FAILURES OF WELDED TITANIUM AIRCRAFT DUCTS

An in-service failure of a thin-walled titanium bleed-air duct from a wide-bodied commercial aircraft has been investigated. Cracking had oc curred in the hear-affected zone (HAZ) adjacent to a circumferential w eld joining two sections of the duct which was manufactured from comme rcially pure (grade 3) titanium sheet. Specimens were cut from the duc t to include an intact weld and tested under known conditions (overloa d, fatigue, sustained loading) for comparison with the failed duct. Me tallographic observations showed that cracking occurred through an aci cular alpha HAZ, and fractographic observations revealed brittle, clea vage-like cracking with occasional areas of fatigue striations for bot h the failed duct and fatigued specimens. These observations, and the absence of sustained-load cracking in test specimens, suggested that t he in-service failure had occurred primarily by fatigue. Observations also indicated that small, cleavage-like cracks had been present in th e ducts prior to service, although whether these cracks were caused by overload tearing, stress-corrosion cracking, hot-salt cracking or fat igue was not clear. Other possible causes of cleavage-like cracking, e .g. the presence of hydrides as proposed for previous failures, contam ination of HAZs by oxygen/nitrogen, are discussed. Possible ways of pr eventing further failures are then outlined.