| Bridges are the infrastructure for improving transportation efficiency and national economy.According to statistics,more and more Bridges have been put into use in China,with more than 900,000 Bridges in active service and under construction.However,due to the impact of the working environment and service time,its safety faces serious challenges.In order to ensure the safety of bridge structure,avoid disastrous accidents and protect people’s life and property safety,it is urgent to carry out online health monitoring of bridge structure.Acoustic emission monitoring technology,with its advantages of fast response speed,wide monitoring range and high sensitivity,can realize real-time and dynamic monitoring of large-scale structural damage,and has innate advantages in bridge health monitoring.However,long-span Bridges are mainly composed of steel trusses,concrete box beams and supports,etc.Due to the different materials of steel trusses and concrete box beams,there are obvious differences in the acoustic emission signals generated during the damage process,resulting in the overall monitoring accuracy of long-span Bridges.Therefore,aiming at the acoustic emission characteristics of concrete and metal materials,the effects of PZT volume fraction and aspect ratio on the mechanical and electrical properties of type 1-3 piezoelectric composites were studied by finite element analysis.The mechanical and electrical properties of type 1-3 piezoelectric composites with different PZT volume fraction and aspect ratio were prepared by cutting and pouring method.A series of type 1-3 acoustic emission sensors were prepared with type 1-3 piezoelectric composite material as the sensor piezoelectric element and epoxy resin as the backing layer.The characteristic parameters of acoustic emission and waveform information were determined and calibrated.The relationship between the age,strength and distance of concrete and the acoustic emission characteristic signals was established by using the prepared type 1-3 acoustic emission sensor,and the correlation between the corrosion degree of steel plate and the acoustic emission characteristic signals was defined.Based on this,the positioning characteristics of acoustic emission signal sources of different materials were studied.The specific research results are as follows:（1）A type 1-3 piezoelectric composite was prepared by cutting and pouring method.The influence of structural parameters such as column integral number and aspect ratio on the piezoelectric,dielectric and acoustic properties of the type 1-3 piezoelectric composite was investigated by finite element analysis and experimental verification.The results show that with the increase of PZT volume fraction,the conductance value of 1-3 piezoelectric composites increases,the peak shape narges,the resonant frequencies are around 210 k Hz,and the vibration modes are mainly thickness modes.The relative dielectric constantεrincreases and shows a linear trend.The piezoelectric strain constant d33increases,the piezoelectric voltage constant g33decreases,and the electromechanical coupling coefficient Ktof thickness fluctuates between 0.64 and 0.74.Mechanical quality factor Qmincreased from 6to 61.7.With the increase of the width to height ratio of PZT ceramic column,the conductivity value increases and the peak deformation widens,the vibration mode is mainly thickness mode,and the resonant frequency increases from 208 k Hz to 255 k Hz.εrand d33gradually decreased,g33was almost unaffected by the aspect ratio,fluctuating slightly between 35.5 m2·C-1and 37.5 m2·C-1,Ktshowed a decreasing trend,varying between 0.69and 0.75,and Qmgradually decreased.It corresponds to the result of finite element analysis.Therefore,type 1-3 piezoelectric composite material with PZT volume fraction of 56.0%and aspect ratio of 0.2 is selected as the piezoelectric element of acoustic emission sensor.（2）The type 1-3 acoustic emission sensor was prepared by using 1-3 piezoelectric composite material as the piezoelectric element,epoxy resin as the backing layer and Al2O3ceramic sheet as the matching layer.The results show that the energy and amplitude decrease with the increase of the distance between the acoustic emission signal source and the acoustic emission sensor,and the linear correlation is large.Due to the addition of cement and tungsten powder,the maximum acoustic emission voltage is increased,and the signal attenuation is faster.The acoustic impedance of epoxy/tungsten powder/cement with backing layer is the largest,which is 3.9 MRal,the wave absorption ability is the strongest,and the acoustic reflection ability is the smallest.Therefore,the 1-3 acoustic emission sensor with backing layer of epoxy/tungsten powder/cement has the widest bandwidth,ranging from 41-280 k Hz,and the response is smooth in the calibration results,which is consistent with the results of the lead breaking test on the aluminum plate.The results show that the type 1-3 acoustic emission sensor with epoxy/tungsten powder/cement backing layer has good consistency and stable amplitude,and has good acoustic emission signal acceptance ability,and can be used for the positioning and monitoring of various material damage sources.（3）This paper also designed three kinds of concrete test blocks with different theoretical strengths,C30,C40 and C50,and pasted 1-3 acoustic emission sensors with epoxy/tungsten powder/cement backing layer on the surface of concrete.The influence of concrete age change on acoustic emission characteristics was studied.The research results showed that:With the increase of age,its compressive strength gradually increases,and the propagation speed of sound wave in the sample gradually increases.The lead breaking signal energy of C30 concrete at the distance of 5 cm from the sensor increases with the increase of age.In each age,the energy and amplitude of lead break signal decrease with the increase of distance,and the amplitude changes more linearly with distance.The energy change of 5 to 10 cm is greater than the energy change of 10 to 25 cm.The energy and amplitude of C40 and C50concrete also have the above rules.With the increase of the design intensity,the maximum value of the waveform received by the acoustic emission sensor gradually increases.With the increase of age,its amplitude gradually increases and the acoustic emission characteristic frequency gradually widens,and all tend to be stable after 14 days of age.This finding provides a theoretical basis for evaluating the state of concrete and improving the monitoring accuracy of concrete.（4）The metal accelerated corrosion experiment was designed by applying applied DC voltage method,and the metal corrosion process was monitored in real time by using type 1-3acoustic emission sensing.Based on the monitoring results,the correlation between the metal corrosion process and acoustic emission characteristic signals was constructed.The research results showed that the acoustic emission amplitude of the corrosion process was mainly concentrated in 45-55 d B.In the early stage,the acoustic emission energy is small,and the corrosion signal energy increases slightly with the accelerated corrosion experiment,and the abnormal energy increases when the corrosion time reaches 100,000-120,000 s.There are a lot of impact signals in the early and middle stage of corrosion.Combined with the cumulative impact number of acoustic emission,the corrosion process of steel plate can be divided into three stages:early depassivation,continuous corrosion and the loss of rust products.（5）Based on the above research,the acoustic emission location monitoring of concrete and metal is studied by optimizing sensor layout and acoustic wave propagation characteristics in each material.The results show that the propagation speed of acoustic signal in aluminum plate is 2982.1 m/s,and the linear positioning error is less than 10%when the simulated damage source is 150 mm away from the sensor.The acoustic signal propagates at2334.4 m/s in C30 concrete test.When the simulated damage source is 150 mm away from the sensor,the linear positioning error is less than 5%,which is better than the positioning accuracy in aluminum plate. |
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