Already in the early 1980's, it has been conjectured that the pulsed ultrasonic polar scan (P-UPS) provides a unique fingerprint of the underlying mechanical elasticity tensor at the insonified material spot. Until now, that premise has not been thoroughly investigated, nor validated, despite the opportunities this would create for NDT and materials science in general. In this paper, we report on the first-ever implementation of an inverse modeling technique on the basis of a genetic optimization scheme in order to extract quantitative information from a P-UPS. We validate the optimization approach for synthetic data, and apply it to experimentally obtained polar scans for annealed aluminum, cold rolled DC-06 steel as well as for carbon fiber reinforced plastics. The investigated samples are plate-like and do not require specific preparation. The inverted material characteristics show good agreement with literature, micro-mechanical models as well as with results obtained through conventional testing procedures.

Abstract:In the early 1980's, the ultrasonic polar scan (UPS) technique was developed to assess the fiber direction of composites in a nondestructive way. In spite of the recognition by several researchers as being a sophisticated and promising methodology for nondestructive testing (NDT) and materials science, little advance was made during the following 30 years. Recently however, the UPS technique experienced a strong revival and various modifications to the original UPS setup have been successfully implemented. This revival has exposed several powerful capabilities and interesting applications of the UPS technique for material characterization and damage assessment. This paper gives a short historical overview of the UPS technique for characterizing and inspecting (damaged) fiber-reinforced plastics. In addition, a few future research lines are given, which will further expand the applicability and potential of the UPS method to a broader range of (damaged) materials, bringing the UPS technique to the next level of maturity.

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A novel approach is presented for the ultrasonic determination of the elastic constants in plate-like structures of an orthotropic material (e.g. composites) using a time-of-flight version of the pulsed ultrasonic polar scan (TOF P-UPS). A forward numerical model of the TOF P-UPS is coupled to an inversion algorithm, based on the genetic optimization principle, targeting the determination of the orthotropic elastic parameters, and the quality of the inversion is demonstrated for synthetic data representative for composites. The advantage of the new approach is that the presented TOF P-UPS inversion method does not require a priori knowledge about the symmetry class of the material, nor about the orientation of the main axes of symmetry. Furthermore, the TOF P-UPS inversion method yields an accurate characterization of the orthotropic elasticity tensor, even when applied to composite plates with small frequency-thickness ratios in which the traditional bulk wave approaches no longer hold. Finally, the robustness of the TOF P-UPS inversion method is demonstrated for noisy data by evaluating the results for a range of signal-to-noise ratios.

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