MS thesis abstract - Ingham, Michel
| Author: | Ingham, Michel |
| Degree: | Masters of Science |
| SERC #: | 2-98 |
| File type: | PDF, 6859 kB |
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Microdynamics and Thermal Snap Response of Deployable Space Structures
Due to the size constraints imposed by the payload bays of carrier spacecraft, future precision space structures (e.g. interferometric telescopes) will undoubtedly require some form of on-orbit deployment mechanism, including joints or hinges which will introduce nonlinearity to the structure. Results are presented from a two-part experimental investigation of the microdynamic response of nonlinear structures, to both mechanical and thermal excitation sources. In the first experiment, the dynamic response of a deployable truss at sub-microstrain levels of vibration is characterized in terms of modal parameters. The test article is subjected to steppedsine sweeps through its fundamental flexible modes over a range of excitation amplitudes. Highsensitivity piezoceramic strain sensors are used in conjunction with a lock-in amplifier to measure the truss response from tens of microstrain down to one nanostrain. The natural frequency and damping ratio are computed from the frequency response functions, using a circle fit method. Results show that the values of the modal parameters are strain-dependent at high response amplitudes, and strain-independent at low amplitudes. It is inferred that, at microdynamic levels of excitation, the internal loads needed to overcome the joint friction are not attained. The nonlinear mechanisms in the structure are thus not activated, resulting in a linear truss response. In the second experiment, the phenomenon of thermal snap, or creak, is investigated. Thermal snap is a disturbance which occurs when thermally-induced stress in a statically indeterminate structure is suddenly released via a slip internal to a joint or other frictional mechanism. A representative deployable truss is suspended in a thermal chamber, where its temperature is cycled between -30°C and 50°C, in order to determine whether thermal snap occurs in such a structure. High-bandwidth accelerometers distributed across the truss are used as the primary sensors for detecting structural events. Thermal snaps are found to occur during the thermal transients, before steady-state is achieved throughout the truss. The truss response to the impulsive and broadband disturbances is characterized in both the time and frequency domains. The transient response exhibits telltale signs of structural behavior, including multi-mode or dominant-mode excitation, and reasonable modal damping in the time decay.