Effects of ultrasonic impact treatment on fatigue behavior of naturally exfoliated aluminum alloys

This paper presents the results of an investigation into the effects of an ultrasonic impact treatment (UIT) on the fatigue behavior of naturally exfoliated 7075-T6511 alloys. The 7075-T6511 specimens were fabricated from C-141 aircraft upper wing skin panels. A non-destructive ultrasonic inspection was carried out to determine the depth of the exfoliation corrosion damage. Some corroded specimens were treated using UIT. Fatigue tests were carried out using the naturally exfoliated and UIT treated specimens. With the aid of scanning electron microscopy (SEM), fractographic analyses were carried out on the fatigue specimens to determine the crack nucleating mechanism. The preliminary results suggested that the UIT had a potential to extend the fatigue lives of the naturally exfoliated 7075-T6511 specimens to some extent, depending on the level of exfoliation. For the lightly exfoliated specimen, the UIT changed the crack nucleating mechanism from intergranular cracking in exfoliation to fretting in the grips, which then resulted in a much longer fatigue life. For the severely exfoliated specimen, the UIT improved the fatigue life slightly, but the crack still nucleated from intergranular cracking, which could not be removed by the UIT process.