| Concrete is one of the important structural materials in civil engineering, a large number of application in high-rise buildings, bridges, dams, tunnels, roads and other civil and military infrastructure. So the safety of concrete structure exists hidden dangers which will directly threaten the national property security, as well as the lives of the people. So online and real-time health monitoring of concrete structures is very necessary. Discussing about ultrasonic nondestructive testing technology can completely realize the safety monitoring of concrete structure. Now commonly-used ultrasonic sensor shell is made by metal encapsulation layer. When ultrasonic sensor is used in the outside surface of concrete structure, it is not easy to be coupled with concrete. Environment which has a major influence makes serious misalignment of the test signal. When ultrasonic sensor is embedded in the concrete and is monitored, it has poor compatibility with concrete, meanwhile, it is susceptible to corrosion and has the adverse impact on the stability of concrete structure. And now commonly-used piezoelectric ultrasonic sensor adopts the piezoelectric element which is piezoelectric ceramic chip or flake piezoelectric composite material, more use of its thickness vibration mode. These make piezoelectric ultrasonic sensor forms the single side directivity which is difficult to receive and send ultrasonic signals in different directions. In order to overcome various disadvantages above, this thesis is dedicated to developing a embedded and radial direction ultrasonic sensor. It has good compatibility with concrete. When it is embedded in concrete structure, receiving and sending ultrasonic signals in different directions can be realized. This thesis mainly involves the preparation of radial direction piezoelectric ultrasonic sensor and its basic application in monitoring concrete defects.(1) The preparation of the radial direction ultrasonic sensor. This thesis mainly studies the design mechanism of the radial direction ultrasonic sensor to improve the preparation technology of the sensor. First of all, the function of the sensor body namely the piezoelectric element is selected. The impedance performance of three different sizes of PZT-52 piezoelectric circular tube and influence factors of the size are discussed. In study, using the size of 24 mm × 20 mm × 15 mm (outside × diameter × high) of PZT-52 piezoelectric ceramic pipe as the piezoelectric elements of transmission type radial direction ultrasonic sensors and using the size of 25 mm × 22 mm x 14 mm of PZT-41 piezoelectric ceramic pipe as piezoelectric elements of receiving type radial direction ultrasonic sensors have good performance. Second, the selection of the matching layer and backing material is dicussed, using the mixture which includes cement,epoxy resin mass ratio of 1:1 as matching layer materials of radial direction piezoelectric ultrasonic sensors.(2) The performance of the radial direction piezoelectric ultrasonic sensor research, including transmission type radial direction ultrasonic sensors and receive type radial direction ultrasonic sensors.For transmission type radial direction ultrasonic sensors, first the effect of the matching layer is analyzed. Five different thickness sensors are prepared. Through the analysis of study we can find that when matching layer thickness is 11 mm, both the amplitude and peak-peak of the transmission type radial direction ultrasonic sensor are the biggest, the emission intensity of the sensor is the largest, which is in line with the quarter-wave theory. The influence of backing layer on emission type radial direction ultrasonic sensors is studied. To add the backing material to sensors, to a certain extent, the sensitivity of the emission type sensors is reduced, but the bandwidth of sensors is increased. Finally, the directivity of emission type radial direction ultrasonic sensors is studied. It is concluded that transmission type radial direction ultrasonic sensors in the radial direction of 360° can realize uniform emission ultrasonic signals.For the performance analysis of receiving type radial direction ultrasonic sensors, the following conclusions:when the thickness of the matching layer is 11.5 mm, the sensitivity of sensors receiving ultrasonic signals is the highest, which completely conforms to the quarter-wave theory of the sensor matching layer. After increasing the backing layer, the sensitivity of receiving type radial direction ultrasonic sensors falls slightly. But the acoustic attenuation performance of the sensor increases greatly, to a certain extent, the bandwidth of the sensor is increased. Finally the receiving directivity of receiving type radial direction ultrasonic sensors is studied, it is concluded that the receiving type radial direction ultrasonic sensor in the radial direction of 360° can realize uniform receiving ultrasonic signals.(3) The application of the radial direction ultrasonic sensor in concrete nondestructive testing is carried out some preliminary discussion and analysis.First the thickness test of concrete by the radial direction ultrasonic sensor is studied. By emission type and receiving type of radial direction ultrasonic sensors embedded into the concrete, under the different thickness of concrete, gathering parameters like the ultrasonic wave, sound velocity, amplitude, and frequencies, it is concluded that the radial direction ultrasonic transducer can better reflect changes of the concrete thickness.Emission type and receiving type of radial direction ultrasonic sensors are dived into concrete pile through certain plans, respectively in different direction of the preparation of concrete crack, gathering parameters like the ultrasonic wave, sound velocity, amplitude and frequencies, it is concluded that the radial direction ultrasonic sensor is sensitive to crack. The use of radial direction piezoelectric ultrasonic sensors for detection of concrete damage is able to realize and can achieve a wide range of applications... |
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