Fatigue Crack Nucleation and Growth in Recrystallized Annealed Ti‐6Al‐4V Combat Aircraft Structures

ABSTRACT In the Swiss Air Force (SAF) F/A‐18C/D Hornet fleet, recrystallized annealed Ti‐6Al‐4V was selected for the wing carry through bulkheads to meet their demanding operational usage profile requirements that included increased airframe strength and toughness. This is in contrast to the aluminum alloy AA7050‐T7451 wing carry‐through bulkheads in all other global fleets. The Defence Science and Technology Group, in partnership with RMIT University, RUAG, and armasuisse, completed a full‐scale durability and damage tolerance test to support Swiss F/A‐18C/D life extension activities. This test program included the development of 24 quantitative fractography reports examining instances of both naturally occurring fatigue cracking and fatigue cracking from artificially induced damage at analytical hotspots for the SAF to refine its fatigue life prediction of the center‐barrel. This paper presents the key findings from the quantitative fractography reports where nucleating discontinuities, for naturally occurring fatigue cracks, are presented along with crack growth rates. The results here show that there can be significant discrepancies between analytically predicted crack growth rates and those achieved in a full‐scale test article. This test program nonetheless provided the significant evidence necessary for the SAF to be able to extend the life of center‐barrel wing carry‐through bulkheads for their aircraft fleet.