A small defect under the skin in the welded conjunction of two rotor bodies of a steam turbine suggested to analyze the possibility of crack propagation during the different operating conditions of the turbine. Stresses and strains in the location of the defect at rated load and during start up transients as well as during a cooling transient from rated conditions have been calculated using a 2D fem model exploiting axial symmetry in order to check the stresses that could propagate the crack according to mode I (axial stresses) or according to mode II (shear stresses).
The geometry of the crack has been introduced in the 3D model of the rotor, the mesh in the surrounding of the crack has been consistently refined and the stresses have been recalculated in the most un-favorable thermal situation This way it was possible to evaluate numerically the stress intensity factors both for axial and shear stresses, that are used for evaluating the propagation velocity combining mode I and mode II.
Finally, assuming that the crack had propagated radially and circumferentially to reach the skin of the rotor, that means assuming a transverse crack with a relative depth of 3.5% the vibrational behavior of the cracked shaft has been calculated with an appropriate methodology, in order to check the shaft vibrations which could be measured in correspondence of the bearings by the monitoring system of the turbo-group.