Chapter 7 - Foreign Object Damage

Abstract

Foreign object damage (FOD) involves the damage caused by objects ingested into turbine engines. These events typically take place upon takeoff and landing. Ingested objects, which involve a range of soft and hard materials of all sizes and shapes, cause damage when they strike rotating blades, static vanes, or other parts of the engine where they cause a reduction in strength of the component. Such events are difficult to prevent although methods, such as the use of screens is unsuccessfully attempted at times in the past. A greater concern arises when the component that is damaged is also undergoing some type of forced or resonant vibration that can lead to high-cycle fatigue (HCF) in the damaged area. The problem of FOD is somewhat related to fatigue of notches, but is much more complicated because it involves additional aspects. The severity of a notch, from a geometric point of view, is characterized by a stress concentration factor, kt, which, under FOD, can vary significantly in service. Different types of damages are illustrated in this chapter involving a wide variation in geometry and corresponding values of kt. Robust engineering methods are needed to predict the influence of these geometric variables on HCF threshold stresses.