Ray tracing boundary value problems: simulation and SAFT reconstruction for ultrasonic testing
(2016)
The application of advanced imaging techniques for the ultrasonic inspection of inhomogeneous anisotropic materials like austenitic and dissimilar welds requires information about acoustic wave Propagation through the material, in particular travel times between two Points in the material. Forward ray tracing is a popular approach to determine traveling paths and arrival times but is ill suited for inverse problems since a large number of rays have to be computed in order to arrive at prescribed end points.
In this contribution we discuss boundary value problems for acoustic rays, where the ray path between two given points is determined by solving the Eikonal equation. The implementation of such a two Point boundary value ray tracer for sound field simulations through an austenitic weld is described and its efficiency as well as the obtained results are compared to those of a forward ray tracer. The results are validated by comparison with experimental results and commercially available UT simulation tools.
As an application, we discuss an implementation of the method for SAFT (Synthetic Aperture Focusing Technique) reconstruction. The ray tracer calculates the required travel time through the anisotropic columnar grain structure of the austenitic weld. There, the formulation of ray tracing as a boundary value Problem allows a straightforward derivation of the ray path from a given transducer Position to any pixel in the reconstruction area and reduces the computational cost considerably.
The synthetic aperture focusing technique (SAFT) is an imaging technique commonly used in ultrasonic inspection. In order to apply SAFT to the inspection of austenitic welds, the inhomogeneous anisotropic nature of the weld structure has to be taken into account. A suitable approach to accomplish this, is to couple the SAFT-algorithm with a ray tracing program (RT-SAFT). While SAFT-imaging of cracks in austenitic welds by use of ray tracing has been carried out before, all attempts so far were limited to longitudinal cracks which usually allows a treatment as 2-dimensional problem. In case of transverse cracks, a full 3-dimensional ray tracing is necessary in order to perform a SAFT-reconstruction. In this paper, we give an outline of our attempts to reconstruct images of transverse cracks in austenitic welds, utilizing 3-dimensional ray tracing and a layered structure model derived from an empirical model of grain orientations in welds. We present results of this RT-SAFT on experimental data taken from transverse cracks in different austenitic welds, which show that size and position of the cracks can be estimated with good accuracy, and compare them to images obtained by assuming an isotropic homogeneous medium which corresponds to the application of the classical SAFT-algorithm.
Up to now there is no sufficient technique to detect transverse cracks in austenitic and dissimilar welds which recently are of increasing interest in the integrity surveillance of nuclear power plants as well as in quality control of longitudinally welded pipes. Weld inspection by interpretation of single A-scans will lead to erroneous results due to effects caused by anisotropy and in worst case might leave flaws undetected. Therefore, imaging techniques such as the synthetic aperture focusing technique (SAFT) should be used. If the SAFT algorithm is applied on data taken from austenitic welds, the inhomogeneous, anisotropic structure of these welds has to be taken into account in order to properly attribute amplitudes measured in A-scans to the corresponding coordinates in the region of interest. While this has been investigated in the past, all attempts so far were limited to the imaging of longitudinal cracks which requires a less complicated setup than the imaging of transverse cracks. In this paper we give an outline of our attempts to reconstruct images of transverse cracks in different welds. For this purpose a SAFT program based on ray tracing and a layered structure weld model derived from an empirical model of grain orientations in welds are used. The results of the image reconstruction on experimental data are shown and compared to images obtained by assuming an isotropic homogeneous model. Root reflection and crack tip echo are clearly visible which allows an estimation of size and position of the crack with good accuracy.
SAFT-Rekonstruktion für die Querfehlerprüfung in austenitischen Schweißnähten und Mischnähten
(2013)
Die Untersuchung austenitischer Schweißnähte ist eine Herausforde-rung für die zerstörungsfreie Prüfung mit Ultraschall, bei der die Anwendung her-kömmlicher Prüftechniken häufig keine zufriedenstellenden Ergebnisse liefert. Die Ursache dafür ist das inhomogen-anisotrope Gefüge im Schweißnahtbereich, das zu einer Verzerrung des Schallfeldes und damit zu Schwierigkeiten bei der Auffindbarkeit und bei der Größenbestimmung von Fehlern führt. Eine neue Her-ausforderung ist die Detektion von Querfehlern in Rohrleitungen des Primärkreises von Kernkraftwerken, aber auch in längsnahtgeschweißten, plattierten Rohrleitun-gen, die zunehmend an Bedeutung gewinnt. Dabei können die Prüfköpfe auf Grund einer unbeschliffenen Decklage meist nicht direkt auf die Naht aufgesetzt werden. Vielmehr muss eine SE-Anordnung der Köpfe verwendet werden, bei der durch die längeren Schallwege der negative Einfluss des austenitischen Gefüges noch ver-stärkt wird.
Im vorliegenden Beitrag wird eine RT-SAFT-Prüftechnik vorgeschlagen, mit der die Fehlerdetektion deutlich verbessert werden konnte. Dabei wurde die SAFT-Rekonstruktion mit einem Ray-Tracing-Ansatz zur Bestimmung der Schalllaufwege und -laufzeiten und mit einem Schweißnahtmodell kombiniert. Das Schweißnaht-modell repräsentiert die Kornorientierung und die elastischen Konstanten in einer diskretisierten Form und macht so die komplexe Nahtstruktur für die Berechnung zugänglich.
Die Aufnahme der Messdaten erfolgt mit zwei Gruppenstrahlerprüfköpfen in V-Anordnung, die beidseitig der Schweißnaht geführt werden sind.
Im Beitrag werden die Prüfanordnung und das Rekonstruktionsverfahren vor-gestellt und die Ergebnisse von experimentellen Untersuchungen an austenitischen Schweißnähten und Mischnähten mit künstlich eingebrachten Testfehlern präsen-tiert. Die Ergebnisse der direkten Fehlergrößen- und -lagebestimmung werden mit den Resultaten konventioneller Prüftechniken verglichen.
The inspection of austenitic and dissimilar welds using ultrasound demands for sophisticated testing techniques. The application of reconstruction methods like the Synthetic Aperture Focusing Technique (SAFT) on the measurement results provides an appropriate approach for defect characterization and sizing. Nevertheless, the reconstruction algorithm has to consider the aniso-tropic wave propagation inside the inhomogeneous weld material. In recent years the detection of transverse cracks has become increasingly important for ensuring the structural integrity of pipes in the primary circuit of nuclear power plants or longitudinally welded, cladded pipes. However, relia-ble inspection techniques are hardly available. In this particular case, it is expected that the compar-atively long propagation path of the ultrasonic wave field inside the inhomogeneous weld material enhances the effect of anisotropy and influences the accuracy and the signal-to-noise-ratio of the reconstruction result. In this contribution we suggest an advanced ultrasonic testing technique for detecting and sizing of transversal cracks in austenitic and dissimilar welds. The method applies a SAFT reconstruction algorithm considering the anisotropy and the inhomogeneity. A V-arrangement of the transducers in pitch-catch technique is chosen to avoid a direct coupling on the weld face. The reconstruction algo-rithm is based on an extended 3-dimensional weld model and uses a ray-tracing approach for de-termining the wave propagation paths. Along with the reconstruction algorithm the transducer set-up and experimental results of different specimens with artificial transverse flaws are presented. The availability of the proposed method for crack sizing is assessed in comparison to conventional testing techniques.