Issue 37

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 37 (2016) 193-199; DOI: 10.3221/IGF-ESIS.37.25 193 Focussed on Multiaxial Fatigue and Fracture A plastic stress intensity factor approach to turbine disk structural integrity assessment V. Shlyannikov, A. Zakharov, R. Yarullin Kazan Scientific Center of Russian Academy of Sciences, Russia shlyannikov@mail.ru, alex.zakharov88@mail.ru , yarullin_r@mail.ru A BSTRACT . This study based on a new fracture mechanics parameter is concerned with assessing the integrity of cracked steam turbine disk which operate under startup-shutdown cyclic loading conditions. Damage accumulation and growth in service have occurred on the inner surface of slot fillet of key. In order to determine elastic-plastic fracture mechanics parameters full-size stress-strain state analysis of turbine disk was performed for a quote-elliptical part-through cracks under considering loading conditions. As a result distributions of elastic and plastic stress intensity factors along crack front in slot fillet of key of turbine disk depending on surface crack form are defined. An engineering approach to the prediction of carrying capacity of cracked turbine disk which is sensitive to the loading history at maintenance is proposed. The predictions of the rate of crack growth and residual lifetime of steam turbine disk are compared for elastic and elastic-plastic solutions. It is shown that the previously proposed elastic crack growth models provide overestimate the lifetime with respect to the present one. An advantage to use the plastic stress intensity factor to characterize the fracture resistance as the self-dependent unified parameter for a variety of turbine disk configurations rather than the magnitude of the elastic stress intensity factors alone is discussed. K EYWORDS . Structural integrity; Turbine disk; Plastic stress intensity factor; Quote-elliptical part-through crack. I NTRODUCTION n power steam turbine components there is the possible occurrence of undetected defects that can propagate at each startup-shutdown cycle, and sequence, damage accumulation and growth acceptance criteria have to be defined for the turbine critical zones. The successful lifetime prediction for power engineering turbines required application of fracture mechanics methodology with knowledge of loading history at operation, local stress/strain in concentration zones, static and fatigue material properties, the stress intensity factors for the appropriate crack geometry and crack growth rate characterization for the material. Accumulated experience with respect to turbine discs and blades based on the deterministic and probabilistic approaches has shown [1-6] that advance in fracture technology proceeds best through mutual interaction between analysis and experimental observations. Very few investigations devoted to the nonlinear fracture mechanics analysis exist which refer to structural integrity turbine disc assessment. The typical operation damages in such stress concentration zones are part-through thickness corner cracks. The most approaches to the corner crack growth prediction in the turbine disc under cyclic loading contains simplified models of stress-strain state in the nonlinear region at the crack tip. From this disadvantage is free a nonlinear stress intensity factors introduced by Shlyannikov et al. [7-10] for the conditions of plasticity and creep-fatigue interaction. These studies have focused on the background for the characterization of static and cyclic fracture resistance I