Multiaxial fatigue life prediction for powder metallurgy superalloy FGH96 based on stress gradient effect

Both proportional and non-proportional axial-torsion fatigue tests were conducted on powder metallurgy (PM) superalloy FGH96 tubular specimens and round bar specimens at 650 degrees C. The alloy is widely used to manufacture the aero-engine turbine disk by a process of hot isostatic pressing (HIP). The different stress distributions of thin walled tubular specimen and the round bar specimen under axial-torsion cyclic loading are discussed. A multiaxial equivalent stress gradient factor is defined based on the tensile stress gradient and the shear stress gradient. A modified Zhong-Wang-Wei (ZWW) model considering the effect of the stress gradient is proposed for multiaxial fatigue of FGH96. Comparing to six multiaxial fatigue models, including the maximum effective strain model, the maximum shear strain model, the Fatemi-Socie (FS) model, the Smith-Watson-Topper (SWT) model, the Itoh model and the ZWW model, the predicted multiaxial fatigue lives of FGH96 by the modified ZWW model based on the effect of the stress gradient agreed better with the experimental results.