Bolt State Evaluation Of CFRP Connection Based On Ultrasonic Guided Wave

Carbon Fiber Reinforced Plastic(CFRP)is more and more widely used in various fields because of its excellent performance.In the field of aerospace,rail transportation and other composite critical connection structures,the composite screw hybrid connection structure can improve the fatigue life of the connection structure and prevent sudden failure.Because the failure of the connection structure caused by bolt loosening often leads to serious consequences,the effective evaluation of the state of the composite connection structure is the key to the health monitoring of the structure in service.This paper around the CFRP materials mixed with aluminum alloy material with the plastic screw bolt connection connection structure,based on ultrasonic guided wave nondestructive testing technology,loose bolts to connect structure as the research object,from the guided wave model,the dispersion curve,micro contact,waveguide power and signal processing,etc,to carry out the research,finally be suitable for two kinds of bolt connection structure of loose detection method.The main work contents are as follows:The conduction characteristics of ultrasonic guided waves in two kinds of connected structures were deduced.The dispersion equations of aluminum alloy plate and CFRP plate were established by combining elastic theory and wave equation,and the corresponding dispersion curves were solved.The epoxy resin layer was idealized as a fixed thin layer,and the dispersion equation of c FRP-epoxy resin layer and aluminum alloy plate was deduced according to the boundary conditions between layers.Combining with finite element software,boundary element simulation and characteristic frequency method are used to draw the dispersion curve of three-layer material with adhesive layer.The connecting mode of the connecting structure is determined and the placement of bolts is calculated.Based on the microscopic interface contact theory and the mechanism of sound transmission,the constitutive model of bolt pretightening force,contact surface deformation and guided wave transmission energy was established.Simulation verifies the influence of different contact modes on the transmission energy and distribution of guided waves.The energy index of ultrasonic guided wave receiving by PZT is proposed.Based on the dispersion curve and window function characteristics drawn,the optimal excitation frequency of ultrasonic guided wave signal in the connected structure is determined to be 250 k Hz,and the optimal modulation period is verified to be 5 cycles.Combined with principle and experiment,the S-G filtering denoising method of ultrasonic guided wave signal is studied,and the signal data volume is compressed with 100-point signal resampling algorithm.The center frequency component is extracted based on Empirical Mode Decomposition(EMD).A testing platform for bolt pretightening force of ultrasonic guided wave composite connecting structure was established.Multi-stage torque experiments were conducted on the pretightening force level of single bolt on the structure for pure bolt and mixed rubber screw.Experimental results show that the method of envelope head amplitude,modal waveform time,guided wave signal energy and LAMB＿F coefficient can be used to identify bolt preload level effectively.The influence of the number of loose bolts on the energy of guided wave is studied.The experimental results show that the energy of guided wave signal decreases with the increase of the number of loose bolts.The position identification experiment of single bolt loose in multibolt is carried out,and the position of loose bolt can be determined by LAMB＿F coefficient of two sensor paths.