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Session: Session 2: Fatigue crack growth and life prediction methods  I
Session starts: Monday 26 June, 13:40
Presentation starts: 13:40
Room: Theatre room: plenary

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Ben Main (Defence Science and Technology Group)
Ben Dixon (Defence Science and Technology Group)
Michael Jones (RMIT University)
Simon Barter (RMIT University)

Abstract:
Aluminium alloy (AA) 7085-T7452 is a recent addition to the 7XXX series of aluminium (Al), zinc (Zn), magnesium (Mg) and copper (Cu) high strength aerospace alloys which has applications in primary airframe structure of the Airbus A380 and Lockheed Martin F-35. AA7085-T7452 was developed for large unitized, lightweight airframe structures since its low quench sensitivity and good through-thickness fracture toughness enables forgings of up to 12 inches (305mm) thick. The Defence Science and Technology Group (DSTG), working with RMIT University, have completed several studies concerning small fatigue crack nucleation and growth in this material using specimens with representative production surface finishes and loaded with service representative spectra. This paper presents an overview of observations concerning fatigue crack nucleation and small fatigue crack growth rates (FCGR) in AA7085-T7X in contrast with other 7XXX-T7X alloys. Two influences on the fatigue durability of this alloy were investigated. Firstly, the ‘fatigue crack like effectiveness’ of surface etch pitting arising from the commonly used Type 1C anodising process was assessed by deriving equivalent initial discontinuity size (EIDS) values via a fractography-based method. This work showed etch pitting associated with Type 1C anodising surface treatments is less effective in nucleating fatigue cracks in AA7085-T7X compared to AA7050-T7X. Secondly, FCGRs for physically small or near-threshold fatigue cracks were quantified for AA7085 using fractography-based measurements and these were compared with equivalent measurements for AA7050 and AA7075 in the T7X condition. Here, small crack and near-threshold FCGRs in AA7085 were largely consistent with those for AA7050 and AA7075 T7X materials. These results highlight the authors’ current progress toward their goal of understanding the fatigue properties of AA7085-T7452 well enough to allow accurate fatigue life predictions for structural certification and sustainment of AA7085-T7452 airframe components.