Experimental investigation and analytical description of the damage evolution in a Ni-based superalloy beyond 106 loading cycles

The fatigue life of metallic materials in the very high cycle fatigue (VHCF) regime is characterized by a large scatter of experimental results. The aim of the present work is to investigate the reasons for the scattering of the fatigue life as well as to model its impact statistically. For this purpose crack initiation in the Ni-base superalloy Nimonic 80A was investigated as a function of the number of cycles to failure at different stress amplitudes both in the HCF and the transitional HCF-VHCF regime. According to the experimental observations the fatigue crack initiation depends on the stress amplitude and occurs at twin boundaries and grain boundaries with a large misorientation angle. The application of two analytical approaches for the assessment of the local stress concentration at different grain boundaries is discussed. In particular in the VHCF regime, the misorientation factor developed by Blochwitz et al. (1997), which depends on the misorientation angle between two adjacent grains as well as on the orientation of their boundary with respect to the external load, was used to estimate the stress concentration at the grain boundaries. It was revealed, that the cracks initiate at the grain boundary of maximum misorientation factor calculated for each specimen. Furthermore, the calculated maximum misorientation factor per specimen show a direct relation to the number of cycles to failure and can be used as an additional microstructural information for statistically based fatigue life prediction models.