Fracture of an aircraft's landing gear

The landing gear of an aircraft failed during its landing procedure due to the fracture of the outer cylinder, which is manufactured from an Al-5.7Zn-2.7Mg-0.5Mn-0.5Cu alloy. The material presented an oriented partially recrystallized microstructure, with the unrecrystallized areas showing intense intragranular precipitation. while the recrystallized areas presented fine intergranular precipitation. Fractographic examination revealed that the unstable crack propagation was originated in the fatigue cracks, which were approximately 0.6 mm deep, and occurred mainly by a mechanism of ductile intergranular fracture. Both stable and unstable crack propagation occurred preferentially parallel to the orientation of the microstructure. Fracture toughness K-IC tests confirmed that LT orientation presented K-IC values 60% higher than TL, although it presented comparatively a higher proportion of intergranular fracture. Fractographic observation of K-IC test pieces showed that unstable crack propagation occurred preferentially along the boundaries of recrystallized grains. TL orientation showed additional propagation along the boundaries between recrystallized and unrecrystallized regions. The elongated morphology of primary and secondary precipitates aligned perpendicular to the loading direction is thought to be responsible for the lower fracture toughness observed for TL orientation. The fatigue crack observed in the landing gear was nucleated on the intersection of the aligned microstructure with the free surface, condition that should be avoided by efficient design of structural components. The abnormal loads absorbed by the outer cylinder during the landing procedure, associated with the presence of fatigue pre-cracks, may be concluded as the cause for this failure. (C) 2002 Elsevier Science Ltd. All rights reserved.