Near Field Effects and Estimation of Poisson's Ratio in Impact-Echo Thickness Testing

Based on the polarization and shape of the first symmetric Lamb mode at the zero-group velocity point (S1-ZGV), a novel approach for Impact-Echo thickness testing is proposed. In this new approach all three plate parameters (thickness, velocity and Poisson's ratio) are estimated from the same wave mode (S1-ZGV). A traditional approach based on a measurement of the longitudinal-and Rayleigh wave velocity along the surface is also studied, and is found to be influenced by near field effects leading to an underestimated thickness. In addition, an alternative approach which estimates Poisson's ratio from the frequency ratio of the S1-ZGV and A2-ZGV point is discussed. The three studied approaches are demonstrated in a realistic field case.