## 2 Simulation Methods  
All the simulation methods used are integrated in the CIVA NDT platform [49] for simulation [2] and analysis. The semi-analytical models (GTD, PTD) have been widely described in Refs. [15; 26]. The Physical Theory of Diffraction (PTD) combines GTD (ray theory) and Kirchhoff Approximation (KA--integral method) and applies to arbitrary large scatterers. PTD [25; 26] replaces the Kirchhoff edge diffraction contribution by GTD, the asymptotical approximation of the edge problem solution. The PTD scattered field is the sum of the Kirchhoff scattered field and a modified GTD field, whose diffraction coefficient is the difference between the GTD and Kirchhoff edge diffraction coefficients. Finally, this summation enables building a PTD total field which is spatially uniform, contrary to GTD, which leads to several singularities (shadow boundaries).  
The 3D hybrid FEM model is a coupling method [42] between an FEM scattering model with a ray-based asymptotic field model (the CIVA pencil method for field computation [2]). This hybrid methods consequently consists of simulating the propagation field until the defect vicinity employing the CIVA pencil method [50] and then the scattering of this field from the defect thanks to a numerical approach. The flaw's echographic signal is finally obtained by the way of the [PERSON]'s reciprocity principle [51], which expresses it as a combination of fields from the emitter and receiver with and without a scattering defect. The numerical solver is built upon a \"dedicated\" high-order spectral finite element method [43; 44], which enables us to address 3D configurations, and second, the constitution of the numerical parameters are automatically deduced from the inspection configuration. A theoretical description of the hybrid FEM method is given in Appendix A. Some additional details (meshing around the defect, numerical scheme, etc.) can be found in [41; 52]. Different calculation strategies adopted in the CIVA 2017 release used in this paper depend on the defect's location (embedded or surface breaking) and on the sensibility area defined by the user to reduce the computation time. For breaking flaws, the coupling reciprocity integral [51] is carried out on both the flaw and component surfaces inside the sensibility area (in red in Figure 1b).  
Figure 1: Different coupling strategies used in the CIVA hybrid FEM model for (a) embedded and (b) breaking cracks.  
Finally, the simulation methods utilized here both use the same ray-based asymptotic field model (the CIVA pencil method for field computation [2]) to simulate wave propagation, but they differ in the method they employ to model flaw scattering. The model based on GTD or on the Physical Theory of Diffraction (PTD) is a semi-analytical high frequency model, whereas, for the hybrid CIVA-FEM model, spectral finite elements (using high order polynomials) are used.  
For contact probes, only longitudinal waves are taken into account in the probe by the simulation, and since a coupling gel is used in experiments to make a bond between the probe and the specimen, a solid-solid sliding interface model is used for transmission coefficients at the specimen interface.