95-22134 Ritchie The proposed project is focused on developing a detailed understanding and quantitative models of the physics and fundamental mechanisms of cyclic fatigue damage and crack advance in monolithic ceramics at elevated temperatures. The central aim of the present study is to characterize and model the cyclic fatigue behavior of ceramics at high homologous temperatures to examine the comparable damage and crack advance mechanisms in both air and vacuum environments. The approach will utilize ultrahigh temperatures (up to 1800C) creep and fatigue -crack growth testing facilities to examine subcritical cracking in a range of microstructures in monolithic alumina, silicon nitride and silicon carbide ceramics, chosen to vary grain size and morphology, the crystallinity of the grain-boundary phase, etc., together with detailed microstructural characterization (using both in situ imaging and pre- and post-failure electron microscopy) of the cycle-dependent degradation of shielding mechanisms behind the crack tip and the accumulated microstructural damage by intrinsic creep and fatigue mechanisms ahead of the tip. %%% With the scientific understanding gained, this work is intended to provide guidelines for the design of improved microstructures and compositions for flaw-tolerant ceramics with optimal combinations of low-temperature toughness and elevated-temperature resistance to subcritical fatigue-crack growth. ***
