The post-repair performance of Ti-6Al-4V after foreign object damage

During an aircraft's life, it will experience a variety of Foreign Object Damage (F.O.D.) strikes, some of which involve critical components in engines and undercarriage. The principal repair method for F.O.D. impacts is to grind away the visible surface damage, as long as this grind-out does not exceed limits on acceptable depth. However, F.O.D. also creates damage which may extend below the visible impact, and may therefore remain after the grind-out repair. This paper describes research which explores the extent to which this subsurface remnant damage might affect future in-service performance. It involved producing Ti-6Al-4V fatigue testing specimens which had contained identical or near identical damage and which are then to be subjected to grind-out repair to various depths. This approach allows the effect of any subsurface metallurgical damage or residual stresses to be assessed. The paper also describes the key features of the impact damage created using the standardised spherical impactors adopted, and the production of the fatigue specimens for fatigue testing to evaluate the performance of the as-damaged conditions with that of the various repair conditions to highlight the effect of the FOD on fatigue life.