| Engineering applications of ceramics often involve mixed-mode loading conditions, where both tensile and shear stresses act simultaneously. Thus, the study of fracture of ceramics under the complex mixed-mode (I/II) loading conditions is of considerable practical interest. Though mixed-mode fracture has been investigated for a wide range of ceramic specimens, there are several aspects of mixed-mode fracture that is still unknown. The present study aims to evaluate the mixed-mode fracture parameters in ceramic specimens, perform fractographic studies on the mixed-mode fracture surfaces and derive an appropriate mixed-mode fracture criterion. Mixed-mode fracture was investigated for three materials, namely soda lime silica glass (amorphous, isotropic material), silicon nitride (polycrystalline material) and mica glass ceramic (R-curve material). Crack geometry effects on mixed-mode fracture in soda lime silica glass disks in diametral compression were first studied. The results indicate that the chevron notch type cracks are more sensitive to mode II loading than surface cracks. Mixed-mode fracture studies on silicon nitride disks showed that silicon nitride had greater fracture resistance in comparison to soda lime silica glass even under mixed-mode loading conditions. Mixed-mode fracture in mica glass ceramic, an R-curve material, from indent cracks in flexure showed a rising crack growth resistance behavior. These results indicate that microstructure and crack geometry influences mixed-mode fracture in ceramics.;The existing mixed-mode fracture theories based on the singular stress terms could not adequately explain the mixed-mode fracture of the disks in diametral compression. The conventional minimum strain energy density was modified to incorporate the non singular or T-stress terms. The results from the modified theory could explain the experimental results. The values of the T-stress terms were found to be geometry dependent.;The mixed-mode fracture surfaces were characterized by an absence of the mist region and the presence of distinct hackle markings, termed lances. Quantitative fractography studies showed that fracture mechanics and fractography principles remain the same in both pure mode I and mixed-mode loading conditions. The major practical implication of this work is that fracture stress can be evaluated from the fractographic measurement of the branching radius without any prior knowledge of the loading conditions. (Full text of this dissertation may be available via the University of Florida Libraries web site. Please check http://www.uflib.ufl.edu/etd.html). |
