Abstract:

ESDU 90018 presents the results of over 150 axial load fatigue tests extracted from the literature on the joints under variable amplitude loading (the FALSTAFF loading sequence). A high load transfer joint is one in which more that 30 per cent of the axial load is transferred between members and many joints in aircraft wings fall in that category. Details are given of the seven joint designs tested intended to simulate typical structural features, together with details of the two countersunk proprietary fasteners used. Data for the alloys, which were 7050-T76 and 7050-T7651, are included. Tests were made with both clearance-fit and interference-fit fasteners and in some cases the holes were also treated to induce compressive stresses. The effects of these differences are discussed. Despite being tested under axial load, in some designs due to the asymmetric distribution of stresses a bending moment, termed secondary bending, was induced in the region of the fasteners. Using strain gauges the secondary bending was measured and the results are presented in terms of the peak stress, including that due to secondary bending, plotted against FALSTAFF cycles. In addition, results are tabulated for the percentage of load transferred and the measured values of secondary bending. It is noted that to use the results for other joints, secondary bending must be taken into account, and that caution must be observed if the results are applied to joints different in geometry or size from those tested.