A comparison of fatigue-crack propagation behavior in sheet and plate aluminum-lithium alloys

The influence of wrought product form on the fatigue-crack propagation resistance of aluminum-lithium alloys was examined; specifically, results on the growth kinetics of long (greater than 10 mm) fatigue cracks in peak-aged AlLiCuZr 2090-T8X and AlLiCuMgZr 2091-T8 alloys, fabricated as sheet and plate, are presented as a function of microstructure, load ratio and specimen orientation. Contrary to popular belief, it was found that fatigue-crack growth rates at equivalent stress intensity levels are significantly faster and less dependent on specimen orientation in sheet than in plate. Such differences are attributed to the prominent role of crack-tip shielding during fatigue in these alloys, resulting from crack deflection and consequent crack closure from wedging of fracture-surface asperities, which microstructurally is related to variations in the degree of recrystallization, grain morphology and texture between the two product forms.