Modeling of ray paths of head waves on irregular interfaces in TOFD inspection for NDE

Abstract

The TOFD (Time of Flight Diffraction) technique is a classical ultrasonic inspection method used in ultrasonic non-destructive evaluation (NDE). This inspection technique is based on an arrangement of two probes of opposite beam directions and allows a precise positioning and a quantitative evaluation of the size of cracks contained in the inspected material thanks to their edges diffraction echoes. Among the typical phenomena arising for such an arrangement, head waves, which propagate along the specimen surface and are chronologically the first waves reaching the receiver, are notably observed. Head wave propagation on planar surfaces in TOFD configurations is well known. However, realistic inspection configurations often involve components with irregular surfaces, like steel excavated specimens. Surface irregularity is responsible for numerous effects on the scattering of bulk waves, causing the melting of surface and bulk mechanisms in the head wave propagation. In order to extend the classical ray approach on these complex cases, a generic algorithm of ray tracing between interface points (GIRT) has been designed. With respect to time of flight minimization (i.e. the Generalized Fermat's Principle), ray paths can be computed by GIRT for different natures of waves scattered by the complex surfaces or by flaws. The head wave fronts computed by GIRT are notably in good agreement with FEM simulated results. This algorithm, based on pure kinematic analysis of waves propagation, represents a first step in the future development of a complete ray theory for head waves simulation on irregular interfaces.

Recent modelling advances for ultrasonic TOFD inspections

“…By modeling the complex wave interactions with the surfaces thanks to this ray approach, GIRT is thus able to correctly predict the head wave time of flight and to interpret the complex waves propagation at the vicinity of an irregular interface. Indeed, comparisons between GIRT and FEM simulations of the head wave fronts near the receiver surface have been previously presented in [5]. A good agreement is obtained between GIRT and FEM results, which validates the GIRT algorithm.…”

“…In [5], complete interpretation and simulation of the head wave propagation mechanisms for irregular surfaces are proposed. This approach leads to the development of a generic algorithm of ray tracing between interface points (called Generic Interface Ray Tracing/GIRT) which is able to compute the travel path of head waves in specimens inspected by TOFD technique.…”

2D size, position and shape definition of defects by B-scan image analysis

“…Similarly to the case of the waves diffracted by the crack tip [6,7,13], the perturbation shows a parabolic-like pattern in the B-scan. In general, the defect affects the shape of the perturbation then, analyzing this last, the defect characteristics can be determined.…”

“…A technique of particular importance in the analysis of defects in plates is the Time Of Flight Diffraction (TOFD) [4][5][6][7][8][9]. This technique allows to determine the presence of cracks in the material and to determine the position and length of the crack even in T components of complex geometry [10][11][12][13]. To facilitate the automation of the analysis, TOFD B-scan maps with selected wave types can be obtained by optimizing the layout of the laser system.…”

Acoustic Scattering Models from Rough Surfaces: A Brief Review and Recent Advances

“…Head waves led to recent researches [70,[86][87][88] and a modification [89] of the Kirchhoff approximation is henceforth proposed for head waves account. Head waves on irregular surfaces can also be modelled [90].…”