Failures of welded titanium aircraft ducts

Abstract An in-service failure of a thin-walled titanium bleed-air duct from a wide-bodied commercial aircraft has been investigated. Cracking had occurred in the heat-affected zone (HAZ) adjacent to a circumferential weld joining two sections of the duct which was manufactured from commercially pure (grade 3) titanium sheet. Specimens were cut from the duct to include an intact weld and tested under known conditions (overload, fatigue, sustained loading) for comparison with the failed duct. Metallographic observations showed that cracking occurred through an acicular α HAZ, and fractographic observations revealed brittle, cleavage-like cracking with occasional areas of fatigue striations for both the failed duct and fatigued specimens. These observations, and the absence of sustained-load cracking in test specimens, suggested that the in-service failure had occurred primarily by fatigue. Observations also indicated that small, cleavage-like cracks had been present in the ducts prior to service, although whether these cracks were caused by overload tearing, stress-corrosion cracking, hot-salt cracking or fatigue was not clear. Other possible causes of cleavage-like cracking, e.g. the presence of hydrides as proposed for previous failures, contamination of HAZs by oxygen/nitrogen, are discussed. Possible ways of preventing further failures are then outlined.