Effects of load ratio and frequency on the fatigue behavior of 7050 aluminum alloys in salt water

Corrosion fatigue (CF) experiments in 3.5 wt% NaCl solution have been performed on peak-aged (T6) and overaged (T73) 7050 aluminum alloys in the short transverse (ST) orientation as a function of load ratio and frequency. Results indicated that an increase in load ratio resulted in an increase in fatigue crack growth rate (FCGR) and reduction in fatigue life for both tempers. The CF life (number of cycles to failure) of smooth-surface specimens in both tempers was also decreased as a result of reducing the frequency. However, as the frequency was decreased, the FCGRs were increased at intermediate and high stress intensity factor range (ΔK) regions but decreased at low ΔK region. The reverse frequency dependence of FCGR was attributed to the greater oxide-induced crack closure effect at low ΔK region for low frequency. In contrast to the longer fatigue lives of 7050-T6 over -T73 smooth-surface specimens under a low load ratio and high frequency, 7050-T73 smooth-surface specimens exhibited longer fatigue lives than 7050-T6 ones under a high ratio and low frequency. This reverse trend in resistance to CF for smooth surface between 7050-T6 and -T73 alloys could be attributed to the greater SCC resistance possessed by T73 temper under high mean stress and low frequency, even though more extensive and larger corrosion pits were generated in the T73 temper.