JOINT DAMPING AND NONLINEARITY IN DYNAMICS OF SPACE STRUCTURES

The presence of joints can strongly affect the dynamics of space structures in weightlessness, especially if the joints are numerous, of low stiffness, or nonlinear. Analysis of the effect of linear joint characteristics on the vibration of a free-free, three-joint beam model shows that increasing joint damping increases resonant frequencies and increases modal damping, but only to the point at which the joint gets "locked up" by damping. This behavior is different from that predicted by modeling joint damping as proportional damping. The maximum amount of passive modal damping obtainable from the joints is greater for low-stiffness joints and for modal vibrations where large numbers of joints are actively participating. A joint participation factor is defined to study this phenomenon. Analysis of the nonlinear three-joint model, with cubic springs at the joints, shows classical single-degree-of-freedom nonlinear response behavior at each resonance of the multiple-degreeof- freedom: nondoubling of response for a doubling of forcing amplitude, multiple solutions, jump behavior, and resonant frequency shifts. These properties can be concisely quantified by characteristics backbone curves, which show the locus of resonant peaks for increasing forcing amplitude. Modal coupling due to joint nonlinearity is also exhibited. Nonlinear effects are emphasized, as damping effects were, when joint activity is high, such as for low-stiffness joints, for high-amplitude vibrations, and for modes with a high joint participation factor. © 1990 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.