Failure analysis of a fuel control pressure tube from an aircraft engine

Multiple fracture cases of fuel control pressure tubes from aircraft engines have been reported. The aim of the present study is to determine the cause of this tube fracture. The studied set was composed by the mentioned tube, a welded connecting pipe, where the fracture has been produced, and a union nut. The fracture has been produced in one most critical zones of the tube, in a region next to the supporting body of the union nut to the connector. A visual examination and with a stereo microscope of the tube fracture surface has been carried out in order to determine the macrofractographic features. The results revealed a plastic macrodeformation of the tube, a damaged surface and signs of a possible corrosion process. The material has been chemical, mechanical and microstructural characterized. The results confirmed that it was within specifications. Fracture surface was also inspected by scanning electron microscopy to determine the microfractographic characteristics in order to find out the failure mechanism involved in the fracture. Fatigue striations, which are typical from a progressive fracture by a fatigue mechanism, have been observed. The origin of the fracture has been placed in defects located in the outer wall of the tube, leading to a final overload fracture.