Influence of residual stresses on high cycle fatigue strength of Ti–6Al–4V subjected to foreign object damage

The role of residual stresses in the high cycle fatigue (HCF) strength of Ti–6Al–4V subjected to foreign object damage (FOD) was evaluated on simulated airfoil and rectangular geometries. Both real and simulated impacts were conducted using spherical projectiles launched at 300 m/s and quasi-static chisel indentation, respectively. The spheres used were 1 mm diameter glass beads while the quasi-static indentor had a radius of 1 mm. The airfoil specimens had leading edge (LE) radii of either 0.13 or 0.38 mm and were indented at 30° to the airfoil LE. The rectangular plates were 1.25 mm thick and were indented quasi-statically at 0°. All specimens subjected to FOD were subsequently tested in uniaxial HCF at a frequency of 350 Hz using a step loading procedure to determine the fatigue limit corresponding to 107 cycles. Before the HCF testing, half of the specimens were stress relief annealed to remove residual stresses. Results indicate that stress relief generally improves the fatigue limit stress, indicating that tensile residual stresses are present after both quasi-static or dynamic indentation. For the dynamic impacts, the craters formed have less influence on the fatigue strengths than that predicted from conventional notch fatigue analysis.