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Test setup for analyzing the electrical resistance during fatigue loading for metastable austenite AISI 304L and its diffusion-brazed joints
(2025-01-08) Sauer, Lukas Maximilian; Otto, Johannes Leon; Ziman, Jonas A.; Starke, Peter; Walther, Frank
The measurement of the electrical resistance of specimens based on the established direct current potential drop (DCPD) method is a widely utilized methodology for the detection of damage mechanisms in the field of crack initiation and propagation and change in microstructural details. These include, e.g., dislocation density, void volume fraction, and micro- and macro-cracks. Given the necessity to consider additional factors influencing the electrical resistance, e.g., specimen geometry and temperature, ex-situ measurement techniques are frequently employed through interruption of fatigue testing. However, ex-situ investigations may result in unintended influences, such as changes in contacting, and analyze only discrete states limiting the characterization possibilities and result interpretation. Accordingly, in-situ electrical resistance measurements were employed in this study to characterize the microstructural changes during fatigue with cyclic creeping. To quantify and compensate the effects of geometry, temperature, and deformation-induced austenite-martensite transformation on the electrical resistance during fatigue loading, a complex experimental setup was developed which includes several measurement systems. The combination of strain measurement and potential drop enables a direct transfer of measured strain to electrical resistance. The method was applied and evaluated on high-temperature diffusion-brazed joints with a metastable austenite as base material and Ni-based filler metal. Finally, the change in microstructure was evaluated through electron channeling contrast imaging (ECCI) analyses at different load cycles.
Amtliche Mitteilungen der Technischen Universität Dortmund Nr. 13
(Technische Universität Dortmund, 2025-05-19)
Amtliche Mitteilungen der Technischen Universität Dortmund Nr.
(Technische Universität Dortmund, 2025-05-15)
Prozess- und Wirkungsevaluation zweier Schulstraßen in Dortmund
(2025-05-09) Brockhaus, Theresa; Westhoff, Norina; Scheiner, Joachim
Enhancing EPR capabilities: From 19F-ENDOR refinement to extreme condition measurements and sensitivity improvements
(2025) Schumann, Simon Lennard; Kasanmascheff, Müge; Clever, Guido
Electron paramagnetic resonance (EPR) spectroscopy is a technique with many different application fields. It is gaining popularity in medicine, material science, and biochemistry. As EPR was further established in other research fields, several new methodologies arose. Over the years, methods have been developed to detect interactions between two paramagnetic centers and a paramagnetic center and a magnetic nucleus. These diverse methodologies allow for structural and function analysis through distance measurements and coupling analysis.
The need for higher precision measurements of minimal distances grew, and methods were developed and employed to satisfy this need. This thesis modifies 19F ENDOR measurements for very short distances from 94 GHz to 34 GHz, enhancing the technique's accessibility for a broader scientific audience. It also investigates DNA G-quadruplexes (GQ), which are critical to essential biological processes such as telomerase maintenance and gene expression. This research showcases the successful application of the 19F-ENDOR methodology at 34 GHz, overcoming the limitations posed by the complexity and scarcity of higher-frequency spectrometers. Notably, the approach retains sensitivity and orientational resolution, enhancing our understanding of GQs and expanding the methodological toolbox for studying other macromolecules.
Furthermore, analyzing biological processes sometimes means looking outside the established boundaries. In some cases, life exists in extreme environments that are not easily reproduced in lab scenarios, like high-pressure deep-sea environments, and are not always reliant on abundant amounts of substances; in some cases, a low amount of molecules can already change biological function. Both of these edge cases are not easily accessible for EPR spectroscopy. A robust high-pressure EPR setup for pressures up to 4 kbar was constructed and tested during this thesis. This not only allows for basic EPR experiments but also opens the door to the full variety of dipolar spectroscopy methods available in EPR by following an out-of-spectrometer approach. This allows the application to be independent of the later spectrometer setup, simplifying the application drastically. Additionally, a high-sensitivity resonator with an extra large sample entrance for microwave and radio frequency double resonance experiments was built and established to allow for measurements of very low-concentration samples that were not feasible in a timely manner with commercially available resonators.