Filtern
Dokumenttyp
Schlagworte
- SAFT (5)
- Ray tracing (4)
- Analytic signal (2)
- Austenitic welds (2)
- Guided waves (2)
- Transverse cracks (2)
- 3D ray tracing (1)
- Anisotropic Material (1)
- Anisotropic austenitic steel (1)
- Austenitische Schweißnähte (1)
Die Rekonstruktion von Materialfehlern aus gemessenen
HF-Bildern mit Hilfe der Synthetic Aperture Focussing
Technique (SAFT) ist ein bildgebendes Verfahren der
zerstörungsfreien Werkstoffprüfung. Die Auswertbarkeit
der gewonnenen Ergebnisse wird dabei maßgeblich durch
die Darstellung der mittels SAFT-Rekonstruktion zusammengestellten
Daten bestimmt, welche zunächst nur
in Form positiver und negativer Amplitudenwerte vorliegen.
Die Aufbereitung dieser Daten erfolgt durch Bildung der
Einhüllenden der Amplituden aus dem zugehörigen analytischen
Signal. Die Erweiterung dieses, nur im eindimensionalen
Fall eindeutig definierten, Konzepts auf
zweidimensionale Signalverläufe erfolgt dabei in Form einer
Mittelung über die Betragsquadrate aller möglichen
mit Single-Orthant-Spektren (SO-Spektren) zu bildenden
analytischen Signale. Es wird gezeigt dass dies
der Verwendung einer Approximation als separables Signal
entspricht und für den zweidimensionalen Fall eine
Möglichkeit vorgeschlagen Signale anhand ihrer partiellen
und totalen Hilberttransformationen auf Separabilität und damit Anwendbarkeit dieser Näherung zu testen.
Die Ergebnisse der so durchgeführten Bildverbesserung
werden am Beispiel einer zweidimensionalen SAFT-Rekonstruktion
an verschiedenen Reflektorarten dargestellt.
The Synthetic Aperture Focusing Technique (SAFT) is an algorithm applied in non-destructive ultrasonic testing which provides an image of flaws within a specimen. The image is reconstructed from A-scans measured at different positions. Reliable evaluation of the images obtained by the SAFT-algorithm, however, depends on the representation of the reconstructed data, which is initially given in terms of positive and negative local values only. A suitable way of processing this data for evaluation is to calculate the envelope, which can be achieved by means of the analytic signal. The extension of this concept to the multidimensional case is neither trivial nor unique and although extensive work on this subject has been carried out in the past, a correct envelope calculation in multidimensional data remains difficult since it depends on an additional condition, namely the separability of the signal. In this paper, the concept of analytic signals with single-quadrant spectra is applied to process 2-dimensional data obtained by the SAFT-algorithm. Furthermore, we present a procedure to overcome the limitations of that approach by selecting local magnitude values from a number of rotated frames after evaluating the signals separability in each frame, which is briefly validated against synthetic and experimental data.
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.
Ultrasonic examination of anisotropic inhomogeneous austenitic welds is challenging, because of the columnar grain structure of the weld leads to beam skewing and splitting. Modeling tools play an important role in understanding the ultrasound field propagation and optimization of experimental parameters during the ultrasonic testing of austenitic welds as well as the interpretation of the test results. In this contribution, an efficient theoretical model based on the ray tracing concepts is developed to calculate the ultrasonic fields in inhomogeneous austenitic welds quantitatively. The developed model determines the ultrasound fields by taking into account the directivity of the ray source, the inhomogenity of the weld as well as ray transmission coefficients. Directivity of the ray source in columnar grained austenitic materials (including layback orientation) is obtained in three dimensions based on Lamb's reciprocity theorem. Ray energy reflection and transmission coefficients at an interface between two general columnar grained austenitic materials are calculated in three dimensions. The ray tracing model predictions on inhomogeneous austenitic weld material are compared against those from CIVA, a commercial non-destructive modeling and simulation tool. The ultrasonic modeling tools in CIVA are based on semi-analytical solutions. For beam propagation simulation, a so-called 'pencil method' is used, which involves modeling the probe as a set of individual source points, each radiating 'a bundle' of diverging rays into the medium and integrating those elementary contributions. Inhomogenity in the weld region is approximated by mapping the grain orientations on weld macrograph. Simulation results for ultrasonic field profiles for an austenitic weld are shown to be in good agreement with the corresponding experimental results.
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.
Quantitative evaluation of ultrasonic C-scan images in homogeneous and layered anisotropic austenitic materials is of general importance for understanding the influence of anisotropy on wave fields during ultrasonic non-destructive testing and evaluation of these materials. In this contribution, a three dimensional ray tracing method is presented for evaluating ultrasonic C-scan images quantitatively in general homogeneous and layered anisotropic austenitic materials. The directivity of the ultrasonic ray source in general homogeneous columnar grained anisotropic austenitic steel material (including layback orientation) is obtained in three dimensions based on Lamb's reciprocity theorem. As a prerequisite for ray tracing model, the problem of ultrasonic ray energy reflection and transmission coefficients at an interface between (a) isotropic base material and anisotropic austenitic weld material (including layback orientation), (b) two adjacent anisotropic weld metals and (c) anisotropic weld metal and isotropic base material is solved in three dimensions. The influence of columnar grain orientation and layback orientation on ultrasonic C-scan image is quantitatively analyzed in the context of ultrasonic testing of homogeneous and layered austenitic steel materials. The presented quantitative results provide valuable information during ultrasonic characterization of homogeneous and layered anisotropic austenitic steel materials.
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.
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.
In this paper, a method to determine the complex dispersion relations of axially symmetric guided waves in cylindrical structures is presented as an alternative to the currently established numerical procedures. The method is based on a spectral decomposition into eigenfunctions of the Laplace operator on the cross-section of the waveguide. This translates the calculation of real or complex wave numbers at a given frequency into solving an eigenvalue problem. Cylindrical rods and plates are treated as the asymptotic cases of cylindrical structures and used to generalize the method to the case of hollow cylinders. The presented method is superior to direct root-finding algorithms in the sense that no initial guess values are needed to determine the complex wave numbers and that neither starting at low frequencies nor subsequent mode tracking is required. The results obtained with this method are shown to be reasonably close to those calculated by other means and an estimate for the achievable accuracy is given.