Topological imaging of bounded elastic domains. Application to non-destructive testing of welds

This study is part of the in-service inspection of the welds of fourth generation nuclear reactors, in order to contribute to the nuclear reactors, in order to contribute to the safety demonstration. The anisotropic and heterogeneous structure of austenitic stainless steel multi-pass welds makes their inspection by ultrasound difficult. Thus, in order to correctly interpret the measured signals and, eventually, to characterize the potential defects, a description of the weld is used. The orientation of the grains and the elasticity constants constitute the a priori knowledge introduced in the Topological Energy method for the resolution of the inverse imaging problem. The study carried out is divided into two parts: i) the development of the method in the boundary environment and its comparison with the Matched Field Processing, which is a close method established in the frequency domain, then ii) its application to the case of real welds. The extension of the Topological Energy method to isotropic and homogeneous boundary media aims at taking advantage of the multiple reflections between the defect and the walls. For this purpose, several solutions of the numerical propagation problem, obtained for different boundary conditions (Neumann or Dirichlet), are judiciously associated in order to select the diffraction echoes carrying information. Thus, depending on the type of defect to be imaged (hole or notch) specific topological energies are defined. They increase the quality of the image and reduce the importance of artifacts resulting from multiple reflections. The principle of the technique is introduced analytically before being validated, first by numerical simulations and then from from experimental data. In a second step, the method is applied to the complex environment of welding. The numerical results obtained confirm the relevance of the approach for defect detection. The procedure is also experimentally tested on model welds with and without notches in order to evaluate the localization performances. However, due to the variability of the structure, the quality of the image can be degraded depending on the case study. The possibility of generating arbitrary sources in situ allows to overcome this difficulty.