Thickness Measurements of Polyamide Specimens using Fluidic Oscillator as a Frequency Modulated Air-Coupled Ultrasonic Transducer
- Ultrasonic inspection, especially contact and immersion methods, have become established techniques for non-destructive testing in civil engineering (NDT-CE). However, the requirement for coupling agents renders these methods cumbersome in practice due to the potential for surface defacement and an arduous measurement process. Consequently, air-coupled ultrasonic transducers have been proposed as an alternative to address these concerns. These (alternatives), however, come with their own challenges, such as a significant impedance mismatch between the air and the specimen as well as the susceptibility of time of flight (TOF) measurements to ambient conditions. Recently, fluidic oscillators have been developed for in-air sound generation to alleviate impedance mismatch and improve the overall robustness, which is an important consideration of NDT-CE applications. The advantage of fluidic oscillators lies in their ability to generate an ultrasonic signal via oscillating air jet excitedUltrasonic inspection, especially contact and immersion methods, have become established techniques for non-destructive testing in civil engineering (NDT-CE). However, the requirement for coupling agents renders these methods cumbersome in practice due to the potential for surface defacement and an arduous measurement process. Consequently, air-coupled ultrasonic transducers have been proposed as an alternative to address these concerns. These (alternatives), however, come with their own challenges, such as a significant impedance mismatch between the air and the specimen as well as the susceptibility of time of flight (TOF) measurements to ambient conditions. Recently, fluidic oscillators have been developed for in-air sound generation to alleviate impedance mismatch and improve the overall robustness, which is an important consideration of NDT-CE applications. The advantage of fluidic oscillators lies in their ability to generate an ultrasonic signal via oscillating air jet excited by a self-actuated fluctuation. Using air as the operating medium provides a supplementary benefit of lowering measurement durations in addition to a substantial drop in acoustic impedance losses with air acting both as an actuation as well as a coupling medium. Thus, TOF measurements using such fluidic transducers are expected to enable faster characterization of specimen parameters.
In that regard, material characterization of polyamide is carried out using ultrasound generated by fluidic devices. Additionally, the mass flow rate of the operating fluid is observed to have a significant effect on the frequency of the fluidic oscillator. Accordingly, the frequency of the oscillation is modulated by varying the supply pressure, leading to a varying flow rate, which in turn facilitates a robust and reliable assessment of the TOF through the material by the generated chirp signals. Furthermore, such frequency modulations effectively improve the signal-to-noise ratio (SNR). Along with a comprehensive study of these new fluidic transducers, preliminary measurement findings are presented, including the material characterization of polymers using air-coupled, frequency-modulated ultrasound.…