Failure analysis of an aircraft auxiliary power unit air intake door

A remarkable number of auxiliary power unit (APU) air intake doors of the C27-J aircrafts consisting in A356-T6 casting aluminium alloy were found prematurely cracked during post-flight inspections. All the cracks were localized in correspondence of the inner side ribs and affected by extroversion. Examination of the fracture surfaces by using electron microscopy revealed the cracks propagated in accordance with a high cycle fatigue mechanism induced by in-service typical loads, such as vibrations and air turbulent flows. The doors were found to be confirmed to the chemical, metallographic and hardness requirements for the selected material. The stress induced by these loads was also locally increased by sub surface shrinkage cavities located in correspondence of the ribs. As a consequence of that, new doors, in terms of design as well as material, have been introduced, thus obtaining the necessary functional improvements.