Fatigue failure of a hydraulic filter head

An in-service failure of a hydraulic filter installed on a combat aircraft has been investigated. Cracking had occurred at the filter head and no sign of plastic deformation was found. Chemical examination and micro-hardness measurement revealed that the filter head material was Al 6061-T6. But the base material of the randomly chosen filters from the field was Al 2024-T4. So the wrong material selection was confirmed. Metallography showed microstructural anomalies which contributed to the early fracture of hydraulic filter head. A surface examination of the fracture region revealed striations, and it was confirmed that the fracture mode was fatigue. At the initiation site there were corrosion pits providing initiation sites for fatigue cracks. The corrosion pits and microstructural anomalies might be generated by applying the manufacturing processes designed for Al 2024 to the wrong material (Al 6061). It was verified by fatigue analysis that with pre-existing defects and/or flaws, the fracture should occur far earlier than the expected cyclic life. The right selection of the material can be the most important remedial action in this case. (C) 2002 Elsevier Science Ltd. All rights reserved.