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- Air-coupled ultrasound (4)
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Development of an Accurate and Robust Air-Coupled Ultrasonic Time-of-Flight Measurement Technique
(2022)
Ultrasonic time-of-flight (ToF) measurements enable the non-destructive characterization of material parameters as well as the reconstruction of scatterers inside a specimen. The time-consuming and potentially damaging procedure of applying a liquid couplant between specimen and transducer can be avoided by using air-coupled ultrasound. However, to obtain accurate ToF results, the waveform and travel time of the acoustic signal through the air, which are influenced by the ambient conditions, need to be considered. The placement of microphones as signal receivers is restricted to locations where they do not affect the sound field. This study presents a novel method for in-air ranging and ToF determination that is non-invasive and robust to changing ambient conditions or waveform variations. The in-air travel time was determined by utilizing the azimuthal directivity of a laser Doppler vibrometer operated in refracto-vibrometry (RV) mode. The time of entry of the acoustic signal was determined using the autocorrelation of the RV signal. The same signal was further used as a reference for determining the ToF through the specimen in transmission mode via cross-correlation. The derived signal processing procedure was verified in experiments on a polyamide specimen. Here, a ranging accuracy of <0.1 mm and a transmission ToF accuracy of 0.3μs were achieved. Thus, the proposed method enables fast and accurate non-invasive ToF measurements that do not require knowledge about transducer characteristics or ambient conditions.
Air-coupled ultrasonic (ACU) testing has proven to be a valuable method for increasing the speed in non-destructive ultrasonic testing and the investigation of sensitive specimens. A major obstacle to implementing ACU methods is the significant signal power loss at the air–specimen and transducer–air interfaces. The loss between transducer and air can be eliminated by using recently developed fluidic transducers. These transducers use pressurized air and a natural flow instability to generate high sound power signals. Due to this self-excited flow instability, the individual pulses are dissimilar in length, amplitude, and phase. These amplitude and angle modulated pulses offer the great opportunity to further increase the signal-to-noise ratio with pulse compression methods.
In practice, multi-input multi-output (MIMO) setups reduce the time required to scan the specimen surface, but demand high pulse discriminability. By applying envelope removal techniques to the individual pulses, the pulse discriminability is increased allowing only the remaining phase information to be targeted for analysis. Finally, semi-synthetic experiments are presented to verify the applicability of the envelope removal method and highlight the suitability of the fluidic transducer for MIMO setups.
Ultrasonic testing is a widely applied measurement method in materials research and medicine. Commonly, a transducer is coupled to the specimen directly or via a liquid coupling agent. While reducing acoustic transmission losses significantly, this procedure is time-consuming and cannot be used for sensitive specimens. Air-coupled ultrasound is a viable alternative in such cases, although suffering from very high acoustic transmission losses between transducer, air and specimen. The recently introduced fluidic transducer (FT) generates ultrasound by utilizing the instability of a supersonic air jet switched inside a fluidic amplifier. Since only air is used as the working medium and no vibrating surfaces are used for ultrasound generation, the transducer is able to efficiently generate large acoustic pressure amplitudes. The resulting acoustic field shares its directivity with the ejected high-velocity air jet. Thus, the acoustic energy needs to be redirected from the jet axis in order to make the fluidic transducer applicable to sensitive specimens. In this study, the effectivity of using sonic crystals (SCs) for this redirection is investigated using acoustic and flow measurements. SCs are air-permeable while being reflective to large acoustic frequency bands. It was shown that both a defect waveguide and a mirroring strategy successfully redirected the acoustic field from the air jet. Furthermore, the interaction of flow and SC showed strong acoustic quenching if the SC was placed too close to the FT outlet. Blockage of the jet entrainment due to the SC may result in slightly higher off-axis flow velocities locally, which should be considered in sensitive applications.
The majority of ultrasonic devices used for non-destructive testing in civil engineering require contact with the surface of the concrete (specimen), which significantly increases the time required for the measurement. This makes it impractical for extensive investigation of large-scale structures such as bridge decks, foundations, or tunnels.
In a pioneering approach, fluidic oscillators are used as contact free ultrasonic sources to overcome the aforementioned limitations. These robust and cost-effective actuators require only pressurised air and are ideally suited for harsh environments. At a constant supply pressure, they generate a continuous mono-frequent actuation signal. Further, varying the supply pressure via a fast pressure regulator was found to generate a frequency modulated signal which enabled time-of-flight measurement with an added advantage of increased signal to noise ratio. To demonstrate the feasibility of this novel idea of non-contact ultrasound, the results of the initial tests are presented.
Hydration is the exothermic reaction between anhydrous cement and water, which forms the solid cement matrix of concrete. Being able to evaluate the hydration is of high interest for the use of both conventional and more climate-friendly building materials. The experimental monitoring is based on temperature or moisture measurements. The first needs adiabatic conditions, which can only be achieved in laboratory. The latter is often measured comparing the weight of the material sample before and after oven drying, which is time-consuming. This study investigates the moisture content of two cement-based and two calcium sulphate based mixtures for the first 90 days by using the calcium carbide method and oven drying at 40 °C and 105 °C (Darr method). Thereby, the amount of chemically bound water is determined to derive the degree of hydration. The calcium carbide measurements highly coincide with oven drying at 40 °C. The calcium carbide method is therefore evaluated as a suitable alternative to the time-consuming Darr drying. The prompt results are seen as a remarkable advantage and can be obtained easily in laboratory as well as in the field.