Airborne non-contact and contact broadband ultrasounds for frequency attenuation profile estimation of cementitious materials

Ultrasonic detection method based on flexible capillary water column arrays coupling

Conventional water immersion ultrasonic testing faces limitations due to factors such as environmental conditions, workpiece dimensions, corrosion, and resource wastage. Contact-based coupling methods, which employ coupling media or specific coupling structures, offer a convenient approach to coupling acoustic waves and reduce signal attenuation. However, these methods are time-sensitive and lack adaptability to uneven surfaces, particularly when dealing with workpieces featuring subtle undulations, resulting in significant signal decay. This paper presents an ultrasonic coupling method based on a flexible capillary water column array. By employing a stable and flexible water column array within the micro-channels as the coupling medium, stable contact-based transmission of ultrasonic signals is achieved. The influence of water column array unit dimensions and array structures is explored through theoretical analysis and experimentation, demonstrating lower energy attenuation compared to reductions in water column area. Notably, the tests revealed the method's adaptability at oblique angles below 20°, which surpasses the performance of submerged detection at similar angles. This research presents an innovative and stable approach for contact-based ultrasonic coupling testing, particularly in scenarios involving dynamic contact scanning between ultrasonic waves and workpieces.

Enhancing the spectral signatures of ultrasonic fluidic transducer pulses for improved time-of-flight measurements

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.

Airborne ultrasound transmission through circular damages in polymer plates

Polymer tanks made of Polypropylene (PP) sub-assemblies are commonly used for their ease of formability, surface and optical quality. The tightness of an assembly is classically assessed by pressurizing the vessel and measuring a pressure drop after a given period. In order to avoid this long and imprecise method, active ultrasound methods can be envisioned, but should be carefully designed in order to derive the proper transducer configurations, frequencies and assess the performances of the method in terms of repeatability and detectability. In this article, a thermoviscous Finite Element Model (FEM) is derived in order to predict the effect of realistic damages on the measured transmitted acoustic field in bonded polymer joints. Three damage scenarios are considered, namely through holes, flat-bottom holes and internal voids that may impair the tightness and durability of a polymer assembly. Numerical results in terms of on-axis sound pressure spectra and directivity diagrams are presented and verified experimentally on a flat panel. These results allow the derivation of design rules for the active inspection of polymer jointed structures.

Comparative Study of Coupling Techniques in Lamb Wave Testing of Metallic and Cementitious Plates

Lamb waves have emerged as a valuable tool to examine long plate-like structures in a faster way compared to conventional bulk wave techniques, which make them attractive in non-destructive testing. However, they present a multimodal and dispersive nature, which hinders signal identification. Oblique incidence is one of the most known methods to generate and receive Lamb waves and it is applied in different experimental arrangements with different types of sensors. In this work, several setups were conducted and compared to determine the optimal ones to launch and detect ultrasonic Lamb waves, especially in non-homogeneous specimens. The chosen arrangements were contact with angle beam transducers, immersion in a water tank, localised water coupling using conical containers and air coupling. Plates of two different materials were used, stainless steel and Portland cement mortar. Theoretical and experimental dispersion curves were compared to verify the existence of Lamb modes and good correspondence was achieved.