Low Cycle Fatigue Experiments on Turbogenerator Steels and a New Method for Defining Confidence Bands

The 26 NiCrMoV 14 5 and 18Mn18Cr steels have wide applications in the manufacturing of rotors and coil retaining rings. Current studies in the literature mainly regard their metallurgical properties, while little information is available about their cyclic and fatigue behavior. There is also little information regarding possible anisotropic behavior along the tangential and radial forming directions of the aforementioned components. The cited items were tackled by running an experimental campaign with specimens machined from real components in the tangential and radial directions. The experimental procedure was very careful from the points of view of methodology, of strain control and of fracture evaluation. Static, cyclic and low cycle fatigue curves were determined. The data were processed for the computation of curve parameters. While an original statistical analysis inspired by ASTM E739 – 10 was conducted to determine hyperbolic confidence bands “wrapped around” the entire fatigue curves, relating total strain amplitudes to lives. To tackle the question of anisotropy, a novel formulation of the Analysis of Variance was applied to compare the fatigue curves of both materials along the two forming directions.