A new nonlinear-modulation acoustic technique for crack detection

A new nonlinear-moduation method for crack detection is discussed. It is based on the so-called cross-modulation effect consisting of the transfer of modulation from an intensive, initially slowly amplitude-modulated stronger (pump) excitation to the probe signal. Advantage of this technique is a very flexible independent choice of the frequencies for both carriers and the modulation, since their ratio may be rather arbitrary. This in its turn makes possible to effectively use the sample resonances in order to achieve the necessary level of the pump excitation and to ameliorate conditions for detection of the modulation sidelobes for the probe wave. Unlike harmonic-generation methods the initial nonlinear distortions of the pump and probe excitations (e.g. due to nonlinearities in the electronics) are not critical for this technique. The performed test experiments indicated high sensitivity of the new technique.