With the development and application of composite material technology, the application of air coupled ultrasonic imaging detection technology in various fields such as aerospace has been increasing year by year. In the application of various composite materials, the use proportion of various foam and honeycomb structures also increases. Due to the significant sound attenuation of these materials, coupled with the fact that the transduction efficiency of air coupled ultrasound technology itself is much lower than that of conventional ultrasound detection technology. There is an urgent need for a detection technology and means that can meet the requirements of non-contact ultrasonic detection and compensate for detection sensitivity, thereby expanding the application scenarios of air coupled ultrasonic detection technology and making non-contact imaging detection of high attenuation composite materials possible. Taking glass fiber polyurethane foam board as an example, this paper tests and verifies a new technology and method to comprehensively improve the sensitivity of air coupled ultrasonic testing.

Sample thickness 100 mm, density approximately 130 Kg/m ^ 3

The experiment uses an AIRSCAN+dual channel air coupled ultrasound imaging detection system, which has a 400Vpp square wave pulse train excitation function and supports pulse compression technology. The frequency bandwidth supports two versions: 50 KHz-12 MHz or 5 KHz - over 1 MHz, and the sampling frequency provides 100 MHz @ 14 bits or over 25 MHz MHz@14 Two types. Simultaneously supporting three-axis encoders and providing passive preamplifiers, the overall performance noise ratio is extremely high. In this experiment, multiple methods including conventional air coupled ultrasonic testing, pulse compression technology testing, and wavelet denoising technology testing were used to detect the same sample and position multiple times. The advantages of various techniques were analyzed by analyzing the transmission signal wave height.

200kHz probe 4-period square wave pulse conventional air coupled ultrasonic testing technology for raw transmission signal

Transmission signal of 200kHz probe after adding LFM pulse compression technology

Transmission signal of 200kHz probe after adding LFM+WDN pulse compression and noise reduction technology

Air coupled C-scan imaging of glass fiber polyurethane foam plate using pulse compression and wavelet denoising technology

summary：

Conventional air coupled ultrasonic testing technology cannot effectively penetrate high attenuation materials like those mentioned in the article. After using pulse compression technology, the transmitted signal can be observed, and the transmitted wave height increases from 0% to 58.3%. After using pulse compression technology and wavelet denoising technology, the transmitted signal increases from 0% to 85.8%. From this, it can be seen that pulse compression technology and wavelet denoising technology can greatly improve the sensitivity of air coupled ultrasonic testing, providing an effective technical solution for non-contact imaging detection of high attenuation materials.