Ultrasonic dry-coupling detection with gradient acoustic impedance match layer

Ultrasonic dry coupling technology eliminates the need for traditional liquid coupling agents and is critical for detecting advanced materials or coatings that cannot be contaminated by liquid coupling agents. In the conventional dry-coupling technique, sound waves propagate within a multilayer medium of water, dry-coupling material (rubber), and the material being examined (steel). The serious impedance mismatch between multilayer media results in very low ultrasonic transmission efficiency, and the maximum transmission coefficient of sound intensity is only 0.12. To improve the acoustic transmission performance, this paper proposes a dry coupling layer with gradient variation of acoustic impedance, whose impedance varies with an exponential law in the thickness direction. In the first stage, the propagation laws of acoustic waves in multilayer media with varying acoustic impedance gradients were investigated. Boundary impedance mismatch conditions were introduced, and the effects of different acoustic impedance mismatches on the acoustic properties of dry coupling layers were investigated based on acoustic transmission theory analysis. In the second stage, based on the optimized parameters after numerical analysis, the gradient composites were fabricated by centrifugal method, and dry-coupling detection experiments were carried out. Experimental results confirmed that the dry coupling layer with an exponential change in acoustic impedance improved the sound propagation efficiency by 80 % and the −6dB bandwidth of the pulse-echo signal by 20 % compared to rubber.