Damping characteristics of composite petal structure for an 8-m diameter telescope at cryogenic temperature

摘要

Concerns have been raised in the engineering community about the potentially extremely low levels of structural damping in structures at cryogenic temperatures. Experiments conducted on material coupons have shown that material damping at those temperatures can be orders of magnitude lower than that at room temperature. Whether structural damping in built-up structures at those temperatures can be that low is unknown, but if it was, the telescope resonances could exacerbate microdynamics originating from the structure itself and residual vibrations propagating from the instrument module to the telescope. Since the effect of those vibrations might not be compensated for optically, the observatory might not meet its wavefront and jitter error budgets. The structural damping characteristics of built-up structures in the micrometer to nanometer regime and at cryogenic temperatures are to a large extent unknown. Characterization on structures traceable to future flight designs is therefore necessary to develop an understanding of these characteristics, as well as devise means to mitigate those effects. To address those concerns and to reduce the technical risks in these areas, Lockheed Martin tested the dynamics characteristics of its Single Petal Testbed (SPT) flight-like petal structure at full-scale, from room temperature down to -175C (98K). The SPT was designed by the Lockheed Martin Advanced Technology Center and fabricated by Programmed Composites Inc. Significant changes in dynamics characteristics with temperature were observed, but primarily in mode shapes as opposed to modal frequencies and modal dampings. The modal damping remained fairly constant throughout the temperature range and, to the extent changes could be detected, the trends were more towards an increase than a decrease in damping at 98K, which was highly unexpected. A detailed analysis of these results extracted from dynamics tests conducted during the cool down portion of the last thermal cycle is presented in this report. The levels of damping observed in the built-up petal structure are 10 to 20 times higher than those measured by Marie Levine at JPL on all-composite coupons of the petal panels provided by Lockheed Martin.