Baseline-free characterisation of a delamination using nonlinear vibro-acoustic modulation

Conventional non-destructive evaluation techniques based on linear response are less sensitive to micro-damage than large defects. Nonlinear ultrasonics has recently attracted strong interest as offering the potential of baseline-free early damage detection. In the present work we employ the nonlinear vibro-acoustic technique, where a structure is simultaneously subjected to a low frequency and a high frequency excitation in order to generate nonlinear vibrational responses for the characterisation of delamination damage. In the presence of structural damage such as cracks and delamination, nonlinear mechanisms such as contact acoustic nonlinearity are activated, resulting in the generation of sideband frequency components that are not generated in a pristine structure. Though extensive qualitative analysis has been performed to assess vibro-acoustic modulation techniques for detecting the presence of defects, there is still a lack of understanding of the spatial sensitivity of the nonlinear response and hence of the prospects for locating and sizing damage. The results of the present study demonstrate that sideband frequency components generated by a delamination defect can be used to characterize the damage.