Browsing by Author "Bossuyt, Sven, Prof., Aalto University, Department of Mechanical Engineering, Finland"

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    Localized deformation in spent nuclear fuel disposal canisters
    (Aalto University, 2019) Forsström, Antti; Konetekniikan laitos; Department of Mechanical Engineering; Advanced Manufacturing and Materials; Insinööritieteiden korkeakoulu; School of Engineering; Bossuyt, Sven, Prof., Aalto University, Department of Mechanical Engineering, Finland; Hänninen, Hannu, Prof., Aalto University, Department of Mechanical Engineering, Finland
    Spent nuclear fuel disposal in copper canisters in a deep geologic repository will be the main disposal method of spent nuclear fuel in Finland and Sweden. The long timespans involved in the disposal bring certain uncertainties, for example, when considering localized deformations and environmental factors, such as corrosion and hydrogen effects on the mechanical properties. The canisters are sealed by friction stir welding (FSW), which results in strong welds, but the variation in cross-weld microstructure may lead to localized deformations during the long disposal of the canisters. Thus, localization of plastic deformation in the canister welds was studied by digital image correlation (DIC). The FSW welding method has recently been modified by argon shielding gas to reduce oxidation during welding. When compared to the old weld, which was welded without shielding gas, the new weld exhibits considerably less oxide particles, and the weld microstructure is more uniform. However, DIC testing indicates more pronounced localization of plastic deformation on the weld boundary on the lid side of the canister. This is attributed to the combined effect of softer lid material and stronger weld material, which leads to worse mechanical performance in terms of strain localization. To minimize this effect, the strength mismatch between different parts of the copper shell should be minimized. In addition to localized deformations, studies were conducted on hydrogen effects on mechanical properties of the copper welds and the cast iron insert. Hydrogen charging of the copper FSW welds indicates that oxide particles trap considerable amounts of hydrogen, but it did not compromise the ductility of the copper welds. However, the cast iron insert of the canisters exhibits a significant sensitivity to hydrogen. In the context of detecting localized deformations by DIC, the trade-off between spatial resolution and noise level of the measurement is critical. Thus, methods for quantifying that trade-off, as well as for improving the DIC data quality were developed. Patterning of copper, by utilizing copper oxide and a photolithographic method, enables the measurement of large deformations in copper, as well as detection of small deformations due to improved data quality. The improvements were quantified by extracting the noise level and autocorrelation length of the noise, which allows the evaluation of precision and spatial resolution of the DIC measurement based on the DIC data itself, and does not require imposing known displacements beforehand.
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    Understanding ice fracture using Digital Image Correlation - From microstructural crack arrest to comparison with the visco-elastic fictitious crack model
    (Aalto University, 2024) Ahmad, Waqas; Bossuyt, Sven, Prof., Aalto University, Department of Mechanical Engineering, Finland; Konetekniikan laitos; Department of Mechanical Engineering; Marine and Arctic Technology; Insinööritieteiden korkeakoulu; School of Engineering; Tuhkuri, Jukka, Prof., Aalto University, Department of Mechanical Engineering, Finland
    Different parameters influence the fracture of ice. These include variation in microstructure of ice and experimental conditions such as loading rate and temperature etc. Conventionally fracture of ice has been studied using linear variable differential transducers (LVDTs) and lasers to record deformation and acoustic emissions to study the propagation of cracks in ice. This thesis discusses the use of Digital Image Correlation (DIC) in ice fracture mechanics to achieve higher spatial resolution and study the influence of the grains on a propagating crack. Analysis of S2 saline and freshwater ice experiments was done by developing a DIC postprocessing method that used cross-correlation of the Heaviside function to find a propagating crack tip. Moreover, a quantitative method was developed to find the size of the deformation zone ahead of the crack by using the decrease in the stiffness (slope of the force-COD plot) as a measure of deformation near the initial crack tip. Saline ice fracture experiment were conducted using load-control configuration. The thin section analysis coupled with crack propagation history obtained from DIC showed that the microstructure influenced the crack arrest events. Moreover, along with intermittent crack growth, localized stable crack growth was observed from the full field data which was also linked with the microstructure. Crack growth was intergranular during stable crack growth while transgranular growth was observed during unstable propagation. The upper bound of the fracture process zone (FPZ) ahead of the crack tip was also measured for floating freshwater ice experiments. This deformation zone (DZ) included both the permanent deformations in the FPZ and the elastic deformations. Comparison of the measured results with the visco-elastic fictitious crack model (VFCM) showed that the estimated FPZ size from VFCM depends on the nature of crack growth. The model overestimated the size of the FPZ if the specimen failed without intermittent crack growth. The specimens that failed with intermittent crack growth resulted in similar magnitude of lengths for the FPZ and DZ. However, VFCM underestimated the width of the FPZ compared to the critical crack opening displacement measured from DIC. Lastly, the accuracy of the calculated FPZ size using VFCM was found to be dependent on the accuracy of the crack tip opening displacement (CTOD).