PSI - Issue 82

Marko Katinić et al. / Procedia Structural Integrity 82 (2026) 220 – 226 M. Katinić, P. Konjatić/ Structural Integrity Procedia 00 (2026) 000–000

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2. Short description of the collar fracture The scheduled shutdown of the turbine was in September 1998. The turbine was restarted in October 1998. After about 40 days of normal operation, the turbine had to be shut down due to a sudden increase in axial displacement of the rotor from 0.4 mm to 0.58 mm and increased noise in the front bearing housing. After cooling the turbine, the front bearing housing was opened and a sever fracture of the thrust collar was observed with the naked eye. The control of the thrust collar was carried out with a dye penetrant test, which confirmed the fracture of the collar (see Fig. 1). It is clearly visible that the fracture occurred at the radius of the shaft-collar transition along the entire circumference and on both sides of the collar.

Fig. 1. Fracture appearance as obtained by penetrant testing.

After removing the rotor from the turbine casing, an ultrasonic inspection was performed to determine the size and position of the observed crack. The largest estimated depth of the crack in the rotor shaft was 18 mm. 3. Root cause analysis of the collar fracture The thrust collar was an integral part of the turbine rotor shaft, as can be seen in Fig. 2 (Technical documentation (1998)). It can be seen that the shaft-collar transition radii were only 0.5 mm. As is well known, the transition radius represents a stress raiser and the smaller the radius, the higher the stress at the root of the radius.

Fig. 2. Cross-sectional drawing of the original thrust collar.

Numerical stress analysis was performed in the area of these shaft-collar transition radii. The analysis was performed using the finite element method (FEM) implemented in the specialized program Ansys 2023 R1. Due to the axial symmetry of the geometry and loading, only a quarter of the shaft with the collar was modeled. The material properties of 21CrMoV5-11 were associated with the geometry of the model. The geometry of the model was discretized with a coarse mesh of 10-node higher order 3D finite elements type SOLID187. In order to obtain the most