Study On The Development Of Smart Bolt And The Monitoring Method Of Connection Status Based On ZnO Piezoelectric Film

Because of the advantages of simple connection,convenient disassembly,low price and wide application range,bolts have become indispensable connectors in equipment manufacturing industry,and are widely used in aerospace,automobile,ship,railway and other fields.However,when connecting the internal mechanism of the aircraft turbine engine,the bolts are subject to high temperature,strong vibration and alternating load for a long time,which is very easy to cause connection failure and cause air disaster.For this reason,this paper proposes a new type of smart bolt"bolt+piezoelectric thin film sensor（PTFS）",which solves the problem that the original smart bolt cannot be normally monitored and tested due to the failure of the adhesive layer.The theoretical model of smart bolt,finite element method（FEM）simulation analysis,smart bolt preparation and electromechanical impedance/admittance test are studied.The main contents and conclusions are as follows:（1）Based on the piezoelectric constitutive equation and the elastic mechanics equation of the bolt head surface,the theoretical model of smart bolt is established.On the basis of considering the friction between the bolt and the connecting plate and the friction between the thread pairs,a mathematical model between the torque of the smart bolt and the output charge is established.The amount of charge output by the smart bolt under different torques is expounded theoretically,and the equivalent circuit model of the output voltage of the smart bolt is constructed.At the same time,the electromechanical impedance/admittance model of the smart bolt is established through the dynamic modeling analysis of the interaction between the ZnO PTFS and the bolt.It can be seen from the model that when the bolted connection state changes,the impedance or admittance will also change;When the stiffness of the monitored bolt changes,the stiffness ratioη（ω）will change,and the impedance or admittance will also change.Two kinds of intelligent bolt mathematical models established in this paper are the theoretical basis for judging bolt looseness.（2）The FEM analysis model of smart bolt is established based on COMSOL Multiphysic software.Based on the analysis of the working principle of PTFS,an smart bolt structure is designed;According to the design standard of GH4169 superalloy bolt,the maximum torque of the bolt is calculated,and six groups of torques are selected to analyze the axial stress of the bolt and the stress on the bolt head surface.Through the simulation analysis of the impedance characteristics of lead zirconate titanate ceramics（PZT）,aluminum nitride（Al N）and zinc oxide（ZnO）,the piezoelectric material was selected as ZnO.Through the simulation analysis of piezoelectric material and electrode shape,size and thickness,it is concluded that the piezoelectric material is circular in shape;The diameter of piezoelectric material is 10 mm,and its thickness is 1μm;The electrode diameter is 6 mm,and its thickness is 200 nm.Then,the coupling field between PTFS and bolt is analyzed by FEM.The scanning frequency range is roughly 0.6 MHz～1.0 MHz,and there is a strong admittance peak near 0.972 MHz.When the excitation voltage frequency range is 0.972 MHz～0.98 MHz,the fitting analysis of the change of the peak electrical admittance of the PTFS under different torques shows that there is a good linear relationship between the torque of the smart bolt and the peak frequency of the admittance,and there is a similar characteristic between the axial preload of the bolt and the peak frequency of the admittance.（3）Preparation and characterization of ZnO piezoelectric films based on GH4169superalloy steel substrate.In this paper,the sputtering power,argon oxygen ratio and pressure of DC pulsed magnetron sputtering system are studied.The crystal orientation,surface morphology and composition of the samples were analyzed by means of X-ray diffraction（XRD）,scanning electron microscopy（SEM）and energy dispersive spectroscopy（EDS）,and the preparation process parameters under the current conditions were optimized;The adhesion between the film and the substrate was tested with the REVETEST scratch test system;The piezoelectric coefficient d₃₃ of the sample film was tested with a piezoelectric tester.When the sputtering power was 150 W,the argon oxygen ratio was 25:10,and the sputtering pressure was 0.7 Pa,the sample had the best surface morphology and microstructure.At the same time,the sample had the largest adhesion,and the piezoelectric coefficient d₃₃ was 4.57p C/N at the maximum.（4）A new type of smart bolt based on a ZnO PTFS is proposed.ZnO PTFS was prepared on the surface of the GH4169 superalloy bolt with optimized sputtering process parameters.Firstly,according to the structure of the PTFS,the fixture of the smart bolt is designed and manufactured,and the mechanical mask is prepared.Then,the piezoelectric layer,the insulating layer,and the electrode layer were characterized.Finally,the performance of the PTFS is tested,including the static characteristics,dynamic characteristics,stability analysis,and the bonding performance between the sensor and the substrate.The results show that the average value of the longitudinal piezoelectric coefficient d₃₃ of the PTFS is 1.36 p C/N,the average sensitivity is 19.77 m V/N,and the maximum repeatability error is 4.03%,and the binding force between the PTFS and the substrate is 4646 m N.（5）A method of monitoring bolt looseness by using smart bolt voltage output and electromechanical impedance change is proposed.During the assembly of smart bolts,the stress and strain of the bolt head will change.According to the enterprise standard of GH4169bolts,the maximum pre-tightening force of the bolts after tightening can be determined,and the maximum torque can be obtained according to the bolt tightening torque model.Take a series of safety torque values to test the output voltage,calibrate it,and summarize the looseness law.In addition,the frequency translation method is used to test the output impedance of the piezoelectric sensor at each torque value.The impedance curve under each torque shows a trend of translation.With the increase of bolt torque,the peak frequency of the admittance curve decreases approximately linearly.Based on the analysis of the impedance frequency experimental data using the Root Mean Square Deviation（RMSD）,the accuracy is lower than that of the frequency translation method.The results show that the looseness of bolts can be monitored by the methods of smart bolt voltage output and electromechanical impedance change.