Linkage between Ductile Fracture and Extremely Low Cycle Fatigue of Inconel 718 Under Multiaxial Loading Conditions

Abstract

Ductile fracture and extremely low cycle fatigue (ELCF) [1] are two common failure modes in aircraft engines and turbomachinery designs [2]; however, the linkage between these two failure modes under multi-axial loading conditions has never been systematically studied. Inconel 718 (IN718) is one type of high temperature alloys widely used in turbomachines. Specially designed specimens and tests were used to achieve desired multi-axial loading conditions. Two groups of tests were conducted: (a) round bar specimens with different notches; (b) plane strain specimens. Similar types of tests were conducted for IN718 under both types of failure modes (ductile fracture and ELCF). It is found that the ductile fracture of IN718 under multi-axial loading conditions is strongly dependent on stress triaxiality, but weakly dependent on the Lode angle parameter [3], A 3D fracture locus was calibrated using modified Mohr-Coulomb (MMC) criterion proposed by Bai and Wierzbicki [4], It is found that the same phenomenon of stress state dependency exists in the ELCF, which need to be addressed. The mechanism linkage between these two failure modes was explored.