Stress redistribution and stress triaxiality effect on the fatigue life of notched Ni-based single crystal superalloy at 760℃

The multiaxial stress state caused by geometric discontinuities will result in significant differences in the fatigue performance compared to the uniaxial stress state. It is crucial importance to research the notch fatigue behaviors of nickel-based single crystal (Ni-SX) superalloys at high temperature. This study investigated the impact of stress redistribution and stress triaxiality on the fatigue life of notched Ni-based single crystal superalloy at 760℃. The high temperature fatigue tests were carried out on the specimens under different stress ratios and the stress amplitudes under the same life conditions were obtained. A novel anisotropic damage model, incorporating the influence of stress triaxiality is proposed coupled with crystal plasticity theory. Based on experimental findings, an evaluation of the test parameters was conducted.The results showed that Mises stress, maximum principal stress, normal stress of principal stress along the loading direction, maximum principal strain, and strain along the loading direction were inadequate in reflecting the significant variations of samples observed among different stress ratios. However, the stress triaxiality at the unloading valley exhibited a high sensitivity to changes in fatigue life. To evaluate the fatigue life of notched specimens under different stress ratios, an improved Basquin's type life prediction model was proposed, utilizing the stress triaxiality at the unloading valley. The proposed model demonstrates a favorable normalization trend for the fatigue life of notched specimens subjected to various stress concentration factor (SCF) conditions.