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Eingeladener Vortrag
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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.
Kurzfassung
Röntgen-Computertomographie (CT) ist eine industriell eingeführte Technik zur zerstörungsfreien Fehlererkennung. Seit einiger Zeit werden industrielle CT-Anlagen auch zur Geometriebestimmung, d.h. für dimensionelle Messungen eingesetzt. Bei diesen Messungen treten Messabweichungen auf, die wegen komplexer Einflussgrößen derzeit nur schlecht quantifiziert werden können. Die Größe der beobachteten Messabweichungen erschwert zum Teil noch die weitere Anwendung der CT als Messtechnik. Das momentan mit CT messbare Teilespektrum wird bei dickwandigen Bauteilen durch die fehlende Verfügbarkeit von Röntgendetektoren für hohe Energien (speziell Flächendetektoren) beschränkt. Im folgenden wird ein Forschungsprojekt zwischen 10 deutschen Industrieunternehmen und Instituten und zwei deutschen Bundesanstalten (Physikalisch-Technische Bundesanstalt (PTB) und Bundesanstalt für Materialforschung und -prüfung) vorgestellt, dass die Weiterentwicklung der CT als dimensionelle Messtechnik zur Aufgabe hat. Das Projekt hat eine Laufzeit von drei Jahren (Start 01.Okt. 2003). Es werden hier die Projektziele und erste Ergebnisse vorgestellt.
Corrosion of steel reinforcement in concrete exposed to chloride containing environments is a serious problem in civil engineering practice. Electrochemical methods, e.g., potential mapping, provide information whether the steel reinforcement is still passive or depassivation has been initiated. By applying such techniques no information on the type of corrosion, its extent and distribution of corrosion products is available. Particular the corrosion progress is a significant problem. Especially in the case of macrocell corrosion in reinforced concrete structures, the development at the anode cannot be separated into corrosion damage resulting from macrocell corrosion or self-corrosion. Until now also in laboratory tests it is impossible to collect such information without destroying specimens after electrochemical testing was performed. To overcome this problem it was tried to study the steel surface within the mortar specimens by X-ray tomography (CT). Within the scope of these investigations it could be shown, that X-ray tomography is suitable to make corrosion pits and their development visible which are embedded in a mortar with a cover thickness of about 35 mm. In this publication the time-dependent corrosion damage of reinforced steel is documented by X-ray tomography.
Investigations of corrosion on small cathodes in reinforced concrete - observations and suggestions
(2009)
Corrosion of steel reinforcement in concrete exposed to chloride containing environments is a serious
problem in civil engineering practice. The standards and guidelines contain only minimum
requirements regarding concrete quality and concrete cover. In very critical cases, for example the
ingress of chlorides, the application of different coatings is currently the only possibility to protect the
reinforced concrete structures. In this case the modelling of reinforcement corrosion would be more
practical. One sub-project of a DFG-research project (FOR 537) is currently performed at the Federal
Institute for Materials Research and Testing (BAM). Aim of the sub-project is the description of
corrosion propagation and the characterization of the corrosion on specimens with small cathodes by
electrochemical parameters. The electrochemical parameters are determined by suitable
electrochemical measurements, considering concrete parameters as well as different environmental
conditions.
The fabric of mineral liner materials that had been exposed to organic compounds over a 12-year period was investigated as part of a wider research project. Macromorphological and micromorphological changes in the fabric were identified using computed tomography and polarisation microscopy.
Special care was taken to ensure artefact free sampling and sample preparation, in particular, the drying method used, has a substantial influence on the quality of the thin sections.
Fabric changes due to contaminant permeation over several years are, by and large, relatively small and their intensity is material specific. Silty clay CML1, in particular, contains a somewhat greater number of fissures and voids in comparison with the original material. The superposition of various processes in both test procedures and sample preparation may lead to fabric changes which can impede interpretation of the results.
Micro-computed tomography (µCT) provides quantitative three-dimensional information of bone around titanium implants similar to classical histology. The study, based on an animal model, using cuboid-shaped biofunctionalised Ti6Al4V implants with surrounding bone after 4 weeks, is performed using 3 µCT-systems with X-ray tubes, one synchrotron-radiation-based µCT-system (SRµCT), and classical histology. Although the spatial resolution of the µCTsystems is comparable, only the results of SRµCT agree with results of classical histology. The X-ray tube sources give rise to huge artefacts in the tomograms (interface scattering, beam hardening), which impaired the quantitative analysis of bone up to about 200 µm from the implant surface. Due to the non-destructive character of µCT the specimens can be subsequently examined by classical histology
without restriction. The quantitative comparison of bone formation uncovers the strong dependence of the detected amount of newly formed bone from the selected slice. This
implies the necessity of 3D analysis. SRµCT and classical histology prove that surface modifications of the titanium implant significantly influence the bone formation. Using SRµCT, the preparation artefacts due to cutting and polishing are excluded.
