| The behavior of grain bridges in aluminum oxide under a few cycles of tension-tension fatigue was studied in this dissertation research. Experiments at room temperature and 800°C established the existence of a residual crack opening displacement (COD) due to grain pull-out. In addition, at room temperature a “pivot” effect occurred during unloading resulting in the fracture of intact elastic grain bridges. Tests at 800°C indicated that the “pivot” effect did not occur at high temperatures. However, the basic grain bridge mechanics were unchanged at high temperature with the partial relaxation of residual thermal strains resulting in a lower crack closure stress (CCS) at the peak load. A preliminary finite element analysis of low cycle fatigue experiments successfully modeled the micro-mechanical behavior of frictional bridges. The evolving finite element model showed that the release of stored elastic distortional energy upon grain pull-out resulted in an unrecovered COD. The “second generation” version of the model was used in conjunction with an iterative process to determine the number of intact elastic bridges and geometrically interlocking grains. This “second generation” model showed the presence of “butting” from distorted, partially and fully pulled-out grains responsible for the residual COD at unloading. Due to the residual COD, wear from frictional grain sliding is limited at a load ratios >0. Wear from contact at “butting” grains remains a possibility as a fatigue degradation mechanism at load ratios <0. The results of the micro-mechanical models also provided further directions for new material research, especially the creation of “tailored” materials and components. |
