Abstract
Abstract The near-threshold crack extension mechanisms of long cracks in a commercial aluminum alloy in peak-aged and overaged condition were evaluated at a stress ratio of R = −1. Tests were performed both in a conventional resonant testing machine, using flat dog-bone specimens with through-wall cracks and on an ultrasonic fatigue testing system using hourglass-shaped samples with surface cracks. After introducing the initial crack, the crack was propagated at a nominally constant Kmax with values in the range of the initially determined threshold. The primary precipitates were found to act as microstructural barriers causing a local pinning of the crack front independent of the testing procedure and hence keeping the crack from extending continuously. This effect depends on the orientation of the initial crack with respect to the rolling texture which, in turn, determines the spatial distribution of the primary precipitates and the grain morphology. Overaging had a tendency to enhance the pinning potential of the primary precipitates.
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