| The admittance model of the substrate-Piezoelectric element coupling system is the theoretical basis for the Electro-mechanical impedance(EMI)technique.However,the substrate factors involved in the existing models are relatively single,and cannot fully reflect the influence of the substrate on admittance.This article proposes a new admittance model based on the analysis of the interaction between the Piezoelectric element and the substrate during the admittance measurement process.Then,factors of the Piezoelectric element and the substrate that would affect the admittance are studied,also with two application cases.The detailed works are as follows.(1)By analyzing the interaction between the Piezoelectric element and the substrate during the admittance measurement process,factors of the substrate that would affect the admittance are found.Then a new admittance model for the substrate-Piezoelectric element coupling system is proposed regarding of the factors including the structural dynamic stiffness coefficient,the displacement ratio,the attenuation coefficient and the propagating distance.By comparing with three existing models,it was found that the proposed model has a comprehensive consideration of the influence of the substrate and the ultrasonic,and can better reflect the changes in the admittance signatures before and after bonding.(2)Factors of the Piezoelectric element are studied based on the admittance model proposed,including the electrode distribution,size,and shapes.Experiment and numerical simulation are employed with single variable control.The electrode distribution is focused on the electrode leading influence.For the size and shapes,PZT patches with three different diameters or five shapes are selected.With a baseline extraction method,the changes in the admittance signatures before and after bonding are compared.The results show that electrode leading only produces a few tiny local peaks.The size and shapes mainly determine the distribution of the local peaks.The area of the Piezoelectric element has a more strong influence on the admittance than the shapes.(3)Factors of the substrate are studied based on the admittance model proposed,including the structure boundary,materials,size,and adhesive.The structure boundary includes a comparison of square and circular boundaries and different boundary distances(by cutting an acrylic plate).For the materials,ten different materials are selected in the experiment(three kinds)and numerical simulation(ten kinds).The size and adhesive are mainly studied based on the experiment of the existing literature.The results show that the structure boundary has an affection on the strength of the local peaks.The elastic modulus of the materials and the cross-section area mainly determine the frequency shift of the resonance peaks.The larger the elastic modulus or the cross-section area,the larger the frequency shifts.For passive materials,the frequency shift is about-15 k Hz～15 k Hz,while for active materials,the frequency shift is about 40 k Hz～80 k Hz.(4)Based on the admittance changes with different factors and the feature of the damage index,a strategy of the optimal frequency range selection is proposed by experiment in which artificial damages(drilling holes)are made on plates with different materials.The damage sensitiveness of different shapes is studied by an experiment in which artificial damages(drilling holes)are made on a circular steel plate with different PZT patches bonded together.The strategy of the optimal frequency range selection is that frequencies centering at the first resonance frequency of the coupling system(>5 k Hz),and a small frequency band with large intervals for passive materials,but a small frequency band with small intervals or a large frequency band with large intervals for active materials.Among different shapes,the circular patch is of the best damage sensitiveness.The rectangle patch has more sensitivity in its width direction than its length direction. |
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