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A B S T R A C T Fatigue tests were carried out on 2024-T351, thickness 1.6 mm, central hole specimens containing pins installed with five different interference-fit levels. Tests clearly demonstrated the beneficial effect of interference fit on fatigue resistance, up to the maximum value examined, 2.5%. A three-dimensional (3D) finite-element model was used in order to characterize the stress field around the hole. A large specimen, with a 40-mmdiameter hole filled with interference-fit pin, was instrumented by strain gauges and statically tested in order to check FEM results. A very good correlation existed between measured and numerically evaluated strains. FEM results demonstrated the well-known effect of interference-fit fasteners on reducing stress ranges. By increasing the interference level, the stress range was practically unchanged, while the mean stress decreased.Interference-fit produces a biaxial stress state, which must be taken into account for fatigue evaluation. In the present case, a simple criterion, based on hoop strain, predicted the fatigue results quite well with the exception of open hole fatigue test results, which were overestimated.Keywords biaxial stresses; experimental measurements; finite-element analysis; interference fit. N O M E N C L A T U R EK f = fatigue stress concentration factor i = interference R = stress ratio = S min /S max S = nominal applied stress S eq = equivalent uniaxial stress S min = minimum stress in fatigue cycle S max = maximum stress in fatigue cycle S x = radial stress S y = longitudinal stress I N T R O D U C T I O NThe fatigue resistance of mechanical joints in aircraft structures is a problem not completely solved and continuously studied because, nearly always, the fatigue resistance of aircraft structures corresponds to the fatigue resistance of the joints contained in these structures. Limiting our attention to mechanical joints, so many different elements are used that even their classification is difficult. A possible distinction can be made between solid rivets, which are plastically deformed during installation, and threadedCorrespondence: A. Lanciotti.

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Since Friction Stir Welding has been identified as a key technology for primary aerospace structures, fatigue crack growth testing and modelling is required. However, there is still a complete lack of information regarding the fatigue crack propagation in friction stir welds under variable amplitude loading and flight loading conditions. Experimental investigations have been performed on centre cracked 4 mm thick AA2024-T3 base metal and FSW specimens: regarding the FSW samples, the crack was placed 5 mm out of the weld centre, in the most critical part of the joint: the coupons have been tested under simple variable amplitude load sequences, and under a standardized flight-simulation load history, Falstaff. The fatigue crack propagation was then predicted using widespread aerospace fracture mechanics software packages. Interaction effects and internal residual stresses were firstly separately simulated and than combined in order to evaluate the ability to predict the fatigue crack propagation on FSW welded structures under service loading conditions.