Study Of Monitoring The Bolt Looseness Based On Laser Ultrasonics

Bolts would be loose after assembled,it affected the safety of steel structure buildings,mechanical equipment,aircraft and other equipment.At present,the study of monitoring bolt looseness was mainly carried out by using PZT or piezoelectric probe,this method excited and collected signal at a single point,it was affected by noise and the temperature of environment,it may affect the determination of bolt looseness.And for some parts with narrow edges,piezoelectric sensors couldn't be arranged,so it could not be detected.In order to solve the above problems,this thesis presented a method to monitor bolt looseness by using laser ultrasonic method.The laser scanning system was used to measure the acoustic field of the tested sample.Wavelet packet decomposition method and power spectral density method were used to calculate the energy of guide wave,and based on this,the looseness of bolt was characterized.First,based on Hertz contact theory,the relationship between the bolt torque and the actual contact area of the bolt joint surface was studied during the elastic,elasticplastic and plastic deformation stages of the asperity,the mathematical model between bolt torque and actual contact area was established.The dissipation mechanism of ultrasonic guide wave between bolted joint surface was studied,and the method for evaluating bolt looseness based on guide wave energy was proposed.Second,the finite element simulation analysis of guide wave monitoring bolt connection state was completed through COMSOL Multiphysics simulation software.The surface roughness Ra of sample which was clamped by bolt used in the experiment was chosen as the parameter.Based on W-M function,the sample surface asperity was molded in 2D,converted the bolt torque into axial force F,applied load was carried out on the sample,taken displacement on the sample surface to excited guide wave signal,calculating the average horizontal displacement on the line to collect the guide wave signal.The wavelet packet decomposition method and power spectral density method were used to calculate the energy of guided wave and characterize the looseness of bolt.Finally,the laser-ultrasonic experimental platform was used to conduct experiments to monitor the bolt looseness.The maximum amplitude map of ultrasonic guide wave propagated on the surface of bolted specimen was obtained by LUVI scanning and image processing program.The wavelet packet decomposition method and power spectral density method were used to calculate the energy of guided wave and characterize the looseness of bolt.In the experiment,three groups of plates with different roughness were used as samples to prove the effectiveness of the laser ultrasonic method to monitor the bolt looseness.The least square method was used to fit the relationship curve between bolt torque and guide wave energy,and the slope of the fitting line was taken as the sensitivity of data processing method to compare the advantages and disadvantages of these two methods.The optimal method was selected,fitting the relation curve between the bolt torque and the energy of guide wave,and establishing the mathematical model between the bolt torque and the guide wave energy.Through theoretical derivation,finite element method and experimental means,the theoretical and experimental basis for using guide wave energy to characterize the bolt looseness was laid,which has important scientific significance and engineering application value.