Issue 37

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 37 (2016) 193-199; DOI: 10.3221/IGF-ESIS.37.25 194 materials and structures using one unified parameter in the form of the plastic and creep SIF’s, which take into account both in-plane and out-of-plane constraint effects at fracture. The present work provide an appropriate theoretical and numerical investigations based on the limited experimental data substantially assist fracture mechanics technology in application especially to part-through surface cracks fatigue life predictions for rotating components of power steam turbines. The paper also concentrates on the residual life assessment aspect in greater detail of nonlinear fracture mechanics using power steam turbine disc as a case study. S UBJECT FOR STUDY AND MATERIAL PROPERTIES otor in power steam turbine experience both cyclic and sustained centrifugal and thermal loads due to the nature of the rotor operating cycles. The methodology describing in the present study is applied to a 100 MW steam turbine rotor shown in Fig.1a. The turbine disc is loaded, in general, by thermal and mechanical stresses because it is operated at c.a. 550ºC and 3000 rpm. Subject for analysis is the disk of 22nd stage turbine rotor with central bore and through-thickness key (Fig.1b). Operation damage in the form of corner crack with length on the free surface a =12 mm and depth along shaft thickness b = 23mm was detected in the slot fillet of key in the disc of 22nd stage (Fig.1c). This stage is operated at about 140°C therefore the influence of temperature is not take into account. For the lifetime calculations, the following operating data were assumed: total operation time is 236607 hours, number of starts is 204. The material of turbine disc is Steel 34ХН3МА which main mechanical properties are listed in Tab. 1. a) b) c) Figure 1 : 100 MW steam turbine rotor and disk of 22nd stage with operation damage. Material Young modulus E, (MPa) Poisson’s ratio  Yield stress σ 0 , (MPa) Ultimate stress σ f , (MPa) Strain hardening exponent n Strain hardening coefficient α Steel 34ХН3МА 196363 0.3 790 992 7.49 2.39 Table 1 : Main mechanical properties. E LASTIC - PLASTIC STRESS FIELDS IN TURBINE DISK rior to lifetime predictions, a 3D FE model of the 22nd stage turbine disc section generated was available and used for the purpose of this study. As shown in Fig.2, the turbine disc and blades, as well as the rivets incorporated in the orignal 3D model. The startup power conditions was chosen in stress analysis as it represents the most severe combination of temperature and rotor speed in the operation profile. Due to the planting disc onto the turbine shaft with a tightness, on the inner surface of the disc bore occur radial stresses with the magnitude of 50MPa, which are also taken into account in the calculations. R P