A Fracture Mechanics Based Methodology for Fatigue Life Prediction of Single Crystal Nickel Based Superalloys

A comprehensive fracture mechanics based life prediction methodology is presented for FCC (Face Centered Cubic) single crystal components, based on computation of stress intensity factors (SIFs), and modeling the crystallographic fatigue crack growth process, under mixed-mode loading conditions. The 3D finite element numerical procedure presented for computing SIFs for anisotropic materials under mixed-mode loading is very general, and not just specific to FCC single crystals. Stress intensity factors for a Brazilian Disc (BD) specimen are presented for the crack on the {111} plane in the 〈101〉 and 〈121〉 directions, which represent the primary and secondary slip directions. Variation of SIFs as a function of thickness is also presented. Modeling of the crystallographic fatigue crack growth (FCG) behavior is performed by using the resolved shear stress intensity coefficient (RSSIC), K_rss. This parameter is sensitive to the grain orientation and is based on the resolved shear stresses on the slip planes at the crack tip, which is useful in identifying the active crack plane as well as predicting the crack growth direction. A multiaxial fatigue crack driving force parameter, ΔK_rss, was quantified, which can be used to predict the FCG rate and hence life in single crystal components subject to mixed mode fatigue loading.