Investigation On Air-coupled Ultrasonic Transducer And Its Application

Air-coupled ultrasonic non-destructive testing technology is a new non-destructive testing technology using air as the transmission medium.It has been widely used to perform inspections on a variety of materials such as food,drugs,and aerospace composite materials.The key to detection technology is the high-performance air-coupled ultrasonic transducer,which directly affects the overall performance of air-coupled ultrasonic non-destructive testing equipment.Therefore,it is of great practical significance to research high-performance air-coupled ultrasonic transducers and corresponding detection applications.In this paper,we aim to research high-performance air-coupled ultrasonic transducer and its application in surface cracks detection.This research is carried out the following three aspects:1.The optimization design and fabrication of flat-plate air-coupled ultrasonic transducer.According to the technical difficulties of the air-coupled ultrasonic transducer,we constructed 1-3 type piezoelectric composites and low acoustic impedance matching materials,and completed the optimal design of matching layer thickness for double matching layers structure air-coupled ultrasonic transducer by experiments.Based on this,we fabricated 200 kHz and 440 kHz flat-plate air-coupled ultrasonic transducers with sensitivity and bandwidth of-23 dB,11.4%and-33.5 dB,23.4%,respectively.2.The optimization design and fabrication of multi-element focused air-coupled ultrasonic transducer.In order to increase detection accuracy,we designed a multi-element focused air-coupled ultrasonic transducer.Firstly,we studied the acoustic field distribution characteristics of multi-element focused air-coupled ultrasonic transducers and analysed the influence of different parameters on focusing characteristics of multi-element focused air-coupled ultrasonic transducer by numerical simulation.Next,we fabricated a 440 kHz six-element focused air-coupled ultrasonic transducer based on our simulation results.The actual measured focal length,focal width and focal depth of the six-element focused air-coupled ultrasonic transducer are 41.44 mm,1.14 mm and 20.30 mm,respectively,and its sensitivity and bandwidth are-50 dB and 20.2%,respectively.3.The application of air-coupled ultrasonic non-destructive testing technology in surface cracks detection.Firstly,by employing a pulse reflection method,we detected surface cracks of different widths and depths.Secondly,we proposed to detect micro surface cracks of materials using an ultrasonic scattering method and conducted the relevant evaluations through multiple groups of experiments.In the ultrasonic scattering evaluation experiments,the surface cracks of materials can be successfully detected by adjusting the angle between the incident direction of the ultrasonic wave and the surface cracks direction.In this paper,surface cracks with a depth of 0.08 mm and a width of 0.03 mm was able to be successfully detected.The overall detection resolution is 1.2 mm.These results show that micro surface cracks can be effectively detected by combining air-coupled ultrasonic non-destructive testing technology and ultrasonic scattering method.In conclusion,the air-coupled ultrasonic transducer developed in this paper has highly sensitivity,and the multi-element focused air-coupled ultrasonic transducer has a small focal spot to obtain a relatively high detection resolution,demonstrating the potential application value in the non-destructive testing field.