A theoretical and numerical study on the mechanics of vibro-acoustic modulation.

Vibro-acoustic modulation (VAM) is a form of a non-destructive testing technique used in nonlinear acoustic methods for the detection of defects. It comprises of exciting the structure with a dual frequency sinusoidal signal and studying the interaction of this wave with the underlying defect. In this work a theoretical study on the mechanics of VAM is presented for a generic material body. The roles of different types of defect on the response of the material are analyzed. The theoretical analysis shows the origins of the nonlinear frequencies in the form of higher harmonics and sidebands commonly observed in the output response of VAM excitation. In addition, the analysis provides insights on the relationships between the magnitudes of the nonlinear responses and those of the input vibrations, and on the physical origins of the nonlinear responses. For a physical visualization of the nonlinear vibrations associated with the theory a finite element analysis of VAM is also performed. The model looks into the plausibility of using VAM for the mapping of damage in physical structures. The model is also used to investigate the effects of the defect size and defect depth on the nonlinear mechanism of VAM.