Aircraft engine bleed system tubes: Material and failure mode analysis

The failures of T-ducts made of Ti A40 with 50 mm in diameter and 0.52 mm in thickness, used in the pneumatic system of a commercial aircraft, are examined as part of a research program aimed to improve the development of such systems. The investigation included the fractographic analysis of the burst parts, microstructure and internal duct surface observations and mechanical tests performed in samples taken from the failed ducts and from unused material. The failed duct fracture surfaces showed typical features of transgranular fatigue cracking. Delamination and oxide particle deposits were found in their internal surfaces. Tensile and fatigue tests revealed some effects of aging, but with no evidence of embrittlement. The fracture mechanisms were the same for the failed ducts and for the unused material. The internal surface damage, associated to the geometry of the duct and to the solid-solution hardening increases the local stress concentration. These facts suggest that cracks had initiated at multiple sites around the inside surface, so that further increments of cracking occurred on subsequent cyclic pressurization during service life until the occurrence of sudden final fast fracture.