Mixed Mode Fracture Mechanics and Crack Growth Analysis in Steam Turbine Discs for Remaining Life Assessment under Thermal Stresses and Mechanical Loading
Abstract
The long‑term performance and structural integrity of steam‑turbine discs are strongly influenced by crack‑growth behavior under combined thermal stresses and mechanical loading. In this study, a comprehensive analytical framework based on finite element analysis (FEA), mixed‑mode fracture‑mechanics evaluation (Modes I, II, and III), and fatigue crack‑growth modeling using the Paris law is presented. The results indicate that Mode II plays the dominant role in the stress‑intensity distribution and crack‑growth behavior of the investigated turbine disc, while Modes I and III also contribute and cannot be neglected. Therefore, safety assessment based solely on Mode I analysis may lead to inaccurate evaluations of structural integrity. The calculated effective stress‑intensity factor was compared with the fracture toughness of 12Cr steel, showing that the turbine disc currently operates within a safe margin against instantaneous fracture; however, gradual fatigue crack growth under thermo‑mechanical loading conditions remains possible. The crack‑growth analysis further demonstrates that a significant portion of the fatigue life is spent during the stable slow‑growth phase as the crack propagates from small initial sizes to intermediate lengths. The proposed fracture‑mechanics‑based approach provides a reliable tool for condition monitoring, inspection planning, and remaining‑life assessment of steam‑turbine discs, thereby contributing to improved operational safety and maintenance decision‑making.