Aircraft landing gear failure: fracture of the outer cylinder lug

摘要

The landing gear of an aircraft failed during landing due to the fracture of its outer cylinder attachment lug, which is manufactured from an aluminium alloy following British Specification DTD 5104 A. The material presented an oriented microstructure composed of bands of recrystallized and unrecrystallized grains. The unrecrystallized areas showed intense intragranular precipitation, while the recrystallized and unrecrystallized grains. The unrecrystallized areas showed intense intragranular precipitation, while the recrystallized areas presented large primary precipitates and fine intergranular precipitation. Pit cavities associated with intergranular corrosion were observed along the perimeter line of the attachment bore surface and consumed preferentially the recrystallized areas of the microstructure. Fracto-graphic examination revealed that the unstable crack propagation was originated in these stress raisers. The fracture mode is preponderantly ductile intergranular, with areas showing ductile fracture composed of small dimples. The results indicated that both corrosion and intergranular ductile crack propagation were favoured by the orientation of the microstructure, which was perpendicular to the free surface. Finally, the abnormal loads absorbed by the outer cylinder during the landing procedure associated with corrosion cavities located in a critical area may be concluded as the cause for this failure.