The Strength Analysis And Progressive Damage Research For Countersunk Composite Bolted Joints

With excellent features such as high strength and stiffness,great corrosion and fatigue resistance,composite has played an increasingly important role in design and manufacture of aircraft.In the design of aircraft structure,the number of fasteners and components is greatly reduced with the development of the structural integrity design,and the connection structure will transmit a larger load.Therefore,the connection structure becomes more critical.In the composite connections,not only high stress concentration caused by the shape of the material and structure discontinuity,but also anisotropy arose from stack sequence of composites makes the connection structure more complex.The countersunk bolt can reduce the hole stress while having good aerodynamic shape,which can also prevent the damage expanding around the hole.Therefore,it has a very wide application in the aircraft connection structure.In this paper,the research on single-lap countersunk composite-titanium alloy bolted joints is performed through the establishment of two progressive damage models with single-bolt and double-bolt.The improved Tserpes failure criterion is adopted,which take failure forms such as matrix damage,fiber fracture,and delamination under tensile or compressive conditions into account.Meanwhile,shear fracture of the countersunk bolt is taken into account as a form of joint final failure.In addition,the modified stiffness deterioration criterion is used.Based on the final failure judgement which the failure elements extend to the edge or the end of the composite laminate,local compression damage is added to make it more specific and more accurate.Using the secondary development language(APDL)of ANSYS,the above criteria are programmed and embedded in the main program to realize parametric modeling.The accuracy of the simulation model is verified through the comparative analysis of test data and simulation results.The effect of joint parameters such as stack sequence,countersink depth,tightening torque,plate width and end distance on the connection strength and initial damage is specifically discussed.After optimization analysis with software MATLAB,the optimal parameters of the joint are finally elected.Visualization analysis of the progressive damage process for the joints with the optimal parameters is conducted.Furthermore,comparing the damage process of layers with the angle of 0 °,± 45 ° and 90 ° of the single-bolt joint from the initial damage to the final failure.As the double-bolt joint,a further analysis of the layers with different locations along the thickness of the composite plate is conducted when comparing the ply orientation.