Analysis of premature failure of aircraft hydraulic pipes

Abstract

Premature failure of aircraft metallic hydraulic pipes was reported frequently and almost all pipes failed in a similar fashion. The failure of pipes during flight depleted hydraulic pressure and put aircraft at potential risk. Failure analysis of a fractured pipe and a cracked hydraulic pipe was carried out. Fatigue striations on the fractured surfaces were observed during scanning electron microscopy (SEM). The fracture initiated from the constrained (inlet) end as crack nucleated under the metallic sleeve on the pipe surface. Metallic sleeves are crimped at the pipe ends to help these connect with other hydraulic system components. On removal of metallic sleeves, stress concentration sites in the form of circumferential grooves (depth 35 ± 7 µm, radius 76 ± 2 µm) were observed. SEM of cracked pipe also confirmed the crack nucleation from circumferential groove under the metallic sleeve. Fluctuating hydraulic pressure caused by random movement of control surfaces produced cyclic loading at the constrained (inlet) end. Forces generated by hydraulic pressure on pipe bends were obtained computationally and were validated analytically by solving impulse-momentum equations. Computational stress analysis of the pipe, based on calculated fluid forces, revealed that maximum Von Mises stress increased by ~ 123% in the presence of circumferential groove at the constrained (inlet) end. Higher stress levels associated with circumferential grooves at constrained end nucleated early cracks, which propagated under fluctuating hydraulic pressure till pipes failed prematurely.