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The impact of very high cycle fatigue (VHCF) load conditions on the microstructure of specimens consisting of nodular cast iron is analyzed by means of micro-computed tomography (µCT) utilizing both monochromatic synchrotron radiation and polychromatic X-ray tube radiation. Using 3D µCT, the microstructure in the region of the smallest cross-sections of shouldered round specimens is imaged in different stages of the VHCF loading. By digital image correlation (DIC) of these tomograms strain fields are analyzed three-dimensionally. Strain levels in the range of a few percent were detected. It is proven that a localization of strain allows to predict the site of the crack which precedes and induces the macroscopic failure of the specimens.
In civil engineering, the corrosion of steel reinforcements in structural elements of concrete bares a risk of stability-reduction, mainly caused by the exposure to chlorides. 3D computed tomography (CT) reveals the inner structure of concrete and allows one to investigate the corrosion with non-destructive testing methods. To carry out such investigations, specimens with a large artificial crack and an embedded steel rebar have been manufactured. 3D CT images of those specimens were acquired in the original state. Subsequently three cycles of electrochemical pre-damaging together with CT imaging were applied. These time series have been evaluated by means of image processing algorithms to segment and quantify the corrosion products. Visualization of the results supports the understanding of how corrosion propagates into cracks and pores. Furthermore, pitting of structural elements can be seen without dismantling. In this work, several image processing and visualization techniques are presented that have turned out to be particularly effective for the visualization and segmentation of corrosion products. Their combination to a workflow for corrosion analysis is the main contribution of this work.
During the last decades, X-ray (micro-)computed tomography has gained increasing attention for the description of porous skeletal and shell structures of various organism groups. However, their quantitative analysis is often hampered by the difficulty to discriminate cavities and pores within the object from the surrounding region. Herein, we test the ambient occlusion (AO) algorithm and newly implemented optimisations for the segmentation of cavities (implemented in the software Amira). The segmentation accuracy is evaluated as a function of (i) changes in the ray length input variable, and (ii) the usage of AO (scalar) field and other AO-derived (scalar) fields. The results clearly indicate that the AO field itself outperforms all other AO-derived fields in terms of segmentation accuracy and robustness against variations in the ray length input variable. The newly implemented optimisations improved the AO field-based segmentation only slightly, while the segmentations based on the AOderived fields improved considerably.
Additionally, we evaluated the potential of the AO field and AO-derived fields for the separation and classification of cavities as well as skeletal structures by comparing them with commonly used distance-map-based segmentations. For this, we tested the zooid separation within a bryozoan colony, the stereom classification of an ophiuroid tooth, the separation of bioerosion traces within a marble block and the calice (central cavity)-pore separation within a dendrophyllid coral. The obtained results clearly indicate that the ideal input field depends on the three-dimensional morphology of the object of interest. The segmentations based on the AO-derived fields often provided cavity separations and skeleton classifications that were superior to or impossible to obtain with commonly used distance-map-based segmentations. The combined usage of various AO-derived fields by supervised or unsupervised segmentation algorithms might provide a promising target for future research to further improve the results for this kind of high-end data segmentation and classification. Furthermore, the application of the developed segmentation algorithm is not restricted to X-ray (micro-)computed tomographic data but may potentially be useful for the segmentation of 3D volume data from other sources.
This poster presentation gives an overview of the great potential of X-ray micro computed tomography (CT) to cast light on the evolution of the microstructure in construction materials. Prevention of damage is of major economic and social importance in the development of suitable construction materials such as concrete and asphalt. Therefore a non-destructive testing method such as CT is an appropriate tool for visualization of the inner structure. Its combination with other test methods allows understanding the damage processes such as crack propagation or corrosion. We show examples of internal structure analyses on a wide range of materials: Automatic 3D crack detection and the visualization of corrosion products inside of steel reinforced concrete, pore and shape analysis of lightweight aggregates and the visualization of deformation of high-pressure loaded aerated concrete specimens, distribution of aggregates inside concrete, and determination of the surface of porous asphalt core samples. Segmented structures serve, e.g., as input data for simulation of transport phenomena or virtual load tests.
