Options
Laserthermoschockversuche - Durchführung und Auswertung für unterschiedliche Hochleistungskeramiken
Citation Link: https://doi.org/10.15480/882.1221
Publikationstyp
Doctoral Thesis
Publikationsdatum
2014
Sprache
German
Author
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2014-11-11
In this thesis, a laser beam thermal shock rig was fundamentally enhanced to realize heating-up thermal shock tests on different advanced ceramics. The testing technique of the rig was extended with respect to testing media, i.e. testing in vacuum is possible now in addition to air. Heating takes place continuously and very rapidly by a laser beam, which moves helically from the center of the sample to the outside.
The measurement of the time and space resolved temperature distribution was realized by using a high performance infrared camera. The investigation of four different advanced ceramics with dissimilar optical and thermal properties represented a challenging task regarding measuring instrumentation. A procedure to apply pyrometric high-speed temperature measurements to samples with high thermal gradients as well as wavelength and temperature dependent transmission and absorption behavior was developed and is presented in detail. This includes the generation of a universal calibration procedure as well as the correction of the measured data with respect to different noise effects and measuring artifacts. The temperature profile can be determined and evaluated with good accuracy by this approach. The temperature is recorded on the complete specimen surface and the temperature increase in the laser spot is detected as well. The additional installation of the acoustic emission measurement allows the exact determination of the moment of specimen failure. With these advancements, ceramics with strong variations in optical and thermal properties can be investigated now in different media.
The induced temperature gradient results in the development of compressive stresses in the hot sample interior and of tensile stresses in the cold rim. Damage of the sample occurs when reaching a critical failure relevant stress. The laser spot represents the rupture cause due to a local rising of the temperature and, consequently, a stress rise close to its dwell point. The determination of the resulting stresses was extended for the first time from a one dimensional approach calculating the tangential and radial stresses to a two dimensional calculation, which is based on the Finite-Element-Method.
The scatter of the failure stress was statistically determined and assessed. The statistics of Weibull, which is accepted as a state-of-the-art approach, was adapted and further developed, respectively, for the requirements of thermal shock tests. The loaded volume of each sample during the experiment is subjected to a spatio-temporal inhomogeneous stress condition, which is taken into account in terms of an envelope. This represents an expansion of the statistics of Weibull, which assumes a constant sample volume. In addition to the failure relevant stresses, it is possible now to calculate the stored elastic energy at failure, which correlates very well with the observed fracture pattern.
Based on the calculated stress state and the stored elastic energy, different failure criteria were derived. After a contrasting juxtaposition of these failure criteria of the investigated advanced ceramics, the most suitable criterion was proposed, which can be applied with justifiable effort.
Thus, a universal method to perform and evaluate media dependent thermal shock experiments on ceramics is available now. Such thermal shock resistance data as well as their scatter can be used to design ceramic components.
The measurement of the time and space resolved temperature distribution was realized by using a high performance infrared camera. The investigation of four different advanced ceramics with dissimilar optical and thermal properties represented a challenging task regarding measuring instrumentation. A procedure to apply pyrometric high-speed temperature measurements to samples with high thermal gradients as well as wavelength and temperature dependent transmission and absorption behavior was developed and is presented in detail. This includes the generation of a universal calibration procedure as well as the correction of the measured data with respect to different noise effects and measuring artifacts. The temperature profile can be determined and evaluated with good accuracy by this approach. The temperature is recorded on the complete specimen surface and the temperature increase in the laser spot is detected as well. The additional installation of the acoustic emission measurement allows the exact determination of the moment of specimen failure. With these advancements, ceramics with strong variations in optical and thermal properties can be investigated now in different media.
The induced temperature gradient results in the development of compressive stresses in the hot sample interior and of tensile stresses in the cold rim. Damage of the sample occurs when reaching a critical failure relevant stress. The laser spot represents the rupture cause due to a local rising of the temperature and, consequently, a stress rise close to its dwell point. The determination of the resulting stresses was extended for the first time from a one dimensional approach calculating the tangential and radial stresses to a two dimensional calculation, which is based on the Finite-Element-Method.
The scatter of the failure stress was statistically determined and assessed. The statistics of Weibull, which is accepted as a state-of-the-art approach, was adapted and further developed, respectively, for the requirements of thermal shock tests. The loaded volume of each sample during the experiment is subjected to a spatio-temporal inhomogeneous stress condition, which is taken into account in terms of an envelope. This represents an expansion of the statistics of Weibull, which assumes a constant sample volume. In addition to the failure relevant stresses, it is possible now to calculate the stored elastic energy at failure, which correlates very well with the observed fracture pattern.
Based on the calculated stress state and the stored elastic energy, different failure criteria were derived. After a contrasting juxtaposition of these failure criteria of the investigated advanced ceramics, the most suitable criterion was proposed, which can be applied with justifiable effort.
Thus, a universal method to perform and evaluate media dependent thermal shock experiments on ceramics is available now. Such thermal shock resistance data as well as their scatter can be used to design ceramic components.
Schlagworte
Thermoschock
Laser
Hochleistungskeramik
Weibull
mechanische Eigenschaften
DDC Class
530: Physik
More Funding Information
Doktorandenprogramm der Bundesanstalt für Materialforschung und -prüfung
Loading...
Name
Dissertation_Druckversion.pdf
Size
6.93 MB
Format
Adobe PDF