Several test series on crack propagation in light alloy specimens were performed at a positive mean stress and axial loading. Results are presented on the following aspects: (1) The frequency effect was studied by comparing the crack rate at a low and a high frequency, viz. about 20 and 2000 c/min respectively. Some fractographical observations were made. (2) The effect of varying the stress amplitude and applying high peak loads on the crack propagation was studied in comparison with constant-amplitude data. (3) The results of stiffened panels were compared with the results of a complete structure and the results of small sheet specimens. At the end of the paper the conclusions are summarized.
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