Micromechanisms of Major/Minor Cycle Fatigue Crack Growth in Inconel 718

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Abstract
Fatigue crack growth tests under major/minor cycle loading were conducted on Inconel 718 at 649°C using center-cracked tension specimens, M(T). The loading waveform used consisted of a major cycle loading (1 Hz) and a hold at the major cycle maximum load, with a superimposed minor cycle loading (10 Hz) during the hold period. Crack growth data were obtained for a range of major and minor cycle stress intensities. Constant load amplitude tests were also run at various R-values to determine the effect of R on major/minor cycle fatigue. Detailed fractographic investigation was conducted on selected specimens using a scanning electron microscope to study the associated micromechanisms. Creep processes as indicated by an intergranular fracture mode were found to be dominant at lower minor cycle stress intensity ranges and higher values of R for the minor load. Transition to fatigue-dominant mechanisms as indicated by a transgranular fracture mode occurred at higher stress intensity ranges and lower R-values for the minor load. The minor cycle stress intensity range at which this transition occurred varied with the major cycle stress intensity range.