Precision Simulation of UT Defect Echo Scattering

The aim of ultrasonic testing is not only to locate defects but to size them for further evaluation of the component’s condition. One approach to size defects is to compare the defect echoes with echoes from reference reflectors e.g. disk shaped reflectors. In this work we simulate elastodynamic scattering of a plane wave under different angles of incidence using the grid-based “Elastodynamic Finite Integration Technique” (EFIT) combined with “Auld’s Reciprocity Theorem” (ART), which simulates all wave modes present in materials with linear wave propagation. All grid-based simulations necessarily are affected by discretization errors resulting in inaccuracies in scattered echoes. On the other hand the duration of the simulation is proportional to the grid size to the power of four (space+time) so that – in the interest of computability - grid size must not be chosen finer than necessary to archive the targeted accuracy. In our approach the deviation between real reflector surface and discretized reflector surface is minimized to achieve an optimized. This fact and the use of the ART leads to feasible simulation times and memory demand. Therefore, the total amplitude inaccuracy due to numerical errors and perturbations in the simulation is determined to less than 1%. To study the angle dependency of reflectivity, we rotate the incidence angle of a plane wave relative to the defect instead of rotating the defect. This avoids inducing roughness of the discretized defect surface due to staircase. In this context we investigate two different stimulation methods to excite a longitudinal plane wave in the far field. First, we use a stimulation via coupling plane, but this induces a backward propagating wave which leads to additional reflections at the simulation borders. This can be avoided by stimulation using a precalculated snapshot of the wave field.