Three-dimensional Sound Field Computation and Optimization of the Delamination Detection based on the Re-Radiation Method

Marhenke, Torben; Leibniz Universitat Hannover; Germany

Marhenke, T.; Leibniz Universität Hannover; Germany
Sanabria, S.J.; ETH Zürich, Computer Vision Laboratory (CVL); Switzerland
Twiefel, J.; Leibniz Universität Hannover; Germany
Furrer, R.; Swiss Federal Laboratiories for Materials Science and Technology (Empa); Switzerland
Neuenschwander, J.; Swiss Federal Laboratiories for Materials Science and Technology (Empa); Switzerland
Wallaschek, J.; Leibniz Universität Hannover; Germany

ID: ECNDT-0002-2018
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Ultrasonic measuring methods are increasingly being used for non-destructive material testing to determine the condition of materials (e.g. crack, delamination or mechanical properties). Especially, air-coupled ultrasound (ACU) testing has seen a surge in development in recent years. The great advantage of this method is that no additional coupling medium has to be used between the transducer and the receiver.
In order to characterize or calibrate these ACU transducers, an accurate understanding of sound propagation is necessary. For this microphones, refracovibrometry or Schlieren-method are often used. The disadvantages of these methods are the laborious measurements in order to determine the sound field characteristics. For this reason reconstruction methods are used to determine the sound field propagation with a lower measurement effort. These are often based on the piston theory. This model has the disadvantage that only symmetrical sound fields can be reconstructed and limitations with respect to the quantitative pressure calculation exist. In this paper, a new method (re-radiation) for three-dimensional sound field determination is described, for which the pressure distribution must be measured in only one plane. It is based on the analytical solutions of the Rayleigh Sommerfeld equations and on time reversal mirrors. Thus, it is possible to determine a forward as well as a backward solution, starting at the measured sound field plane. In addition to characterize airborne sound fields, the re-radiation method was used to optimize the delamination detection. In the case of ACU delamination detection, a minimum distance must be kept between the material and the receiver, which is due to high temperatures or vapors from the sample and to prevent reflections. Therefore, the minimum resolution of the delamination size is severely restricted. By means of the re-radiation, it was possible to reconstruct the sound field directly above the plate. This improved the detectable delamination size.