Pyrolysis and fire behaviour of a phosphorus polyester (PET-P-DOPO) have been investigated. The glycol ether of the hydroquinone derivative of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide was used as a reactive halogen-free flame retardant in PET-P-DOPO. PET-P-DOPO is proposed as an alternative to poly(butylene terephthalate) (PBT) with established halogen-free additives. It exhibits a high LOI (39.3%) and achieves V-0 classification in the UL 94 test. Three different mechanisms (flame inhibition, charring and a protection effect by the intumescent char) contribute to the flame retardancy in PET-P-DOPO and were quantified with respect to different fire risks. The fire load was reduced by 66% of the PBT characteristic. The reduction is the superposition of the relative reduction due to flame inhibition (factor 0.625) and charring (factor 0.545). The peak of heat release rate (pHRR) was reduced by 83% due to flame inhibition, charring and the protection properties of the char (factor 0.486). The strength of all three mechanisms is in the same order of magnitude. The intumescent multicellular structure enables the char to act as an efficient protection layer. PBT flame-retarded with aluminium diethylphosphinate was used as a benchmark to assess the performance of PET-P-DOPO absolutely, as well as versus the phosphorus content. PET-P-DOPO exhibits superior fire retardancy, in particular due to the additional prolongation of the time to ignition and increase in char yield. PET-P-DOPO is a promising alternative material for creating halogen-free flame-retarded polyesters.
Die Computertomographie (CT) hat sich als dimensionelle Messmethode in der industriellen Produktion etabliert. Durch Messungen an speziellen Prüfkörpern ist es möglich, quantitative Informationen über die metrologischen Eigenschaften dimensioneller Messungen zu erhalten. In dieser Arbeit wird einen Aluminium-Guss-Prüfkörper (120 mm x 90 mm x 60 mm) benutzt, mit dem – in der vorliegender Form – auch der Einfluss von Materialien unterschiedlicher Schwächung auf dimensionelle CT-Messungen untersucht werden kann. Der Prüfkörper ist in vier Segmente zerteilt, so dass auch innenliegende, raue Gussfreiformflächen der taktilen Koordinatenmessung zugänglich sind. Durch adaptierte Rubinkugeln als Referenzstrukturen können die CT-Messdaten in taktile Referenz-Koordinatensysteme überführt werden. Um den Einfluss von Materialien unterschiedlicher Schwächung auf das dimensionelle Messen mit CT zu untersuchen, wurden stärker schwächende ZTA-Störkugeln (Zirconia Toughened Alumina) mit zur Ausrichtung geeigneter Symmetriebrechung in der Nähe von ausgewählten taktilen Messstellen angebracht.
Zur Bewertung der Messabweichungen dienen taktile Referenzmessungen an den ZTA-Kugeln sowie taktile Messungen an den Freiformflächen des Gussteils, wobei durch ein iteratives Antastverfahren eine verbesserte Erfassung der Geometrie auch bei starken lokalen Oberflächenkrümmungen sichergestellt wird.
Phasenkontrastmessungen haben sich als hilfreich bei der
Kantenfindung von Strukturen mit nur geringem Absorptionskontrast erwiesen.
Solche Messungen können besonders gut mit monochromatischer Röntgenstrahlung
hoher räumlicher Kohärenz durchgeführt werden, wie sie bei der Synchrotron-
Computertomographie (SY-CT) benutzt wird. Nachteilig wirken sich allerdings die
Phasenverschiebung des Synchrotronstrahls über das Strahlprofil und damit die
Kantenverbreiterung in den zu untersuchenden Strukturen aus. Daher ist ein
dimensionelles Messen mit normalen Rekonstruktionsverfahren nicht möglich. In
der vorliegenden Arbeit werden neue Filterverfahren zur Korrektur der
Phasenverschiebung unter Beibehaltung des erhöhten Kontrastes bei der
Rekonstruktion angewandt, um ausgewählte einkomponentige Referenzobjekte
hinsichtlich der dimensionellen Messeigenschaften zu untersuchen. Als Referenz
werden rückgeführte Messungen mit taktilen Koordinatenmessmaschinen
herangezogen. Die Untersuchungen wurden im freien Strahl durch Variation des
Abstandes zwischen Objekt und Detektor am Strahlrohr der BAM am Helmholtz-
Zentrum Berlin für Materialien und Energie - Speicherring BESSY II durchgeführt.
Analyzing damages at concrete structures due to physical, chemical, and mechanical exposures need the application of innovative non-destructive testing methods that are able to trace spatial changes of microstructures. Here, the utility of three different crack detection methods for the analysis of computed tomograms of various cementitious building materials is evaluated. Due to the lack of reference samples and standardized image quality evaluation procedures, the results are compared with manually segmented reference data sets. A specific question is how automatic crack detection can be used for the quantitative characterization of damage processes, such as crack length and volume. The crack detection methods have been integrated into a scientific visualization system that allows displaying the tomography images as well as presenting the results.