Rayleigh wave second harmonic generation and mixing for probing depth-varying acoustic nonlinearity in thermally damaged steel

This study compares the results of two nonlinear ultrasonic Rayleigh wave techniques for investigating depth-dependent acoustic nonlinearity in thermally damaged steel. Both methods leverage the frequency-dependent penetration depth of Rayleigh waves to probe near-surface microstructural degradation associated with decarburization. The first method employs second harmonic generation over a broad frequency range from 0.4 to 2.2 MHz for the primary wave. The second uses nonlinear mixing of two counter-propagating Rayleigh waves that generate a bulk longitudinal wave at the sum frequency. For parameters measured by both techniques, the relative contrast between damaged and intact steel increases with frequency, as the half primary wavelength compares to the thickness of the decarburized layer. Differences between the two techniques are discussed. These findings show the feasibility of using multi-frequency second harmonic generation and Rayleigh wave mixing to non-invasively measure depth-varying elastic nonlinearity in steel.