Role of foreign-object damage on thresholds for high-cycle fatigue in Ti-6Al-4V

The increasing incidence of military aircraft engine failures that can be t raced to high-cycle fatigue (HCF) has prompted a reassessment of the design methodologies for HCF-critical components, such as turbine blades and disk s. Because of the high-frequency vibratory loading involved, damage-toleran t design methodologies based on a threshold for no crack growth offer a pre ferred approach. As impact damage from ingested debris is a prime source of HCF-related failures, the current study is focused on the role of such for eign-object damage (FOD) in influencing fatigue crack-growth thresholds and early crack growth of both large and small cracks in a fan blade alloy, Ti -6Al-4V. FOD, which was simulated by the high-velocity (200 to 300 m/s) imp act of steel spheres on a flat surface, was found to reduce markedly the fa tigue strength, primarily due to earlier crack initiation. This is discusse d in terms of four salient factors: (1) the stress concentration associated with the FOD indentation, (2) the presence of small microcracks in the dam aged zone, (3) the localized presence of tensile residual hoop stresses at the base and rim of the indent sites, and (4) microstructural damage from F OD-induced plastic deformation. It was found that no crack growth occurred from FOD impact sites in this alloy at Delta K values below similar to 2.9 MPa root m, i.e., over 50 pct higher than the "closure-free", worst-case th reshold value of bK(TH) = 1.9 MPa root m, defined for large cracks in bimod al Ti-6Al-4V alloys at the highest possible load ratio. It is, therefore, c oncluded that such worst-case, large fatigue crack thresholds can, thus, be used as a practical lower-bound to FOD-initiated cracking in this alloy.