Foreign-object damage and high-cycle fatigue: role of microstructure in Ti–6Al–4V

The objective of this study was to evaluate the influence of microstructure on the susceptibility to high-cycle fatigue (HCF) failure in Ti–6Al–4V following foreign-object damage (FOD), specifically by comparing a fine-grained bi-modal microstructure with a coarse grained lamellar microstructure. FOD was simulated by high-velocity impacts of steel spheres on a flat surface. This caused a marked reduction in the smooth-bar fatigue strength in both microstructures, primarily because of the premature initiation of fatigue cracking resulting from the stress concentration associated with damage site and FOD-induced microcracking. The FODinitiated microcracks were found to be of a size comparable with microstructural dimensions, and on subsequent fatigue loading were seen to propagate at applied stress-intensity levels below K 1 MPa m, i.e. a factor of roughly two less than the ‘worstcase’ threshold stress-intensity range in Ti–6Al–4V for a crack of large size compared to microstructural dimensions (a ‘continuumsized’ crack). A rational approach against HCF failures from such microcracks is proposed for the fine-grained bi-modal microstructure based on the Kitagawa–Takahashi diagram. For the bi-modal microstructure, the Kitagawa–Takahashi diagram provides a basis for describing the threshold conditions for FOD-induced HCF failures, in terms of the stress concentration corrected smooth-bar fatigue limit for small crack sizes and the worst-case threshold for larger continuum-sized cracks. However, this approach was found to be less applicable to the coarse grained lamellar microstructure, primarily because of low small-crack growth resistance relative to its higher smooth-bar fatigue limit.  2001 Elsevier Science Ltd. All rights reserved.