Lamb wave basis for impact-echo method analysis

The impact-echo method has been developed over the past 20 years and is now widely used in the nondestructive evaluation of concrete. However, some practical issues remain unresolved, such as the physical basis for the empirical correction factor (β) used to obtain thickness mode frequency. A new approach based on guided wave theory is proposed in this paper: That the impact-echo resonance in plates corresponds to the zero-group-velocity frequency of the S₁ Lamb mode. A numerical model is developed, verified by experiment, and then shown to adequately simulate the dynamic response of a concrete plate. Using this model the thickness resonance mode is identified and found to accurately match that particular Lamb mode in terms of shape and frequency. New values for β based on the Lamb mode model are computed and dependence on material Poisson's ratio is demonstrated.