The influence of Poisson's ratio on the excitability of the first higher symmetric (S1) zero-group velocity Lamb mode.

The impact echo (IE) method is widely used in non-destructive testing of concrete plate like structures but has proved difficult to apply on asphalt pavements. Observed low IE resonance amplitudes on asphalt have been attributed to larger material damping but the present study shows that it may also be an effect from a larger Poisson's ratio. The IE frequency corresponds to the stationary resonance of the S1 Lamb mode at zero-group velocity (ZGV). It has been previously shown that the S1 ZGV Lamb mode only exists below a Poisson's ratio of 0.45. [Prada et al., J. Acoust. Soc. Am. 124, 203-212 (2008).] The present study investigates the magnitude of the excitability and damping ratio of the S1 ZGV mode as a function of Poisson's ratio. The frequency response from a point source is predicted using a finite element model. Results show that the excitability decreases with increasing Poisson's ratio. The excitability of the S1 ZGV mode is about twice as high at a Poisson's ratio of 0.18 compared to 0.30, which are representative values for concrete and asphalt, respectively. The modal damping ratio increases with Poisson's ratio, causing an overall lower IE resonance amplitude at higher values of Poisson's ratio