Dynamic testing of an inflatable, self-supporting, unpressurized thin-film torus

Dynamic testing of an inflatable thin-film torus structure with regular formed convex dome surface features has been performed to evaluate structural natural frequencies, mode shapes, and modal damping ratios. The structure presented two unique challenges with respect to modal testing. First, it is extremely lightweight, flexible, and highly damped. Second, the thin film that provides the integrity to the structure is not smooth and flat, but has a pattern of hexagonal dome structures formed into it that increases the local stiffness of the thin film to the point that the structure is self-supporting in the gravity environment of Earth when the internal pressure is released. The dynamic testing was performed in this self-supporting state. In the modal test a loudspeaker provided acoustic excitation, and a laser displacement sensor was used to measure the vibration response at various points on the torus surface. The acoustic excitatipn and the laser displacement measuring technique were chosen because they are all noncontact methods that avoid mass loading of the structure as would happen if accelerometers and a shaker were used. Three in-plane and three out-of-plane modes were extracted using this approach. The experimental results indicate that the noncontact modal data-acquisition approach for extremely lightweight structures is suitable and effective. The modal identification procedure found modes that are analogous to elastic ring modes as well as widely spaced repeated modes.