In-situ SEM study of fatigue micro-crack initiation and propagation behavior in pre-corroded AA7075-T7651

Fatigue micro-cracking behavior in pre-corroded AA7075-T7651 is analyzed by in-situ scanning electron microscopy (SEM) observation and SEM-based fractography. The experimental results show that (i) fatigue microcracks nucleated mainly from inclusions at the specimen notch and from corrosion pits on the specimen surface, (ii) pre-corrosion severity degree and maximum stress level significantly affected the extent and time of microcrack initiation. Corrosion pits are shown to play a crucial role in fatigue micro-crack propagation, due to crack deflection (light corrosion) and crack coalescence (severe corrosion). The growth rate of minor cracks was suppressed by pre-dominant rapid propagation of the main crack.