Short Cracks Initiated In Al 6013-T6 with the FIB-Technology

Kurzfassung

The fatigue crack growth behaviour of short corner cracks in the Aluminium alloy Al 6013-T6 was investigated. The crack growth tests were performed according to ASTM E647 with M(T) and SENT specimens under computer control on closed loop servo-hydraulic testing machines at room temperature in laboratory air. The load wave form was sinusoidal at test frequencies of 30-60 Hz. Corner cracks were introduced into the specimens at one side of the rivet hole by compression loads (R=20). The notch was cut with the focussed ion beam technology. The length of the precracks (including the notch) was smaller than 100µm. Results of the tests with short corner cracks were compared with the long crack test data. The AFGROW simulation of the da/dN- K curves is based on long crack growth data. The K-solution used in the simulation was for a corner crack at a hole. The K-factors for the experimental data were calculated with the same formula as in AFGROW, but for the crack growth rates the optically measured crack lengths were used. Three important differences can be seen in this comparison: 1. Short cracks grow at K values below the threshold for long cracks at the same stress ratio R. 2. The difference in the crack propagation rate between the experimental data and the simulated data becomes bigger at smaller R-ratios. 3. The threshold value for short crack growth is just a little bit smaller than the threshold for long cracks at the highest stress ratios reachable in experiments.