ON THE AMPLITUDE DYNAMICS AND CRISIS IN RESONANT MOTION OF STRETCHED STRINGS

An N-mode truncation of the equations governing the resonantly forced nonlinear motions of a stretched string is studied. The external forcing is restricted to a plane, and is harmonic with the frequency near a linear natural frequency of the string. The method of averaging is used to investigate the weakly nonlinear dynamics. By using the amplitude equations, which are a function of the damping and the frequency of excitation. it is shown that to O(e), only the resonantly forced mode has non-zero amplitude. Both planar (i.e. lying in the planar of forcing) and non-planar constant solutions are studied and amplitude-frequency curves are determined. For small enough damping, solutions in the non-planar branch become unstable via a Hopf bifurcation and give rise to a branch of periodic solutions in the amplitude-frequency plane. This branch exhibits several period-doubling bifurcations, but does not directly result in the formation of a chaotic attractor. At lower values of damping, many other branches of periodic solutions exist. A series of bifurcations leads to the formation of chaotic attractors in some of these solution branches. Various types of interactions between the different solutions are found to result in many interesting phenomena including, the formation of a homoclinic orbit and chaos quenching. These results are discussed in the context of the Sil'nikov mechanism near a homoclinic orbit for a saddle-focus. Results from the investigations of the averaged system are interpreted for the truncated string system using the averaging theory and the theory of integral manifolds. Numerical investigations with the single mode truncation of the non-autonomous string system show that there is a good correspondence even between chaotic solutions of the averaged system and those of the original system.