Cracking Analysis on Oil Inlet Pipe of Hydraulic Load Unit of Aircraft Rudder

The oil inlet pipe of the hydraulic load unit of an aircraft rudder cracked. In order to find out the cracking cause of the pipe, macroscopic and microscopic observation, chemical composition analysis, metallographic structure examination were carried out, and the preparation process of the pipe was analyzed. The results indicate that the failure mode of the oil inlet pipe is fatigue cracking. The reason for the fatigue cracking is that an original crack was formed in the cold rolling/pulling process, and then an area without fracture characteristics was formed after heat treatment and acid pickling process. The fatigue crack originated linearly from the edge of the area without fracture characteristics. In addition, there exist intergranular cracks both at the inner and outer surface of the oil inlet pipe, which were formed in the process of acid pickling of the pipe.