21st Congress of International Council of the Aeronautical Sciences, Melbourne, Australia, 13-18 September, 1998
Paper ICAS-98-5.2.2

THE INFLUENCE OF CORROSION ON THE FATIGUE AND FRACTURE BEHAVIOUR OF 7050-T76 ALUMINIUM ALLOY SPECIMENS CONTAINING COLD EXPANDED HOLES

Glinos N., Wagstaff P. G., Cook R.*
Kingston University, United Kingdom; *DERA, United Kingdom

Keywords: corrosion, fatigue, fracture, aluminium alloy, cold expanded holes

A test programme to assess the influence of corrosion on the fatigue performance of cold expanded fastener holes has been carried out at Kingston University on specimens of 7050-T76 aluminium alloy. Previous work on specimens subjected to prior atmospheric corrosion showed a 75% reduction in fatigue life when compared to non-corroded material due to cracks initiating from corrosion pits and causing premature failure. Tests at stress levels close to the fatigue limit of cold expanded specimens (l75MPa) produced failure due to cracks originating from corrosion pits remote from the hole. At stress levels above 200MPa failures originated from the edge of the hole. The current investigation examines the influence of the degree of corrosion on this transition. Tests at an applied stress close to the fatigue limit on specimens subjected to corrosion in 0.35% NaCI solution show that after a minimum period of 7 days in a salt bath, failure took place from cracks originating at corrosion pits. A microscopic investigation has been carried out to define the characteristics of these corrosion pits. Tests were also carried out on specimens subjected to more severe corrosion (3.5% NaCI solution) with cold expansion performed prior to or after exposure to the corrosive environment. Cold expansion prior to corrosion exposure gave on average 66% better fatigue performance compared with specimens cold expanded after corrosion. These tests simulate the difference between the use of cold expansion for manufacture and for repair procedures.

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