Leading edges of STOA Ti-6Al-4V simulated-blade samples were ballistically impacted. SEM damage-characterization established crater dimensions, material loss, cracks and extrusions, followed by step-loading axial HCF to determine the HCF strength loss. Initiation was always at the exit side of the impact crater, along the crater centerline. Sectioned specimens revealed subsurface adiabatic shear bands and surface impact transformation layers with fine, beta-transformed structure. These bands contained micro-cracks, tears and some shrinkage porosity, indicating a high level of localized heating during impact. Extrusions, loss of material and impact-induced cracks did not correlate well with the reduction in fatigue strength. Still, a moderate correlation was found between crater depth and loss of fatigue strength. A fatigue limit stress model based on notch fatigue analysis predicted the fatigue loss quite well for craters less than 0.4 mm deep, but underestimated it for deeper craters, indicating that another mechanism, such as microstructure damage, played an important role.
展开▼
