Micromechanical Stress Concentrations in Two-Phase Brittle-Matrix Ceramic Composites

A quantitative investigation was conducted on the effect of micromechanical stress concentrations on the strength of two-phase brittle-matrix ceramic systems. The materials consisted of a continuous brittle matrix containing dispersions with elastic properties different from those of the matrix. A soda borosilicate glass was used as the matrix and the dispersions consisted of spherical alumina particles 60μ in diameter and spherical pores 60μ in diameter. Stress concentrations were varied by measuring the strength of the composite under uniaxial and biaxial tensile stress conditions. The experimental results showed that micromechanical stress concentrations strongly affect the macroscopic strength of the composite. Under biaxial tensile stress, additions of either alumina microspheres or spherical porosity to the glass matrix resulted in a decrease in strength equal to the maximum calculated stress concentration factor. Under uniaxial tensile stress conditions, however, the reduction in strength for the glass-alumina system was negligible. The glass-porosity system gave a reduction in uniaxial strength which was not equal to the maximum calculated stress concentration factor. Experimental results suggest that differences in strength of brittle multi-component systems under uniaxial and biaxial stress states can be attributed in part to micro-structural features. On the basis of the experimental work, a hypothesis is developed relating the relative size of the region in the glass matrix over which stress concentrations act to the size of the Griffith flaws responsible for failure. This hypothesis is extended to the effect of porosity on the strength of polycrystalline brittle ceramic materials.