Fractography of fatigue crack propagation in 2024-T3 and 7075-16 aluminum alloys in air and vacuum

Electron fractography and transmission electron microscopy were used to study fatigue crack propagation processes in 2024-T3 and 7075-T6 aluminum alloys in vacuum and air. There was evidence that crack growth occurs cycle-by-cycle in vacuum, but only for fa tigue in air was it possible to relate dislocation substructure band spacings, immediately below the fracture surface, to cyclic crack growth. A discussion of crack propagation mechanisms suggested that the Tomkins and Biggs model of striation formation comes closest to explaining the fractographic features.