Multi-frequency ultrasonic NDE for early defect recognition and imaging

The nonlinear response of flaws results in frequency changes which can be used for NDE (nondestructive evaluation). We report on the observation and NDE application of the ultimate nonlinear spectrum which (beyond the well-known ultra-harmonics and modulation side-bands) comprises sub-harmonics, ultra-subharmonics and ultra-frequency pairs. The experiments include 20-40 kHz intense acoustic wave excitation and local spectrum measurements with a scanning laser interferometer over the 1 MHz frequency range. The new modes produced reveal the distinctive features of the nonlinear resonance mechanism: hysteresis, bistability and avalanche-like dynamics. Unlike that of the excitation frequency, their spatial distribution is confined strictly around the sources of nonlinearity (defects) and is used for the multi-frequency C-scan imaging of flaws that are invisible in the mono-frequency NDE regime. The series of images for various frequency lines is applied to remove background noise and to enhance the object to be identified by linear and nonlinear image processing. The results obtained for the fractured defects (impacts, cracks, delaminations, welded-joints) in a number of materials (CFRP, C-C/SiC, etc.) and components demonstrate evident benefits of the cumulative multi-frequency NDE for defect recognition and imaging.