Inspection de blocs d'acier forgé de grand volume par ultrasons multi-éléments

Abstract

The objective of this Ph.D. project was to inspect large size forged steel blocks using a phased array transducer. The first part of this work focused on the material. From a metallurgical point of view, it was observed that the studied block was 99% bainitic and that the grain size was varying from 70 μm to 700 μm between the surface and the core. From the ultrasonic point of view, it was shown that the block could be considered as locally isotropic and homogeneous. Group velocity measurements coupled with densities calculated from empirical formulas were used to determine the impact of mechanical parameters on ultrasonic propagation velocities. It was shown that Young's modulus and density were the most important parameters for group velocity, whereas phase velocity was linked to the grain size. The measurements were then used in a CIVA software model to compute the best parameters of a phased array probe optimized for the inspection of large size forged steel blocks. It was demonstrated that commercial probe available were not adapted to the large size problem. Based on these simulations, a first prototype probe with 8-element of 9.5 × 22.5 mm was designed and used to perform imaging of circular defects in a 776 mm block. It was observed that the limiting parameter was the lateral resolution and that the measurements were in very good agreement with the simulations. Finally, a 32-element probe was realized and used on circular defects and notches in a 776 mm block. Different emission sequences were then tested in order to improve imaging performance. The plane wave sequence which activates simultaneously multiple elements was compared to full-matrix capture (FMC) sequences. It was observed that signal-to-noise ratio (SNR) increased for the plane waves but resolutions were worse. This result confirms that the energy transmitted in the material is greater for the plane waves compared to the FMC sequences. Finally, a new sequence based on the Hadamard matrix was proposed. This element activation scheme combines plane waves which maximize the energy transmitted into the material, with the FMC sequences which have a high resolution. Finally, the images generated using the Hadamard method showed a good compromise a SNR gain of 10 dB and a low-resolution loss.

Date	17 Oct 2018
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Pierre Bélanger (Supervisor) & Mohammad Jahazi (Co-supervisor)