Characterization of metal’surface treatment intensity with ultrasound requires a deep knowledge of material attributes. In particular, residual stress characterization with surface acoustic waves (Rayleigh waves) or bulk waves demands an awareness for nonlinear elastic properties. In this article several experimental works for the determination of acoustoelastic constants/third-order elastic constants (AEC/TOEC) for bulk and surface waves for Inconel 718 and Titanium 6246 alloys are compared. In addition, a new method for in-situ TOEC determination is proposed and preliminary numerical simulation are shown.