Multi-mode nonlinear resonance ultrasound spectroscopy for defect imaging: an analytical approach for the one-dimensional case.

A nonlinear version of the resonance ultrasound spectroscopy (RUS) theory is presented as an extension of the RUS formalism to the treatment of microdamage characterized by nonlinear constitutive equations. General analytical equations are derived for the one-dimensional case, describing the excitation amplitude dependent shift in the resonance frequency and the generation of harmonics resulting from the interaction between bar modes due to the presence of either localized or volumetrically distributed nonlinearity. Solutions are obtained for classical cubic nonlinearity, as well as for the more interesting case of hysteresis nonlinearity. The analytical results are in excellent quantitative agreement with numerical calculations from a multiscale model. Finally, the analytical formulas are exploited to infer critical information about damage position, degree of nonlinearity, and width of the damage zone either from the shifts in resonance frequency occurring at different excitation modes, or from the shift and the harmonics predicted at a single mode. Unlike other techniques, the multi-mode-nonlinear RUS method does not require a spatial scan to locate the defect, as it lets different excitation modes, with different vibration patterns, probe the structure. Two general methods are suggested for inverting experimental data.