Lamb wave evaluation of concrete plates

This thesis is about the application of Lamb waves for non-destructive evaluation of plate-like concrete structures under one-sided access test condition using a portable equipment consisting of an accelerometer and impact hammer. With this type of equipment and based on array signal processing, the impact-echo (IE) method can be extended with measurements of surface waves; this extension allows an evaluation with Lamb waves. For both the conventional impact-echo method as well as evaluations based on Lamb waves, results in the literature report a systematically underestimated plate thickness. This error is investigated and reproduced in a numerical study and further verified in a measurement. A main source of uncertainty is related to the estimation of a longitudinal wave velocity from first arrivals. To reduce this uncertainty, a new approach for estimation of Poisson’s ratio is proposed. The new approach is based on the amplitude polarization of the S1-ZGV Lamb mode: a through-thickness representative estimate of Poisson’s ratio is thereby obtained leading to an overall improved estimation of the Lamb wave plate parameters.
A new technique based on 2D arrays is also presented. In an example, the technique is used to map (image) the variation of phase velocity for the A0 Lamb mode in the lateral plane of the plate; that is, the technique is in this case used to assess the material and plate thickness homogeneity. Compared to ultrasonic reflection imaging methods, no limitation to a specific set of operating frequencies (transducers) exists since the 2D array technique is based on a full wave field dataset with wide frequency bandwidth. Variation of material properties transversally through the thickness may also exist. For improved understanding about zero-group velocity (ZGV) modes under this type of material condition, two cases with continuously varying acoustic bulk wave velocities are investigated. Results show that a zero-group velocity mode exists with the similar robustness and detectability for both cases: non-destructive applications based on zero-group velocity modes are possible also for these two material variation cases.