Special load histories are adopted to obtain information about the behavior of the moving crack tip during the increasing and decreasing part of a load cycle. It is associated with the crack opening and closure of the crack tip. Secondly, modern SEM techniques are applied for observations on the morphology of the fractures surfaces of a fatigue crack. Information about the cross section profiles of striations are obtained. Corresponding locations of the upper and the lower fracture surface are also explored in view of the crack extension mechanism. Most experiments are carried out on sheet specimens of aluminum alloys 2024-T3, but 7050-T7451 specimens are also tested in view ofa different ductility of the two alloys.

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Presented in this paper are results of an experimental investigation on the rivet flexibility and load transmission in a riveted lap joint representative for the aircraft fuselage. The test specimens consisted of two aluminium alloy Alclad sheets joined with three rows of rivets. Rivet flexibility measurements have been performed under constant amplitude fatigue loading using several methods including two novel optical techniques developed by the present authors. The axial forces in the sheets required to determine the rivet flexibility have been derived from strain gauge measurements. In order to eliminate the effect of sheet bending the strain gauges have been bonded at the same locations on the outside and faying surface of the sheet. Variations of the rivet flexibility during the fatigue loading as well as the dependence of the rivet flexibility and load transmission on the rivet squeeze force have been studied. The experiments enabled to evaluate the usefulness of various techniques to determine the rivet flexibility. It was observed that although the measured flexibility was identical for both outer rivet rows, the load transfer through either of these rows was different. Previous experimental results of the authors suggest that the non-symmetrical load transfer distribution through the joint is associated with large differences between the rivet hole expansion in the sheet adjacent to the driven rivet head and the sheet under the manufactured head. It has been concluded that commonly used computation procedures according to which the load transfer is only related to the rivet flexibility may lead to erroneous results.@en