Bolt Looseness Monitoring Method Based On Piezoelectric Response

The new possibilities offered by the use of smart materials,such as piezoelectric material(PZT),have accelerated developments of structural identification and structural health monitoring(SHM)in the last decade.The electro-mechanical impedance(EMI)technique,which is used as self-sensing piezo-impedance transducers have made huge progress.The technique uses a PZT patch bonded to the structure to monitor structures and the electro-mechanical impedance signature extracted from the patch is used for damage detection.This research sets out the use of piezoelectric sensors for real-time monitoring of bolted joint loose.The sensitivity to local damage of the electromechanical coupling effect between a piezoelectric transducer and a host structure is utilized for the Electromechanical(EM)impedance-based damage detection techniques.However,the method applied on largescale structure present impedance mismatch due to the impedance of the host structure becoming excessively large compared to that of the PZT.A dual PZT configuration was developed to overcome this issue by recent researchers on the subject.In this study,the dual PZT impedance measurement technique is used for monitoring bolt loose,which is composed of two separate lead zirconate titanate(PZT)segments.The research presents the background theory of the electromechanical impedance technique and outlines its implementation into the health monitoring of bolted connections.The formulation of a simplified electro-mechanical(EM)impedance 1D model of a smart material bonded to a host structure is given to understand the EM impedance signatures under the effect of damage.The numerical feasibility of the dual PZT for monitoring bolt loose is carried out by a finite element modeling of two different bolted joint connections.The simulations were conducted by applying a dynamic loading(vibration)under different bolt preload conditions,the admittance is measured(inverse of the impedance)from the signal of the PZT(Lead zirconate titanate)patches which are bonded on the bolted joint.A wide frequency range is studied to find a sensitive frequency band for the piezoelectric impedance.Several peaks in the sensitive frequency band are investigated to establish the effect of the preload on the impedance frequency.Results show that the bolted joints are affected by the tightness of the bolt(preload),the surface condition and frequency of applied excitation.Frequencies left shift,peak amplitude decreases,and the emergence of peaks in the frequency band are observed as the bolt preload decreases.The frequencies of the real parts of the admittance signature present large different results in the presence of damage.Therefore,the method presents a good application prospect for monitoring bolt loose.The effect of excitation frequency and the effect of the temperature were observed over a wide frequency range.The higher excitation frequencies are optimal for sensing purposes.Through the impedance signatures,a linear relation was fitted to evaluate the temperature effect.The gradient of an impedance signature increases with increasing temperature.