Small fatigue crack nucleation and growth in aluminium alloy 7085-T7452
Creators: name: Main, Benjamin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400101
subject: Aerospace materials
classificationCode: 400102
subject: Aerospace structures

description: Aluminium alloy (AA) 7085 is a recent addition to the 7XXX series of aluminium (Al), zinc (Zn), magnesium (Mg) and copper (Cu) high strength aerospace alloys with applications in primary airframe structure of the Airbus A380 and all variants of the Lockheed Martin (LM) F-35. T7452 is the temper and designation of the die-forged product, which 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) thickness. AA 7085 has a lower Mg and higher Zn content than comparable alloys such as 7050. As a relatively new material at the time of publication, there is little fatigue data available in the published literature and none with a focus on the small or near-threshold fatigue crack growth regime where sub-millimetre fatigue cracks spend most of their lives in service structures. Further, there is little research into fatigue crack nucleation mechanisms in this material in its final, surface-finished airframe part production form with applied service loading. This research project examines small fatigue crack nucleation and growth in AA 7085-T7452, comparing it with other 7XXX series alloys. The role of material composition, microstructure and production surface finish treatments in fatigue crack nucleation and growth are given prominence and more accurate fatigue crack growth tools are sought.

This research addresses gaps in the open literature centred on AA 7085-T7452 small fatigue crack nucleation mechanisms and growth rates with an applied, aircraft industry focus. The objectives of this research were to:
i. establish the importance of accurate fatigue crack growth rate data and models in the small crack and near-threshold regime in the context of engineered, AA 7XXX aircraft production parts;
ii. quantify the small and near-threshold fatigue crack growth behaviour of AA 7085-T7452 relative to other 7XXX alloys including AA 7050-T7451 and AA 7075-T7351;  
iii. determine the extent to which microstructure can be significant in influencing small fatigue crack nucleation and growth rates in AA 7085-T7452;
iv. establish the causal factors and significant influences on small fatigue crack nucleation and equivalent initial damage sizes (EIDS) in AA 7085 versus AA 7050 for Type 1C anodised airframe production parts; and to
v. establish the fundamental material mechanisms that give rise to improved resistance to fatigue crack nucleation of AA 7085-T7452 over AA 7050-T7451 in Type 1C anodised material representative of aircraft production parts. 

Significant new knowledge arising from this research project includes:
i. characterisation and validation of small and near-threshold fatigue crack growth rates in AA 7085-T7452 and ranking relative to other AA 7XXX alloys;
ii. an evaluation of the role of microstructure and common aircraft production surface finish treatments on small fatigue nucleation and growth rates in AA 7085-T7452;
iii. development of EIDS distributions for crack nucleating etch pits associated with Type 1C anodising of AA 7085-T7452 and AA 7050-T7451 alloys and establishing the significant role of material microstructure and stress;
iv. material characterisation of crack nucleation sites in Type 1C anodised AA 7085-T7452 and AA 7050-T7451 specimens demonstrating the significance of the population, size, shape and distribution of Al7Cu2Fe intermetallics on fatigue crack nucleation.

While this dissertation identifies various avenues for further inquiry and investigation, it records substantial progress in the understanding of the nucleation and growth of small fatigue cracks in AA 7085-T7452 and how these behaviours may be incorporated into fatigue design and fatigue tools for the certification and sustainment of aircraft structures.