Development of Life Prediction Method of Turbine Blade Using the Theory of Critical Distance

Gas turbine blades mounted dovetail root are subjected to high centrifugal loads and gas forces. This situation causes low cycle fatigue (LCF). Recently, rotating speed and temperature of turbine rotor become higher in order to improve engine performance. To achieve this, it is required to evaluate accurate turbine blade’s LCF life of the contact surface between the blade dovetail root and the disk. However, the estimated blade lives using the peak stress calculated by finite element analysis (FEA) are much shorter than actual life because the stress at contact surface is excessively high. As a result, the blades are designed conservative and the blade’s weight tends to be heavy. Therefore, a more accurate evaluation methodology needs to be established.This study investigates the method to estimate the fatigue strength of dovetail using the theory of critical distance. The theory assumes that fatigue failures would occur due to the representative stress within a specific distance from stress concentration point. Fatigue tests and FEA for the turbine blade dovetail were conducted respectively in this research. The tests were carried out using single crystal nickel-based turbine blades at 600 °C and the fracture lives of dovetail were obtained. FEA was conducted to obtain the stress distributions at dovetail contact surface under testing condition. In this study, the critical distances of the single crystal nickel based alloy were obtained from the notched bar fatigue tests and FEA. Using these results and the theory of critical distance, fatigue lives of dovetail were obtained more accurately.© 2012 ASME