Minimum Solid Area Research Articles

Comparison of stress concentration versus minimum solid area based mechanical property-porosity relations

Stress concentrations due to pore shape are questioned as a fundamental determinant of mechanical property-porosity relations, especially elastic property porosity relations. On the other hand, actual solid load-bearing areas, especially minimum solid areas of porous bodies, clearly are a determinant of mechanical property-porosity effects. The correlation of pore shape-stress concentration effects with elastic properties of ceramics can be explained by the correlation of pore shapes with minimum solid areas.

The Porosity Dependence of Mechanical and other Properties of Materials

AbstractThough the volume fraction (or %) porosity is commonly used as the exclusive porosity variable to describe the porosity dependence of physical properties, it is not sufficient. The average minimum solid area, i.e., of the bond or sintered area between sintering particles, or the minimum web or strut cross-sections is the most appropriate second porosity parameter. Pore shape-stress concentration effects have, at best, limited direct effect on mechanical properties, but generally correlate with minimum solid area effects. Methods of combining effects of different types of porosity within the same body are important and result in quite reasonable descriptions of mechanical properties across a broad range of porosities.

Micromechanisms of crack extension in ceramics

The micromechanisms of crack extension at room temperature in ceramic single crystals are compared with those in bicrystals and in polycrystals. The toughness of polycrystals is found to depend upon their macroscopic ductility, their single crystal cleavage strength, and their intergranular cohesion. For some cubic materials plastic deformation may contribute to the measured toughness, whereas for brittle anisotropic ceramics over a wide range of grain sizes, the intergranular cohesion determines the toughness. The roles of thermal expansion anisotropy, of grain size and shape, and of porosity are considered in some detail, particularly with regard to their effect on lowering the intergranular cohesion. Porosity is of primary importance in determining toughness since the crack seeks the path of minimum solid area, but it can also toughen since it lowers the elastic modulus and increases the compliance. In the same way microcracking lowers the elastic modulus and at the same time increases the absorption of energy by increasing the total area of fracture surface. A process zone of microcracking enables slow crack growth to be stabilized and this is believed to be responsible for the increased toughness found in Al2O3 and Si3N4 to which critical volume fractions of ZrO2 have been added. A further mechanism of toughening occurs in certain partially stabilized zirconias, in which metastable coherent precipitates of tetragonal ZrO2 transform in the stress field of the crack, absorbing energy as they do and giving rise to compressive stress fields which further inhibit the crack. So far it has proved possible to increase the fracture surface energy of carefully prepared and treated alloys by a factor of about five.