The effect of helicoidal cracks on the behaviour of rotating shafts

Fatigue cracks can develop in rotating shafts: their propagation is mainly in planes perpendicular to the axis of rotation, but also slant or helicoidal cracks could appear in industrial machine shafts. In this paper the static elastic behaviour of a shaft affected by a slightly helicoidal crack has been analysed by means of 3D finite element non-linear models, calculating deflections in different load conditions. Differences with respect to shafts affected by planar cracks of same depth and extension are emphasized. In order to simulate the dynamic behaviour of rotating shafts, a simplified model of the elastic behaviour of the cracked shaft has been developed, validated and substituted to the cumbersome 3D model. This model has then been introduced in the model of a power plant turbogroup shaft line in order to analyse its behaviour. The results show the additional vibrations caused by the crack which could be measured in different operating conditions of the group.