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.
The ultrasonic TOFD (Time of Flight Diffraction) Technique is commonly used to detect and characterize disoriented cracks using their edge diffraction echoes. An overview of the models integrated in the CIVA software platform and devoted to TOFD simulation is presented. CIVA allows to predict diffraction echoes from complex 3D flaws using a PTD (Physical Theory of Diffraction) based model. Other dedicated developments have been added to simulate lateral waves in 3D on planar entry surfaces and in 2D on irregular surfaces by a ray approach. Calibration echoes from Side Drilled Holes (SDHs), specimen echoes and shadowing effects from flaws can also been modelled. Some examples of theoretical validation of the models are presented. In addition, experimental validations have been performed both on planar blocks containing calibration holes and various notches and also on a specimen with an irregular entry surface and allow to draw conclusions on the validity of all the developed models.
Edge Diffraction Coefficients around Critical Rays L Fradkin, M Harmer and M Darmon-Towards semi-automated non-destructive evaluation L Fradkin, V Zernov, G Elston et al.-Elastodynamic models for extending GTD to penumbra and finite size flaws A Kamta Djakou, M Darmon and C Potel-Recent citations Frequency control of sol-gel composite films fabricated by stencil printing for nondestructive testing applications Tsukasa Kaneko et al-Curie temperature and high temperature behavior of Pb(Zr,Ti)O 3 /Pb(Zr,Ti)O 3 sol-gel composites Shota Fujimoto et al-This content was downloaded from IP address 34.218.44.141 on 11/05 Abstract. The Time of Flight Diffraction (TOFD) technique is a classical ultrasonic method used in ultrasonic non-destructive evaluation, which allows a precise positioning and a quantitative size evaluation of cracks in the inspected material. Among the typical phenomena arising in the current TOFD inspection, the so-called "head wave" is the first contribution reaching the receiver. The head wave propagation on a planar interface is well known and identified as a critical refraction taking place on the material surface. On irregular surfaces, it has been shown that the head wave results from the melting of surface and bulk waves mechanisms and that surface irregularities are responsible for numerous diffractions of the incident head wave. To simulate such behaviour, a model has been developed using a ray tracing technique based on time of flight minimization (generalized Fermat's principle). It enables the calculation of the ray path and the corresponding time of flight of all waves propagating in the material, including the head wave. To obtain a complete propagation model for these waves (both trajectory and amplitude), the integration of Geometrical Theory of Diffraction (GTD) models is currently performed by coupling them with the ray-based approach discussed above. 1. Introduction Among the various NDE inspections techniques, the Time of Flight Diffraction (TOFD) technique allows a precise and quantitative evaluation of surface breaking or embedded cracks. This technique uses two ultrasonic probes which are mechanically associated and remote from each other. In a contact inspection, the two probes are positioned on the surface of the inspected specimen using a wedge to ensure the impedance adaptation between the probes and the specimen. The first probe emits an ultrasonic pulse in the specimen, and the second probe receives the diffracted signals arising from the specimen. In Fig. 1, a schematic example of a TOFD inspection on a specimen with a planar surface is provided.
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