Effects of thermal transients on cracked shaft vibrations

Thermal transients can cause significant changes in the dynamic behavior of cracked rotors. The thermal expansions of the shafts cause changes of the distribution of stresses and strains, whose effects can give rise to the separation or the contact between portions of the surfaces of transverse cracks. This phenomenon can cause significant changes of the local flexural stiffness of the rotor, in the area close to the cracked section, and of the shaft lateral vibrations. However, this phenomenon must not be confused with the crack breathing, that is the periodic opening and closure of a transverse crack, caused by the machine weight, which occurs over a complete revolution of horizontal shafts. This paper is focused on the study of the effects of thermal transients on cracked shaft vibrations. With regard to this, the results obtained by the analysis of the experimental behavior of a cracked steam turbine are shown and discussed.