Connection State Detection And Loose Recognition For Fasteners On A Stiffened Plate

Stiffened panel is one of the most typical bolts connection fastener in aviation structure. It is widely used in the fuselage and wing of the aircraft. Vibration and impact load often changes bolt connection state, leading to bolt pretightening force reducing or completely disappearance. Bolt loosening failure will damage the integrity, function and safety hazards of structures, even lead to catastrophic accidents. Therefore, detection and recognition of stiffened plate loosening bolts fastening is one of the main contents of aviation structure health monitoring. Aiming at the problem of bolt loosening, this paper proposes adetection and recognition method for bolt loosening strain modal theory combining and fiber Bragg grating sensing technology.Firstly, the development status of state detection and damage identification of the fastener bolt connection at home and abroad is described. Detection and identification methods based on dynamic parameters, mechanical and electrical impedance and acoustic emission technique are compared. On the summary of various methods, this paper adopts the structural modal parameters as the eigenvalue in detection and recognition of stiffened plate bolt fasteners loose.This paper analyzes aerospace fastener connection form and characteristics, introduces several common bolts fastening connection failure mode, and the sensitivity of the modal parameters of stiffened plate fastener structure to bolt loosening. Numerical simulation has been made based on ANSYS software to analyze the modal parameters of stiffened plate structure in different locations of bolt loosening and different degrees of bolt loosening. The simulation results show that strain modal parameters eigenvalue can be very accurate for identification of bolt loosening, but natural frequency and vibration mode is not sensitive to this kind of damage. Then method of testing strain mode of structures is put forward in actual engineering.Sensitivity of strain modal parameters to bolt loose damage is certificated through the experiments. With the increase of bolt looseness, strain modal parameters has no obvious change in the no damage parts, but strain modal parameters changes in great degree in the damage location. Obviously, using fiber Bragg grating sensor to measure the strain response amplitude in the bolts location and doing normalized processing, can be used as an effective index of injury, to identify the location and degree of bolt loosening. Finally, based on the strain measurement data, a BP neural network is developed to simulate the identification of accurate positioning of bolt loose damage.The simulation analysis and experiment results show that: loosen detection method combining fiber Bragg grating sensor and strain modal parameters can be very good for location and degree recognition of stiffened panel bolt loose damage. It has a certain reference value for the analysis and health monitoring of aircraft stiffened panel.