Optimization of Bifrequential Pump Excitation Parameters by Genetic Algorithm in Nonlinear Vibro Acoustic Wave Modulation Technique

The nonlinear Vibro-Acoustic wave Modulation (VAM) is a widely used technique in nonlinear nondestructive testing. Many research, in this area, are devoted to find the best combination of the excitation parameters, especially the right frequencies to the optimization of the damage detection sensitivity. In a previous study, we have proposed a novel method using the optimal command principle for a VAM system. Indeed, this method has permitted to find automatically the best pump frequency maximizing the nonlinear modulation (NM) effects in a multiple scattering sample. The cost function to be optimized thanks to the pump wave frequency is the correlation coefficient (CC) between a reference output signal without pump and an output modulated probe signal with the presence of the pump wave. In the present work, we aim to improve the nonlinear damage detection by exciting simultaneously two resonance modes of the medium. We consider a new excitation waveform for the pump wave, composed by the sum of 2 frequencies, instead of a monochromatic sine wave. We propose, then, to find automatically the frequencies f
_p
1 and f
_p
2 giving the optimal sensitivity for damage detection. The tested sample is an aluminium bar where nonlinear scatterers can be controllably added or removed. For both f
_p
1 and f
_p
2, the genetic algorithm (GA) seems to stuck on one of the resonance frequencies until it converges to the optimal combination. The corresponding values of the CC are quite similar to those obtained by a single pump frequency.