A study of main and secondary resonances in non-linear multi-degree-of-freedom vibrating systems

A uniform study of all types of resonances that can occur in non-linear, dissipative multi-degree-of-freedom systems subject to sinusoidal excitation is presented. The theoretical investigation is based on a harmonic or multi-harmonic solution and the Ritz method. The new approach suggests that non-linear normal mode shape or the so-called “coupled” non-linear mode shapes are those which are retained in resonance conditions, no matter what type of resonancemain, or secondary, periodic or almost-periodic.

By introducing the concept of non-linear normal coordinates the response of an n-degree-of-freedom system is described, to a satisfactory degree of accuracy, by a single coordinate in the case of main or secondary-periodic resonance, or by p coordinates in the case of almost-periodic (combination) resonance with p harmonic components.

Numerical examples indicate good agreement of theoretical and analog computer results and illustrate considerable discrepancies between resonance curves calculated by the commonly used “single linear mode approach” and the suggested “single non-linear mode approach”.