1. Introduction and purpose

For reasons of performance and practical implementation, dynamic shaft lines are not modeled by 3D finite elements but by beam elements, which naturally represent the slenderness of the rotors and which, in linear elasticity, approximates the real behavior of the system at a low cost.

However, it is desired to take into account in the calculation of the dynamics of the shaft lines the impact of a crack on the rotor. While 3D calculation, with fine modeling of crack behavior by a contact algorithm, is conceptually feasible, it is extremely demanding in terms of computer resources. This is why the behavior of the crack is modelled on the usual tree line beam model by creating two nodes on the axis of the cracked rotor at the right of the crack and by introducing a non-linear « law of crack behavior » linking the average rotations of the two nodes representing the lips of the crack to the bending moments applied to the beam.

The law of crack behavior is tabulated from 3D calculations. It is shown that it is not necessary to cover the entire field of \((\mathit{My},\mathit{Mz})\) torques but that a calculation is sufficient for each angle of the bending moment with respect to the crack.