Life Prediction Method of CC and DS Ni Base Superalloys Under High Temperature Biaxial Fatigue Loading

Polycrystalline conventional casting (CC) and directionally solidified (DS) Ni base superalloys are widely used as gas turbine blade materials. It was reported that the surface of a gas turbine blade is subjected to a biaxial tensile-compressive fatigue loading during a start-stop operation, based on finite element stress analysis results. It is necessary to establish the life prediction method of these superalloys under biaxial fatigue loading for reliable operations. In this study, the in-plane biaxial fatigue tests with different phases of $x$ and $y$ directional strain cycles were conducted on both CC and DS Ni base superalloys (IN738LC and GTD111DS) at high temperatures. The strain ratio $ϕ$ was defined as the ratio between the $x$ and $y$ directional strains at 1/4 cycle and was varied from 1 to $−1$⁠. In $ϕ=1$ and $−1$⁠. The main cracks propagated in both the $x$ and $y$ directions in the CC superalloy. On the other hand, the main cracks of the DS superalloy propagated only in the $x$ direction, indicating that the failure resistance in the solidified direction is weaker than that in the direction normal to the solidified direction. Although the biaxial fatigue life of the CC superalloy was correlated with the conventional Mises equivalent strain range, that of the DS superalloy depended on $ϕ$⁠. The new biaxial fatigue life criterion, equivalent normal strain range for the DS superalloy was derived from the iso-fatigue life curve on a principal strain plane defined in this study. Fatigue life of the DS superalloy was correlated with the equivalent normal strain range. Fatigue life of the DS superalloy under equibiaxial fatigue loading was significantly reduced by introducing compressive strain hold dwell. Life prediction under equibiaxial fatigue loading with the compressive strain hold was successfully made by the nonlinear damage accumulation model. This suggests that the proposed method can be applied to life prediction of the gas turbine DS blades, which are subjected to biaxial fatigue loading during operation.