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This work was undertaken as a comparative study on fatigue properties of a typical third generation Aluminum-Lithium alloy, AA2198-T8, to that of a traditional aerospace aluminum-copper alloy, AA2024-T3, with a focus on developing a methodology for reconstructing fatigue crack growth history under two different fatigue loading functions: a) constant amplitude; and b) marker band loading conditions (i.e., using two different R-ratios). Fatigue fracture surface morphologies were examined to determine the effect of alloying and loading direction relative to the specimen's rolling direction on fatigue crack growth behavior. The fracture surface morphologies of the two alloys were markedly different: the AA2024-T3 exhibited substantial meandering fracture surface, with localized fatigue cracks progressing along multiple directions and at different rates. The AA2198-T8 alloy exhibited more uniform but shallower fatigue striations, with numerous micro and macro interlaminar cracks. The effect of constituent size on the localized fatigue crack progression and rate in the two alloys could be discerned directly. Fatigue crack growth rate (da/dN) measurements were taken from the fractographs, using striation based and marker band based analyses, from which fatigue crack initiation cycle was estimated and compared with the ultrasonic inspections. |
