Wissenschaftliche Artikel der BAM
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Das vorgestellte Projekt beschäftigt sich mit den optischen, thermophysikalischen und mechanischen Materialeigenschaften von additiv gefertigten Kunststoffteilen und deren Alterung. Dazu wurden mittels Fused Layer Modeling (FLM) Probekörper aus ABS hergestellt, die anschließend für drei Monate einer künstlichen Bewitterung unterzogen wurden. Die dabei erzeugte definierte Alterung wurde nach drei Zeitabschnitten jeweils mit zerstörungsfreien Methoden charakterisiert. Dabei wurde neben den spektroskopischen Standardmethoden auch die aktive Thermografie eingesetzt. Die Ergebnisse zeigen, dass sich mittels der Thermografie sowohl chemische Veränderungen als auch mechanische Schädigungen (Fehlstellen) darstellen lassen. Die Bewitterung beeinflusst die Teiltransparenz der Proben zumindest im sichtbaren Spektralbereich, was bei der Charakterisierung der Fehlstellen, die mittels optisch angeregter aktiver Thermografie detektiert wurden, berücksichtigt werden muss.
The initial focus of this research was on the development of a general workflow for the documentation and monitoring of historical stained glass windows using structured light scanning. Therefore windows from different churches, time periods and with different corrosion and damage phenomena were scanned before and after conservation measures. To evaluate the execution of the restoration measures the data was compared using 3D inspection software to examine the differences in geometry between the two scans. Various problems had to be solved, for example, how to deal with heavily reflective surfaces and the extreme contrast between light and dark surfaces, as seen in the borders between ‘Schwarzlot’ painting and plain glass. The application of materials for matting the surfaces, such as Cyclododecane spray, was impossible due to the high accuracy of the surface measurement required for 3D inspection. Regarding the contrast differences of the surfaces, the creation of exposure fusions and the use of polarization filters to reduce reflections were tested. In addition to the general problems encountered when recording translucent surfaces, the historical glasses caused additional problems in calculating surface comparisons. For example, the windows have to be moved and turned around several times, both during the conservation process and while scanning, causing deformations of the geometry due to the flexible lead rods allowing a certain degree of movement.
In the present study, the impact of static traffic loading on the slight deflection effects in the concrete structure of an existing bridge has been investigated using distributed fiber optic sensors. In the face of increasing traffic density and severe traffic loading, the results of the load tests on the Amsterdam bridge 705 make an important contribution to the understanding of its structural behavior. The concept of the static loading was based on the use of two 36-ton trucks stopped on the bridge at multiple pre-determined locations. The load applied in this way led to location-dependent small deflection effects recorded as longitudinal strain of the sensing fiber embedded at the underside of the bridge. The measurements were performed with a commercially-available solution based on Tunable Wavelength Coherent Optical Time Domain Reflectometry with the measurement accuracy in the range of 0.5 µm/m.
Die zeitaufgelöste THz-Spektroskopie (THz-TDS) operiert mit elektromagnetischen Impulsen, die ein Amplitudenspektrum im unteren THz-Bereich aufweisen. Die THz-TDS ist in der Lage, dielektrische Volumina zerstörungs- und kontaktfreie zu durchstrahlen. Um eine 3D-THz Bildung zu ermöglichen, wurde für die THz-TDS ein rechnergestütztes Mess- und Rekonstruktionsverfahren (THz-TDSAFT) entwickelt, mit dem es gelingt, unter Verwendung der im Zeitbereich registrierten THz-TDS-Daten das Volumeninnere in Form eines dreidimensionalen Tomogramms zu rekonstruieren. Die THz-TD-SAFT basiert auf die für die Ultraschalltechnik entwickelte Synthetische Apertur Fokussierung (SAFT) bzw. des heuristisch begründeten Algorithmus des Synthetischen Apertur Radars (SAR). Anhand von Beispielen wird die Möglichkeiten der THz-TD-SAFT demonstriert und nachgewiesen, dass mit dem Rekonstruktionsansatz die künstlich eingebauten Fehlstellen bezüglich der Position und Größe geometrisch richtig im Tomogramm abgebildet werden können.
