Crack Initiation and Growth from Pre-corroded Pits in Aluminum 7075-T6 Under Laboratory Air and Salt Water Environments

Small crack growth behavior, including the pit-to-crack transition, of pre-corroded aluminum alloy 7075-T6 was investigated under ambient air and saltwater environments. Through pit and corner crack pits were created prior to constant stress amplitude fatigue testing. The small crack growth life (SCGL) and crack initiation life (CIL) under salt water condition were about 5-8 times less when compared to those under ambient air. The ratio of SCGL to CIL was significantly smaller with large uncertainty (variation) under salt water environment. The small crack growth rates under salt water were 2-12 times larger than corresponding rates under laboratory air at identical stress amplitude and crack length. The crack lengths required to attain Paris region growth rates (Paris region threshold crack lengths) were about 1-3 mm for stress amplitudes less than 45 MPa. Sub-millimeter cracks attained 10⁻⁴ mm/cycle fatigue crack growth rates (FCGR) for stress amplitudes greater than 45 MPa. The crack growth rates of 0.25-mm-long cracks were about 2 times higher for salt water environment when compared to identical stress conditions in ambient air. Empirical crack growth rate versus stress intensity factor range equations similar to Paris law were developed for small crack growth and crack initiation under ambient air and salt water condition, for fatigue crack prognostics applications. For the design applications, a modified Kitagawa-Takahashi (K-T) diagram for corrosion-pitted aluminum 7075-T6 is presented.