Influence of foreign object damage (FOD) on the fatigue life of simulated Ti-6Al-4 V airfoils

The fatigue behavior of Ti-6Al-4 V samples that have undergone foreign object damage (FOD) was investigated. A unique test specimen configuration that replicates the leading edge of a typical fan blade was ballistically shot with 1.0 mm diameter glass spheres to simulate actual FOD. Samples were shot at various incident angles to investigate the influence of impact angle on fatigue strength. Two stress ratios were examined: R=0.1 and 0.5. Attempts were made to correlate the measured damage (macro-/microscopic) to the debit in fatigue strength. The FOD impact sites of fatigue samples were examined prior to testing via a scanning electron microscope (SEM); post-test examinations of the fatigue initiation sites were also performed on selected test samples. Subsurface or internal damage resulting from the FOD was also investigated from precision micro-sectioning of selected samples. Off-angle impacts were found to be more detrimental than head-on (0°) impacts. Fatigue strength losses, some as high as 50%, showed little or no correlation with leading edge thickness or depth of notch.