Metallurgical failure analysis of a cracked aluminum 7075 wing internal angle

• A metallurgical failure analysis of aluminum 7075 wing internal angle in Airbus A-300.
• A crack formed by a process of corrosion fatigue which itself was coming from a number of reasons.
• Surface corrosion is obviously cause of microscopic flaws (and initial cracks) in which high values of stress intensities would be created.
• Incorrect cold working maintenance job brought about scratches in the cracking region.
• Existence of a filet at the vicinity of crack location created high values of stress intensity and stress concentration which can help crack propagation.

Internal angles are used to strengthen Aircrafts center box corners where the wing is attached to the airframe. There are 16 angles in Airbus A300s wing box. On the right side, rear spur, and lower flange area of the center wing box, one of these angles had been cracked with a length of 28 mm. This crack has decreased residual strength of the part under allowed values and resulted to a rupture in the rear spur lower cap. Several reports of the same occurrences in other Airbus A300 air crafts, highlight the importance of finding the causes of this failure. Detailed optical and SEM, plus 4 other metallurgical tests were conducted on the failed angle. Finally, it was concluded that corrosion fatigue was the main reason which itself comes from manufacturing, maintenance, metallurgical, and geometric reasons as were discussed in this study.