Cracked Aircraft Wing Spar

A crack was found in an aircraft main wing spar flange fabricated from 7079-T6 aluminum alloy during a routine nondestructive x-ray inspection after the craft had logged 300 h. Scanning electron microscopy (SEM) revealed an intergranular fracture pattern indicative of stress-corrosion cracking (SCC) and fatigue striations near the crack origin. Visual examination of the crack edge revealed that the installation of the fasteners produced a fit up stress. Further inspection of the opened fracture showed that the crack had been present for some time because a heavy buildup of corrosion products was seen on the fractured surface. Metallographic examination of the flange in the area of fracture initiation showed the presence of end grain exposure, which would promote SCC. Electron optical examination of the fracture clearly showed the flange was cracking by a mixed mode of stress corrosion and fatigue. The cracking was accelerated because of an inadvertent fit up stress during installation. The age of the crack could not be established. However, a reevaluation of prior x-ray inspections in this area would result in some close estimate of the age of the crack. End grain exposure further promoted SCC.