elastic or flexible modes predicted by the same modal analysis are accurate. This paper presents an experimental study to address this question by designing, analyzing, fabricating, and testing a simple cylindrical pressure vessel. A free-free steel cylindrical pressure vessel was modeled with finite elements and modal tested with and without pressure. Modal tap testing was used to extract the structural response and compute frequency response functions which were compared to analytical results to discern the accuracy of the predicted elastic modes of the pressure vessel.