Effect Of Cold Expansion On Fatigue Life Of Metal Laminated Elements

The fatigue of fastener hole is very serious in aircraft structure. Cold expansion is one of the most effective ways to improve the fatigue life of structures, which is commonly used in aircraft anti-fatigue workmanship design. In practical applications, in order to ensure the hole is concentric and to improve efficiency, cold expansion is usually processed by laminated. For the purpose of simulating the fact preferably, it is necessary to study cold expansion of laminated elements.In this paper, the FE model of metal laminated elements which consists of 7075 Aluminum alloy, Ti-6AL-4V Titanium alloy and 2024 Aluminum alloy was built based on 3-D nonlinear finite element method by MSC.Patran/Marc. Different cold working process (split-sleeve expansion and mandrel cold expansion) were applied to the hole of laminated elements, and the residual stress distribution of metal laminated elements composed of Aluminum and Titanium alloys was calculated. The residual stress distribution of laminated elements and single plate shows that tangential residual stress of laminated elements agreed with that of single plate. Compared with mandrel cold expansion process, split-sleeve expansion is more effective. In the meantime, the predicted residual stress distribution agreed with the residual stresses measured by X-ray diffraction method. In addition,the effect of value of expansion and friction coefficient on cold expansion is analysed.Based on residual stress field, the stress and strain of non-cold expansion, mandrel cold expansion and split-sleeve expansion under fatigue load was obtained by FE simulation. The fatigue life under low stress is predicted by stress fatigue method. Meanwhile, the fatigue life under high stress is predicted by strain fatigue method.In addition, different cold working process (split-sleeve expansion and mandrel cold expansion) were applied to the hole of laminated elements. These specimens were then subjected to cyclic loading which included high stress level and low stress level. The experimental results were analysed in order to determine the cold working effect on the fatigue life improvement. Experimental results reveal that split-sleeve expansion is more effective than mandrel cold expansion process in improving fatigue life of fastener hole. The fatigue life under low stress level gets more improvement than that under high stress level. The fatigue life improvement factor of Aluminum alloy is bigger than that of Titanium alloys. Finally, Comparison between the result of experiment and the estimated result shows that they are fitting well.