Effect of Corrosion Damage on Fatigue Behavior of Al 2024

Abstract

Abstract Fatigue crack nucleation from corrosion damage is drawing considerable attention in the aging aircraft research community. Localized corrosion, such as corrosion pits, is a potential stress raiser and provides active sites for fatigue crack initiation. The detailed knowledge of the metrics of early stages of corrosion damage and transition to fatigue cracks is critical to the accurate life prediction of the structure. High strength aluminum alloys are known to be very sensitive to orientation and this could play a critical role in both pitting and the resulting fatigue damage. Fatigue experiments on dog bone specimens of Al 2024-T3 with known pit morphology were conducted to investigate the crack initiation and growth behaviors of cracks nucleating from pits. The corrosion pits were grown electrochemically in a controlled fashion and samples were prepared with pit depth varying from 3 to nearly 130 μm. These pits were characterized using white light interferometer; surface parameters such as average roughness and morphology were determined before fatigue testing. Specimens were aligned in three different directions, L-T, T-L and L-S to investigate the effect of orientation on pitting and fatigue characteristics. The fatigue lives were observed to decrease with increasing pit depth. The fractured surface was analyzed after the fatigue test and stress intensity calculations were made based on these observations. A linear relationship between stress intensity factor and critical pit depth was observed. The crack growth behavior of the crack nucleating from these pits was typical of a short crack. The results provide the details of the critical pit size that can lead to fatigue crack initiation as well as supply information about the growth behavior of cracks nucleating from corrosion pits. An understanding of the role of pitting in crack initiation aids in extending the fatigue life of the aging aircraft.