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A large series of experimental and theoretical studies, many of which were the pioneering ones, was carried out at the IAP RAS to identify the physical mechanisms of a strong (anomalous) acoustic nonlinearity of media having an inhomogeneous microstructure. As a result, high-sensitivity nonlinear acoustic methods for nondestructive control and testing, methods for their industrial application, and appropriate diagnostic instrument prototypes have been developed (V. E. Nazarov, L. A. Ostrovsky, I. A. Soustova, A. M. Sutin, V. Yu. Zaitsev, I. Yu. Belyaeva, A. V. Radostin, L. A. Matveev, and V. V. Kazakov). The physical and rheological models of microinhomogeneous media elaborated at the IAP RAS show that the presence of even very low concentrations of highly compressible defects (e.g., cracks) leads to a strong increase in nonlinearity of the material with an almost constant value of the linear elastic moduli. This basic property can be used for an early (at the stage of initiation) detection of such defects in the samples and engineering constructions, while the conventional linear methods of acoustic testing almost fail. An application of these results, tested in the industrial environment, can be the technology of fatigue crack detection in the axles of railway wheel sets, which was developed in collaboration with the Nizhny Novgorod Branch of the Research Institute of Railway Transport.
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