Short Crack Growth in Al 6013-T6

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 possibility to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into M(T)-specimens at one side of a hole (Ø = 4.8 mm) by compression loads (R=20). The precracks were smaller than 100µm (notch + precrack). A completely new method was used to cut very small notches (10 – 50µm) 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 K values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same K-values and the same stress ratios. A model was created on the basis of constant Kmax-tests on long cracks that predicts the short crack growth behaviour very well.