Industrial computed tomography (CT) today is an important method to analyze defects and to measure the geometry of technical products. The measurement capability of industrial CT often depends on the knowledge of the measurement errors being present. Only with this knowledge, effective corrections are possible by manufacturers, or adapted measurement procedures become possible for the customers. Thus, there is the strong need to have adequate reference standards both for the assessment of general dependencies and for the measurement of task-specific effects. Especially the latter are difficult to assess. This paper shows the application of a versatile dismountable workpiece-near reference body for the use with CT. The reference standard was made from a miniaturized cast aluminium (one-)cylinder head. It can be dismounted into four segments, each featuring reference geometries which enable a registration of CT measured and other reference datasets (e.g. from tactile CMM). The reference standard embodies complex freeform surfaces which are a challenging measurement task for CT and other sensors. Here the application of the reference standard is the study of the influence of material mixes. This is realized by adding pieces of other materials which disturb the mono-material measurement.
Results of the application of the reference standard are presented using data from industrial micro-CT systems and CMM reference data. A special emphasis is given on the workflow of the data processing and the data analysis.
Auswirkungen von Ermüdungsbeanspruchungen auf Struktur und Eigenschaften von Fahrbahndeckenbeton
(2015)
Ausgehend von der Zielsetzung der DFG-Forschergruppe 1498, die Einflüsse auf den AKR-Schädigungsprozess in Fahrbahndecken aus Beton grundlegend zu klären, sind zwei aufeinander aufbauende Untersuchungsschwerpunkte Gegenstand dieses Beitrags. Im ersten Schwerpunkt wurden die prüftechnischen Voraussetzungen für die Charakterisierung der Degradation und damit der Rissbildung im Fahrbahndeckenbeton durch die Vorschädigung infolge von Ermüdungsbeanspruchungen geschaffen. Realisiert wurde dies durch die Implementierung und Optimierung eines Monitoringsystems im zyklischen 4-Punkt-Biegeversuch zur simultanen Erfassung der Rissbildung im großformatigen Schwingbalken. Zusätzlich wurde die Röntgen-3D-Computertomografie für die hochauflösende räumliche quantitative Rissanalyse in den Bohrkernen aus den Schwingbalken ohne und mit Vorschädigung weiterentwickelt. Im zweiten Untersuchungsschwerpunkt wurde mittels vielfältiger Experimente an kleinformatigen Probekörpern aus dem Schwingbalken gezeigt, dass durch die Ermüdungsbeanspruchung die bruchmechanischen Materialkennwerte des Betons zum Teil eine signifikante Veränderung erfahren. Based on the objective of the DFG (German research foundation) funded research group 1498 to clarify the influences on the ASR damage process, this paper deals with two focuses of investigation. In the first focus the technical requirements for the characterisation concerning the degradation and the formation of cracks in concrete pavements by the pre-damage as a result of fatigue loading were created. This was realized by the implementation and the optimisation of a monitoring system in the cyclic four point bending test to capture simultaneously the formation of cracks in the beam under sinusoidal bending load. In addition, the X-ray 3D-computed tomography was enhanced for the high-resolution quantitative spatial crack analysis in the drilling cores extracted of the beam under sinusoidal bending load without and with a pre-damage. Building on the first focus, in the second focus of investigation a significant change of the fracture-mechanical properties of the concrete as a result of fatigue loading has been demonstrated in numerous experiments using small specimens extracted from the beam under sinusoidal bending load.
Bikontinuierliche poröse Keramiken für Filteranwendungen weisen eine komplizierte Mikrostruktur auf, in der Poren und Festkörperphase 'vermischt' sind. Mechanische, thermische sowie Filtereigenschaften können nur präzise bestimmt werden, wenn die Morphologie beider Phasen quantitativ angegeben werden kann. Hier werden unter Anwendung von 3D Computertomographie (CT) und verschiedenen Techniken, die auf der Röntgenrefraktion basieren, die Porosität und die Porenorientierung in keramischen Cordierit-Dieselpartikelfiltern bewertet. Dabei werden zur Auswertung der Tomogramme die Fouriertransformierten sowie eine neu entwickelte richtungssensitive Bildanalyse (DIVA, directional interface variance analysis) angewandt. Sowohl die genannten Messtechniken als auch der statistische Ansatz gestatten die Extraktion optional ortsaufgelöster oder gemittelter Werte. Es ergeben sich sehr gute Übereinstimmungen der experimentell ermittelten Porositätswerte mit Ergebnissen aus Quecksilberintrusionsversuchen als auch der ermittelten Porenorientierung im Vergleich zu publizierten Angaben zur kristallographischen Textur. Dies legt den Schluss nahe, dass die Kenntnis der Grenzflächen (zwischen Poren und Materie) genügt, die morphologischen Eigenschaften solcher Materialien hinreichend zu beschreiben.