A method of non-destructive testing is proposed, which combines the acoustic load of the object of inspection and interferometric control of its surface displacements, formed as a result of an acoustic wave resonance in a hidden defect. The method is aimed at solving the problem of detection and identification of subsurface defects formed in structural elements made of polymer layered composite materials or contain protective paint coatings. At the same time, the defect is considered as an acoustic resonator filled with a medium in which only a longitudinal acoustic wave propagates. Usually such media are air or water. The novelty of the method is that it allows us not only to detect a hidden defect and establish its location, but also to determi¬ne its dimensions. The scheme of the experimental installation is given and the method is described. The research of the detection and identification of subsurface defects that most often occur in practice was conducted with the help of the model of the experimental installation. In products made of polymer layered composites, such defects are unbounded areas between layers during construction, internal damages such as cracks and areas of crumpling during its operation. The results of the experiments proved the effectiveness of the proposed method. The described method is also suitable for detection and identification of blistering of protective paint coatings, provided that they are filled with liquids or gases in which transverse acoustic waves do not propagate.

1. Deo, R.B.; Starnes, J.H.; Holzwarth, R.C. Low-cost composite materials and structures for aircraft applications. In: RTO Meeting Proceedings RTO-MP-069(II). NATO Research and Technology Organisation (RTO), 2003, 1-11.

2. Mrazova, M. Advanced composite materials of the future in aerospace industry. Incas bulletin, 2013, 5.3: 139. https://doi.org/10.13111/2066-8201.2013.5.3.14

3. Riegert, G.; Pfleiderer, K.; Gerhard, H.; Solodov, I.; Busse, G. Modern Methods of NDT for Inspection of Aerospace Structures, In Proceedings of 9th European Conference on NDT, September 2006, Berlin, Germany, We.4.1.4, pp. 1-11.

4. Fomitchov, P.; Wang, L.-S.; Krishnaswamy, S.; Advanced Image-Processing Techniques for Automatic Nondestructive Evaluation of Adhesively-Bonded Structures Using Speckle Interferometry. Journal of Nondestructive Evaluation, 1997, 16, 215-227. https://doi.org/10.1023/A:1021848031529

5. Chuang, K.-C.; Lin, S.-H.; Ma, C.-C. Efficient excitation of transverse vibrational modes using improved configurations of PFCs connected to an isotropic plate, Composite Structures, 2021, 265, 113718. https://doi.org/10.1016/j.compstruct.2021.113718

6. Solodov, I.; Bernhardt, Y.; Kreutzbruck, M. Resonant Airborne Acoustic Emission for Nondestructive Testing and Defect Imaging in Composites. Appl. Sci., 2021, 11, 10141. https://doi.org/10.3390/app112110141

7. Ida, N.; Meyendorf, N. Handbook of Advanced Nondestructive Evaluation, Springer Nature Switzerland AG, 2019. https://doi.org/10.1007/978-3-319-26553-7

8. Kino, G. S. Acoustic Waves: Devices, Imaging and Analog Signal Processing. Prentice-Hall, Inc. A Division of Simon & Schuster Englewood Ch'ffs, New Jersey, 1987.

9. Galagan, R.M. Theoretical foundations of ultrasonic non-destructive testing. National Technical University of Ukraine "Igor Sikorsky Kyiv Polytechnic Institute", Kyiv, 2019. [in Ukrainian]

10. Li, F.; Peng, H.; Sun, X.; Wang, J.; Meng, G. Wave Propagation Analysis in Composite Laminates Containing a Delamination Using a Three-Dimensional Spectral Element Method, Mathematical Problems in Engineering, 2012, 659849. https://doi.org/10.1155/2012/659849

11. Gerhard, H., Busse, G. Two new techniques to improve interferometric deformation-measurement: Lockin and Ultrasound excited Speckle-Interferometry. Osten, W. Ed., Fringe, Springer, Berlin, Heidelberg, 2005, 530-538. https://doi.org/10.1007/3-540-29303-5_70

12. Hosoya, N.; Yoshinaga, A.; Kanda, A.; Kajiwara, I. Non-contact and non-destructive Lamb wave generation using laser-induced plasma shock wave. International Journal of Mechanical Sciences, 2018, 140, 486-492. https://doi.org/10.1016/j.ijmecsci.2018.03.023

13. Wang L.-S.; Krishnaswamy, S. Additive-subtractive speckle interferometry: Extraction of phase data in noisy environments. Optical Engineering, 1996, 35(3), 794-801. https://doi.org/10.1117/1.600649

14. Wiciak, P. Quality Assessment of Composite Materials using Ultrasonic Non-Destructive Testing Methods, UWSpace, 2020. 15. Derusovaa, D.A.; Vavilova, V.P.; Sfarraa, S.; Sarasinid, F.; Druzhinine, N.V. Applying ultrasonic resonance vibrometry for the evaluation of impact damage in natural/synthetic fibre reinforced composites. Polymer Testing, 2018, 68, 70-76. https://doi.org/10.1016/j.polymertesting.2018.03.053

16. Voronyak, T.I.; Kmet', A.B.; Muravs'kyi, L.I. Determination of the 3D fields of displacements by the method of phase-shifting speckle interferometry. Materials Science, 2009, 45(3), 372-377. https://doi.org/10.1007/s11003-009-9201-8

17. Jones, R.; Wykes, C. Holographic and Speckle Interferometry, 2nd ed. Cambridge University Press., 1989, 140-141. https://doi.org/10.1017/CBO9780511622465

18. Muravsky, L.; Kmet', A.; Voronyak, T. Two approaches to the blind phase shift extraction for two-step electronic speckle pattern interferometry. Opt. Eng., 2013, 2(10), 101909. https://doi.org/10.1117/1.OE.52.10.101909

19. Lobanov, L. М.; Muravs'kyi, L. I.; Pivtorak, V. A.; Voronyak, T. I.; Monitoring of the Stressed State of Structural Elements with the Use of Electromagnetic Waves of Optical Range, in: Z. T. Nazarchuk Ed., Technical Diagnostics of Materials and Structures: A Handbook [in Ukrainian], Vol. 3, Prostir-M, 2017, ISBN 978-617-7501-37-3.

20. Rona, A. The acoustic resonance of rectangular and cylindrical cavities. Journal of Algorithms & Computational Technology, 2007, 1(3), 329-356. https://doi.org/10.1260/174830107782424110