Interactions between mechanical and environmental variables for short fatigue cracks in a 2024-T3 aluminum alloy in 0.5M NaCl solutions

Abstract

An investigation of the interactions between mechanical and environmental variables on the shortfatigue-crack growth rate (FCGR) for a 2024-T3 aluminum alloy in 0.5M NaCl solution was carried out. Fatigue-crack growth tests were performed under a constant stress-intensity-factor range (‡K) control using single-edge-cracked tension specimens. The relationship between FCGR and crack length (0.5 to 15 mm) was determined at a cyclic frequency of 10 Hz over six ΔK levels (4,5,6,7, 8, and 10 $MPa\sqrt m$), two load ratios (R) (0.1 and 0.5), and three dissolved oxygen concentrations (0, 7, and 30 ppm). Tests in gaseous environments (namely, high-purity oxygen) were also conducted for comparison. Short-crack effects were observed, with the FCGR in the short-crack regime accelerated by as much as a factor of 2. The observed crack-size effects tend to appear only at the lower loading levels (ΔK<10 $\Delta K < 10MPa\sqrt m$ and R=0.1) and are more pronounced at higher oxygen levels. Fractographic examinations suggested that hydrogen embrittlements is responsible for the environmental enhancement of the FCGR for both short and long cracks in this material/environment system. A transport model was developed to estimate the crack-tip oxygen concentration and to examine its correlation to changes in the FCGR with crack length. The model correctly accounted for the decrease in short-crack effect on the FCGR with crack length under a given mechanical condition at each oxygen level, but did not explain the disappearance of short-crack effects at ΔK≥10 $\Delta K \geqslant 10MPa\sqrt m$.