Material fatigue prognosis with higher-order spectral analysis of nonlinear ultrasonic characteristics

Nonlinear aspects of ultrasonic wave propagation in a specimen that is subjected to increasing intervals of cyclical loadings are characterized. The ultimate goal is to develop an approach that would facilitate monitoring the extent of the deterioration occurring during the precursor to crack formation. The effectiveness of using higher-order spectral analysis tools, and more particularly the bispectrum, in characterizing certain nonlinear aspects of the ultrasonic wave propagation are presented. The results show that the bispectrum is capable of detecting couplings between the fundamental frequency of the exciting burst, its subharmonic, its first harmonic and its sidebands. Differences in coupling levels between these modes are observed as the number of cycles is increased. This suggests that such levels could be used to derive parameters that can be directly related to the fatigue damage state of the material. The results also present a discussion on the shortcomings of using the amplitudes of generated harmonics to determine the fatigue damage state in a specimen.