Failure analysis of assembly drain pipe installed in an aircraft

A fractured drain pipe-port installed in an aircraft was forwarded to the laboratory for analyzing mode and cause of failure. Examination revealed that the drain pipe has failed by fatigue. There was multiple fatigue crack initiation on the outer surface of the tube. After initiation, these cracks have propagated simultaneously to critical lengths before giving way to final static overload failure. No mechanical or metallurgical abnormalities were responsible for the fatigue crack initiation. The material of construction was also found to conform to the specification of ASTM type 321 stainless steel. Analysis showed that the most probable reason for the fatigue crack initiation appears to be residual assembly stresses