Research On Low Frequency Air-coupled Ultrasonic Transducer

Air-coupled ultrasonic non-destructive testing technology is an emerging nondestructive testing technology using air as the transmission medium.It has the characteristics of non-contact,non-immersion,completely non-destructive and high detection precision.As the most critical device in the air-coupled ultrasonic nondestructive testing system,the air-coupled ultrasonic transducer directly affects the overall performance of the non-destructive testing system.Therefore,the research of high-performance air-coupled ultrasonic transducers has important engineering practical significance for speeding the development of non-destructive detection technology.This paper aims to design,fabricate and optimize the high sensitive piezoelectric air-coupled ultrasonic transducer with an operating frequency of about 100 k Hz.In this paper,the basic theory of piezoelectric ultrasonic transducer is systematically analyzed,including the structure and working principle of piezoelectric ultrasonic transducer,the sound pressure distribution and directivity formula of rectangular piston’s radiant far field,the impedance matching theory of transducer and major attenuator factors in aircoupled ultrasonic testing.The theory provides a theoretical basis for the design of the low-frequency air-coupled ultrasonic transducer and the optimal design of the matching layer.We designed 100 k Hz air-coupled ultrasonic transducer with backing structure and without backing structure.In terms of piezoelectric material structural design,we choosed stacks made from two piezoelectric ceramics and double-layer matching structures.In terms of materials,1-3 PZT-epoxy piezoelectric composites and epoxy composite filled with hollow glass-beads were prepared,and transducer samples were made for relevant tests.The experiment completed the optimized design of the matching layer thickness for double-matching layer structure air-coupled ultrasonic transducer,and explored the influence of the matching layer thickness on the frequency characteristics of the transducer.The results show that the sensitivity of the air-coupled transducer can be significantly improved by optimizing the thickness of the matching layer,and the sensitivity of transducers without backing is significantly better than that of transducers with backing.In conclusion,the low-frequency air-coupled ultrasonic transducer developed in this paper is suitable for defect detection of non-uniform materials such as concrete,and has practical values for engineering applications.