Fatigue of air supply manifold support rod in military jet engines

Abstract

Bending fatigue failure occurred in a support rod linking the bleed air supply manifold to the diffuser case in a military jet engine. Other rods were subsequently found cracked. The rods were wrought Inconel 718, operating at approximately 550 °C. The bending was caused by relative movements of two expansion sleeves held together by the rods. Accommodative swiveling of the spherically rounded ends of the rods in the mating seatings of the inner sleeves was prevented by contact pressure, fretting, and binding. The main reason for binding was decomposition of the antigalling compound applied to the contact surfaces. Fractographic analysis and oxidation interference colors enabled interpretation of the cracking history of the broken rod. Elementary stress and fracture mechanics calculations indicated that the maximum fatigue stress was below the stress needed to “unlock” the spherically rounded rod end from its seating.