Selecting optimum air gap length in air-coupled ultrasonic through-transmission testing of products made of polymer materials

In air-coupled ultrasonic non-destructive testing of a number of products (biological objects, products made of chemically active or explosive materials), the amplitude of the electrical signal applied to the transmitting piezoelectric transducer is limited and in some cases cannot exceed the value of the order of U ~ 10—15 V. In this case, the sensitivity of testing is significantly reduced and therefore all possible ways to increase it should be used. First of all, piezoelectric transducers with the highest possible electroacoustic conversion coefficient should be used. In addition, it is necessary to select such an air gap length da between the transmitting transducer and the test object, that ensures the maximum amplitude of the ultrasonic emission signal “at the input” of the product. And since the maximum amplitude of the ultrasonic signal emitted by the transducer is located in the near field of the transducer, it is necessary to select the value da corresponding to the length of the near field of the transmitting transducer in the air, provided that in this case there will be no re-reflections of the emission signals in the air gap. This in turn requires the use of short (broadband) ultrasonic signals and, consequently, the use of ultrasonic broadband piezoelectric transducers. The article shows that the parameters of the matching layers of the air-coupled ultrasonic piezoelectric transducer affect both the bandwidth of the transducer and the spatial characteristics of the transducer, including the position of the acoustic field maximum. It is shown that it is possible to determine the maximum of the ultrasonic broadband signal in the air in order to determine the optimal length of the air gap, at which the ultrasonic signal with maximum amplitude is emitted into the product, by analyzing the correlation distribution of the field of an air-coupled broadband transducer. The results of the experiments are presented, confirming the necessity of providing the optimal length of the air gap between the air-coupled ultrasonic transmitting transducer and the test object to increase the sensitivity of through-transmission testing of simulators of products made of explosive materials.