Polymerbeschichtungen auf Beton bieten einerseits mechanischen Schutz vor Beschädigungen und verhindern das Eindringen von Wasser, gelösten Salzen und CO2 und können andererseits der Oberfläche ein gewünschtes Aussehen geben. Daher werden sie in der Bauindustrie insbesondere bei Brücken und Parkhäusern häufig verwendet. Für die beabsichtigte Wirkung der Beschichtung ist deren Schichtdicke ein entscheidender Faktor und muss daher nach Aufbringung der Schicht und im Betrieb regelmäßig überprüft werden. Üblich sind hier zerstörende Verfahren mit nachträglich erforderlicher Ausbesserung. Im Rahmen eines Vorgängerprojektes wurde an der BAM in Kooperation mit der IBOS GmbH ein zerstörungsfreies Verfahren entwickelt, welches auf aktiver Thermografie basiert. Ein Prototyp mit Halogenlampe und Infrarot-Kamera wurde erprobt.
Das hier beschriebene Folgeprojekt hat das Ziel, dieses Verfahren zu standardisieren, damit es sich in der Industrie als zerstörungsfreies Alternativverfahren zur Schichtdickenbestimmung durchsetzt. Im ersten Schritt wird gezeigt, bei welchen Oberflächenschutzsystemen das Verfahren bereits zuverlässige Schichtdickenmessungen ermöglicht und praxistauglich ist. Anschließend werden weitere Forschungsthemen definiert und bearbeitet, die eine Anwendung für alle Oberflächenschutzsysteme ermöglichen sollen. Besondere Herausforderungen stellen etwa Sandeinstreuungen in der Polymerschicht dar. Auch muss der Frage nach der Anwendbarkeit auf Mehrschichtsysteme nachgegangen werden. Des Weiteren stellt sich die Frage nach dem Einfluss der Beschaffenheit des Betonuntergrunds auf die Anwendbarkeit des Verfahrens. Im zu erarbeitenden Normentwurf sollen diese Untersuchungen die Möglichkeiten, aber auch die Grenzen der aktiven Thermografie zur Schichtdickenmessung von Oberflächenschutzsystemen aufzeigen. Im Beitrag werden die systematischen Untersuchungen bezüglich dieser Fragestellungen dargelegt.
Increasing traffic volume on the one hand and ageing infrastructure on the other hand have created many new challenges for maintenance and structural health monitoring of roads and bridges. In the past, many bridges and road structures have been neglected, often resulting in traffic congestion, road closure, and increased repair costs.
This research is concerned with the development of a system to improve the challenge of maintenance and early detection of damage, particularly moisture penetration and corrosion of steel reinforced concrete components. The objective is to develop a method that will also work after 30 years and longer. Many new IoT solutions are equipped with internal energy storage elements (accumulators or batteries) which are inappropriate here, since most relevant signs of concrete degradation occur after decades, where the functioning of such elements are more than questionable. The presented technology approach uses radio-frequency identification (RFID) and enables connectivity to sensors. It offers the advantage of an passive, completely independent energy supply without any energy storage components.
Since the system should be permanently embedded in concrete, it is crucial to develop a long-term stable device which is adapted to the environmental influences of the structure, e.g., long-term resistance in very alkaline environment of pH 13. In numerous experiments, the robustness of the system was tested and evaluated. Various tests with encapsulations to protect the electronics were performed, and for long-term validation different concrete specimens were instrumented with RFID-sensor-systems. Their operating time is now around two years and investigations for signs of fatigue and damage to the encapsulation and the electronics are ongoing.
