Extraction of Dispersion Curves from a Phased Array based Ultrasonic Polar Scan: Simulation Study

Daemen, Jannes; KU Leuven Kulak; Belgium

Daemen, J.G.S.; University of Leuven; Belgium
Delrue, S.B.M.; University of Leuven; Belgium
Kersemans, M.; Ghent University; Belgium
Martens, A.A.D.; University of Leuven; Belgium
Van Den Abeele, K.E.A.; University of Leuven; Belgium
Van Paepegem, S.; Ghent University; Belgium
Verboven, E.C.; Ghent University; Belgium

ID: ECNDT-0451-2018
Session: PAUT-Signal processing
Room: G2
Date: 2018-06-13
Time: 16:20 - 16:40

Composite materials have anisotropic mechanical properties and therefore demand intricate approaches to characterize their tensorial visco-elasticity and to detect internal defects. The Ultrasonic Polar Scan (UPS) is able to achieve both goals. During an UPS experiment, a material spot is insonified at several oblique incidence angles Ψ(θ,φ), with θ the vertical incident angle and φ the in-plane polar angle, after which the reflected or transmitted signal is recorded. Conventionally, the reflection (R) or transmission (T) amplitudes of the time domain signal are then analysed and used to infer the material properties. However, it would be much more interesting to deduce R or T for multiple frequencies at once. In order to expand the conventional procedure and still conceive a practical experiment, we propose the use of a hemispherical phased array, consisting of small PZT elements, which sequentially fire a broadband pulse for many different incidence angles to an immersed plate. The time-domain reflection signal is then captured by a wide selection of array elements. The frequency content is subsequently retrieved by adding the appropriately time delayed signals of the separate array elements together and performing a Fourier Transform. The plate will effectively act as a frequency filter, and will induce typical patterns in the reflection landscape (θ-f space), which correspond to the conditions for efficient Lamb wave stimulation (dispersion curves). By repeating this process for different polar angles φ, the dispersive behaviour in different directions can be extracted in a straightforward manner. These results can then be coupled to a suitable inversion algorithm in view of inferring the material parameters (e.g. visco-elastic stiffness tensor). In the present contribution, we report on the results of a simulation study of the new implementation which provides a proof of concept for the further development of an experimental phased array based UPS scanner.