Study On Mechanism And Method Of Bolt-connectionstate Monitoring Based On Frequency Variation Of Piezoelectric Impedance

As one of the most basic fastener connection structures,bolt connection is widely used in many fields of national production and life;From small tools to large facilities as well from daily civilian to advanced scientific and technological equipment.Under the long-term impact,vibration and alternating load,the bolted structure will loosen and even break.Once the bolted structure in some key parts is loosened or falls off,it may lead to major safety accidents.In this paper,piezoelectric impedance technology,combined with theoretical analysis,finite element simulation analysis,and experimental verification,systematically studied the monitoring mechanism and method of bolt connection state.The main research contents and main conclusions are as follows:Based on the high-frequency resonance characteristics of piezoelectric materials,the high-frequency theoretical models of the piezoelectric impedance of piezoelectric materials and the coupling system of piezoelectric materials and substrate structures are established.Based on the point-by-point dynamic stiffness theory,the influence of the change of the working state of the matrix structure on its local dynamic characteristics(local resonance frequency of the structure)is revealed,that is,the mechanical impedance changes accordingly when the working state of the matrix structure changes,resulting in changes in the local impedance ratio or stiffness ratio of the structure,which is reflected by the output impedance/admittance spectrum of piezoelectric materials.Through the piezo output impedance/admittance signal,the health state of the structure can be determined.The local stiffness change of the matrix structure only affects the local dynamic characteristics of the matrix structure.It is difficult to detect the change by testing the whole structure;However,when piezoelectric impedance technology uses high-frequency excitation and sensing,it can measure the local dynamic characteristics of the base structure in the installation position,which can effectively detect the initial changes,which is difficult to achieve by other structural health monitoring methods.Based on the wave equation of uniaxial tension-compression one-dimensional members,the functional relationship between the change of axial load and the natural frequency of the structure is derived.The natural frequency of the transverse vibration mode of the structure increases approximately linearly with the increase of the axial tension load(tensile stress)and with the increase of the axial compression load(compressive stress).While the linear frequency decreases,while the longitudinal vibration modal frequency basically remains unchanged.In view of the fact that every vibration mode of the structure will appear in the form of resonance in the piezoelectric impedance/admittance signal,the stress state of the structure can be determined by observing the corresponding frequency shift of the peak and valley of the admittance spectrum in the impedance/admittance spectrum of the piezoelectric coupling system.Monitoring of stress state.Based on the linear relationship between the variation of piezoelectric impedance resonance frequency and the variation of the external load of the structure,the variation of piezoelectric impedance resonance frequency is proposed as the characteristic parameter to monitor the bolt connection state.Aim to the structural characteristics and application environment of bolted joints,three monitoring schemes for bolted joints are designed,i.e.bolt-rod integrated piezoelectric material,bolt-head integrated piezoelectric material,and nut-side integrated piezoelectric material.(a)The state monitoring results of bolted joints with integrated piezoelectric materials in the bolt rod show that the impedance/admittance spectrum resonance frequency of the coupling structure increases approximately linearly with the increase of the axial tension load of the bolt.(b)The experimental results show that the peak resonance frequency of the piezoelectric admittance spectrum decreases linearly with the increase of pre-tightening force,that is,when the bolt loosens and the pre-tightening force decreases,the peak resonance frequency increases linearly.The comparative test results in the clean environment and oil pollution environment also show that oil pollution environment has little effect on resonance peak frequency.Therefore,the bolt joint condition monitoring method based on resonance frequency variation has the strong anti-interference ability.(c)The experimental results of the bolt joint condition monitoring of the integrated piezoelectric material on the side of the nut turned out that under the pretension force,the part of the outer side of the nut near the free end face is in the tensile stress state,and the part near the joint part is in the compressive stress state.Therefore,the peak admittance resonance frequency of the integrated piezoelectric material on the side of the nut is related to the bolt.The size of the preload is related to the integrated position of the piezoelectric chip;However,the linear relationship between the resonance frequency and the change of preload force in impedance spectroscopy agrees with the theoretical analysis.It is also found that the smaller the size of piezoelectric patches,the larger the peak frequency of impedance resonance of the coupling structure.For the same load,the change of resonant frequency with load is more obvious when piezoelectric patches are transversely bonded.When the piezoelectric material is integrated into the bolt rod,the end face of the bolt head and the outer side of the nut to form an intelligent bolt structure,the peak value of resonance frequency in the impedance/admittance spectrum can be used as the eigenvalue to monitor the bolt preload and the connection state online.The method has good anti-noise performance and also a wide application range.