Air-Coupled Ultrasound Time Reversal (ACU-TR) For Subwavelength Nondestructive Imaging

Abstract

Air-coupled ultrasound (ACU) is increasingly used for non-destructive testing (NDT). With ACU, no contact or coupling agent (e.g., water, ultrasound gel) is needed between transducers and test sample, which provides high measurement reproducibility. However, for testing in production, a minimum separation is often necessary between sample and transducers to avoid contamination or transducer damage. Due to wave diffraction, the collimation of the ultrasound beam decreases for larger propagation distances, and ACU images become blurred and show lower defect lateral resolution with increasing sample-transducer separation. This is specially critical for thick composites, where large-size planar sources are used to bridge the large ACU transmission loss with good collimation. In this work, ACU re-radiation in unbounded media is extended to NDT of multi-layered composites. The extended method is named ACU time-reversal (ACU-TR), and significantly improves the defect resolution of ACU imaging. With ACU-TR, the complete pressure distribution radiated by large ACU source is measured with point receivers in one plane arbitrarily separated from the sample. By applying acoustic holography physics, it is then possible to quantitatively reconstruct the pressure field directly at arbitrary sample defect planes, which compensates for undesired diffraction phenomena and improves minimum detectable defect size, thereby achieving subwavelength lateral resolution. We tested the method on complex wood-based composite samples, based on ACU far-field measurements at a separation of 160 mm between sample and receiver transducer. With the proposed method, it is possible to detect surface defects as well as inner defects within composite boards. By using in the future point receiver arrays instead of a scanned microphone, both data acquisition and evaluation can be potentially implemented in real time. Index Terms-air-coupled ultrasound non-destructive testing, wood-based composite materials, gluing defects, inverse problem, time-reversal.

Investigation and Enhancement of the Detectability of Flaws with a Coarse Measuring Grid and Air Coupled Ultrasound for NDT of Panel Materials Using the Re-Radiation Method

“…The test material was a commercially available wooden medium-density fiberboard (MDF) particleboard measuring hight = 800 mm, wide = 800 mm and thick = 20 mm. Pieces of paper were used as imitations of flaws, which is typically done for particleboards [17,22], instead of teflon tape, which is historically used for the inspection of CFRP or GFRP. These were circular with a diameter of 2, 4, 5, 10, and 20 mm ( Figure 1).…”

“…However, in order to obtain the information of a volume sound field and to shorten the measuring time, one possibility is to measure only one plane in a C-scan and to calculate the volume sound field or the sound field of the air layer directly above the panel surface via post-processing methods. This can be done by the re-radiation method [16][17][18][19][20][21][22]. In this way, very well interpretable amplitude images of the air layer directly above the panel surface are generated.…”

“…In this way, very well interpretable amplitude images of the air layer directly above the panel surface are generated. These allow the identification and localization of flaws or the heterogeneities in the materials [17,18,22]. To create a C-scan image with a high detectability of flaws by using a typical measurement grid with a grid point distance of ≤ λ 3 [16][17][18]22], many measurements have to be made, and this is comparatively time-consuming.…”

“…These allow the identification and localization of flaws or the heterogeneities in the materials [17,18,22]. To create a C-scan image with a high detectability of flaws by using a typical measurement grid with a grid point distance of ≤ λ 3 [16][17][18]22], many measurements have to be made, and this is comparatively time-consuming. Nevertheless, with the re-radiation method of Marhenke2019 [22] we were able to identify flaws which are smaller than λ.…”

“…To create a C-scan image with a high detectability of flaws by using a typical measurement grid with a grid point distance of ≤ λ 3 [16][17][18]22], many measurements have to be made, and this is comparatively time-consuming. Nevertheless, with the re-radiation method of Marhenke2019 [22] we were able to identify flaws which are smaller than λ. Therefore, there is a need for techniques that can be applied with the re-radiation method that provide high detectability of flaws in a C-scan image, while requiring less measurement time due to a coarser measurement grid (> λ 3 ).…”