Ivar E. Reimanis

The effect of flaws on the propagation of cracks at bi-materials inferfaces

This study uses the boundary element method to investigate the effects of interfacial flaws on the propagation of a crack at or near an interface between two elastic, isotropic materials. These calculations reveal that flaws tend to deflect cracks approaching the interface from their original trajectory if the distance between the flaw and crack tip is small in relation to the flaw size. It is found that the higher the elastic mismatch parameter, the more crack trajectories are attracted to flaws. Other calculations show that materials with interfacial flaws have a significantly increased tendency to deflect cracks along the interface as compared to defect-free materials.

The fracture behavior of single-crystal Y3Al5O12

Abstract The fracture behavior of single-crystal yttrium aluminum garnet in ambient air has been studied as a function of temperature for two different crystallographic orientations. A Chevron notch specimen geometry was used so that stable crack propagation could be achieved. It was found that the room temperature fracture toughness, K Ic = 1.8 MPa m 1 2 , increased to K Ic = 4.0 MPa m 1 2 at 1600°C. At 1750°C, samples did not fracture even when the load was brought to an equivalent stress intensity factor of 11.3 MPA m 1 2 . The fracture toughness was found not to depend significantly on crystallographic orientation. The increase in toughness with temperature was attributed to the activation of slip systems, which were evaluated by trace analysis. It was found that the degree of plasticity is extremely sensitive to the strain rate. Finally, periodic striations were observed on the fracture surfaces which are believed to be related to periodic chemical or structural fluctuations which arise during the Czochralski growth process.

The Mechanical Properties of a Novel Si 3 N 4 -Amorphous Si 3 N 4 Composite

The hardness and fracture toughness of a model two-phase composite consisting of crystalline Si 3 N 4 particles in a matrix of amorphous Si 3 N 4 are examined. The composite is created by heat treating high purity, partially amorphous CVD Si 3 N 4 in N 2 for various times and temperatures in order to induce crystallization of the a phase. Microindentation tests at temperatures up to 1200 °C are conducted to evaluate the high temperature hardness and fracture toughness. The role of the microstructure is examined using optical and transmission electron microscopy. Finally, the relationship between the microstructure and the mechanical properties is discussed.

TRISO-Coated Fuel Durability Under Extreme Conditions

The PIs propose to examine TRISO-coated particles (SiC and ZrC coatings) in an integrated two-part study. In the first part, experiments will be performed to assess the reaction kinetics of the carbides under CO-CO2 environments at temperatures up to 1800 degree C. Kinetic model will be applied to describe the degradation. Scanning and transmission electron microscopy will be employed to establish the chemical and microstructure evolution under the imposed environmental conditions. The second part of the proposed work focuses on establishing the role of the high temperature, environmental exposure described above on the mechanical behavior of TRISO-coated particles. Electron microscopy and other advanced techniques will be subsequently performed to evaluate failure mechanisms. The work is expected to reveal relationships between corrosion reactions, starting material characteristics (polytype of SiC, impurity concentration, flaw distribution), flaw healing behavior, and crack growth.