Analysis of cracked cooling tubes of air-to-air radiator of a particular type of aircraft

The cooling tubes of both the aircraft have failed by fatigue. Fatigue cracks were found to have initiated at the weld interface either in the corrugated sheet or in the inner skin. After initiation, the cracks have propagated along the weld interface in the circumferential direction. There were no metallurgical abnormalities responsible for the fatigue crack initiation and fatigue appears to be stress related. The nature of fatigue striations suggests that the fatigue was of high cycle-low stress type. Shifting of the position of the mating fracture surfaces in the corrugated sheet indicates that assembly and/or fabrication stresses could be the most probable reason for the fatigue crack initiation