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 traced to high-cycle fatigue (HCF) has prompted a reassessment of the design methodologies for HCF-critical components, such as turbine blades and disks. Because of the high-frequency vibratory loading involved, damage-tolerant design methodologies based on a threshold for no crack growth offer a preferred 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 foreign 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 (200-300 m/s) impacts of steel spheres on a flat surface, was found to reduce markedly the fatigue strength, primarily due to earlier crack initiation. This is discussed in terms of four salient factors: (i) the stress concentration associated with the FOD indentation, (ii) the presence of small microcracks in the damaged zone, (iii) the localized presence of tensile residual hoop stresses at the base and rim of the indent sites, and (iv) microstructural damage from FOD-induced plastic deformation. It was found that no crack growth occurred from FOD impact sites in this alloy at ∆K values below ~2.9 MPa√m, i.e., over 50% higher than the "closure-free", worse-case threshold value of ∆KTH = 1.9 MPa√m, defined for large cracks in bimodal Ti-6Al-4V at the highest possible load ratio. It is therefore concluded that such worst-case, large-crack thresholds can thus be used as a practical lower-bound to FOD-initiated cracking in this alloy. * Currently at Daimler Chrysler Aerospace MTU Munich, 80976 Munich, Germany 2