Air-coupled ultrasound (ACU) is increasingly used for nondestructive testing (NDT). With ACU, no contact or coupling agent (e.g., water and 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 the sample and the 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 especially critical to thick composites, where large-size planar sources are used to bridge the large ACU transmission loss with good collimation. In this work, ACU reradiation in unbounded media is extended to NDT of multilayered 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 (RXs) 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 the ACU far-field measurements at a separation of 160 mm between the sample and the RX transducer. With the proposed method, it is possible to detect surface defects as well as inner defects within composite boards. In the future, by using point RX arrays instead of a scanned microphone, both data acquisition and evaluation can be potentially implemented in real time.

中文翻译：

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空气耦合超声时间反转（ACU-TR），用于亚波长无损成像。

空气耦合超声（ACU）越来越多地用于无损检测（NDT）。使用ACU，在换能器和测试样品之间不需要接触或偶联剂（例如水和超声凝胶），从而提供了高测量重现性。但是，为了在生产中进行测试，通常需要在样品和换能器之间保持最小的距离，以避免污染或换能器损坏。由于波衍射，对于更大的传播距离，超声波束的准直度降低，并且随着样品-传感器间距的增加，ACU图像变得模糊并且显示出较低的缺陷横向分辨率。这对于厚的复合材料尤为关键，在厚的复合材料中，大型平面光源用于以良好的准直度弥合较大的ACU传输损耗。在这项工作中 无限制介质中的ACU辐射扩展至多层复合材料的NDT。扩展方法称为ACU时间反转（ACU-TR），可显着提高ACU成像的缺陷分辨率。使用ACU-TR，可以在与样品任意分离的一个平面上使用点接收器（RX）测量大型ACU源辐射的完整压力分布。通过应用声学全息物理学，可以直接在任意样品缺陷平面上定量重建压力场，从而补偿不希望的衍射现象并改善最小可检测缺陷尺寸，从而实现亚波长横向分辨率。我们基于ACU远场测量对复杂的木质复合样品进行了测试，样品与RX传感器之间的距离为160 mm。利用所提出的方法，可以检测复合板上的表面缺陷以及内部缺陷。将来，通过使用点RX阵列而不是扫描的麦克风，可以实时实现数据采集和评估。