Effect of FOD on fatigue strength of Ti-6Al-4V alloy

In order to investigate the effects of Foreign-object damage (FOD) on fatigue strength of Ti-6Al-4V alloy, the FOD was simulated by shooting a steel ball with a diameter φ = 4.7 mm onto specimen surface at a velocity of 90 m/s using a compressed-air gun facility. Tension-compression fatigue tests clarified that the fatigue limit of the specimen having shot marks (FOD) was 56% lower than that of smooth specimen. The shot mark also decreased the fatigue life of specimens remarkably. Fracture origins of the specimen having shot marks were at the bottom of the shot mark in all specimens. The fatigue limit of the specimen having shot marks was evaluated quantitatively from the viewpoint of the following factors, (1) residual stress distribution inside and vicinity of the shot mark, (2) work hardening related with plastic deformation at the bottom of the shot mark and cyclic softening at the stress amplitude of fatigue limit, and (3) the size of shot mark which acts as a detrimental defect. A narrow beam with size □ 100μm × 100μm using a synchrotron radiation facility at SPring-8 was used for the measurement of the residual stress distribution. The fatigue limit of the specimen having shot marks can be predicted within 10% error by the √area parameter model even in the case the residual stress was ignored.