Experiment on single bolted joints in laminates under multilevel fatigue loading

A series of quasi-static tension experiments and constant amplitude tension-tension fatigue experiments for researching the multilevel fatigue behavior under real cyclic loading were performed on single bolted joints in laminates with the ratio of the width of the laminate to the diameter of the central hole being 3 and the ratio of the distance between the central hole and the end of joint to the diameter of the central hole being 3. Real damage types and failure rules of single bolted joints in laminates were also discussed to provide the basis of stress level selecting of multilevel fatigue experiments.Finally,an experimental program of researching fatigue behavior under two-level fatigue for low-to-high and high-to-low loading sequence was conducted.Fatigue damage index was calculated and analyzed by using the classical linear progressive damage method of Miner rule.The results show that the fatigue damage index is larger than 1 when the loading sequence changes from low-level to high-level.On the contrary,the fatigue damage index is less than 1.