Nonlinearly coupled vibrations of thin-walled elastic beams of open section

An account of certain subharmonic vibrations as observed during a resonant testing of thin-walled beams of monosymmetric open section for coupled torsional and bending vibrations is presented. The phenomenon can be described in terms of the vibrational modes of the beam. When the beam is excited at the resonant frequency of a higher mode, there is a tendency for the lowest mode to be excited, resulting in a high-order subharmonic oscillation. It is found that when such phenomenon occurs, the high-mode frequency is a multiple or near multiple of the fundamental frequency of the beam. Under such condition, the response of the beam consists of a superposition of the response of the high mode (harmonic response) and that of the fundamental mode (subharmonic response). The amplitude of the subharmonic motion is generally much larger than that of the harmonic response.