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

An investigation of the interactions between mechanical and environmental v ariables on the short-fatigue-crack growth rate (FCGR) for a 2024-T3 alumin um alloy in 0.5M NaCl solution was carried out. Fatigue-crack growth tests were performed under a constant stress-intensity-factor range (Delta K) con trol using single-edge-cracked tension specimens. The relationship between FCGR and crack length (0.5 to 15 mm) was determined at a cyclic frequency o f 10 Hz over six Delta K levels (4, 5, 6, 7, 8, and 10 MPa root m), two loa d 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) w ere 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 l evels (Delta K < 10 MPa root m and R = 0.1) and are more pronounced at high er oxygen levels. Fractographic examinations suggested that hydrogen embrit tlement is responsible for the environmental enhancement of the FCGR for bo th short and long cracks in this material/environment system. A transport m odel was developed to estimate the crack-tip oxygen concentration and to ex amine its correlation to changes in the FCGR with crack length. The model c orrectly accounted for the decrease in short-crack effect on the FCGR with crack length under a given mechanical condition at each oxygen level, but d id not explain the disappearance of short-crack effects at Delta K greater than or equal to 10 MPa root m.