Effect of Broaching on High Temperature Fatigue Behaviour in Notched Specimens of Inconel

Notches were machined in specimens of Inconel 718 by a broaching process, where differing broaching runs led to differing extents of sub-surface deformation and surface roughness. Fatigue tests were carried out at 600 ̊C with a trapezoidal loading waveform at 0.25Hz. The broaching process that led to the more severe sub-surface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elasto-plastic finite element model, showed that the workhardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behaviour were seen for the two broached finishes as well as for broached and subsequently polished samples. These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work hardened layer.