Some Special Effects Caused by Cracks

Transverse cracks affect not only bending behaviour of rotating shafts but have effects also on torsional and axial vibrations. Torsional and axial stiffness are reduced when the crack is open. These stiffness are 1X periodical when the crack is breathing and cause parametric excitation when torsion or axial loads are applied. Moreover also coupling between shear loads and torsion and vice versa between torsion loads and bending behaviour exists. These coupling effects develop with open crack and generally disappear with closed crack. In fact when the crack is open or partially open, then the resultant of the tangential stresses due to the applied torque and/or to shear forces is eccentric with respect to the rotor axis: this causes the coupling of bending and torsional behaviour. In addition the 1X opening and closing of the crack, imposed by the bending load, generates also axial 1X displacements.