Metallurgical failure analysis of a cracked aluminum 7075 wing internal angle

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.