Fatigue fracture analysis of an ADF antenna in a military aircraft

The antenna of military aircraft was fractured into two pieces during the flight mission. The fracture occurred in bottom part of antenna. The lower portion of the antenna stayed attached to the fuselage, whereas the upper portion fall off the aircraft. Based on the fractographic examination of the fracture surface, the characteristic beach marks, cleavage-like cracking mode and striation were observed indicating the occurrence of fatigue failure. The fatigue was initiated from very closely grouped corrosion pits located on the external surface of the antenna. Chemical analysis of the corrosion products in these corrosion pits showed the presence of significant amounts of chlorine, sulfur and calcium. Metallographic examination discovered numerous corrosion pits with secondary micro cracks propagating out of them. The nonlinear finite element (FE) analysis show that the antenna is exposed to fully-reversed fatigue loading, inducing cycles of tension and compression on the lateral faces of the antenna. Also, the FE analysis confirmed that the crack was originated in the region with maximum tensile stress in the antenna.