POROSITY EFFECTS ON MACHINING DIRECTION STRENGTH ANISOTROPY AND FAILURE MECHANISMS

The anisotropy in room-temperature flexural strength of ceramics as a result of machining test bar tensile surfaces parallel vs perpendicular to the bar axis was studied for various porous bodies. This shows that fine, relatively homogeneous porosity has no significant effect on such strength anisotropy, implying that such porosity has no significant effect on flaw sizes or shapes, which was also shown by fractography. However, as the size of pores or pore clusters (due to pore heterogeneity) increases, the strength anisotropy diminishes, becoming zero when the pores or pore clusters dominate failure. Logarithm of strength vs porosity (P) plots for the two machining directions followed nearly parallel lines for fine, homogeneous porosity, but have less separation and intersect at lower porosity as pore size or heterogeneity increases. Fracture toughnesses calculated from fractography data for Al2O3 and B of varying porosity levels could be normalized to values at P approximately 0. Thus, extrapolation of strengths to P = 0 is clearly justified for bodies with fine, homogeneous porosity, but may be uncertain in bodies with coarser, or heterogeneous porosity.