Synchrotron Mesodiffraction: A Tool for Understanding Turbine Engine Foreign Object Damage

Aircraft turbine engines routinely experience the ingestion of debris resulting in “foreign object damage” or FOD. Failures associated with foreign object damage have been estimated to cost the aerospace industry $4 billion per year. Often, FOD does not lead to sudden catastrophic failure, yet such damage can dramatically reduce the lifetime of components subjected to cyclic fatigue stresses. Turbine blades, for example, are susceptible to debris strikes and also experience significant fatigue loading. The current study seeks to develop insight into the driving forces and predictability of fatigue failures induced by foreign object damage. Such insight can be used to improve existing design methodologies for turbine engine components and inspection regimens. The material used for this study, a titanium alloy Ti-6Al-4V, is commonly used for blades in the front, low-temperature stages of the engine, where the initial foreign object strikes can occur. Details of the material, heat treatment, and microstructure can be found in Ref. [1]. The lifetime of undamaged Ti6Al-4V is illustrated by the dashed line in Fig. 1. This curve shows typical behavior for metallic materials: as the fatigue stresses are increased, the number of cycles to failure is decreased. The fatigue limit, defined as the stress level at which no failures would occur within 10 7 cycles, is σ max ~525 MPa for the undamaged material. When components are subjected to stresses below this level, the component can be thought of as having an essentially infinite life. However, at a maximum cyclic stress of only 500 MPa, below the nominal fatigue limit, impact-damaged Ti-6Al-4V has a lifetime of <10 5 cycles as shown by