Microstructural Control of Indentation Crack Extension under Externally Applied Stress

Abstract

In many ceramic and glass materials indentation half-penny cracks extend stably under applied stress to over twice their original length before instability—consistent with an ideal identation response. Observations of such extension can be made with custom-designed bending fixtures, allowing crack extension in indented specimens to be observed with an optical microscope during controlled loading. The resulting applied stress vs crack length traces exhibit a concave-down shape, with decreasing derivative prior to instability at which the specimen breaks and the derivative vanishes. Many materials, however, do not exhibit this behavior—a consequence of the influences of reactive environments, lateral cracking, microstructural toughening, surface stresses, phase-transformation trapping and preferred crystallographic cleavage planes. These influences lead to subtantially suppressed or enhanced stable crack extension, asymmetric extension and stick-slip extension. After a brief development of the indentation fracture model, in-situ observations of indentation crack extension in a set of cordierite glass-ceramic composites and a polycrystalline alumina are presented and examined in terms of variability of microstructural interactions giving rise to toughening.