A general Kirchhoff approximation for echo simulation in ultrasonic NDT

The Kirchhoff approximation is commonly used for the modeling of echoes in ultrasonic NDE. It consists in locally approximating the illuminated surface by an infinite plane to compute elastic fields. A model based on this approximation is used in the CIVA software, developed at CEA LIST, to compute echoes from cracks and backwalls. In its current version, it is limited to stress-free surfaces. A new model using a more general formalism has been developed. It is based on reciprocity principles and is valid for any host and flaw materials (liquids, isotropic and anisotropic solids). Experimental validations confirm that this new model can be used for a wider range of applications than the previous one. A second part of this communication deals with the improvement of the Kirchhoff approximation in the aim of predicting diffraction echoes. It is based on an approach called refined Kirchhoff, which combines the Kirchhoff and Geometrical Theory of Diffraction (GTD) models. An illustration of this method for the case of a rigid obstacle in a fluid is given.