Transverse crack modelling and validation in rotor systems including thermal effects

In this paper a model is described that allows to simulate the static behaviour of a transversal crack in a horizontal rotor, under the action of the weight and other possible static loads and the dynamical behaviour of the rotating cracked shaft. The crack “breaths”, i.e. the mechanism of opening and closing of the crack is ruled by the stress acting on the cracked section due to the external loads; in a rotor the stress are time depending with a period equal to the period of rotation, thus the crack “periodically breaths”. An original simplified model is described that allows cracks of different shape to be modelled and thermal stresses to be taken into account, since they may influence the opening and closing mechanism. The proposed method has been validated using two criteria. At first the crack “breathing” mechanism, simulated with the model has been compared with the results obtained by a non-linear 3D FEM calculation and a good agreement in the results has been observed. Then, the proposed model allows the development of the equivalent cracked beam. The results of this model are compared with those obtained by the above said 3D FEM. Also in this case, there is a good agreement in the results. Therefore the proposed crack model and equivalent beam model can be inserted in the finite beam element model used for the rotor dynamical behavior simulation: the obtained equations have time depending coefficients, but they can be integrated in the frequency domain by using the harmonic balance method. The model is suitable for finite beam elements with 6 degrees of freedom per node, in order to account also for torsion vibrations and coupling between torsion and flexural vibrations.