  Vibrations of the Jeffcott rotor are modelled by a three degree of freedom
system including coupling between lateral and torsional modes. The crack in a
rotating shaft of the rotor is introduced via time dependent stiffness with off
diagonal couplings. Applying the external torque to the system allows to
observe the effect of crack "breathing" and gain insight into the system. It is
manifested in the complex dynamic behaviour of the rotor in the region of
internal resonance, showing a quasi--periodic motion or even non-periodic
behaviour. In the present paper report, we show the system response to the
external torque excitation using nonlinear analysis tools such as bifurcation
diagram, phase portraits, Poincare maps and wavelet power spectrum. In the
region of resonance we study intermittent motions based on laminar phases
interrupted by a series nonlinear beats.
