Selecting Optimum Air Gap Length in Air-Coupled Ultrasonic Through-Transmission Testing of Products Made of Polymer Materials

In air-coupled ultrasonic nondestructive 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 a value of the order of U ~ 10–15 V. In this case, the sensitivity of testing is considerably reduced and therefore all possible ways should be used to increase it. 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 \({{d}_{{{\text{air}}}}}\) between the transmitting transducer and the test object that ensures the maximum amplitude of the ultrasonic emission signal “at the input” of the product. 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 \({{d}_{{{\text{air}}}}}\) corresponding to the length of the near field of the transmitting transducer in air, provided that in this case there is no re-reflections of the emission signals in the air gap. In turn, this 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 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 optimum 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.