Failure of Nose Gear Door Bolts
A failed 25 x 32 mm (1 x 1 in.) cadmium-plated 1040 carbon steel countersunk head type nose gear door securing bolt with a common screwdriver slot was examined. Fracture originated at a thread root and propagated across the cross section. The topography of the fracture was excessively rough and more granular than would be expected from pure mechanical fatigue. This indicated an allied corrosion mechanism. Cracks other than the one leading to failure were observed. Metallographic examination of the bolt cross section showed many cracks typical of stress-corrosion damage. It was concluded that the bolt failed by a combination of SCC and fatigue. It was recommended that aerospace-quality fasteners meeting NAS 7104, NAS 7204, or NAS 7504 be used to replace the currently used fasteners.