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The ultrasonic echo technique is a frequently used method in non destructive testing for geometry Determination of concrete building elements. Important tasks are thickness measurements as well as the localization and characterization of built-in components and inhomogeneities.
Currently mainly the synthetic aperture focusing family of techniques (SAFT) is used for imaging. These algorithms have difficulties in imaging steeply dipping interfaces and complicated structures such as steps and lower boundaries of voids. As an alternative two geophysical migration methods, pre-stack Kirchhoff depth migration and pre-stack Reversetimemigration (RTM)were tested in this paper at a reinforced concrete foundation slab. The slab consists of various reinforcement contents, different thicknesses and two pile heads.
In a first step, both methods were evaluated with synthetic 2D data. In the second step, ultrasonic measurement data recorded with shear wave transducers on a line profile on the foundation slab were processed. The use of an automatic scanner simplified the measurements. A comparison of the geophysical migration results with those of SAFT shows, in particular for RTM, a significant improvement in the Imaging of the geometry of the foundation slab. Vertical borders were reconstructed and the location and structure of the lower boundary of the foundation slab were reproduced better. Limitations still exist in imaging the piles below the slab.
Ultrasonic echo testing is widely used in non-destructive testing in civil engineering to investigate concrete structures, to measure thickness, and to locate and characterise built-in components or inhomogeneities. Currently, synthetic aperture focusing techniques are mostly used for imaging. These algorithms are highly developed but have some limitations. For example, it is not possible to image the lower boundary of built-in components like tendon ducts or vertical reflectors. We adopted reverse time migration for non-destructive testing in civil engineering in order to improve the imaging of complicated structures in concrete. By using the entire wavefield, including waves reflected more than once, there are fewer limitations compared to synthetic aperture focusing technique algorithms. As a drawback, the required computation is significantly higher than that for the techniques currently used.
Simulations for polyamide and concrete structures showed the potential for non-destructive testing. The simulations were followed by experiments at a polyamide specimen. Here, having acquired almost noise-free measurement data to test the algorithm, we were able to determine the shape and size of boreholes with sufficient accuracy. After these successful tests, we performed experiments at a reinforced concrete foundation slab. We obtained information from the data by reverse time migration, which was not accessible by traditional imaging. The imaging of the location and structure of the lower boundary of the concrete foundation slab was improved. Furthermore, vertical reflectors inside the slab were imaged clearly, and more flaws were found. It has been shown that reverse time migration is a step forward in ultrasonic testing in civil engineering.
Anwendung der Reverse-Time Migration auf Ultraschall-Echo-Daten in der zerstörungsfreien Prüfung
(2016)
Das Ultraschall-Echo-Verfahren ist eine klassische zerstörungsfreie Prüftechnik zur Bestimmung der Bauteilgeometrie sowie zur Lokalisierung von Einbauteilen und Fehlstellen. Der aktuelle Stand der Technik bei den Abbildungsverfahren ist die SAFT-Rekonstruktion (Synthetic Aperture Focusing Technique). Diese Gruppe von Verfahren weist u.a. Schwierigkeiten bei der Darstellung von steilen Grenzflächen auf und verarbeitet nur die primären Reflexionen am abzubildendem Objekt korrekt. Als Alternative werden seit einiger Zeit Migrationsmethoden aus dem Bereich der Geophysik evaluiert. Am Beispiel eines Stahlbetonfundamentes wurden in einer Vorarbeit die Kirchhoff-Migration und die Reverse-Time-Migration (RTM) getestet.
Die strahlenbasierten Algorithmen der Kirchhoff-Migration und SAFT-Rekonstruktion sind eng miteinander verwandt. Die RTM hingegen basiert auf der numerischen Lösung der vollständigen Wellengleichung. Durch Kreuzkorrelation von zeitlich vorwärts- und rückwärtsmodellierten Wellenfeldern erzeugt die RTM die migrierte Abbildung. Für die Durchführung der RTM wurde ein 2D akustischer Code verwendet.
Beide Migrationsmethoden wurden an auf Basis der akustischen Wellengleichung generierten synthetischen 2D-Daten sowie an realen Ultraschallmessdaten getestet. Letztere wurden mit Scherwellenprüfköpfen an der Fundamentplatte aufgenommen. Ein Vergleich der Migrationsergebnisse mit den Ergebnissen der SAFT-Rekonstruktion zeigte besonders für die RTM eine deutliche Verbesserung in der Abbildung der Bauteilgeometrie. Die Vorstudie an der Fundamentplatte lieferte somit den Nachweis, dass geophysikalische Migrationsverfahren auf reale Ultraschall-Messdaten anwendbar sind. Jedoch zeigten sich vereinzelt starke Artefakte und systembedingt Schwierigkeiten bei der Abbildung dreidimensionaler Strukturen.
Nunmehr wird vom akustischen Code auf einen elastischen Code umgestellt, da die eigentlichen Ultraschallmessungen mit elastischen Wellen erfolgen. In einem ersten Schritt wurde dies mit dem Softwarepaket Madagascar realisiert und an einem einfachen 2D-Modell getestet. Dabei fanden verschiedene Quellanregungen Anwendung. Des Weiteren wurden zwei Abbildungsbedingungen evaluiert. Ergebnisse der Kreuzkorrelation der Verschiebungskomponenten der zeitlich vorwärts-und rückwärtsmodellierten Wellenfelder wurden mit Resultaten der Kreuzkorrelation der Skalar- und Vektorpotentiale beider Wellenfelder verglichen.
The ultrasonic echo technique is widely used in non-destructive testing (NDT) of concrete objects for thickness measurements, geometry determinations and localization of built-in components. To improve ultrasonic imaging of complex concrete structures, we transferred a seismic imaging technique, the Reverse Time Migration (RTM), to NDT in civil engineering. RTM, in contrast to the conventionally used synthetic aperture focusing technique (SAFT) algorithms, considers all wavefield types and thus, can handle complex wave propagations in any direction with no limit on velocity variations and reflector dip. In this paper, we focused on the development, application and evaluation of a two-dimensional elastic RTM algorithm considering horizontally polarized shear (SH) waves only. We applied the elastic SH RTM routine to synthetic ultrasonic echo SH-wave data generated with a concrete model incorporating several steps and circular cavities. As these features can often be found in real-world NDT use cases, their imaging is extremely important. By using elastic SH RTM, we were able to clearly reproduce almost all reflectors inside the concrete model including the vertical step edges and the cross sections of the cavities.We were also capable to show that more features could be mapped compared to SAFT, and that imaging of complex reflectors could be sharpened compared to elastic P-SV (compressional-vertically polarized shear) RTM. Our promising results illustrate that elastic SH RTM has the potential to significantly enhance the reconstruction of challenging concrete structures, representing an important step forward for precise, high-quality ultrasonic NDT in civil engineering.