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

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 MohrCoulomb (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. Introduction Ductile fracture is an important failure mode for many materials and structures including turbomachines. For example, the foreign object damage (FOD) on the blade and casing is a design factor in aircraft engines and turbomachinery under extreme loading conditions [2], Here are two examples of FOD on turbomachines. The first one is bird/ice/hail strike on aircraft engines fan blades and further ingestion into the engine hot sections, which may cause blade out and further damage on engine casing. Each year, bird and other wildlife strikes to aircraft (including engines and fuselage) cause more than $600 million in damage to U.S. civil and military aviation [5], The second one is the bolts and nuts (or other hard bodies) passing screen and ingestion into gas/steam turbines, which may cause damage on high speed rotating blades [3], One critical technique here is the accurate prediction of ductile fracture under complex loading conditions. Extremely low cycle fatigue (ELCF) is another critical failure mode for turbomachinery. For example, the damage caused by frequently turning on and off in gas turbines. It is also one of important failure mechanisms of aircraft engine casings under blade out