Stress/Strain Sensing Property Of Piezoelectric Sensor And Its Application In Health Monitoring Of Concrete

The piezoelectric materials used for structural health monitoring have caused wideconcerns in civil engineering. In this paper, the encapsulation materials were optimized toenhance the mechanical compatibility between piezoelectric sensors and concrete, and thepiezoelectric sensors were prepared by piezoelectric ceramics. The stress/strain sensingproperties of piezoelectric sensors were studied and the stress sensitivities were tested. At thesame time, the quantification relationships between voltages of sensors and strain ofstructures were studied. Meanwhile, stress monitoring of concrete was performed with thepiezoelectric sensors and the feasibility of piezoelectric sensors in the application of structuralhealth monitoring was discussed. The results in this paper are as follows:The encapsulation materials were optimized and the technologies of preparingpiezoelectric sensors were summarized. The experimental results show that the optimized rawmaterials of encapsulation materials are mixtures of cement, epoxy resin, curing agent andalumina powder. The corresponding proportion of raw materials is4:2:0.5:1.6~4:2:0.5:2. Theoptimized encapsulation materials own excellent performance of high strength, highinsulation property, high liquidity before solidification, high elastic modulus and highperformance at different loading frequencies. And the sensors were processed by electricshielding.The stress sensitivities (V/MPa), frequency and repetition characteristics of piezoelectricsensors were studied. The results show that the relationship between output voltages andstress are linear; when the thicknesses of the piezoelectric ceramics keep the same, the stresssensitivities increase with the increasing of ceramic areas; when the areas of piezoelectricceramics keep the same, the stress sensitivities increase with the increasing of thicknesses ofthe ceramics, but the change is not so obvious; the improved encapsulation material canimprove the ability of voltage output under the high-frequency loadings; the repetition error issmall and the sensors can meet the requirements of civil engineering.The strain sensing property of piezoelectric sensor was studied in the low-frequency (0.1Hz~40Hz) vibrations, and the quantification relationships between voltages of sensors and strain of structures were studied. The experimental results show that the piezoelectric sensorsresponse faster than the strain gauges in the condition of quasi-static loadings, the strain staysa fixed value after reaching maximum values, the output voltage decays fast after reachingvoltage peaks. The quasi-static strain sensitivities of sensors are lower than the dynamic strainsensitivities. When the vibration frequencies are increasing, the strain sensitivities ofpiezoelectric sensors show increasing trends. At low-vibration frequencies, the outputvoltages of sensors and strain of beam present a good linear relationship, which satisfies thebasic demands of the application of bridge-low-frequency vibrations.The piezoelectric sensors were embedded in the concrete and the stress conditions ofconcrete were monitored in the compression tests. The results show that the output voltages ofpiezoelectric sensors steadily increase when the loading is increasing. The output voltages ofpiezoelectric sensors are consistent when the loading frequencies increase, which indicate thegood frequency independence of piezoelectric sensors in the application of concrete healthmonitoring. Piezoelectric sensors can reflect the loading frequencies of concrete. In theexperiments, piezoelectric sensors are not affected by the interference of50Hz frequency,which demonstrate that piezoelectric sensors own excellent abilities of resisting interferencesof environments.The piezoelectric sensors were embedded in concrete beams and the concrete was loadedin three point bending test, at the same time the strain gauges were applied in the experiment.The results show that piezoelectric sensors and strain gauges can reflect the rupture failure ofconcrete structures in the process of linear loadings. The piezoelectric sensors can not monitorthe quasi-static loadings of concrete. In cyclic loadings of different frequencies, the responsesof piezoelectric sensors are good, and the changes of output voltages are well correspondingto the strain of gauges and the changes of loadings at different frequencies, whichdemonstrates that the piezoelectric sensors can reflect the stress conditions of concretestructures. and the piezoelectric sensors are feasible to apply in dynamic stress monitoring ofconcrete structures.