Experimental investigation of small fatigue crack growth due to foreign object damage in titanium alloy TC4

The ingestion of runway debris during takeoff, landing and low-flying of aircraft has been a major concern for gas turbine engines. The small craters or notches introduced by particles impacting blades are commonly known as foreign object damage (FOD). The objective of this paper is to identify the initiation and propagation behavior of small fatigue crack originating from FOD crater in titanium alloy TC4. FOD was simulated by firing steel sphere projectile onto the surface of flat specimen at 30°, 60° and 90°. Replication technique based on a two-part silicon mixture was used to observe the initiation and growth process of FOD-initiated crack. The results reveal that all the cracks nucleate from the micronotches at the material pileup of FOD crater rim. The growth process of FOD-initiated crack consists of two stages. In the first stage, the crack propagates merely inside the crater. Once exceeding ~500 µm in length, the crack will grow into the second stage, in which the crack extends rapidly both inside and outside the FOD crater. Moreover, the growth rates of FOD-initiated cracks are lower than naturally-initiated cracks. Of all the three tested impact angles, the 60° impact seemed to lead to the most crucial decline in fatigue resistance. A linear relationship exhibits between the fatigue lifetime and crater cross-section area of FODed specimens.