Effect of Stresses from Thermal Expansion Anisotropy, Phase Transformations, and Second Phases on the Strength of Ceramics

Abstract

Critical flaws were determined by fractography in several ceramic bodies having internal stresses due to thermal expansion anisotropy, phase transformations, or second phases. Fracture energies calculated using the observed flaw sizes and shapes agreed well with those measured by fracture‐mechanics techniques at large flaw sizes. However, at smaller flaw sizes, calculated fracture energies decreased with decreasing flaw size. This decrease is attributed to internal stresses which increasingly contribute to failure as flaw sizes approach the sizes dominated by tensile components of these internal stresses. These components, in turn, are due to statistical variations of grain orientation. The projected contribution of internal stresses to failure is consistent with simple estimates of their levels as the flaw size decreases to about the grain size. Thus, application of fracture mechanics must take into account an increasing contribution of internal stresses as flaw sizes decrease in bodies in which these stresses can exist.