Thickness measurement is an essential operation in non-destructive testing (NDT) for monitoring the integrity of structures operating under severe conditions, such as industrial pipelines or components exposed to high temperatures. Among the various available techniques, ultrasonics remain a preferred solution due to their accuracy, rapid acquisition, and ability to evaluate internal discontinuities within materials.
The electromagnetic acoustic transducer (EMAT) differs from conventional piezoelectric sensors by its couplant-free operation and its ability to generate shear waves. This type of wave, with a shorter wavelength at a given frequency, provides higher spatial resolution for thickness measurement. The EMAT mainly relies on the Lorentz force resulting from the interaction between the induced current density in the specimen and a static magnetic field, enabling the generation and detection of shear waves in a pulse-echo configuration. In ferromagnetic materials, additional contributions from magnetization and magnetostrictive effects also participate in the acoustic wave transduction process.
Finite element simulations were performed to optimize the device design by analyzing the distribution of the static magnetic field and the density of Lorentz forces within the active region. Several magnet and coil configurations were investigated, and the arrangement combining a ferromagnetic core, lateral magnets, and a butterfly-shaped coil proved to be the most efficient, leading to a significant improvement in signal-to-noise ratio (SNR) and acoustic beam directivity.
Prototypes were fabricated using Samarium–Cobalt (SmCo) permanent magnets and a ceramic printed circuit board (PCB) made of aluminum oxide (Al2O3), ensuring mechanical and thermal stability up to 400 °C. Tests conducted on 1018 and 1045 steel samples showed an SNR greater than 17 dB across the entire temperature range, with excellent stability after several thermal cycles and one week of continuous operation at 300 °C.
Finally, the detection of internal defects, such as flat bottom holes (FBH), was successfully achieved, including at high temperature, using a compact and optimized EMAT.
| Date | 11 Dec 2025 |
|---|
| Original language | French |
|---|
| Awarding Institution | - École de technologie supérieure
|
|---|
| Supervisor | Pierre Bélanger (Supervisor) |
|---|
Rollet, P. (Author),
Bélanger (Supervisor),
11 Dec 2025Student thesis: Master's thesis › Master in Engineering: Automated Manufacturing Engineering