Investigation into Corrosion Pit-to-Fatigue Crack Transition in 7075-T6 Aluminum Alloy

Abstract

Transition of corrosion pit to crack under fatigue condition was investigated in high-strength 7075-T6 aluminum alloy. The pit was formed at the edge of a hole in a specimen. Specimen was subjected to a constant stress during the pit formation. Two types of corrosion pit were considered: corner-pit and through-pit. Two sizes were tested for each pit type. Also, the baseline data of cycles to initiate a 250-µm-long crack were established when the corrosion pit was created without any applied stress on the specimen, i.e., Sappl = 0. The cycles to initiate a 250-µm-long crack initially decreased with increasing Sappl relative to the baseline value and then increased with increasing Sappl such that this increase was significant with higher value of Sappl. The transition between this increase and decrease occurred when the Sappl was greater or less than a value which caused the onset of plastic deformation at the root of the pit, respectively. Microstructural analysis showed that this decrease in cycles to initiate the crack was due to microcracks at the pit front which developed at the lower level of Sappl, and the increase was due to plastic deformation at the higher levels of Sappl.