Determination of bolt load distribution ratio of multi-bolted joints is essential for structural strength analysis, and is also the precondition for investigation of stress distribution in critical bolts and neighboring regions of plates. In this paper, the multi-bolted aluminum stepped lap joints from a certain type of aircraft are studied in terms of the bolt load distribution ratio, which is preliminarily estimated by means of the strain gauge electric testing technique. Then, a three-dimensional finite element model, taking into account of the load transferred by the bolt-head, contact friction, bolt clamp-up,material nonlinearity and anti-bending fixture, is constructed with ABAQUS software to simulate the load distribution between bolts for the joints. The calculation results reveal that edge rows carry more amount of load than the inner ones, and the step height is an important influence factor for bolt load distribution ratio of stepped lap joints. The effect of step height on uniformity of bolt load distribution is further analyzed. The analysis shows that during the elastic range, with the increase of step height, the bolt load distribution becomes more even; during the plastic range, with the increase of step height, the bolt load distribution grows even initially and then deteriorates. The configuration of stepped lap joints is optimized based on bolt load distribution and the maximum stress around the bolt-holes. It is shown that for the optimized stepped lap joints, the extreme difference of bolt load distribution ratio is less than 1.1%, and the maximum stress around the serious bolt-holes and the maximum shear stress of bolts decreases by 4.4% and 10.2% of those of the original ones, respectively.
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