No abstract

The experimental investigation were conducted in order to study the dynamic strength of the mechanically fastened composite joints, involving interference-fit joints with blind bolt and sliding-fit joints with high lock bolt. The joint type of the specimens researched was single-bolt, double-lap, which was a standard test configuration in ASTM for composite joints. Experimental results showed that the fatigue performance of the blind bolted joint was better than that of the high-locking bolted joint. The fatigue life of the blind bolted joints was 10-100 times higher than that of the high-locking bolted joint under the same cyclic stress, and the limit fatigue strength of the blind bolted joints was over 50MPa higher than hat of the high-locking bolted joints.

Due to the manufacturing restrictions or design requirements, composite structures are usually connected in the form of butt joint near the wing rib or fuselage frame to solve the problem of discontinuity of load transfer. In this paper, the finite element model of butt joint structure is established by simulating the engineering model of metal fasteners and common joint connected by beam element under the action of in-plane load, and the mechanical simulation analysis of typical butt joint configuration is carried out. In the engineering model, the allowable stress value is used to predict the failure of composite material and the classical strength theory is used to predict the fracture failure of metal fasteners. The result show that the strength prediction obtained by the engineering model is in good agreement with the test results, and the application of this method can improve the calculation efficiency of connection strength with complicated multi-fastened composite structures.

Experimental investigations were conducted to determine the effect of several influencing factors on the fatigue behavior of interference-fit bolted composite joints. The sizes of interference fit, different materials of laps, and stacking sequences of main laminates were separately considered. Tension-compression reversed force/stress ratio, R=-1, was selected to evaluate the fatigue tests. The appropriate levels of fatigue stress were determined by the ultimate bearing strength of the fastener structure obtained from the static tensile tests. The bearing stress and the fatigue life (S-N) data of all specimens were presented and the relationship between influencing factors and fatigue life were obtained. The experimental results showed that the appropriate size of interference fit and the matched laps could improve the fatigue life of composite joint, however stacking sequences of main laminates was not sensitive to the fatigue life of composite joints.

Based on the contact fretting fatigue theory and fatigue tests, the fastener fatigue fracture mechanism of composite large interference fit connection was conducted in this paper. Test results showed that the fatigue fracture mode of composite mechanical connections in large interference fit (1.8% and 3%) were fastener fatigue fracture. Under the impact of cyclic load, fastener fretting wear and damage type of fretting fatigue were generated between blind bolted fastener and composite hole. According to the theory of fretting white layer, the failure mechanism of fastener fatigue fracture was explored, which provided theoretical basis and experimental basis for the design of mechanical connected structure with higher fatigue life.