Ultrasonic methods are used in concrete investigations since decades. While being limited to transmission testing in the laboratory for a while, in-situ echo measurements for structural investigations and condition assessment have made their way into practical application in the past 20 years. However, several challenges remain. On one side, there are technical issues as limitations in depth of penetration, resolution and imaging capabilities. On the other side there are still gaps in validation, standardization and certification, which are limiting the applicability in condition/load capacity assessment.
This review reports a couple of developments which will help to overcome these issues. This includes technical developments as new devices which are easier to handle on site or giving a much deeper penetration depth (e.g. the LAUS device at BAM) as well as improvements in imaging by hardware update (e. g. air coupled ultrasound or coded signals) or new software (e. g. RTM imaging). To foster the application in real world projects we are as well working on standardization by developing new reference specimen with international partners which will ensure world-wide comparability of ultrasonic and other methods and quality assurance codes. Further, non-destructive methods are being used to update probabilistic models used for the reassessment of existing structures to support the structural engineer’s decisions.
At present, to produce renewable energy offshore wind farms play an important role. The available space combined with the more valuable wind conditions make offshore locations very attractive for wind powered energy production. In Europe a significant number of offshore wind farms already exist, especially in the North and Baltic Sea. In future this trend will continue, and further offshore wind farms will be built. The majority of offshore wind turbines are mounted on steel foundation structures. Due to the high-cyclic loading by wind and waves fatigue stress plays a substantial role regarding structural safety.
Besides the consideration of fatigue within the design process, to monitor existing steel structures for potential fatigue cracks during their life time is a major topic and a challenge.
For the structures of the offshore wind turbines are large and partially under water effective reliable methods for the detection of fatigue cracks are required.
This contribution presents investigations on different crack detection methods applied at high-cycle fatigue tests on small-scale welded steel samples as well as on large-scale welded steel components. The tests were conducted at the BAM laboratories. For crack detection mainly three different methods were used and compared. The first method regards to the measurement of strain by conventionally strain gauges. Secondly, the crack luminescence was used as a new and effective optical method for surface monitoring. And finally, crack detection by pressure differentials of the inner and outer section of tubular steel elements was investigated. A comparison study will emphasize the advantages and disadvantages of the different methods and show which of the described methods is potentially more suitable for an application on real offshore wind structures.
The structural performance of many geotechnical systems (e.g. axially-loaded pile foundations), depends on the shearing resistance at the soil interface, which may govern the load bearing capacity of the foundation. Experimental investigations have shown that this interaction is mainly localised within a narrow shear band next to the structure. Under cyclic loading, a contraction of the soil at the interface may arise (net volume loss), possibly leading to a stress relaxation and thus to a reduction of the load bearing capacity (the so-called friction fatigue). Based on the constitutive similarities between soil continua and interfaces, we propose here the adaption of a Generalized Plasticity model for sandy soils for the numerical analysis of interface problems. In this contribution, the results of an experimental campaign for the parameter calibration of the constitutive model are presented. The tests have been conducted with a ring shear device involving different normal stresses, roughness of the steel plates as well as cyclic loading. The new modelling approach shows promising results and has the additional practical advantage that the interface zone and the soil continuum can both be described with the same constitutive model in general boundary value problems.
Investigation of the salinization in marine and offshore environment - test methods and challenges
(2019)
Salinization and contamination of metal surfaces by chloride-containing aerosols is of great importance with regard to corrosion phenomena in the maritime sector and in offshore applications. Especially Offshore Wind Turbines are exposed to extreme corrosive conditions due to high chloride concentrations in the atmosphere and the resulting high chloride deposition rates. It is of great importance to evaluate to what extent salinization of the surface influences the corrosion protection of coatings and pitting occurrence on stainless steels under atmospheric conditions to evaluate the durability of metallic building structures in offshore and marine environments. The evaluation of the scientific literature and regulatory guidelines has shown that there are still many open questions regarding the contamination of metal surfaces by chlorides. This contribution will discuss how salinization of metal surfaces is evaluated and monitored according to current standards and guidelines. Future challenges concerning test methods and the application of evaluated salinization values and deposition rates will be discussed.