Imagerie ultrasonore multivues par cohérence de phase pour la détection et la caractérisation de défauts dans les soudures

Abstract

Ultrasonic nondestructive testing (NDT) methods are widely used in the industry for the detection and characterization of defects on all types of structures and, sometimes, at high speeds. This is for example the case for the inspection of girth welds during the manufacturing of pipelines. When using multi-element probes to produce an image of the inspected part, the resulting image is actually an indirect representation of the geometry and material properties that must be interpreted correctly in order to conclude on the presence of a defect. The latest advances in terms of electronics enable the use of the total focusing method (TFM) in real time to produce a high resolved image of the different regions of interest inside the inspected part. However, this method, although more accurate, has some drawbacks such as being less sensitive to small reflectors and generating a large amount of data during acquisition. Moreover, the increasing use of multi-view imaging, derived from the TFM and which does not rely only the direct path for image reconstruction, is an additional burden for the inspector who must now deal with a very large number of parameters. However, imaging methods based on the signal phase have also been proposed in recent years and could be useful to compensate the disadvantages associated with the TFM. In this work, the properties of the Vector Coherence Factor (VCF), based on the analysis of the in-phase and quadrature components of the measured signal, are leveraged in order to generate images of the region of interest. VCF imaging is based on the same type of delay-and-sum (DAS) beamformers as the TFM. However, it has the advantages of not relying on the signal amplitude, being particularly sensitive to diffractive defects such as crack tips, and allowing easy merging of images from multi-view methods. In this study, VCF imaging is first evaluated for defects sizing and compared to TFM and the time-of-flight diffraction method (TOFD). Then, as VCF imaging relies only on the use of the signal phase, the reduction of the data throughput during acquisition to generate a satisfying image is studied. Finally, the merging of different images from the multi-view implementation of VCF imaging is studied and a statistical threshold is defined to filter the background noise. Experiments were conducted on two sets of samples. The first one was composed of two ¾” thick steel plates with EDM notches at several angles centered at the mid-wall or surface-breaking on the backwall . The second one was a 20 mm thick steel plate with Flat Bottom Holes (FBH) machined on its edge mimicking reflectors used for calibration of a circumferential weld inspection apparatus. A 60-element linear multi-element probe with a center frequency of 7.5 MHz mounted on a 36° Rexolite wedge was used for the different acquisitions. Finally, it is shown that : 1- VCF imaging has similar sizing performances compared to TOFD and better performances than TFM for notches with an angle of 70° or more with respect to the horizontal plane; 2- it is possible to reduce the amount of data required for VCF imaging by almost fifty times by exploiting only the rising or falling edges of the signal; 3- VCF imaging is suitable for simple fusion of images from different views, while maintaining a contrast-to-noise ratio of more than 30 dB, and allowing the definition of a statistical threshold that filters out most of the background noise in the fused image.

Date	21 Nov 2022
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Pierre Bélanger (Supervisor)