Influence of finite size transducers on Ultrasonic Polar Scan recordings and its impact on the determination of the viscoelastic tensor of anisotropic media

Martens, Arvid; 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-0452-2018
Session: Modelling & data processing - UT
Room: J2
Date: 2018-06-13
Time: 15:20 - 15:40

The Ultrasonic Polar Scan (UPS) is well-suited for the inverse characterization of the full elastic tensor of anisotropic media (e.g. for carbon fiber reinforced plastics). During a typical UPS experiment, the reflection and/or transmission coefficient of an ultrasonic signal, steered at a specific material spot, is recorded for all possible incidence angles Ψ(θ,φ) on the hemisphere above the targeted spot. In the inversion procedure, the experimental data are iteratively reproduced by means of a numerical model of the UPS to infer information on the properties of the investigated material. Although, the currently available UPS model bypasses most of the limitations experienced by other ultrasonic models in literature, it disregards the finite size of typical transducers, as it assumes that a single plane wave propagates inside the material. In the present study, the numerical UPS simulation model is extended by incorporating a 2D Gaussian beam profile (instead of a plane wave), which allows to account for 3D effects induced by the finite size transducers. The inclusion of the boundedness of the ultrasonic beam for instance prompts the observation of multidirectional leaky wave fields in the region of a critical angle due to the skewing of waves in anisotropic media. As these phenomena will have a considerable impact on the measured reflected and transmitted signals, a study on the correctness of the UPS based inversion procedure has been performed for both a harmonic and broadband input signal. In the case of a broadband signal, only minor modifications on the inversion results are found when compared to the plane wave model. On the other hand, in the case of a harmonic input signal, the inattention of the boundedness of the ultrasonic beam in the UPS model and in its inversion approach produces significantly wrong inverted stiffness parameters (especially the viscous parameters).