Einfluss der thermischen Aktivierung und der Schwingungsdynamik von Versetzungen auf die thermodynamische Versetzungstheorie
Mechanik
Zusammenfassung der Projektergebnisse
The dynamics of stress-assisted grain boundary oxidation was investigated. It was found that the morphology change of grain boundary oxidation due to dislocation pile-ups alters stress fields of both the matrix and the intrusion, hence, amending the fracture resistance of the materials. In a defect free sample, the growth rate of grain boundary oxidation increases with the applied stress. While this could be opposite if the sample is pre-strained and contains prior dislocation structures, which proposes a mechanism for reconciling the inconsistency in the experimental observation by Kitaguchi et al. 2013. The effective temperature, a dual variable of dislocation entropy, proposed in thermodynamic dislocation theory was assessed using discrete dislocation plasticity which is a lower scale method comparing to the former. The relationship between the field quantities of two theories are derived, and the steady-state effective temperature is calculated using discrete dislocation plasticity by driving system to a saturated dislocation density condition. The result shows good agreement with the Boltzmann formula provided in thermodynamic dislocation theory. Geometrically necessary dislocation density is calculated using Nye’s tensor method and Burgers circuit method in the framework of discrete dislocation theory. The element size dependency in the former approach and the circuit size dependency in the latter approach are accounted for, and an improved method is proposed with which the geometrical necessary dislocations can be identified and their distribution as well as the amount can be found. As examples, cyclic loadings of tension and bending in single and oligocrystals are investigated. A model is developed for Van der Waals materials, graphite at micron scales involving shear cracks. Bending behaviours are described using this model and analysed the experimental data of single cantilever and double cantilever beams. The effective bending stiffness obtained from the model shows a good agreement with experimental data, and the energy release rates for different size of double cantilever beam are calculated.
