Wave mixing with the fundamental mode of edge waves for evaluation of material nonlinearities

Abstract

The measurement of material and geometric nonlinearities is of great interest as these nonlinearities can be linked to mechanical damage and applied stress. Current experimental techniques often utilise the growth rate of the second or mixed harmonics of bulk, Rayleigh, or Lamb waves in the ultrasonic range to conduct measurements. However, there are many challenges in obtaining the nonlinearities from experimental data as all these waves experience spatial or/and frequency dispersion. This paper focuses on the application of the fundamental mode of edge waves (ES0), which is a natural analogue of Rayleigh waves, for the measurement of material nonlinearity. The ES0 wave mode (as well as other modes of edge waves) is guided by the edge of a plate, and it is therefore spatially nondispersive and not affected by the interior of the structure. These features can be important for practical implementations e.g. for the evaluation of fatigue damage and detection of structural defects, which typically initiate from free surfaces. However, this wave mode is weakly dispersive in the frequency domain and has a significant energy decay at high frequencies. Among the outcomes of this paper are the experimental generation of higher order and mixed harmonics with the fundamental mode of edge waves, as well as the evaluation of material nonlinearity. The latter is shown to be independent of the frequency and type of harmonic used.