In this paper, the applicability of a nonlinear wave modulation-based crack monitoring methodology has been experimentally investigated. Experiments using a beam specimen with a low-cycle fatigue crack have been conducted for the purpose of preliminary study, in which two PZT patches attached on the beam have been used as the transducer of high frequency probe wave. When the specimen is subjected to a harmonic loading at low frequencies, it vibrates, and the presence of the crack introduces a nonlinear effect to the vibro-acoustic dynamics resulting an interaction between the low frequency structural vibration and the high frequency probe wave. This nonlinearity is observed as the amplitude and phase modulation of the received probe wave synchronous with the structural vibration. To investigate the relationship between the modulations, the structural vibration and the damage extent, the collected signal at the receiver PZT has been separated into low frequency and high frequency components, the former has been used to obtain the information about the structural vibration, while the latter has been demodulated in amplitude and phase. The demodulated waveforms have been examined as a potential indicator of the crack extent, especially focusing on their higher harmonics. Then, a "modulation surface" constructed from the modulated envelopes and the low frequency components has been proposed, which could provide more detailed view of the nonlinear wave modulation effects induced by the crack development. Several candidates for a damage indicator based on the modulation surface have been presented to demonstrate the usefulness of the modulation surface as a sensitive and promising feature relevant to the damage extent.

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