Multimode TFM imaging for weld inspection

K Sy and O Roy
Eddyfi Europe, 1 rue de Terre Neuve 91940, Les Ulis, France 

In the field of non-destructive testing, total focusing method (TFM) imaging is increasingly recognised for its qualities: realistic images with maximum resolution, thus facilitating the analysis of results and offering new perspectives for the characterisation of defects. The method of constructing the image is based on the calculation of the flight time between the elements of the multi-element transducer and each point of the image to be formed. The paths considered can be direct or with rebound on the back wall.

The images associated with these different paths can then carry different information, sometimes complementary on the same defect, but also artefacts that can distort the interpretation. Multiple paths between elements and points of the image may increase the presence of artefacts. In many cases, TFM imaging is used with direct mode, as for detecting corrosion. For weld inspection, TFM imaging can produce realistic images of cracks using paths with rebound on the back wall. But many paths exist and it is necessary to select relevant paths to avoid artefacts.

The authors have developed a tool named the Specular Echoes Estimator, which allows for the prediction of relevant paths for the cracks with unknown crack orientation. This method was validated on simulated and experimental data. Various experimental results are presented to outline the advantage of the multimode TFM imaging.