Several AISI type 321 stainless steel welded oil tank assemblies used on helicopter engine systems began to leak in service. One failure, a fracture on the aft side of a spot weld, was submitted for analysis. SEM fractography examination revealed fatigue failure. The failure initiated at an overload fracture near the root of the weld and was followed by mode III fatigue crack propagation (tearing) around the periphery of the weld. The initial overload fracture was caused by a high external load, which produced a concentrated stress and fracture at the weld root. The subsequent fatigue fracture was caused by engine vibrations during operation of the aircraft. Fracture characteristics indicated that the fatigue would not have occurred if the initial damage had not taken place.