Fatigue and fracture behaviour of structural Al-alloys up to very long life regimes

Very long life fatigue and near-threshold fatigue crack growth behaviour of 7075 and 6061 Al-alloys in T6 condition were investigated using accelerated fatigue testing at 19.5 kHz on a piezoelectric machine. Significant interfacial voids were observed in the two Al-alloys during crack initiation and early fatigue crack growth process. Fatigue failure in Al-alloys could be ascribed to: the formation of a number of fatigue voids, their growth and coalescence, and the subsequent formation and propagation of macroscopic cracks. Moreover, fatigue crack growth rates of small cracks in these alloys were found to be greater than those of large cracks for the same stress intensity factor range.