Non-destructive ultrasonic testing is utilized widely by industries for quality assurance. For sensitive materials or surfaces, non-contact, non-destructive testing methods are in demand. The air-coupled ultrasound (ACU) is one possible solution. This can be used to investigate large, panel-like objects for delaminations and other flaws. For a high detectability, fine measurement grids are required (typically <λ is used), which results in extremely long data acquisition times that are only practicable for laboratory applications. This paper aimed at reducing the required measurement grid points for obtaining high detectability evaluations. The novel method presented in this paper allows a measurement grid that is much coarser than the resulting grid. The method combines a software refinement of the measured data with the Rayleigh-Sommerfeld diffraction integral for the calculation of the pressure distribution on the object's surface. This result allows the precise prediction of delaminations and flaws in the tested object. The presented method shows a decrease in the total investigation time by up to 98%.

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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.

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