Influence of localized microstructure evolution on second harmonic generation of guided waves

The following work is intended to utilize the potential of nonlinear guided waves to characterize incipient fatigue damage. Since fatigue crack initiation often occurs in regions where there is localized microscale damage, this difficult-to-analyze condition is investigated in simplistic terms. The third order elastic constants are taken to be a function of position in accordance with the plastic strain distribution accrued during a quasistatic tension test. Two specimen geometries are considered along with multiple average plastic strain levels. Finite element analysis is employed to simulate generation of the s2 secondary Lamb mode from the S1 primary Lamb mode in al aluminum plate. The results indicate that even though the S1/s2 mode pair is synchronized and has positive power flux, the second harmonic wave mode does not accumulate linearly with propagation distance due to the inhomogeneous nature of the plate.