Effect of dent geometry on fatigue life of aircraft structural cylinder part

Aircraft structural parts are structure critical parts and they are exposed to various and tedious inspections from the design phase thru the end of their service life. During design phase, taking fatigue under dynamic and static loads, corrosion and aging effects into consideration, detailed calculations are performed on structural parts to be utilized with no problems during their service lives. The areas that are more prone to fail are the critical areas for these parts. The service life of a part is especially assigned according to the critical areas. These areas play the main role to assign the inspection period and the inspection types for the part as well. Critical areas may cause notch sensitivity and vary part to part but rivet/bolt hole edges, sharp edges, threads, necked down areas from elongation are the critical areas in general for a part.In this study, a cylinder case, which is a part of a hydraulic system, has been inspected. The result of the failure analysis study revealed that the part has been defected fatigue crack propagation. The fatigue initiation location has been found to be on the necked down area of the mount of the cylinder case. Neck down method during manufacturing aimed to constraint the ball has been considered to be improper and the main player for the crack initiation. An alternate neck down configuration, which is functionally identical and do not jeopardize the fatigue life of the part, has been recommended.