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 foreig n-object damage (FOD). This study focused on the role of simulated FOD in a ffecting 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 nlm diameter steel spheres on t he 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 p rimarily affected by the stress concentration associated with the FOD inden tation and the presence of small microcracks in the damaged zone (seen only at the higher impact velocities). Furthermore, the effect of residual stre sses and microstructural damage from FOD-induced plastic deformation at the indent sites are assessed in terms of fatigue strength degradation. It is shown that POD-initiated cracks, that are of a size comparable with microst ructural dimensions, can propagate at applied stress-intensity ranges on th e order of Delta K similar to 1 MPa root m, i.e., a factor of roughly two l ess than the "worst-case" threshold stress-intensity range in Ti-6Al-4V for a crack of a size large compared to microstructural dimensions (a "continu um-sized" crack). Correspondingly, for FOD-initiated failures, where the cr itical condition for HCF must be defined in the presence of microstructural ly small cracks, the Kitagawa-Takahashi diagram, with the limiting conditio ns of the stress-concentration corrected 10(7)-cycle fatigue limit and the "worst-case" Delta K-TH fatigue threshold, is proposed as a basis for desig n against FOD-induced HCF failures. (C) 2000 Elsevier Science Ltd. All righ ts reserved.