Novel Fatigue Testing of Extruded Inconel 718

Abstract

This paper presents accelerated fatigue testing results of extruded Inconel 718. Tapered cantilever specimens, that is designed to achieve uniform maximum bending stresses along their gage lengths, were used for high-speed fatigue testing in rotate-bending mode. Random fracture proved the uniformity of bending stress along the gage length. The endurance limits and fatigue lives of Inconel 718 were significantly impacted by material hardness and surface finish. The aged and polished specimens can achieve about one order of magnitude longer in fatigue lives compared to the unaged and as-machined specimens. Optical and scanning electron microscopic examinations showed the microstructure and mechanism of fatigue failure. Brittle particles, shattered during fatigue test, formed internal cracks and voids that linked and fractured the remaining matrix in ductile mode; circumferential cracks would initiate at deep machining marks on the specimen surface, propagate inward until reaching a critical crack length and then fracture catastrophically. A polished specimen, having no machining mark and smooth surface finish, exhibited multiple and random crack origins that propagated simultaneously inward till fracture. The resulted fatigue data were compatible with other published data for the wrought Inconel 718.