Low cycle biaxial fatigue behavior of direct aged Nickel-based 718 superalloy

摘要

In recent years, significant advances in the fabrication process of nickel-base supperalloy leading to grain size reduction have been made in order to improve fatigue properties of aircraft turbine discs. Indeed, a change in particle size affects the initiation mode of fatigue cracks as well as the fatigue life of the material. The present study aims to investigate the fatigue behavior of a newly developed nickel-based superalloy under biaxial-planar loading. Low Cycle Fatigue (LCF) tests are performed at different stress ratios to study the influence of the multiaxial stress state on the fatigue life of the material. Full-field displacement and strain measurements as well as crack initiation detection are obtained using Digital Image Correlation (DIC) techniques. Results related to different load ratios are presented and an appropriate biaxial lifetime prediction is given. Crack detection, strain amplitude and number of cycles to crack initiation vs. triaxial stress ratio for each loading case are mentionned. From fractographic investigations by scanning electron microscopy it is found that the mechanism of fatigue crack initiation does not depend on the triaxial stress ratio and that most fatigue cracks initiate from subsurface carbides.