Effects of FOD on the fatigue crack initiation of ballistically impacted titanium-aluminum(6)-vanadium(4) simulated engine blades

Nicks and dents caused by foreign object damage (FOD) in leading edges of turbine engine blades often provide crack initiation sites that may grow under high cycle fatigue (HCF) conditions and lead to reduced life. Leading edge specimens simulating the geometry of these engine blades were fabricated from STOA Ti-6Al-4V. The leading edges were then ballistically impacted with small steel spheres and the subsequent damage was characterized with SEM according to the crater dimensions (depth and length), material lost, cracks and extrusions.; A few specimens were sectioned after impact and SEM of the area beneath the impacts revealed adiabatic shear bands as well as a surface impact transformation layer with a fine transformed microstructure. These shear bands contained micro-cracks, tears and some shrinkage porosity, which indicated the magnitude of the localized high temperatures achieved during the impact.; The remaining specimens were subjected to tension-tension axial HCF testing using a step loading fatigue procedure. Fatigue fracture initiation always occurred at the exit side of the impact crater and at the crater center. Extrusions, loss of material and cracks caused by the impact did not correlate well with the normalized fatigue strength. However, SEM of the fractured surfaces beneath the impacts revealed adiabatic shear band traces as well as a surface impact transformation layer exhibiting a fine texture at the fracture initiation sites.; Only a moderate correlation existed between the depth of the impact crater and the normalized fatigue stress. No correlation existed between the crater length and the normalized fatigue strength. The failure of several craters with the smaller crater depth strongly suggested that factors not related to the crater geometry were involved. The fatigue limit stress model was able to predict the fatigue limit for craters less than 0.4 mm deep. However, this model underestimated the fatigue strength loss for deeper craters, indicating that another mechanism, such as microstructural damage played an important role. Considering the presence of the transformed microstructure and adiabatic shear bands, the use of slower strain rate methods for producing FOD on the leading edge of a test specimen may need to be re-evaluated.