Fatigue failure of a stainless steel wires used in a hydraulic pressure line

A Boeing B767-300 experienced a right engine fire during taking off. The inspection of the right engine’s fuel system revealed a failure in the turbine case cooling (TCC) air valve actuator hydraulic pressure line, a flexible component made of AISI 304 stainless steel wire braid. The failure occurred in an area of broken wires in the braid that was located at the edges of a kink in the line associated with a bending. The examination of the broken ends of the wires by means of the use of field emission scanning electron microscope (FESEM) revealed fractures features that were indicative of fatigue failure, with very fine fatigue striation spacing, thus indicating at least 1000 cycles to failure. The energy dispersive X-ray (EDX) analysis did not reveal any defects acting as initiation source. Rather, the failure resulted from the permanent deformation of the line, thus developing a stress concentration at the kink. Following these results, new safety recommendations were suggested and subsequently adopted.