Nonlinear modulation methods of structural damage detection based on dissipative nonlinear effects

The nonlinear vibro-acoustic response of solid samples containing quite a small amount of defects can be anomalously high in magnitude compared to the case of undamaged intact solids. Functional dependencies of the nonlinear effects often exhibit rather interesting behavior which cannot be explained by manifestations of conventional nonlinear elasticity or hysteretic nonlinearity of the material. In this paper, experimental evidences of another, essentially nonlinear-dissipative mechanism responsible for nonlinear vibro-acoustic interactions in defect-containing solids are presented. In particular, experimental results on nonlinearity-induced cross-modulation of a high-frequency (HF) f=15...30 kHz signal by a low-frequency (LF) F=20-60 Hz vibration in an aluminium plate with a small single crack are reported. Comparison with a reference sample (the identical plate without a crack) has proven that the presence of such a small defect can be easily detected due to its nonlinear manifestations. It is demonstrated that under proper choice of the sounding signal parameters, the effect level can be so pronounced that the amplitude of the modulation sidelobes originated due to the nonlinearity exceeds the amplitude of the fundamental harmonic of the HF signal. Similar modulation effects were observed in polycrystalline metal samples using a new variant of modulation technique exploited amplitude-modulated intensive HF excitation instead of the LF vibration used in the first experiment. Main features of the observed phenomena are pointed out, their physical explanation based on a nonlinear-dissipative mechanism is suggested and some simulation results are also presented. 1 also at the Institute of Applied Physics RAS, 46 Uljanova Str., Nizhny Novgorod, 603600, Russia, e-mail: [email protected]