Short cracks initiated in Al 6013-T6 with the focused ion beam (FIB)-technology

Abstract The fatigue crack growth behaviour of short corner cracks in the Aluminium alloy Al 6013-T6 was investigated. The aim was to determine the crack growth rates of small corner cracks at a stress ratio of R  = 0.1, R  = 0.7 and R  = 0.8 and to find a possible way to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M ( T ) – specimens, at one side of a hole (O = 4.8 mm) by cyclic compression ( R  = 20). The precracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a focussed ion beam. The results of the fatigue crack growth tests with short corner cracks were compared with the long fatigue crack growth test data. The short cracks grew at Δ K -values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same Δ K -values and the same stress ratios. A model was created on the basis of constant K max -tests with long cracks that gives a good and conservative estimation of the short crack growth rates.