The two simple mechanical models, i.e. pendulum and two-well potential system, introduced the Readers to fundamental theoretical concepts and essential chaotic phenomena that arise in nonlinear, dissipative oscillators, driven by periodic force. In both systems, the principal resonance curves were bowed to the left, towards the lower frequency, that is, their nonlinearity had a softening property. One may ask: what about oscillators with a hardening type of restoring force characteristics? Indeed, the oscillators with hardening elastic nonlinearity may also exhibit chaotic motion. Recall that Ueda s strange attractors were first found in the Duffing system with this type of nonlinearity.

However, the systems with hardening type elastic nonlinearity are not as useful in the task of brief introduction to the chaotic dynamics as those with softening nonlinearity. The problem is that, in the classical Duffing system, the strange attractors discovered by Ueda appear in very small regions of system parameters. It would be hard to reveal such fundamental phenomena as routes to chaos or crisis of chaotic attractors, even in a very precise computer experiment.

Since the discovery of the chaotic phenomena in the dissipative forced nonlinear oscillators was made with the use of an analog computer, the researchers faced an essential problem: did the irregular solutions occur only in the computational simulations, or did they really exist in the physical world around us? No wonder that, at the next stage of investigations, attention was focused on physical experiments. In the field of mechanics, it was the experiment performed by F.Moon...