Micromechanisms of cyclic and environmentally-assisted subcritical crack growth in ceramic-matrix composites. Final report, 1 Oct 88-30 Sep 91

The future use of ceramics for advanced structural applications represents an important precursor to potential major improvements in design performance for high temperature, corrosion and wear resistance applications. The limited use of such brittle materials to date has primarily been attributed to their inherently low toughness and lack of defect tolerance. Scientific research in the last decade, however, has resulted in major advances in the toughening of ceramics by such mechanisms as whisker reinforcement, transformation and microcrack toughening. However, very recent work has shown that such toughened ceramics and composites, contrary to conventional wisdom, may become susceptible to cyclic fatigue. The intent of this program was to study and model the physics and fundamental micromechanisms of environmentally-assisted and principally cyclic fatigue crack-growth processes in several classes of ceramic-matrix composites, chosen to reflect different primary toughening mechanisms. Based on initial studies, the central hypothesis of this work is that the mechanisms of crack-tip shielding, used to enhance fracture toughness, can lead to degradation in crack-growth resistance under cyclic loading.