Abstract: Composites have been widely used as the main load-bearing structures′ material in aircraft, and among various structures, stepped topology has been chosen to become the main form of fuselage repairing and connection due to the easy implementation. Although a quantity of literature has given the fatigue performance of single-lap of metal, composite and two hybrids, the fatigue durability of the special multi-lap for high-efficiency load-transferring structure is seldom studied. Besides, the fatigue failure mechanism is still not illustrated adequately. In this research, the experiments on composite stepped-lap adhesively bonded structure, which is made under the process of mold making, prepreg clipping and laying firstly, autoclave solidification secondly, step surface brushing adhesive thirdly and finallycutting into specimens, were conducted. The cyclic tensile load of stress ratio 0.1 was selected in the experiment, and the visible cracks and propagation of fatigue testing process were detected through the experiments. All the specimens have been completely fractured at the end of the experiment in order to obtain the total life span. According to the linear curve fitted by S-N discrete based on the testing data, the fatigue life decreases linearly as the stress level increases. After visible cracks being found, the composite bonded structure only has 10% residual cracks expansion life until being completely fractured, which implies that the component must be repaired or replaced once visible cracks appear. For the cracking mechanism, composites matrix shear-mode damage dominates the fatigue initiation damage, while cohesive failure, adhesive failure, matrix crack, and delamination composed the fracture modes after stepped-surface fracture appearing.

Key words: composites, stepped-lap, adhesive, fatigue, cracks propagation