Abstract
The vibro-acoustic modulation (VAM) technique is probably the most widely used nonlinear method for crack detection. The VAM method is based on the effect of modulation of high-frequency acoustic waves by a low-frequency vibration. The intensity of the modulation is related to the severity of the damage and has been used so far as a damage index. The damage index simply based on the amplitude of the first side bands in the spectral domain often leads to controversial results about the severity of the damage. In this work, the nonlinear characteristics of the vibro-modulation were systematically investigated by employing time-frequency analysis based on the Zhao-Atlas-Marks (ZAM) distribution. The results of the analysis show that the amplitude of the sideband components is modulated by the low frequency vibration and the modulation amplitude depends on the size of the crack. Based on the obtained results, a new damage index was defined in relation to the strength of the modulation. The new damage index is more sensitive and robust and correlates better with crack size compared to the index based on the amplitude of the sidebands.
Highlights
Structures with inhomogeneities or defects exhibit strong nonlinear vibrational and acoustical effects
The phenomenon of vibroacoustic modulation (VAM) is usually measured in the frequency domain and it is manifested as sidebands around the carrier peak of the ultrasound wave at frequencies equal to the sum and difference of the excitation frequencies and their integer multiples
The primary aim of the present work is to investigate vibroacoustic modulation in the time-frequency domain by employing time-frequency analysis based on the Zhao-AtlasMarks (ZAM) distribution, which has the advantage of significantly reducing cross-terms between signal components, through its cone-shaped kernel function
Summary
Structures with inhomogeneities or defects exhibit strong nonlinear vibrational and acoustical effects. Strong nonlinear effects were observed in structures with cracks. These effects include the generation of higher harmonics and intermodulation of a high-frequency acoustic wave by a low-frequency vibration [1] and provide the foundation for developing different techniques for nondestructive testing. The vibroacoustic modulation (VAM) method is based on the fact that a high-frequency ultrasound probing wave propagating in a structure is modulated by a low-frequency vibration. The modulation is generated by the nonlinear interaction of waves caused by the presence of the crack. Modulation effects have been observed in several applications. Ekimov et al [5]
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