Conception d'une sonde acoustique électromagnétique pour la détection de l'épaisseur minimale d'une surface corrodée

Abstract

Detection and sizing of corrosion are critical issues across many fields, such as oil and gas or petrochemical industries. Indeed many structures are only partially accessible; this is particularly the case for pipelines at support locations. Direct-non-destructive testing techniques for thickness measurement can not therefore be applied. Ultrasonic guided waves have the particularity of being able to propagate over long distances. For example, it is possible to obtain an accurate map of the thickness of a region that is not accessible by tomography, but this process requires specific probes and precise positioning. Additionnaly, the post-processing of data is difficult, both in terms of computing time and needed resources. It has been shown, in previous studies, that wall thickness gauging can be tackled using the existence of cut-off frequency-thickness products of high order shear horizontal guided wave modes. Indeed, a reduction of thickness of a waveguide has the effect of creating a low-pass mechanical filter for high order modes. When the thickness decreases and the frequency-thickness product falls below the cut-off threshold of a mode, the mode can no longer propagate and is either reflected or converted to another mode. This technique therefore makes it possible to determine the minimum thickness of the inspection zone. The efficiency of the process depends greatly on the number of excited modes. There are two elements to control this number: the frequency and the probe. An increase in the frequency will allow a generation of more modes and thus an increase in the resolution, but also an increase in the attenuation of the ultrasonic waves, which will limit the maximum inspection distance. The two types of probes most suited to a controlled emission of a multimodal wave packet are phased-array piezoelectric probes and electromagnetic-acoustic transducers (EMAT). In previous studies, the excitation was done using phased-array probes. The objective of this project was to design an EMAT allowing the excitation of a certain number of modes. Dimensions and number of magnets to be used were determined by finite element simulations with COMSOL and POGO software. This made it possible to produce a probe facilitating the separation of the modes, as well as a control of the directivity of the probe so that it can be sensitive to small defects. Experimental results showed that the thickness of an aluminum plate with corroded areas can be measured with a 2 mm resolution. The main contribution of this work is a successful implementation of the experimental technique with an EMAT, while previous studies used piezoelectric transducers.

Date	12 Dec 2018
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Pierre Bélanger (Supervisor)