The active control of vibrations of composite beams by parametric stiffness modulation

The paper addresses an active control of the resonant vibrations of composi te beams performed by a parametric stiffness modulation. A sandwich beam co mposition with the continuous core is considered. The stiffness modulation is introduced by some fairly small changes in an orientation of elements of the microstructure of a core ply. The controlled vibrations are those of t he dominantly flexural type excited by a transverse force acting at a low r esonant frequency, whereas the stiffness modulation is performed at a compa ratively high frequency identified by the resonance of a mode of the domina ntly shear type. This difference in time scales of the controlled vibration s and the input signal facilitates a use of the method of direct partition of motion that predicts an existence of the modal interaction between the l ow-frequency and the high-frequency motions due to so-called vibrational fo rces. It is shown that such a parametric control can provide a significant favourable shift of the first eigenfrequency of a controlled beam (the one subjected to the stiffness modulation) from its nominal value for an uncont rolled beam. Heavy fluid loading conditions are accounted for as well as ma terial losses in a structure. Then instead of analysis of eigenfrequencies, a problem of forced vibrations is posed and the forced frequency-amplitude response is analysed. It is demonstrated that although heavy fluid loading reduces resonant frequencies of forced vibrations, the suggested mechanism of control remains valid in these cases. (C) 2000 Editions scientifiques e t medicales Elsevier SAS.