Influence of foreign-object damage on crack initiation and early crack growth during high-cycle fatigue of Ti–6Al–4V

The objective of this work is to provide a rationale approach to define the limiting conditions for high-cycle fatigue (HCF) in the presence of foreign-object damage (FOD). This study focused on the role of simulated FOD in affecting the initiation and early growth of small surface fatigue cracks in a Ti–6Al–4V alloy, processed for typical turbine blade applications. Using high-velocity (200–300 m/s) impacts of 3.2 mm diameter steel spheres on the flat surface of fatigue test specimens to simulate FOD, it is found that the resistance to HCF is markedly reduced due to earlier crack initiation. Premature crack initiation and subsequent near-threshold crack growth is primarily affected by the stress concentration associated with the FOD indentation and the presence of small microcracks in the damaged zone (seen only at the higher impact velocities). Furthermore, the effect of residual stresses and microstructural damage from FOD-induced plastic deformation at the indent sites are assessed in terms of fatigue strength degradation. It is shown that FOD-initiated cracks, that are of a size comparable with microstructural dimensions, can propagate at applied stress-intensity ranges on the order of ΔK∼1 MPa √m , i.e., a factor of roughly two less than the “worst-case” threshold stress-intensity range in Ti–6Al–4V for a crack of a size large compared to microstructural dimensions (a “continuum-sized” crack). Correspondingly, for FOD-initiated failures, where the critical condition for HCF must be defined in the presence of microstructurally small cracks, the Kitagawa–Takahashi diagram, with the limiting conditions of the stress-concentration corrected 107-cycle fatigue limit and the “worst-case” ΔK TH fatigue threshold, is proposed as a basis for design against FOD-induced HCF failures.