ИСТИНА

In this paper the method of nonlinear laser scanning vibrometry is used for diagnostics of thin cylindrical resonators with flexural Lamb waves. Resonators are made of the plate 1.2 mm thick of polycrystalline aluminium alloy D16. Latent defects and residual stresses in the plate as well as in the manufactures resonators causes nonclassical elastic nonlinearity. Structural nonlinearity was found during a study of frequency dependence of one of the resonator’s natural modes on the amplitude of the flexural Lamb waves in it (the fast dynamics effect). The resonant frequency, and hence the flexural Lamb waves velocity decreases linearly with Lamb waves amplitude increasing. Furthermore, the threshold frequency dependence on the oscillation amplitude was found. This behaviour of the resonant frequency is typical for media with nonclassical nonlinearity. Due to the strong dispersion of flexural Lamb waves velocity, the spectrum of the natural oscillations of the resonator is not equidistant and as a result, the harmonics of the fundamental Lamb mode are not the same as the natural frequencies of its higher modes. Therefore, due to break of energy and momentum conservation law, it is hard to observe generation of such harmonics. To investigate the resonant nonlinear phenomena in resonators with residual stresses and artificially created defect (crack), we used a vibromodulation technique. The low-frequency (Ω = 344 Hz) and highfrequency (ω = 75,39 kHz) flexural Lamb waves were excited simultaneously; these frequencies coincide with normal modes of the resonator. Due to the nonlinear interaction between these oscillations, it became possible to observe also oscillations on the combination frequencies (ω ± nΩ, n = 1,2,3) which were visualized by a laser vibrometer. The local increase in their amplitude was recorded in areas with residual deformations and in the crack mouth. The studies were supported by the Russian Science Foundation (project N◦14-22-00042).