Mechanical and chemical aspects of corrosion fatigue of a 2024-T3 aluminum alloy in the short crack regime

The primary objective of this study was to quantitatively investigate the effects of mechanical loading and environment on the enhancement of crack growth rate for 2024-T3 aluminum alloy in aqueous environments. The effect of crack size on growth rates in the chemically short fatigue crack regime was specially emphasized.; Experiments were performed to study the crack growth response of 2024-T3 alloy in aqueous environments, using single-edge-cracked tension (SEC(T)) specimens tested under constant stress intensity factor range ({dollar}Delta{dollar}K) conditions. The relationship between crack growth rate and crack length (0.5 to 15.0 mm) was determined at 10 Hz cyclic frequency over six {dollar}Delta{dollar}K levels (4, 5, 6, 7, 8, and 10 MPa-m{dollar}sp{lcub}1/2{rcub}){dollar}, two load ratios (0.1 and 0.5), and three dissolved oxygen concentrations (0, 7, and 30 ppm). Experiments in gaseous environments (namely, high-purity oxygen and water vapor) were also conducted for comparison.; Chemically short crack effects were observed, with crack growth rates in the short crack regime accelerated by as much as a factor of two. Short crack behavior was determined to depend strongly on the mechanical driving force {dollar}Delta{dollar}K, load ratio R, and dissolved oxygen concentration. The observed crack size effect tends to appear at lower load ({dollar}Delta{dollar}K and R) levels and is more pronounced at higher oxygen levels.; Fractographic examinations of specimens indicated no noticeable differences in the fracture modes of short and long cracks. In addition, fracture mechanisms appeared to be identical for specimens tested in water vapor and in aqueous environments. The fracture surface produced in oxygen, on the other hand, showed relatively large deformation and featureless regions. The fundamental working hypothesis that hydrogen embrittlement is the cracking mechanism for 2024-T3 alloy in aqueous environments is supported by the fractographic studies. The observed higher growth rates of short cracks are associated with the acceleration in kinetics of electrochemical reactions caused by the oxygen around the crack tip, rather than by a change in cracking mechanism. In other words, the decrease in fatigue crack growth rate with crack length is associated with the depletion in dissolved oxygen at the crack tip.; A model was developed to estimate the crack-tip oxygen concentration and correlate it with the change in crack growth rate with crack length using theoretical and numerical approaches. The model accounts for the supply (by diffusion and convection) and consumption (on crack walls) of oxygen, and predicts a consistent trend for crack growth behavior under different mechanical loading in each environment. However, the limited predictive capabilities of the model for the absence of short crack effects at higher {dollar}Delta{dollar}K levels require further studies in the future.