Zero group velocity mode nonlinear ultrasonics for fatigue crack detection

This short communication reports an enhanced nonlinear ultrasonics methodology leveraging zero group velocity (ZGV) modes for fatigue crack detection. Conventional nonlinear ultrasonics has been widely investigated as a promising tool for monitoring incipient damage due to its high sensitivity. Nevertheless, nonlinear features are vulnerable and could be easily covered by other nonlinearity sources from electronic instruments, actuator bonding layers, and distributed material nonlinearity. To address such challenges, this short communication exploits the ZGV mode to amplify the second harmonic signal features at fatigue cracks. ZGV modes are endowed with unique characteristics of a zero-value group velocity with a finite wavenumber, introducing a localized resonance and confining the wave energy in the vicinity of a fatigue crack, which will greatly enhance the signal nonlinear features. This short communication develops as follows: firstly, theoretical fundamentals of ZGV Lamb modes are laid out for preparing the investigation. Then, the half-ZGV frequency is selected for wave actuation, targeting at generating the second harmonic at the ZGV resonance. Finite element simulations demonstrate that the half-ZGV frequency actuation triggers the ZGV resonance at the fatigue crack, thus considerably enhancing the second harmonic features compared to the standard actuation. Furthermore, the superb caliber of such a method is further demonstrated by several peculiar characteristics of the nonlinear ZGV generation from temporal, spectral, and spatial perspectives. This communication finishes with concluding remarks and suggestions for future work.