MODEL FOR TOUGHNESS CURVES IN 2-PHASE CERAMICS .2. MICROSTRUCTURAL VARIABLES

The fracture mechanics analysis of Part I is here extended to consider the effects of volume fraction and scale of second-phase particles on the toughness-curve properties of ceramic-matrix composites. Increasing these variables enhances the flaw tolerance of the material, but only up to certain limits, beyond which bulk microcracking occurs. These limits define domains of damage accumulation and potential strength degradation by microcrack coalescence. In the familiar approximation of elliptical crack-wall profiles, we show that the principal effects of increasing volume fraction (or expansion mismatch) and particle size is to enhance the slope and scale of the T-curve, respectively. We also derive expressions for the microcracking limits and use these expressions to construct a simple design diagram for characterizing the effects of microstructural variation on mechanical behavior. Indentation-strength data on Al2O3/Al2TiO5 composites over a range of volume fractions and particles sizes are used to demonstrate the severe loss in mechanical integrity that can be suffered on entering the microcracking domains.