Effects of FOD on fatigue strength of 7075-T6 hourglass specimens

The term "Foreign Object Damage (FOD)" refers to the damage associated with the impact of particles on aircraft engine components that causes changes in fatigue strength as a result of induced stress concentrations, residual stresses and microstructural changes. This paper presents the Finite Element (FE) models created to study the stress field induced by the impacts of a steel sphere on a 7075-T6 hourglass specimen. The impacts occur at the minimum cross section, in the normal and oblique directions. The results reveal that in the event of an oblique impact the axial tensile stresses are greater and closer to the crater created by the impact. The superposition of the residual axial stresses to the bending stresses by fatigue test allows to identify the areas in which the maximum stresses are reached and therefore crack initiation is expected.