Nonlinear acoustics for nondestructive testing is still an emergent technique in complex materials with localized cracks because linear effects can be cancelled using specific symmetries for the pre- and post-signal processing. The local nonlinear signature is accessible even in complex aeronautic structure by using the symmetry properties: i. e. time reversal invariance and pulse inversion associate to chirp-coded excitation. Excitation Symmetry Analysis Method [Dos Santos et al., JNLM, 2007] which is an extension of the pulse inverted method for extraction of third order nonlinearity is applied experimentally for the measurement of relative level of nonlinearity on a airplane steel bracket, on which visible crack was created by fatigue tests. This pre-processing method has been coupled to a 100-250 kHz bandwidth chirp-coded ultrasonic excitation, realized at various amplitude (A, A/2, 31/2A/2, etc.), and performed with DAKEL IDK09 200 kHz PZT sensors. Signal processing based on correlation measurements, allow an extraction of the nonlinear signature versus fatigue level. Experiments and data processing have been conducted with a 12 bit DAKEL-DTR system in the bandwidth 80-800 kHz and compared with a continuous 4 channel acoustic emission device.

Abstract:The scratch test, as probably the most widespread technique for assessment of the adhesive/cohesive properties of a film-substrate system, fully depends on reliable evaluation based on assessment of critical loads for systems' failures. Traditionally used evaluation methods (depth change record and visual observation) may sometimes give misleading conclusions about the failure dynamics, especially in the case of opaque films. Therefore, there is a need for another independent evaluation technique with the potential to complete the existing approaches. The nondestructive method of acoustic emission, which detects the elastic waves emitted during film cracking and delamination, can be regarded as a convenient candidate for such a role even at nano/micro scale. The strength of the combination of microscopic observation of the residual groove and depth change record with the acoustic emission detection system proved to be a robust and reliable approach in analyzing adhesion/cohesion properties of thin films. The dynamics of the gradual damage taking place during the nano/micro scratch test revealed by the combined approach is presented for SiC and SiCN thin films. Comparison of critical load values clearly reflects the higher ability of the AE approach in detecting the initial material failure compared to the visual observation.

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