Damage Monitoring Of CFRP Bolted Joint Based On Flexible Eddy Current Sensing Film

Because of the excellent performance of Carbon Fiber Reinforced Polymer(CFRP),it has been widely used in aerospace industry.To meet the demands of inspection,transportation and maintenance,large CFRP structure needs to arrange some separation surfaces,utilized to connect two or more pieces effectively,in the process of aircraft CFRP structure manufacturing.Bolted joint is one of the most common ways for connection of aircraft CFRP structure,but it is easy to cause damage around hole edge during loading due to stress concentration effect,which could affect to the integrity of structure.Due to bolted joint of CFRP structure usually with lager thickness,complicated damage of hole-edge,and hard to detect/monitor,this paper designed three kinds of flexible eddy current sensing film bonded on bolt of bolted joint structure innovatively,based on FPC technology,applying active current to coil to realize monitoring different kinds of damage of bolted joint of CFRP structure.This paper,from easy to difficult,from metal joint structure to CFRP joint structure,optimized to design flexible eddy current sensing film and proved it feasibility and effectiveness of hole-edge damage monitoring by simulation and experiment.1.In order to monitor the radial crack extension of hole-edge of the CFRP structure,a double rectangular coil space structure is designed.Active coil and passive coil covered with the entire thickness of joint structure.Using FEM,the result show that:in the metal joint structure,with the expansion of radial crack,induced voltage of the passive coil increased;in the CFRP joint structure,induced voltage of the passive coil are the same with metal joint structure,but the change rate is relatively small.Under laboratory conditions to establish monitoring system of radial crack extension of metal joint structure and hole-edge extrusion of CFRP joint structure,analysis of sensing film on the hole-edge damage monitoring result,the experimental results are consistent with simulation results.The feasibility and effectiveness of this eddy current sensing film to monitor the damage along the radial expansion of CFRP structure are verified.2.In order to monitor axial crack extension of bolted joint of CFRP structure,A new kind of coil,single active coil and passive coil array,is designed.Active coil covered the entire thickness of bolted joint structure,and passive coils are uniformly distributed along the bolt axial direction.Using FEM,the result show that:due to eddy current skin effect,passive coil is only sensitive to corresponding layer material,not to other axial layer.In the experiment of monitoring different layer by this array film,the results are same with simulation.The feasibility and effectiveness of this eddy current array sensing film to monitor the axial position of bolted joint of CFRP structure are verified.3.In order to monitor delamination of bolted joint of CFRP structure,some rectangular coils connected forming flexible eddy current sensing film are designed.Its coil route direction is along the direction of the screw axis mainly,which is perpendicular to the layered plane.The simulation results show that induced voltage of the passive coil increase slightly due to the expansion of the delamination damage,and this kind of eddy current film realized monitoring the delamination of CFRP structure theoretically.But the experiment needs to be further verified.This paper results for further study of CFRP bolted joint structure damage of real-time monitoring has important academic value,to ensure the safety of aircraft CFRP structure,and has important practical significance and application value to avoid major safer accidents caused by bolted joint structure failure.