Material Degradation During the Stressed Oxidation of CMCs

The characterization and understanding of damage initiation and evolution in SiC/SiC ceramic matrix composites (CMCs) is critical to improve modeling efforts and design guidelines for these high impact materials. Through the support of this fellowship and collaboration with NASA GRC, microscale damage accumulation was investigated in two systems of SiC/SiC minicomposites of different architectures. A combination of mechanical testing with acoustic emission (AE) inside a scanning electron microscope was used to map surface damage at high spatial resolution and compare it to bulk damage accumulation. This combination of in situ damage monitoring enabled the characterization of the evolution of matrix crack density and crack opening displacements and enabled accurate mapping of local damage events to their generated AE. It was found that AE is a strong predictor of transverse matrix crack density, and also likely captures small contributions from secondary mechanisms such as debonding and sliding. The sensitivity of AE was examined using a modeling framework that leveraged simplifications of specimen geometry and assumptions of damage progression to compare point-by-point AE accumulation with estimations of damage accumulation in unidirectional CMCs. This framework resulted in an AE-based estimate of fiber failure evolution that agreed with micromechanics-based predictions when the AE contributions of secondary mechanisms were included. Finally, in situ matrix crack opening displacements (CODs) were characterized, and variations within specimens and across minicomposite systems were related to key microstructural and damage accumulation differences. Experimentally obtained CODs were compared to COD predictions from the literature, and it was found that the accuracy of the model predictions principally depended on consideration of the (i) progressive nature of fiber failure prior to specimen failure; and (ii) variations between the initial crack opening rate (upon matrix crack formation) and the subsequent crack opening rate (upon continued axial loading). While the initial intent of the proposal was to ultimately perform stressed oxidation of CMCs, the focus ended up being placed on the room temperature multi-modal characterization of SiC/SiC minicomposites due to the large amount of data that was generated through those experiments and the utility of that data in supporting modeling of damage progression. Those studies and the understanding of damage mechanisms they provided have prepared a foundation for subsequent research using similar SiC/SiC minicomposites in stressed oxidation testing.