A FATIGUE-CRACK-GROWTH-BASED ANALYSIS OF 2-STEP CORROSION-FATIGUE TESTS

A method for the analysis of two-step fatigue level sequences is propo sed and compared with experimental results. Two-step loading tests of the aluminum alloy 2017-T4 in 3% sodium chloride solution have been ca rried out in conjunction with a replica technique used to monitor the growth of fatigue cracks. Fatigue cracks were nucleated at corrosion p its 10-15 mu m in size, and crack growth rather than crack initiation was found to take up the major portion of the fatigue lifetime in thes e tests. The results could therefore be analyzed on the basis of the f ollowing constitutive relation for fatigue crack growth: da/dN = A(Del ta K-eff - Delta K-effth)(2) This analysis was simplified since the in fluence of transients in the crack growth rate induced upon change in load level was found to be minimal. The approach provides a rational b asis for dealing with load-sequence effects.