Lamb Wave Phase Velocity Imaging of Concrete Plates with 2D Arrays

In the nondestructive evaluation of concrete structures, ultrasonic techniques are considered to be more capable than low-frequency techniques such as the impact-echo method. This is especially true with the recent development of ultrasonic transducers, synthetic apertures, and results in an image form, and because low-frequency techniques are usually limited in their evaluation to the frequency of one single resonant mode. With the aim of reducing this gap in capabilities, we present a 2D array and wide-frequency bandwidth technique for Lamb wave phase velocity imaging. The presentation involves a measurement on a newly cast concrete plate using a hammer and an accelerometer as an example. The key concept of the technique is the use of 2D arrays that record a full wave field response over a limited surface subdomain within the complete measurement domain. Through a discrete Fourier transform, a spectral estimate is obtained for the 2D array in the frequency-phase velocity domain. The variation of the phase velocity is then mapped using a stepwise movement of the 2D array within the complete measurement domain. With two different types of 2D arrays, the variation of the phase velocity for the A0 Lamb mode is mapped and displayed in a polar and image plot, and low variation is observed for both cases. This result verifies the expected condition of a homogenous material and plate thickness and, more importantly, highlights the potential of wide-frequency bandwidth techniques based on full wave field data.