An Application of Metal Plasticity in Finite Element Modeling to Predict the Low-Cycle Fatigue Life of a High-Pressure Steam Turbine Casing

In the current market for large steam turbines, customers increasingly want to aggressively cycle their equipment to accommodate electrical grids that include fluctuating supplies of green energy. Increased and aggressive cycling leads to higher probability of low-cycle-fatigue cracking and provides motivation for the design of new steam turbines that are robust enough to withstand this demanding working environment yet still meet the operational and cost expectations of potential customers. ASME BPVC Section III Subsection NH provides a calculation for fatigue damage assessment using either an elastic method or an inelastic method. This paper describes how the inelastic method can be applied to large steam turbines — calculating low-cycle fatigue damage by using commercial finite element software and plastic material models to directly determine elastic-plastic strains throughout the cycle, rather than approximating them using the results of an elastic analysis. The inelastic method is applied to a steam turbine casing during startup cycles — the total strain through the cycle is calculated directly by the elastic-plastic finite element analysis (FEA) then the delta equivalent total strain is calculated using equations in Subsection NH. For comparison, an elastic method is applied to the same analysis — the maximum elastic stress is calculated by the linear-elastic FEA then the delta equivalent total strain is approximated using Neuber’s rule. The inelastic method calculates a smaller delta equivalent total strain, which leads to significantly increased fatigue life. This more sophisticated method could lead to steam turbine components with less cost, more durability, and better performance. This paper also discusses some issues in using the inelastic method, such as shakedown and ratcheting.